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Membrane Pore Formation: Reviews
Sun, P. B. and B. Sun (2003). "Effect of compatibility of PVC/P2 alloy system on membrane structure and performance." Ann N Y Acad Sci 984: 267-78.
The effects of the secondary polymer component (P(2)) on poly(vinyl chloride) (PVC)/P(2) alloy membrane structure and performance were systematically investigated. A series of P(2) with varying compatibility with PVC used in this study included vinyl chloride-vinyl acetate copolymer (VC-co-VAc); copolymer of vinyl chloride, vinyl acetate-maleic anhydride copolymer (VC-co-VAc-co-MAL); isobutylene-maleic anhydride copolymer (IB-co-MAL); poly(methyl methacrylate) (PMMA); poly(vinylidene dichloride) (PVDC); and styrene-acrylonitrile copolymer (SAN). Alloy membranes were prepared by means of solution blending-phase inversion technique. On the basis of our experimental results, the compatibility of PVC/P(2) was proved to be the most critical factor affecting the alloy membrane structure and performance. Systems with good compatibility, such as PVC/VC-co-VAc, are more suitable for preparing membranes with small pore size; whereas systems with partial compatibility, such as PVC/PMMA, are more favored for the formation of large-pore membranes.
Ng, A. W., K. M. Wasan, et al. (2003). "Development of liposomal polyene antibiotics: an historical perspective." J Pharm Pharm Sci 6(1): 67-83.
PURPOSE: The purpose of this review article is to review the development of a number of liposomal polyene antibiotics. BACKGROUND: In the past thirty years, the increase in life-threatening pre-systemic and systemic fungal infections within cancer, diabetic and AIDS patients have reached alarming proportions. A number of antifungal agents have been developed to combat this problem. In particular, polyene antibiotics such as Amphotericin B (AmB) and Nystatin (Nys) have remained the most effective and widely used agents in the treatment of these infections. However, their administration is limited by dose-dependent toxicities. One such dose-limiting toxicity is renal toxicity. Polyene antibiotic-induced renal toxicity is believed to be mediated by the drug anchoring to cholesterol within the mammalian cell membrane, resulting in pore formation, abnormal electrolyte flux, decrease in adenosine triphosphate (ATP), and eventually a loss of cell viability. CONCLUSION: In the 1980s and 90s a number of promising lipid-based AmB and Nys formulations were developed to overcome these toxicities. This article will review the development of these liposomal polyene antibiotics.
Mettenleiter, T. C. (2003). "Pathogenesis of neurotropic herpesviruses: role of viral glycoproteins in neuroinvasion and transneuronal spread." Virus Res 92(2): 197-206.
Neuroinvasion by herpesviruses requires entry into nerve endings in the periphery, transport to the cell body, replication in the cell body, axonal transport to the synapse and transneuronal viral spread. Entry occurs after receptor binding by fusion of virion envelope and cellular plasma membrane followed by microtubuli-assisted transport of capsids to the nuclear pore. By transneuronal spread, the virus gains access to synaptically linked neuronal circuits. A common set of herpesvirus glycoproteins is involved in entry and direct viral cell-cell spread. However, both processes can be distinguished by involvement of additional viral components. Interestingly, transneuronal spread appears to be functionally linked to intracytoplasmic formation of mature virions. This review will focus on the importance of herpesvirus envelope glycoproteins for infection of neurons and transneuronal spread, and their influence on viral pathogenesis.
Cole, D. S. and B. P. Morgan (2003). "Beyond lysis: how complement influences cell fate." Clin Sci (Lond) 104(5): 455-66.
Complement is a central component of the innate immune system involved in protection against pathogens. For many years, complement has been known to cause death of targets, either indirectly by attracting and activating phagocytes or directly by formation of a membrane pore, the membrane attack complex. More recently, it has been recognized that complement may cause other 'non-classical' effects that may not directly be aimed at killing of pathogens. Products of complement activation collaborate with the adaptive immune system to enhance responses to antigens. The membrane attack complex of complement, apart from lysing cells, can also trigger diverse events in target cells that include cell activation, proliferation, resistance to subsequent complement attack and either resistance to, or induction of, apoptosis. Various complement products play important roles in signalling for clearance by phagocytes of apoptotic self cells. Here we review some of these non-classical activities of complement and stress the roles that they may play in maintaining the integrity of the organism.
Alouf, J. E. (2003). "Molecular features of the cytolytic pore-forming bacterial protein toxins." Folia Microbiol (Praha) 48(1): 5-16.
The repertoire of the cytolytic pore-forming protein toxins (PFT) comprises 81 identified members. The essential feature of these cytolysins is their capacity to provoke the formation of hydrophilic pores in the cytoplasmic membranes of target eukaryotic cells. This process results from the binding of the proteins on the cell surface, followed by their oligomerization which leads to the insertion of the oligomers into the membrane and formation of protein-lined channels. It impairs the osmotic balance of the cell and causes cytolysis. In this review the molecular aspects of a number of important PFT and their respective encoding structural genes will be briefly described.
Zakharov, S. D. and W. A. Cramer (2002). "Insertion intermediates of pore-forming colicins in membrane two-dimensional space." Biochimie 84(5-6): 465-75.
The formation of integral membrane voltage-gated ion channels by the initially soluble C-terminal channel polypeptide (CP) of the pore-forming colicins is a fruitful area for studies on membrane protein import. The dependence of CP import on specific membrane parameters can be better understood using liposomes and planar membranes of defined lipid composition. The membrane surface and interfacial layer provide special conditions for the transition of a pore-forming colicin from the soluble to the integral membrane state. The colicin E1 CP is arranged in the membrane interfacial layer as a conformationally mobile helical array that is extended far more in the two dimensions parallel to the membrane surface than in the third dimension perpendicular to it. The alpha-helical content of CP(E1) increases by approximately 30% upon binding to the membrane. The sequence of kinetically distinguishable events in the CP(E1)-membrane interaction is binding, unfolding to a subtended area of 4200 A(2), helix extension, and insertion, the last three events overlapping in their time course ( approximately 10 s(-1)). The extension into two dimensions and the interaction with the membrane surface may explain the reversible denaturation and refolding of secondary structure that occurs after boiling of the CP-membrane complex. Although DSC showed the presence of helix-helix interactions in the membrane-bound state, the change in secondary structure and the extended surface area argue against a molten-globule intermediate in the CP-membrane interaction. However, the surface-bound state is mobile, as surface conformational mobility is a necessary prerequisite for insertion of CP trans-membrane helices into the bilayer. The requirement for this surface protein mobility, described by "thermal melting" FRET experiments, may provide the explanation for the precipitous decrease in the voltage-gated CP channel formation at high values of surface potential of planar bilayer membranes. Thus, the membrane interfacial layer, with the CP backbone situated near the acyl chain carbonyls, provides a favorable environment for the structure changes necessary for the transition from the soluble to the membrane-inserted state.
Weiger, T. M., A. Hermann, et al. (2002). "Modulation of calcium-activated potassium channels." J Comp Physiol A Neuroethol Sens Neural Behav Physiol 188(2): 79-87.
