Cooperative All-Or-None Recruitment of Synaptotagmin C2AB on Single Vesicles Explains Why Ca2+ Regulates the Amplitude of SNARE Mediated Vesicle Fusion

Abstract

Synaptotagmin-1 (syt) has been identified as the principle determinant of synchronous release of neurotransmitters at the synapse. The clear correlation between the Ca2+-sensitivity of release and the Ca2+-dependent binding of different syt mutants to negatively charged phospholipid membranes (in the form of small unilamellar vesicles) is in strong favor of this hypothesis.Both syt wt and C2AB have been exhibiting an enigmatic behavior in all in vitro studies reporting Ca2+regulated vesicle fusion, namely they predominantly, or in some cases exclusively, modulated fusion amplitudes and not fusion kinetics. This means that contrary to intuition syt and Ca2+ do not increase the probability of vesicle fusion. Instead they increase the total fraction of vesicles that are available for fusion through a yet unknown mechanism. Interestingly, this phenotype is reminiscent of the observation that in vivo Ca2+influx in addition to changing fusion kinetics increases the size of the RRP. Here we demonstrate that this striking phenotype originates from the highly cooperative binding of syt to membranes that results to an all-or-none recruitment at the single vesicle level1-4 which is regulated by Ca2+.