Complexin 1 And Synaptotagmin 1 Compete For Membrane Binding In A Pip2 Dependent Manner

One of the main proposed functions of complexin (Cpx) is to inhibit neurotransmitter release in the absence of Ca2+, while synaptotagmin 1 (Syt1) overcomes this inhibition in the presence of Ca2+ to drive synchronous neurotransmitter release. Although this regulatory function is indispensable for efficient membrane fusion and proper synaptic communication, the molecular mechanisms that underly this regulation remain controversial and elusive. Previous work from our lab shows that Cpx simultaneously binds both acceptor SNAREs and membranes, where the membrane interactions occur via the terminal domains in Cpx in a highly curvature-sensitive manner. In a fusion assay, where the target membrane contains only acceptor SNARE complex consisting of syntaxin and SNAP25, these interactions inhibit lipid mixing by lowering the affinity of synaptobrevin to the acceptor SNAREs. The Ca2+ sensor, Syt1 couples Ca2+ influx to membrane fusion and it has also been reported to modulate membrane curvature and like Cpx to bind preferentially to more highly curved membranes. We tested the idea that Syt1 might compete with Cpx at the membrane interface and might modulate fusion through its membrane interaction. In a binding assay, using total internal reflection microscopy, we find that Cpx competes with Syt1 in either the presence or absence of the acceptor SNARE complex, but does so only when PIP2 is present in the membrane. Both Cpx and Syt1 exhibit higher membrane affinity in the presence of PIP2, and we speculate that the competition for the membrane interface may in part be due to the ability of these proteins to sense and modulate membrane curvature.