Potassium currents play a critical role in action potential repolarization, setting of the resting membrane potential, control of neuronal firing rates, and regulation of neurotransmitter release. The diversity of the potassium channels that generate these currents is nothing less than staggering. This diversity is generated by multiple genes (as many as 100 and perhaps more in some creatures) encoding the pore-forming channel alpha subunits, alternative splicing of channel gene transcripts, formation of heteromultimeric channels, participation of auxiliary (non-pore-forming) beta and other subunits, and modulation of channel properties by post-translational modifications and other mechanisms. Prominent among the potassium channels are several families of calcium activated potassium channels, which are highly selective for potassium ions as their charge carrier, and require intracellular calcium for channel gating. The modulation of one of these families, that of the large conductance calcium activated and voltage-dependent potassium channels, has been especially widely studied. In this review we discuss a few selected examples of the modulation of these channels, to illustrate some of the molecular mechanisms and physiological consequences of ion channel modulation.
Waterhouse, N. J., J. E. Ricci, et al. (2002). "And all of a sudden it's over: mitochondrial outer-membrane permeabilization in apoptosis." Biochimie 84(2-3): 113-21.
Identification of pro-apoptotic activities for a variety of proteins normally resident in the mitochondrial inter-membrane space has substantiated the role of mitochondria as integral to the apoptotic process. Cytochrome c is involved in apoptosome formation and caspase activation, SMAC/Diablo deregulates the inhibitor of apoptosis proteins, apoptosis-inducing factor may play a role in chromatin condensation and release of other proteins such as adenylate kinase may adversely affect cellular metabolism and contribute to the death of a cell if the downstream apoptotic pathway is blocked. It is still unclear how these proteins are released from the mitochondria. Recent advances in our knowledge of mitochondrial outer-membrane permeabilization and the consequences of this event on mitochondria will be discussed.
Russo, L. M., G. L. Bakris, et al. (2002). "Renal handling of albumin: a critical review of basic concepts and perspective." Am J Kidney Dis 39(5): 899-919.
Biochemical and physiological processes that underlie the mechanism of albuminuria are completely reassessed in this article in view of recent discoveries that filtered proteins undergo rapid degradation during renal passage and the resulting excreted peptide fragments are not detected by conventional urine protein assays. This means that filtered protein and/or albumin levels in urine have been seriously underestimated. The concept that albuminuria is a result of changes in glomerular permeability is questioned in light of these findings and also in terms of a critical examination of charge selectivity, shunts, or large-pore formation and hemodynamic effects. The glomerulus appears to function merely in terms of size selectivity alone, and for albumin, this does not change significantly in disease states. Intensive albumin processing by a living kidney occurs through cellular processes distal to the glomerular basement membrane. Failure of this cellular processing primarily leads to albuminuria. This review brings together recent data about urinary albumin clearance and current knowledge of receptors known to process albumin in both health and disease states. We conclude with a discussion of topical and controversial issues associated with the proposed new understanding of renal handling of albumin.
Raulin, J. (2002). "Human immunodeficiency virus and host cell lipids. Interesting pathways in research for a new HIV therapy." Prog Lipid Res 41(1): 27-65.
It has been reported in the literature that biological membranes arising from HIV-induced cell fusion, as well as syncytium formation between infected and non-infected cells and those involved in transduction, viral DNA nuclear import and virion budding from the host cell, are all made of proteins, a phospholipid (P) bilayer and cholesterol (C). However, the P/C molar ratio is higher in the retroviral envelope than in the plasma membrane where they originate, and higher than in the nuclear envelope. Mechanisms are described which elucidate this puzzling fact, as well as cholesterol-dependent leakage and pore formation during cell fusion. Fatty acylation of viral and host cell proteins is required to direct them to membranes. Detergent-insoluble microdomains enriched in cholesterol and sphingolipids, termed either DIGs (detergent-insoluble glycolipid-enriched complexes), DRMs (detergent resistant membranes), TIFFs (Triton-insoluble floating fractions) or GEMs (glycolipid-enriched membranes), function as platforms for attachment of proteins in the process of signal transduction. HIV-SUgp120 (HIV-surface glycoprotein), T-cell receptor (TCR)-CD4+ and co-receptors promote aggregation of these lipid "rafts" which concentrate the Src family tyrosine kinases SFKs (PTK, Lyn, Fyn, Lck), GPI (glycosyl phosphatidylinositol)-anchored proteins, and phosphatidylinositol kinases PI(3)K and PI(4)K, inducing cell signalling. HIV-SUgp120 transduces the activation signal and provokes the formation of polyunsaturated fatty acid (PUFA) metabolites, i.e. the prostaglandin PGE2 suppressor of immune function and inhibitor of cytotoxic T-lymphocyte (CTL) proliferation, while PGB2 activates SFKs and increases mRNA expression, as well as NFkappaB (nuclear transcription factor) translocation to nucleus. HIV nuclear import, DNA integration, chromatin template capacity may be mediated by the lipid environment. The lipid-enriched microdomains from which HIV-1 buds, may explain the high level of cholesterol and sphingolipids in the viral envelope, since host cell rafts become a viral coat. HIV-1 infection induces alteration of cellular lipids: (1) shift in phospholipid synthesis to neutral lipids associated with the viral load, polyunsaturated fatty acid (PUFA) peroxidation, and n-3 deficiency with deregulation of cytokines and PPAR-gamma (peroxisome proliferator-activated receptor-gamma), and (2) alloimmune phospholipid antibody production in which antibodies to cardiolipin and to phosphatidylserine are most prevalent, due to the destruction of mitochondrial membranes and progression of lymphocyte apoptosis. The current highly active anti-retroviral therapy, including both viral reverse transcriptase (RT) inhibitors (NRTIs and NNRTIs, nucleoside and non-nucleoside RT inhibitors) and protease inhibitors (PIs), induces side-effects in the long term. Lipodystrophy (LD), consists of peripheral lipoatrophy associated with central fat accumulation (called "crixbelly" and "buffalo hump"), insulin resistance, elevation of very low density lipoproteins, decrease in high density lipoproteins and inhibition of adipocyte differentiation. LD syndrome appears to be induced by PIs that inhibit GLUT4, glucose transporter isoform, and by NRTIs which provoke mitochondrial failure. New therapeutic strategies assessed: (1) inhibition of the viral integrase and/or HIV entry into cells through natural products or their derivatives, (2) inhibition of HIV-1 entry into macrophages pretreated with Gram-negative bacterial lipopolysaccharide, (3) vaccination with multi-lipopeptides, i.e. sequences of HIV-1 peptides with CD4+ T-cell and B-cell epitopes, modified by adding a lipid tail to one end, which produce HIV-specific CTL and multispecific immune responses in most of the vaccinated subjects and (4) stimulation of antiviral drug activity with lipid-prodrugs targeting viral RT, polymerase, integrase, or aspartyl-protease.
Radi, R., A. Cassina, et al. (2002). "Peroxynitrite reactions and formation in mitochondria." Free Radic Biol Med 33(11): 1451-64.
