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In synaptic terminals, complexin can suppress spontaneous mini release and promote Ca2+-triggered synchronized neurotransmitter release.Here through single-particle microscopy imaging, we show that complexin-1 also reduces spontaneous fusion in the same assay.Moreover, distinct effects of several complexin-1 truncation mutants on spontaneous and Ca2+-triggered fusion closely mimic those observed in neuronal cultures.The very N-terminal domain is essential for synchronization of Ca2+-triggered fusion, but not for suppression of spontaneous fusion, whereas the opposite is true for the C-terminal domain.By systematically varying the complexin-1 concentration, we observed differences in titration behavior for spontaneous and Ca2+-triggered fusion.Taken together, complexin-1 utilizes distinct mechanisms for synchronization of Ca2+-triggered fusion and inhibition of spontaneous fusion.