Fusion pore

Clustering of a certain number of CD4 and coreceptor molecules is presumed to be necessary for the efficient HIV-1 Env-mediated fusion pore formation. It has been proposed that four to six CCR5 molecules (91) and three CD4 binding events are needed to induce fusion between the viral and host cell membranes (92). Both CD4 (93) and chemokine receptors can form functional dimers (94) in the plasma membrane. It was proposed that formation of CD4 dimers, mediated by a disulfide bond between the cysteine residues of the D2 domain, might enhance HIV-1 entry and infection (95,96). In contrast, others have provided... 

Markosyan RM, Cohen FS, Melikyan GB. HIV-1 envelope proteins complete their folding into six-helix bundles immediately after fusion pore formation. Mol Biol Cell 2003 14(3) 926—938. 

CaMKI and CaMKII) Dynamin-1 GTPase required for endocytosis that is phosphorylated by protein kinase C and dephosphorylated by calcineurin upon membrane depolarization and binds to AP2. Important for budding and fusion pore closure. 

However, an alternative pathway that bypasses clathrin-mediated endocytosis and EEs appears to be available as well. This model of endocytosis known as kiss and run or its variant kiss and stay have attracted increasing interest in recent years [74] (Fig. 9-9B). Kiss and run has been directly demonstrated with dense-core granules in neuroendocrine cells [84, 85], and this model would explain some observations that are not readily accommodated by the classical pathway. The kiss and run model proposes that neurotransmitters are released by a transient fusion pore, rather than by a complete fusion with integration of the synaptic vesicle components into the plasma membrane. Synaptic membrane proteins never lose their association and the vesicle reforms when the pore closes. As a result, the empty vesicle can be refilled and reused without going through clathrin-mediated endocytosis and sorting in the EEs. 

Breckenridge, L. J. and Aimers, W. Currents through the fusion pore that forms during exocytosis of a secretory vesicle. Nature 328 814-817,1987. 

Discharge of the contents into the cleft through the fusion pore. 

The arrival of the action potential at the presynaptic terminal opens voltage-dependent Ca ion channels in the plasma membrane so that the Ca ions enter the cytosol down their concentration gradient. This results in activation of a Ca -binding cytosolic or a membrane protein. This facilitates movement of the vesicles to the membrane and formation of a fusion pore through which the neurotransmitter is discharged into the synaptic cleft (i.e. exocytosis). This occurs within about 0.1 ms of the arrival of the depolarisation (Figure 14.8). The process of exocytosis lasts for only a short time, since the Csl ion concentration in the cytosol is rapidly lowered due to the ion extrusion from the cell (Appendix 14.3). 

Figure 14.8 Simple diagram of release of neurotransmitter and recycling of the vesicles in presynaptic neurone. After exocytosis, the membrane recycles to form a new vesicle which is re-filled with neurotransmitter. The Ca ion binding protein may control packaging, formation of fusion pore and release of neurotransmitter.

E. Borrego-Diaz, M. E. Peeples, R. M. Markosyan, G. B. Melikyan, and F. S. Cohen, Completion of trimeric hairpin formation of influenza virus hemagglutinin promotes fusion pore opening and enlargement, Virology, 316 (2003) 234-244. 

In addition to functioning as Ca2+-sensors for vesicle exocytosis, synaptotag-mins may be involved in vesicle endocytosis, particularly the decision between kiss-and-run versus full exocytosis. Such a role would be economical in linking fusion-pore opening (which is triggered by Ca2+-binding to synaptotagmin) to fusion-pore expansion or contraction, but the precise mechanisms involved have not yet been explored. 

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Femandez-Femandez JM, Abogadie FC, Milligan G et al (2001) Multiple pertussis toxin-sensitive G-proteins can couple receptors to GIRK channels in rat sympathetic neurons when expressed heterologously, but only native G(i)-proteins do so in situ. Eur J Neurosci 14 283-92 Femandez-Peruchena C, Navas S, Montes MA et al (2005) Fusion pore regulation of transmitter release. Brain Res Brain Res Rev 49 406-15... 

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Fernandez-Peruchena, C., Navas, S., Montes, M.A., et al. Fusion pore regulation of transmitter release. Brain Res. Rev. 2005,49 406-415. 

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