Mitochondria constitute a primary locus for the intracellular formation and reactions of peroxynitrite, and these interactions are recognized to contribute to the biological and pathological effects of both nitric oxide ((*)NO) and peroxynitrite. Extra- or intramitochondrially formed peroxynitrite can diffuse through mitochondrial compartments and undergo fast direct and free radical-dependent target molecule reactions. These processes result in oxidation, nitration, and nitrosation of critical components in the matrix, inner and outer membrane, and intermembrane space. Mitochondrial scavenging and repair systems for peroxynitrite-dependent oxidative modifications operate but they can be overwhelmed under enhanced cellular (*)NO formation as well as under conditions that lead to augmented superoxide formation by the electron transport chain. Peroxynitrite can lead to alterations in mitochondrial energy and calcium homeostasis and promote the opening of the permeability transition pore. The effects of peroxynitrite in mitochondrial physiology can be largely rationalized based on the reactivities of peroxynitrite and peroxynitrite-derived carbonate, nitrogen dioxide, and hydroxyl radicals with critical protein amino acids and transition metal centers of key mitochondrial proteins. In this review we analyze (i) the existing evidence for the intramitochondrial formation and reactions of peroxynitrite, (ii) the key reactions and fate of peroxynitrite in mitochondria, and (iii) their impact in mitochondrial physiology and signaling of cell death.
O'Sullivan, L., R. P. Ross, et al. (2002). "Potential of bacteriocin-producing lactic acid bacteria for improvements in food safety and quality." Biochimie 84(5-6): 593-604.
Lactic acid bacteria (LAB) have been used for centuries in the fermentation of a variety of dairy products. The preservative ability of LAB in foods is attributed to the production of anti-microbial metabolites including organic acids and bacteriocins. Bacteriocins generally exert their anti-microbial action by interfering with the cell wall or the membrane of target organisms, either by inhibiting cell wall biosynthesis or causing pore formation, subsequently resulting in death. The incorporation of bacteriocins as a biopreservative ingredient into model food systems has been studied extensively and has been shown to be effective in the control of pathogenic and spoilage microorganisms. However, a more practical and economic option of incorporating bacteriocins into foods can be the direct addition of bacteriocin-producing cultures into food. This paper presents an overview of the potential for using bacteriocin-producing LAB in foods for the improvement of the safety and quality of the final product. It describes the different genera of LAB with potential as biopreservatives, and presents an up-to-date classification system for the bacteriocins they produce. While the problems associated with the use of some bacteriocin-producing cultures in certain foods are elucidated, so also are the situations in which incorporation of the bacteriocin-producer into model food systems have been shown to be very effective.
Michel-Briand, Y. and C. Baysse (2002). "The pyocins of Pseudomonas aeruginosa." Biochimie 84(5-6): 499-510.
Pyocins are produced by more than 90% of Pseudomonas aeruginosa strains and each strain may synthesise several pyocins. The pyocin genes are located on the P. aeruginosa chromosome and their activities are inducible by mutagenic agents such as mitomycin C. Three types of pyocins are described. (i). R-type pyocins resemble non-flexible and contractile tails of bacteriophages. They provoke a depolarisation of the cytoplasmic membrane in relation with pore formation. (ii). F-type pyocins also resemble phage tails, but with a flexible and non-contractile rod-like structure. (iii). S-type pyocins are colicin-like, protease-sensitive proteins. They are constituted of two components. The large component carries the killing activity (DNase activity for pyocins S1, S2, S3, AP41; tRNase for pyocin S4; channel-forming activity for pyocin S5). It interacts with the small component (immunity protein). The synthesis of pyocins starts when a mutagen increases the expression of the recA gene and activates the RecA protein, which cleaves the repressor PrtR, liberating the expression of the protein activator gene prtN. R and F-pyocins are derived from an ancestral gene, with similarities to the P2 phage family and the lambda phage family, respectively. The killing domains of S1, S2, AP41 pyocins show a close evolutionary relationship with E2 group colicins, S4 pyocin with colicin E5, and S5 pyocin with colicins Ia, and Ib.
Kuvichkin, V. V. (2002). "DNA-lipid interactions in vitro and in vivo." Bioelectrochemistry 58(1): 3-12.
The data on lipid-nucleic interactions and their role in vitro and in vivo are presented. The results of study of DNA-lipid complexes in absence and in presence of divalent metal cations (triple complexes) are discussed. The triple complexes represent the generation of cellular structures such as pore complexes of eucaryotes and "Bayer's junctions" of procaryotes. The participation of triple complexes in the formation of structure of bacterial and eucaryotic nucleoid and nuclear matrix is analysed. A model of formation of triple complexes and cellular structures and their role in DNA-lipid interactions are discussed.
Hoffmann, A., U. Pag, et al. (2002). "Combination of antibiotic mechanisms in lantibiotics." Farmaco 57(8): 685-91.
Recent studies on the mode of action have revealed exciting features of multiple activities of nisin and related lantibiotics making these peptides interesting model systems for the design of new antibiotics (Molec. Microbiol. 30 (1998) 317; Science 286 (1999) 2361; J. Biol. Chem. 276 (2001) 1772.). In contrast to other groups of antibiotic peptides, the lantibiotics display a substantial degree of specificity for particular components of bacterial membranes. Mersacidin and actagardine were shown to bind with high affinity to the lipid coupled peptidoglycan precursor, the so-called lipid II, which prevents the polymerisation of the cell wall monomers into a functional murein sacculus. The lantibiotics nisin and epidermin also bind tightly to this cell wall precursor; however, for these lantibiotics the binding of lipid II has two consequences. Like with mersacidin blocking of lipid II inhibits peptidoglycan biosynthesis; in addition, lipid II is used as a specific docking molecule for the formation of pores. This combination of lethal effects explains the potency of these peptides, which are active in nanomolar concentration. Other type-A lantibiotics are believed to also use docking molecules for pore formation, although identification of such membrane components has not yet been achieved.
Heuck, A. P. and A. E. Johnson (2002). "Pore-forming protein structure analysis in membranes using multiple independent fluorescence techniques." Cell Biochem Biophys 36(1): 89-101.
A large number of transmembrane proteins form aqueous pores or channels in the phospholipid bilayer, but the structural bases of pore formation and assembly have been determined experimentally for only a few of the proteins and protein complexes. The polypeptide segments that form the transmembrane pore and the secondary structure that creates the aqueous-lipid interface can be identified using multiple independent fluorescence techniques (MIFT). The information obtained from several different, but complementary, fluorescence analyses, including measurements of emission intensity, fluorescence lifetime, accessibility to aqueous and to lipophilic quenching agents, and fluorescence resonance energy transfer (FRET) can be combined to characterize the nature of the protein-membrane interaction directly and unambiguously. The assembly pathway can also be determined by measuring the kinetics of the spectral changes that occur upon pore formation. The MIFT approach therefore allows one to obtain structural information that cannot be obtained easily using alternative techniques such as crystallography. This review briefly outlines how MIFT can reveal the identity, location, conformation, and topography of the polypeptide sequences that interact with the membrane.
Hechard, Y. and H. G. Sahl (2002). "Mode of action of modified and unmodified bacteriocins from Gram-positive bacteria." Biochimie 84(5-6): 545-57.
