Synaptotagmin: A multifunctional protein in the synaptic vesicle cycle

Synaptotagmin is a synaptic vesicle protein whose cytosolic domain contains two C2 domains, C2A and C2B. Synaptotagmin is critical for synaptic transmission, and is implicated in synaptic vesicle docking, priming, Ca2+-sensing to trigger fusion and endocytosis. To further define synaptotagmin's role in the synaptic vesicle cycle, I examined three Drosophila synaptotagmin mutants: (1) a synaptotagmin null (sytnull) mutant, (2) a C2B Ca2+-binding motif mutant, and (3) a C2B polylysine motif mutant.; Although synaptic transmission is nearly abolished in syt null mutants, the mutation does not cause gross morphological changes. Indeed, sytnull mutants can survive to adulthood. However, synaptic vesicles, including docked vesicles, are decreased at sytnull active zones. syt null terminals also accumulate large, membranous structures. These experiments support a role for synaptotagmin in maintaining a population of synaptic vesicles in the nerve terminal, synaptic vesicle docking, Ca 2+-sensing to trigger fusion, and endocytosis.; Synaptic transmission is more severely disrupted in C2B Ca 2+-binding motif mutants than it is in sytnull mutants. However, unlike sytnull mutants, synaptic ultrastructure is normal in Ca2+-binding motif mutants. Thus, this motif is not involved in maintaining synaptic vesicles at nerve terminals, synaptic vesicle docking or endocytosis. Instead, the near elimination of synaptic transmission recorded in these mutants is likely because the protein cannot bind Ca2+ by its C2B domain, strongly supporting a role for synaptotagmin in Ca2+-sensing.; The polylysine motif in synaptotagmin's C2B domain is also critical for synaptic transmission. In vitro, this motif interacts with numerous presynaptic proteins. Thus, the synaptic transmission defect in these mutants may be due to a disruption of any of these interactions. Here I present data implicating this motif in synaptic vesicle recycling. I demonstrate that the motif is not involved in endocytosis, but does play a role prior to vesicle fusion. Furthermore, the polylysine motif mutants have a decreased release probability. These results are consistent with the hypothesis that the polylysine motif is involved in synaptic vesicle priming.; These studies demonstrate that synaptotagmin is a multifunctional protein in the synaptic vesicle cycle, playing important roles in vesicle docking, priming, Ca2+-sensing and endocytosis via distinct molecular interactions mediated by its various motifs.