Calcium-dependent oligomerization of synaptotagmins I and II - Synaptotagmins I and II are localized on the same synaptic vesicle and heterodimerize in the presence of calcium

Synaptotagmins constitute a large family of membrane proteins characterized by their distinct distributions and different biochemical features. Geneti c evidence suggests that members of this protein family are likely to funct ion as calcium sensors in calcium-regulated events in neurons, although the precise molecular mechanism remains ill defined, Here we demonstrate that different synaptotagmin isoforms (Syt I, II, and TV) are present in the sam e synaptic vesicle population from rat brain cortex. In addition, Syt I and II co-localize on the same small synaptic vesicle (SSV), and they heterodi merize in the presence of calcium with a concentration dependence resemblin g that of the starting phase of SSV exocytosis (EC50 = 6 +/- 4 mu M). The a ssociation between Syt I and Syt II was demonstrated by immunoprecipitation of the native proteins and the recombinant cytoplasmic domains and by usin g fluorescence resonance energy transfer (FRET), Although a subpopulation o f SSV containing Syt I and TV can be isolated, these two isoforms do not sh ow a calcium-dependent interaction, These results suggest that the self-ass ociation of synaptotagmins with different calcium binding features may crea te a variety of calcium sensors characterized by distinct calcium sensitivi ties. This combinatorial hypothesis predicts that the probability of a sing le SSV exocytic event is determined, in addition to the gating properties o f the presynaptic calcium channels, by the repertoire and relative abundanc e of distinct synaptotagmin isoforms present on the SSV surface.