The antibiotic activity of bacteriocins from Gram-positive bacteria, whether they are modified (class I bacteriocins, lantibiotics) or unmodified (class II), is based on interaction with the bacterial membrane. However, recent work has demonstrated that for many bacteriocins, generalised membrane disruption models as elaborated for amphiphilic peptides (e.g. tyriodal pore or carpet model) cannot adequately describe the bactericidal action. Rather, specific targets seem to be involved in pore formation and other activities. For the nisin and epidermin family of lantibiotics, the membrane-bound cell wall precursor lipid II has recently been identified as target. The duramycin family of lantibiotics binds specifically to phosphoethanolamine which results in inhibition of phospholipase A2 and various other cellular functions. Most of the class II bacteriocins dissipate the proton motive force (PMF) of the target cell, via pore formation. The subclass IIa bacteriocin activity likely depends on a mannose permease of the phosphotransferase system (PTS) as specific target. The subclass IIb bacteriocins (two-component) also induce dissipation of the PMF by forming cation- or anion-specific pores; specific targets have not yet been identified. Finally, the subclass IIc comprises miscellaneous peptides with various modes of action such as membrane permeabilization, specific inhibition of septum formation and pheromone activity.
Heales, S. J. and J. P. Bolanos (2002). "Impairment of brain mitochondrial function by reactive nitrogen species: the role of glutathione in dictating susceptibility." Neurochem Int 40(6): 469-74.
Mitochondrial enzymes involved in energy metabolism display varying degrees of sensitivity towards reactive nitrogen species such as peroxynitrite (ONOO-). With regards to the electron transport chain, cytochrome oxidase appears particularly sensitive. Inhibition of this component may lead to an increase in mitochondrial superoxide formation, exacerbation of cellular oxidative stress and further mitochondrial damage. Impairment of the electron transport chain may lead to a loss of membrane potential, ATP deficiency, opening of the permeability transition pore and the release of factors capable of initiating apoptosis. Reduced glutathione will react, via a number of diverse reactions, with reactive nitrogen species and hence is capable of limiting mitochondiral damage. Loss of brain glutathione may therefore be an important factor in those neurological conditions in which there is evidence of excessive nitric oxide formation and mitochondrial damage.
Gilbert, R. J. (2002). "Pore-forming toxins." Cell Mol Life Sci 59(5): 832-44.
Pore-forming toxins are widely distributed proteins which form lesions in biological membranes. In this review, bacterial pore-forming toxins are treated as a paradigm and discussed in terms of the structural principles on which they work. Then, a large family of bacterial toxins, the cholesterol-binding toxins, are analyzed in depth to provide an overview of the processes involved in pore formation. The ways in which the cholesterol-binding toxins (cholesterol-dependent cytolysins) interact with membranes and form pores, the structure of the monomeric soluble and oligomeric pore-forming states, and the effects of the toxin on membrane structure are discussed. By surveying the range of work which has been done on cholesterol-binding toxins, a working model is elaborated which reconciles two current, apparently diametrically opposed, models for their mechanism.
Duche, D. (2002). "The pore-forming domain of colicin A fused to a signal peptide: a tool for studying pore-formation and inhibition." Biochimie 84(5-6): 455-64.
Pore-forming colicins are plasmid-encoded bacteriocins that kill Escherichia coli and closely related bacteria. They bind to receptors in the outer membrane and are translocated across the cell envelope to the inner membrane where they form voltage-dependent ion-channels. Colicins are composed of three domains, with the C-terminal domain responsible for pore-formation. Isolated C-terminal pore-forming domains produced in the cytoplasm of E. coli are inactive due to the polarity of the transmembrane electrochemical potential, which is the opposite of that required. However, the pore-forming domain of colicin A (pfColA) fused to a prokaryotic signal peptide (sp-pfColA) is transported across and inserts into the inner membrane of E. coli from the periplasmic side, forming a functional channel. Sp-pfColA is specifically inhibited by the colicin A immunity protein (Cai). This construct has been used to investigate colicin A channel formation in vivo and to characterise the interaction of pfColA with Cai within the inner membrane. These points will be developed further in this review.
Deutsch, C. (2002). "Potassium channel ontogeny." Annu Rev Physiol 64: 19-46.
Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.
Asokan, A. and M. J. Cho (2002). "Exploitation of intracellular pH gradients in the cellular delivery of macromolecules." J Pharm Sci 91(4): 903-13.
Most cellular components such as the cytoplasm, endosomes, lysosomes, endoplasmic reticulum, Golgi bodies, mitochondria, and nuclei are known to maintain their own characteristic pH values. These pH values range from as low as 4.5 in the lysosome to about 8.0 in the mitochondria. Given these proton gradients around a neutral pH, weak acids, and bases with a pKa between 5.0 and 8.0 can exhibit dramatic changes in physicochemical properties. These compounds can be conjugated as such to macromolecules or incorporated into polymeric or liposomal formulations to promote the efficient cellular delivery of macromolecules. Mechanistically, the carrier molecules can facilitate favorable membrane partition, membrane fusion, transient pore formation, or membrane disruption. Drug carriers equipped with such pH-sensitive triggers and switches are able to significantly enhance the cellular delivery of macromolecules in vitro. However, the successful application of these molecules for efficient delivery in vivo requires the design of noncytotoxic, nonimmunogenic, serum compatible and biochemically labile carriers, systematic analysis of their mechanisms of action, and extensive animal studies.
Williams, A. J., D. J. West, et al. (2001). "Light at the end of the Ca(2+)-release channel tunnel: structures and mechanisms involved in ion translocation in ryanodine receptor channels." Q Rev Biophys 34(1): 61-104.
RyR and InsP3R are Ca(2+)-release channels. When induced to open by the appropriate stimulus, these channels allow Ca2+ to leave intracellular storage organelles at an astonishing rate. Investigations of the ion-handling properties of isolated RyR channels have demonstrated that, at least in comparison to voltage-gated channels of surface membranes, these channels display limited powers of discrimination between physiologically relevant cations and this relative lack of selectivity is likely to contribute to the ability of Ca(2+)-release channels to maintain high rates of cation translocation without compromising function. A range of ion-handling properties in RyR are consistent with the proposal that this channel functions as a single-ion channel and theoretical considerations indicate that the high rates of ion translocation monitored for RyR would require the pore of such a structure to be short and possess a large capture radius. Measurements of the dimensions of regions of RyR involved in ion conduction and discrimination indicate that this is likely to be the case. In each monomer of RyR/InsP3R, residues making up the last two trans-membrane spanning domains and a luminal loop linking these two helices contribute to the formation of the channel pore. The luminal loops of both RyR and InsP3R contain amino acid sequences similar to those known to form the selectivity filter of K+ channels. In addition the luminal loops of both Ca(2+)-release channels contain sequences that are likely to form helices that may be analogous to the pore helix visualised in KcsA. The correlation in structural elements of the luminal loops of RyR/InsP3R and KcsA has prompted us to speculate on the tertiary arrangement for this region of the Ca(2+)-release channels using the established structure of KcsA as a framework.
Tweten, R. K., M. W. Parker, et al. (2001). "The cholesterol-dependent cytolysins." Curr Top Microbiol Immunol 257: 15-33.
