The C2B Domain of Synaptotagmin-1 and Complexin Reduce the Asynchronous Release Activation

Abstract

Nerve-evoked vesicle fusion has been kinetically separated in two components, synchronous and asynchronous. The Ca2+-sensor Synaptotagmin binds the SNARE-complex and drive fusion. Complexin co-evolve with Synaptotagmin, bind the complex and speed the exocytosis. Synaptotagmin controls the Ca2+-dependency of quantal release and synchronicity where Complexin cooperate in this function. Neuronal disruption of Synaptotagmin leaves an asynchronous release with a slight Ca2+-cooperativity suggesting the presence of an asynchronous Ca2+-sensor. The asynchronous sensor is thought to mediate the asynchronous releases observed during burst of activity, low Ca2+ or replacing Ca2+ by Sr2+. Here we investigated the asynchronous release at the Drosophila glutamatergic synapses. We monitor the differential effects of Ca2+, Sr2+ and Cd2+ in the evoked release modifying the expression of Synaptotagmin and Complexin at the nerve terminals. In addition, we explored the C2A and C2B Ca2+ binding domains mutants of Synaptotagmin. Our data shows that the asynchronous/synchronous release depends inversely with the level of Synaptotagmin and Complexin. Moreover, C2B but not C2A domain of Synaptotagmin reduces the asynchronous/synchronous release. Our work suggests that Complexin may be increasing the accessibility of Synaptotagmin for the SNARE complex by the C2B domain reducing the asynchronous/synchronous release. The Cd2+-sensibility was consistent with the Ca2+-dependency in the release probability discarding major alteration in calcium influx.