Endocytotic role of NDK, Phosphins and Dynamin

Reliable neurotransmitter release requires the presence of sufficent numbers of synaptic vesicles. The process of synaptic vesicle endocytosis (SVE) is coordinated by a group of proteins called dephosphins. The current set of seven known dephosphins are nerve treminal proteins with little or no structural hoimology but satisfy two essential criteria: they are essential for SVE and they are rapidly and coordinately dephosphorlyated in response to a calcium influx through voltage-dependent calicium channels. The calicium signal is mediated by calmodulin (CaM) and calcineurin. Each dephosphin plays an essential role in overlapping phases or segments of the cycle. The four phases of SVE are nucleation, invagination, fission and uncoating. In this diagram the internalization of the vesicle is the uncoating step. In order to illustrate the complexity of this process the initial display of the protein is labelled with a name and a number. This number is then used at the decloaking or uncoating step to show the release and seperation of the various factors. Each step in the process has a different set of complex components. The components are shown in highlighted boxes above. The nucleation phase initiates with the activation of CaM/Calcineurin and the subsequent dephosphorylation of AP180. AP180 and clathrin are then recruited to the membrane by AP2 and PtdIns(4,5)P2 (comma shaped molecule in the diagram). Nucleation is completed by the addition of epsin and eps15. Invagination proceeds with the formation of the amphiphysin1/2 heterodimer and its addition to the maturing vesicle surface complexes. The final components, dynamin and synaptojanin, are recruited to the budding vesicle by the amphiphysin1/2 heterodimer. Dynamin forms a complete ring around the vesicle neck and completes fission via its PtdIns(4,5)P2 stimulated GTPase activity. After internalization the vesicle is quickly uncoated, in a process believed to be mediated by synaptojanin, and is accompnied by the disassembly of clathrin.

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