In view of the recent studies on the CDCs, a reasonable schematic of the stages leading to membrane insertion of the CDCs can be assembled. As shown in Fig. 3, we propose that the CDC first binds to the membrane as a monomer. These monomers then diffuse laterally on the membrane surface to encounter other monomers or incomplete oligomeric complexes. Presumably, once the requisite oligomer size is reached, the prepore complex is converted into the pore complex and a large membrane channel is formed. During the conversion of the prepore complex to the pore complex, we predict that the TMHs of the subunits in the prepore complex insert into the bilayer in a concerted fashion to form the large transmembrane beta-barrel, although this still remains to be confirmed experimentally. Many intriguing problems concerning the cytolytic mechanism of the CDCs remain unsolved. The nature of the initial interaction of the CDC monomer with the membrane is currently one of the most controversial questions concerning the CDC mechanism. Is cholesterol involved in this interaction, as previously assumed, or do specific receptors exist for these toxins that remain to be discovered? Also, the trigger for membrane insertion and the regions of these toxins that facilitate the [figure: see text] interaction of the monomers during prepore complex formation are unknown. In addition, the temporal sequence of the multiple structural changes that accompany the conversion of the soluble CDC monomer into a membrane-inserted oligomer have yet to be defined or characterized kinetically.
Shimizu, T. and H. Hayashi (2001). "[Molecular mechanism of membrane pore formation with cholesterol binding cytolysin: streptolysin O and perfringolysin O]." Tanpakushitsu Kakusan Koso 46(4 Suppl): 532-9.
Oheim, M. (2001). "Imaging transmitter release. I. Peeking at the steps preceding membrane fusion." Lasers Med Sci 16(3): 149-58.
Over the recent year we have witnessed considerable advances in the study of neurotransmitter release. This progress has been severalfold as different techniques have allowed us to characterise many steps along the process of exocytosis, membrane fusion, formation of the fusion pore, and have given insight in the kinetics of release and membrane re-uptake. Patch clamping provided quantitative measurements of the capacitance changes as the membrane of the secretory vesicle is added to the surface of the cell during secretion, and the change in the opposite direction when membrane is retrieved back into the cell during exocytosis. Carbon-fibre microelectrodes have measured electrochemically the release of oxidisable transmitters into the extracellular space. Differential interference contrast microscopy has given us spatially resolved images of the cell surface during exocytosis; real-time images that are suggestive of bubbles breaking the surface of a boiling pot of water. The interest in novel techniques stems from the fact that existing approaches can provide only indirect evidence on the steps preceding membrane fusion. The vesicular dynamics just beneath the plasma membrane are out of the reach of capacitance measurements or amperometric detection. What we have needed is a tool that would allow us to look just below the cell surface. This much-needed tool appears to be evanescent-wave microscopy. This review describes how laser microscopy can be used to study exocytosis at single-vesicle resolution. A companion paper deals with the practical aspects of evanescent-wave imaging.
McClane, B. A. (2001). "The complex interactions between Clostridium perfringens enterotoxin and epithelial tight junctions." Toxicon 39(11): 1781-91.
Clostridium perfringens enterotoxin (CPE) is responsible for the diarrheal symptoms of C. perfringens type A food poisoning and antibiotic-associated diarrhea. The CPE protein consists of a single 35 kDa polypeptide with a C-terminal receptor-binding region and an N-terminal toxicity domain. Under appropriate conditions, CPE can interact with structural components of the epithelial tight junctions, including certain claudins and occludin. Those interactions can affect tight junction structure and function, thereby altering paracellular permeability and (possibly) contributing to CPE-induced diarrhea. However, the tight junction effects of CPE require cellular damage as a prerequisite. CPE induces cellular damage via its cytotoxic activity, which results from plasma membrane permeability alterations caused by formation of a approximately 155 kDa CPE-containing complex that may correspond to a pore. Thus, CPE appears to be a bifunctional toxin that first induces plasma membrane permeability alterations; using the resultant cell damage, CPE then gains access to tight junction proteins and affects tight junction structure and function.
Haller, T., K. Pfaller, et al. (2001). "The conception of fusion pores as rate-limiting structures for surfactant secretion." Comp Biochem Physiol A Mol Integr Physiol 129(1): 227-31.
It is well established that the release of surfactant phospholipids into the alveolar lumen proceeds by the exocytosis of lamellar bodies (LBs), the characteristic storage organelles of surfactant in alveolar type II cells. Consequently, the fusion of LBs with the plasma membrane and the formation of exocytotic fusion pores are key steps linking cellular synthesis of surfactant with its delivery into the alveolar space. Considering the unique structural organization of LBs or LB-associated aggregates which are found in lung lavages, and the roughly 1-microm-sized dimensions of these particles, we speculated whether the fusion pore diameter of fused LBs might be a specific hindrance for surfactant secretion, delaying or even impeding full release. In this mini-review, we have compiled published data shedding light on a possibly important role of fusion pores during the secretory process in alveolar type II cells.
Duclohier, H. and H. Wroblewski (2001). "Voltage-dependent pore formation and antimicrobial activity by alamethicin and analogues." J Membr Biol 184(1): 1-12.
Dietrich, P., D. Sanders, et al. (2001). "The role of ion channels in light-dependent stomatal opening." J Exp Bot 52(363): 1959-67.
Stomatal opening represents a major determinant of plant productivity and stress management. Because plants lose water essentially through open stomata, volume control of the pore-forming guard cells represents a key step in the regulation of plant water status. These sensory cells are able to integrate various signals such as light, auxin, abscisic acid, and CO(2). Following signal perception, changes in membrane potential and activity of ion transporters finally lead to the accumulation of potassium salts and turgor pressure formation. This review analyses recent progress in molecular aspects of ion channel regulation and suggests how these developments impact on our understanding of light- and auxin-dependent stomatal action.
Bratton, S. B. and G. M. Cohen (2001). "Apoptotic death sensor: an organelle's alter ego?" Trends Pharmacol Sci 22(6): 306-15.
Caspases are intracellular cysteine proteases that are primarily responsible for the stereotypic morphological and biochemical changes that are associated with apoptosis. Caspases are often activated by the apoptotic protease-activating factor 1 (APAF-1) apoptosome, a complex that is formed following mitochondrial release of cytochrome c in response to many death-inducing stimuli. Both pro- and anti-apoptotic BCL-2 family members regulate apoptosis, primarily by their effects on mitochondria, whereas many inhibitor of apoptosis proteins (IAPs) regulate apoptosis by directly inhibiting distinct caspases. Exposure of cells to chemicals and radiation, as well as loss of trophic stimuli, perturb cellular homeostasis and, depending on the type of cellular stress, particular or multiple organelles appear to 'sense' the damage and signal the cell to undergo apoptosis by stimulating the formation of unique and/or common caspase-activating complexes.
Arechaga, I., A. Ledesma, et al. (2001). "The mitochondrial uncoupling protein UCP1: a gated pore." IUBMB Life 52(3-5): 165-73.
