Differential regulation of synchronous versus asynchronous neurotransmitter release by the C2 domains of synaptotagmin 1.

Synaptic vesicle fusion at many synapses has been kinetically sep- arated into two distinct Ca2-dependent temporal components consisting of a rapid synchronous phase followed by a slower asyn- chronous component. Mutations in the synaptic vesicle Ca2 sensor Synaptotagmin 1 (Syt 1) reduce synchronous neurotransmission while enhancing the slower asynchronous phase of release. Syt 1 regulation of vesicle fusion requires interactions mediated by its tandem cytoplasmic C2 domains (C2A and C2B). Although Ca2* bind- ing by Syt 1 is predicted to drive synchronous release, it is unknown if Ca2 interactions with either C2 domain is required for suppres- sion of asynchronous release. To determine if Ca2 binding by Syt 1 regulates these two phases of release independently, we performed electrophysiological analysis of transgenically expressed Syt 1 mu- tated at Ca2 binding sites in C2A or C2B in the background of Dro- sophila Syt 1-null mutants. Transgenic animals expressing mutations that disrupt Ca2 binding to C2A fully restored the synchronous phase of neurotransmitter release, whereas the asynchronous component was not suppressed. In contrast, rescue with Ca2-binding mutants in C2B displayed little rescue of the synchronous release component, but reduced asynchronous release. These results suggest that the tandem (2 domains of Syt 1 play independent roles in neurotrans- mission, as Ca2 binding to C2A suppresses asynchronous release, whereas Ca2 binding to C2B mediates synchronous fusion. [ABSTRACT FROM AUTHOR]