The uncoupling protein UCP1 is a member of a superfamily of homologous proteins formed by the mitochondrial metabolite transporters. Although they act in vivo as carriers, under specific experimental conditions some of these transporters have been shown to behave as channels. This dual transport operation suggests that these carriers are likely to be formed by two differentiated functional and structural domains. The kinetic model termed "single binding center gated pore" is well suited to understand the behaviour of these carriers. It proposes that in the protein core there must exist a hydrophilic translocation pore whose access is controlled by gates. It is highly likely that the hydrophilic channel is formed by the transmembrane alpha-helices and that loops contribute to the formation of the gates. UCP1 is regulated physiologically by fatty acids and purine nucleotides. Nucleotides maintain the proton conductance inhibited while fatty acids act as cytosolic second messengers of noradrenaline to active UCP1. Based on photoaffinity labeling and mutagenesis data, we propose a structural model for the localization of the binding site. The nucleotide enters through a gate in the cytosolic side and binds deep inside the protein. The three matrix loops contribute to the formation of a hydrophobic binding pocket that would accommodate the purine moiety. Three arginine residues (in helices II, IV, and VI) would interact with the phosphate groups. His214 and Glu190 have been involved in the pH regulation of the nucleotide binding but because they are on the cytosolic side of the protein, we propose that their state of protonation will determine the access of the nucleotide to the binding center.
Vieira, H. L., D. Haouzi, et al. (2000). "Permeabilization of the mitochondrial inner membrane during apoptosis: impact of the adenine nucleotide translocator." Cell Death Differ 7(12): 1146-54.
Mitochondrial membrane permeabilization can be a rate limiting step of apoptotic as well as necrotic cell death. Permeabilization of the outer mitochondrial membrane (OM) and/or inner membrane (IM) is, at least in part, mediated by the permeability transition pore complex (PTPC). The PTPC is formed in the IM/OM contact site and contains the two most abundant IM and OM proteins, adenine nucleotide translocator (ANT, in the IM) and voltage-dependent anion channel (VDAC, in the OM), the matrix protein cyclophilin D, which can interact with ANT, as well as apoptosis-regulatory proteins from the Bax/Bcl-2 family. Here we discuss that ANT has two opposite functions. On the one hand, ANT is a vital, specific antiporter which accounts for the exchange of ATP and ADP on IM. On the other hand, ANT can form a non-specific pore, as this has been shown by electrophysiological characterization of purified ANT reconstituted into synthetic lipid bilayers or by measuring the permeabilization of proteoliposomes containing ANT. Pore formation by ANT is induced by a variety of different agents (e.g. Ca(2+), atractyloside, thiol oxidation, the pro-apoptotic HIV-1 protein Vpr, etc.) and is enhanced by Bax and inhibited by Bcl-2, as well as by ADP. In isolated mitochondria, pore formation by ANT leads to an increase in IM permeability to solutes up to 1500 Da, swelling of the mitochondrial matrix, and OM permeabilization, presumably due to physical rupture of OM. Although alternative mechanisms of mitochondrial membrane permeabilization may exist, ANT emerges as a major player in the regulation of cell death. Cell Death and Differentiation (2000) 7, 1146 - 1154
Tran Van Nhieu, G., R. Bourdet-Sicard, et al. (2000). "Bacterial signals and cell responses during Shigella entry into epithelial cells." Cell Microbiol 2(3): 187-93.
Shigella invades epithelial cells by inducing cytoskeletal reorganization localized at the site of bacterial-host cell interaction. During entry, the Shigella type III secretion apparatus allows the insertion of a pore that contains the IpaB and IpaC proteins into cell membranes. Insertion of this complex is thought to allow translocation of the carboxy-terminus moiety of IpaC, but also of other Shigella effectors, such as IpaA, into the cell cytosol. IpaC triggers actin polymerization and the formation of filopodial and lamellipodial extensions dependent on the Cdc42 and Rac GTPases. IpaA, on the other hand, binds to the focal adhesion protein vinculin and induces depolymerization of actin filaments. IpaA and the GTPase Rho are not required for actin polymerization at the site of bacterial contact with the cell membrane, but allow the transformation of the IpaC-induced extensions into a structure that is productive for bacterial entry. Rho is required for the recruitment at entry foci of ezrin, a cytoskeletal linker required for Shigella entry, and also of the Src tyrosine kinase. The Src tyrosine kinase activity, which is required for Shigella-induced actin polymerization, also appears to be involved in a negative regulatory loop that downregulates Rho at the site of entry.
Schroth-Diez, B., K. Ludwig, et al. (2000). "The role of the transmembrane and of the intraviral domain of glycoproteins in membrane fusion of enveloped viruses." Biosci Rep 20(6): 571-95.
Fusion of enveloped viruses with their target membrane is mediated by viral integral glycoproteins. A conformational change of their ectodomain triggers membrane fusion. Several studies suggest that an extended, triple-stranded rod-shaped alpha-helical coiled coil resembles a common structural and functional motif of the ectodomain of fusion proteins. From that, it is believed that essential features of the fusion process are conserved among the various enveloped viruses. However, this has not been established so far for the highly conserved transmembrane and intraviral sequences of fusion proteins. The article will focus on the role of both sequences in the fusion process. Recent studies from various enveloped viruses strongly imply that a transmembrane domain with a minimum length is required for later steps of membrane fusion, i.e., the formation and enlargement of the aqueous fusion pore. Although no specific sequence of the TM is necessary for pore formation, distinct properties and motifs of the domain may be obligatory to ascertain full fusion activity. However, with some exceptions, the intraviral domain seems to be not required for fusion activity of viral fusion proteins.
Sablon, E., B. Contreras, et al. (2000). "Antimicrobial peptides of lactic acid bacteria: mode of action, genetics and biosynthesis." Adv Biochem Eng Biotechnol 68: 21-60.
A survey is given of the main classes of bacteriocins, produced by lactic acid bacteria: I. lantibiotics II. small heat-stable non-lanthionine containing membrane-active peptides and III. large heat-labile proteins. First, their mode of action is detailed, with emphasis on pore formation in the cytoplasmatic membrane. Subsequently, the molecular genetics of several classes of bacteriocins are described in detail, with special attention to nisin as the most prominent example of the lantibiotic-class. Of the small non-lanthionine bacteriocin class, the Lactococcus lactococcins, and the Lactobacillus sakacin A and plantaricin A-bacteriocins are discussed. The principles and mechanisms of immunity and resistance towards bacteriocins are also briefly reported. The biosynthesis of bacteriocins is treated in depth with emphasis on response regulation, post-translational modification, secretion and proteolytic activation of bacteriocin precursors. To conclude, the role of the leader peptides is outlined and a conceptual model for bacteriocin maturation is proposed.
Rosenau, F. and K. Jaeger (2000). "Bacterial lipases from Pseudomonas: regulation of gene expression and mechanisms of secretion." Biochimie 82(11): 1023-32.
Lipases from Pseudomonas bacteria are widely used for a variety of biotechnological applications. Overexpression in heterologous hosts like Escherichia coli failed to produce enzymatically active lipase prompting to study the molecular mechanisms underlying the regulation of lipase gene expression and secretion. The prototype lipase from P. aeruginosa is encoded in a bicistronic operon which is transcribed from two different promotors, one of which depends on the alternative sigma factor RpoN (sigma(54)). Recently, a two-component regulatory system was identified as an element controlling transcription of the lipase operon. P. aeruginosa lipase is secreted via a type II pathway. The cytoplasmic prelipase contains a 26 amino acid N-terminal signal sequence mediating secretion across the inner membrane via the Sec-machinery. In the periplasm, lipase folds into an enzymatically active conformation assisted by its specific intermolecular chaperone Lif and by unspecific accessory folding catalysts including Dsb-proteins which catalyze the formation of a disulfide bond. Enzymatically active and secretion-competent lipase is finally transported through a complex secretion machinery consisting of 12 different Xcp-proteins of which XcpQ forms a pore-like structure in the outer membrane allowing the release of lipase into the extracellular medium. Biotechnologically important lipases from Burkholderia glumae and P. alcaligenes also use such a type II secretion pathway whereas lipases from P. fluorescens and Serratia marcescens, which lack a typical signal sequence are secreted via a type I pathway. Future challenges to produce Pseudomonas lipases may include artificial up-regulation of lipase gene transcription and construction of more efficient expression strains in which both folding and secretion of lipase are optimized.
Reusch, R. N. (2000). "Transmembrane ion transport by polyphosphate/poly-(R)-3-hydroxybutyrate complexes." Biochemistry (Mosc) 65(3): 280-95.
Transmembrane ion transport, a critical process in providing energy for cell functions, is carried out by pore-forming macromolecules capable of discriminating among very similar ions and responding to changes in membrane potential. It is widely regarded that ion channels are exclusively proteins, relatively late arrivals in cell evolution. Here we discuss the formation of ion-selective, voltage-activated channels by complexes of two simple homopolymers, namely, inorganic polyphosphates (polyPs) and poly-(R)-3-hydroxybutyrates (PHBs), derived from phosphate and acetate, respectively. Each has unique molecular characteristics that facilitate ion selection, solvation, and transport. Complexes of the two polymers, isolated from bacterial plasma membranes or prepared from the synthetic polymers, form voltage-dependent, Ca2+-selective channels in planar lipid bilayers that are selective for divalent over monovalent cations, permeant to Ca2+, Sr2+, and Ba2+, and blocked by transition metal cations in a concentration-dependent manner. Recently, both polyP and PHB have been found to be components of ion-conducting proteins: namely, the human erythrocyte Ca2+-ATPase pump and the Streptomyces lividans potassium channel. The contribution of polyP and PHB to ion selection and/or transport in these proteins is yet unknown, but their presence gives rise to the hypothesis that these and other ion transporters are supramolecular structures in which proteins, polyP, and PHB cooperate in forming well-regulated and specific cation transfer systems.
Ogiso, T. and T. Tanino (2000). "[Transdermal delivery of drugs and enhancement of percutaneous absorption]." Yakugaku Zasshi 120(4): 328-38.
This paper describes 1) the drug delivery through the skin to produce systemic effects, 2) the enhancement of percutaneous absorption by absorption enhancers, heating and complex formation, 3) the mechanism for the enhancement effect by enhancers, 4) the percutaneous absorption of peptides, and 5) the pharmacokinetic analysis for percutaneous absorption. 1,3-Dinitroglycerin, indomethacin (IND) and many drugs were efficiently absorbed via rat and rabbit skins in the presence of some enhancers, and using a microporous membrane therapeutic plasma concentrations were maintained for a long time. Enhancement of percutaneous absorption by the complex formation with fatty acid was observed for propranolol (PL) in vitro and in vivo. Heating at 42-45 degrees C also enhanced the percutaneous absorption dramatically, with decreased activation energies. The following mechanisms for the enhancement effect by enhancers were found: a) an increase in the fluidity of the stratum corneum lipids and reduction in the diffusional resistance to permeants, b) the removal of intercellular lipids and dilation between adherent cornified cells, c) an increase in the thermodynamic activity of drugs in vehicles, d) the exfoliation of stratum corneum cell membranes, the dissociation of adherent cornified cells and elimination of the barrier function. Peptides such as enkephalin, elcatonin and insulin were effectively absorbed through the skin in the presence of some enhancers and specific inhibitors, with no proteolytic degradation. The pharmacokinetic model with two parallel absorption processes, lipidic and aqueous pore transport pathways, in skin could adequately describe the percutaneous absorption of IND, PL and valproic acid. With peptides, a kinetic model including zero-order input rate, first-order permeation rate and first-order degradation rate was able to describe well the steady-state flux of peptides.
Nir, S. and J. L. Nieva (2000). "Interactions of peptides with liposomes: pore formation and fusion." Prog Lipid Res 39(2): 181-206.
Leakage from liposomes induced by several peptides is reviewed and a pore model is described. According to this model peptide molecules become incorporated into the vesicle bilayer and aggregate reversibly or irreversibly within the surface. When a peptide aggregate reaches a critical size, peptide translocation can occur and a pore is formed. With the peptide GALA the pores are stable and persist for at least 10 minutes. The model predicts that for a given lipid/peptide ratio, the extent of leakage should decrease as the vesicle diameter decreases, and for a given amount of peptide bound per vesicle less leakage would be observed at higher temperatures due to the increase in reversibility of surface aggregates of the peptide. Effect of membrane composition on pore formation is reviewed. When cholesterol was included in the liposomes the efficiency of inducation of leakage by the peptide GALA was reduced due to reduced binding and increased reversibility of surface aggregation of the peptide. Phospholipids which contain less ordered acyl-chains and have a slightly wedge-like shape, can better accommodate peptide surface aggregates, and consequently insertion and translocation of the peptide may be less favored. Demonstrations of antagonism between pore formation and fusion are presented. The choice of factors which promote vesicle aggregation, e.g., larger peptides, increased vesicle and peptide concentration results in enhanced vesicle fusion at the expense of formation of intravesicular pores. FTIR studies with HIV-1 fusion peptides indicate that in systems where extensive vesicle fusion occurred the beta conformation of the peptides was predominant, whereas the alpha conformation was exhibited in cases where leakage was the main outcome. Antagonism between leakage and fusion was exhibited by 1-palmitoyl-2-oleoylphosphatidylglycerol vesicles, where the order of addition of peptide (HIV(arg)) or Ca(2+)dictated whether pore formation or vesicle fusion would occur. The current study emphasizes that the addition of Ca(2+), which promotes vesicle aggregation can also reduce peptide translocation in isolated vesicles.
Nieva, J. L. and T. Suarez (2000). "Hydrophobic-at-interface regions in viral fusion protein ectodomains." Biosci Rep 20(6): 519-33.
In this chapter we shall describe how to apply the hydrophobicity-at-interface scale, as proposed by Wimley and White [Wimley, W. C. and White, S. H. (1996) Nature Struct. Biol. 3:842-848], to the detection of amino acid sequences of viral envelope glycoproteins putatively engaged in interactions with the target membranes. In addition, a new approach will be briefly introduced to infer the bilayer location at equilibrium of membrane-partitioning sequences. The use of these new procedures may be important in describing the molecular mechanism leading to the formation of a fusion pore by viral glycoproteins.
Lentz, B. R., V. Malinin, et al. (2000). "Protein machines and lipid assemblies: current views of cell membrane fusion." Curr Opin Struct Biol 10(5): 607-15.
Protein machines and lipid bilayers both play central roles in cell membrane fusion, a process crucial to life. Recent results provide clues to how both components function in fusion. Recent observations suggest a common mechanism by which very different fusion machines (from lipid-enveloped viruses and synaptic vesicles) may function to produce compartment-joining pores. This mechanism presumes that fusion proteins act as machines that use stored conformational energy to assemble closely juxtaposed lipid bilayers, bend these to form fusion-competent structures, stabilize unfavorable lipid structures and destabilize a committed intermediate to drive fusion pore formation.
Hertle, R. (2000). "Serratia type pore forming toxins." Curr Protein Pept Sci 1(1): 75-89.
The Serratia marcescens hemolysin represents a new type of hemolysin and has been studied in great molecular detail with regard to structure, activation and secretion. It has nothing in common with the pore forming toxins of E. coli type (RTX toxins), the Staphylococcus aureus alpha-toxin or the thiol activated toxin of group A beta-hemolytic streptococci (Streptolysin O). Studies on erythrocytes, eukaryotic cells and artificial black lipid membranes, have shown that the mechanism of pore formation of ShlA is different form other pore forming toxins. The S. marcescens hemolysin proteins ShlB and ShlA exhibit protein sequence homologues in Proteus mirabilis, Haemophilus ducreyi, Edwardsiella tarda and Erwinia chrysantemi. Furthermore, sequence motifs present in ShlA and Shlb have been shown to be important for activity and secretion of the S. marcescens hemolysin. Thus, the S. marcescens hemolysin forms the prototype of a new class of hemolysins and of a new secretory mechanism. The uniqueness of this new mechanism is underlined by the fact that activation of ShlA by ShlB strictly requires phosphatidylethanolamine as a cofactor. New data implicate a conformational change in ShlA during activation. In addition, ShlA not only forms pores in erythrocytes but also in fibroblasts and epithelial cells. The cytotoxic action of ShlA is mainly determined by lysis of infected cells in vitro. In sublytic doses, as will normally be the situation in vivo, ShlA exerts additionally effects which are currently under investigation. The knowledge of the structure, activation, secretion and mode of action of S. marcescens hemolysin has implications for proteins, related in sequence or in mode of secretion and activation.
Ferri, K. F., E. Jacotot, et al. (2000). "Mitochondrial control of cell death induced by HIV-1-encoded proteins." Ann N Y Acad Sci 926: 149-64.
In most examples of physiological or pathological cell death, mitochondrial membrane permeabilization (MMP) constitutes an early critical event of the lethal process. Signs of MMP that precede nuclear apoptosis include the translocation of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria to an extra-mitochondrial localization, as well as the dissipation of the mitochondrial transmembrane potential. MMP also occurs in HIV-1-induced apoptosis. Different HIV-1 encoded proteins (Env, Vpr, Tat, PR) can directly or indirectly trigger MMP, thereby causing cell death. The gp120/gp41 Env complex constitutes an example for an indirect MMP inducer. Env expressed on the plasma membrane of HIV-1 infected (or Env-transfected) cells mediates cell fusion with CD4/CXCR4-expressing uninfected cells. After a cell type-dependent latency period, syncytia then undergo MMP and apoptosis. Vpr exemplifies a direct MMP inducer. Vpr binds to the adenine nucleotide translocator (ANT), a mitochondrial inner membrane protein which also interacts with apoptosis-regulatory proteins from the Bcl-2/Bax family. Binding of Vpr to ANT favors formation of a non-specific pore leading to MMP. The structural motifs of the Vpr protein involved in MMP are conserved among most pathogenic HIV-1 isolates and determine the cytotoxic effect of Vpr. These data suggest the possibility that viruses employ multiple strategies to regulate host cell apoptosis by targeting mitochondria.
Dunant, Y. and M. Israel (2000). "Neurotransmitter release at rapid synapses." Biochimie 82(4): 289-302.
The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca(2+) and they are critically involved in determining the local Ca(2+) microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca(2+) channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15-25 large quanta, or 150-200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca(2+)-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.
Dietl, P. and T. Haller (2000). "Persistent fusion pores but transient fusion in alveolar type II cells." Cell Biol Int 24(11): 803-7.
The release of vesicle contents following exocytotic fusion is limited by various factors including the size of the fusion pore. Fusion pores are channel-like, narrow structures after formation and proceed through semi-stable states ('fusion pore flickering'), unless they fully expand (full fusion) or close again (transient fusion). Partial release of vesicle contents may occur during transient fusion, which was described to last between milliseconds and seconds, depending on the size of the vesicle. We studied fusion pores in a slow-secreting lung epithelial cell (type II cell) using fluorescence staining of vesicle contents (surfactant) and fluorescence recovery after photobleaching (FRAP). Surfactant is a lipidic material, which is secreted into the alveolar lumen to reduce the surface tension in the lung. We found release of surfactant to be a slow process, which can last for hours. Accordingly, fusion pores in these cells are stable structures, which appear to be a barrier for release. FRAP measurements suggest that transient fusions occasionally take place in these long-lasting fusion pores, resulting in partial release of surfactant into the extracellular space. These data suggest that postfusion mechanisms may regulate the amount of secreted surfactant.
Crompton, M. (2000). "Mitochondrial intermembrane junctional complexes and their role in cell death." J Physiol 529 Pt 1: 11-21.
A mitochondrial complex comprising the voltage-dependent anion channel (outer membrane), the adenine nucleotide translocase (inner membrane) and cyclophilin-D (matrix) assembles at contact sites between the inner and outer membranes. Under pathological conditions associated with ischaemia and reperfusion the junctional complex 'deforms' into the permeability transition (PT) pore, which can open transiently, allowing free permeation of low Mr solutes across the inner membrane. This may be a critical step in the pathogenesis of lethal cell injury in ischaemia and reperfusion. Moreover, it is argued, the degree of pore opening may be an important determinant of the relative extent of apoptosis and necrosis under these conditions. In addition, mitochondria are the major sites of action of Bax and other apoptotic regulatory proteins of the Bcl-2 family. These proteins control a mitochondrial amplificatory loop in the apoptotic signalling pathway in which cytochrome c and other apoptogenic proteins of the mitochondrial intermembrane space are released into the cytosol. There are indications that the junctional complex, or components of it, may also mediate the action of Bax, but in a way that does not involve PT pore formation.
Billington, S. J., B. H. Jost, et al. (2000). "Thiol-activated cytolysins: structure, function and role in pathogenesis." FEMS Microbiol Lett 182(2): 197-205.
Members of the thiol-activated family of cytolysins are involved in the mechanism of pathogenesis of a number of Gram-positive species. While they are pore-forming toxins, their major pathogenic effects may be more subtle than simple lysis of host cells, and may include interference with immune cell function and cytokine induction. Crystal structure, electron microscopy, mutagenesis and antibody binding studies have led to the modeling of a novel mechanism of pore formation, encompassing membrane-binding, membrane insertion and oligomerization. Despite their designation as thiol-activated cytolysins, it is now clear that thiol activation is not an important property of this group of toxins.
Abrami, L., M. Fivaz, et al. (2000). "Adventures of a pore-forming toxin at the target cell surface." Trends Microbiol 8(4): 168-72.
The past three years have shed light on how the pore-forming toxin aerolysin binds to its target cell and then hijacks cellular devices to promote its own polymerization and pore formation. This selective permeabilization of the plasma membrane has unexpected intracellular consequences that might explain the importance of aerolysin in Aeromonas pathogenicity.