Polybasic patches in both C2 domains of Synaptotagmin-1 are required for evoked neurotransmitter release

Synaptotagmin-1 (Syt1) is a vesicular calcium sensor required for synchronous neurotransmitter release. It is composed of a single-pass transmembrane domain linked to two tandem C2 domains (C2A and C2B) that bind calcium, acidic lipids, and SNARE proteins that drive fusion of the synaptic vesicle with the plasma membrane. Despite its essential role, how Syt1 couples calcium entry to synchronous release is not well understood. Calcium binding to C2B, but not to C2A, is critical for synchronous release and C2B additionally binds the SNARE complex. The C2A domain is also required for Syt1 function, but it is not clear why. Here we asked what critical feature of C2A may be responsible for its functional role, and compared this to the analogous feature in C2B. We focused on highly conserved poly-lysine patches located on the sides of C2A (K189-192) and C2B (K324-327). We tested effects of charge-neutralization mutations in either region (Syt1K189-192A and Syt1K326-327A) side-by-side to determine their relative contributions to Syt1 function in cultured cortical mouse neurons and in single-molecule experiments. Combining electrophysiological recordings and optical tweezers measurements to probe dynamic single C2 domain-membrane interactions, we show that both C2A and C2B polybasic patches contribute to membrane binding, and both are required for evoked release. The readily releasable vesicle pool or spontaneous release were not affected, so both patches are specifically required for synchronization of release. We suggest these patches contribute to cooperative binding to membranes, increasing the overall affinity of Syt1 for negatively charged membranes and facilitating evoked release. Significance StatementSynaptotagmin-1 is a vesicular calcium sensor required for synchronous neurotransmitter release. Its tandem cytosolic C2 domains (C2A and C2B) bind calcium, acidic lipids, and SNARE proteins that drive fusion of the synaptic vesicle with the plasma membrane. How calcium-binding to Synaptotagmin-1 leads to release and the relative contributions of the two C2 domains is not clear: unlike C2B, calcium-binding to C2A is not critical for evoked release, yet both domains are needed for Syt1 function. Combining electrophysiological recordings from cultured neurons and optical tweezers measurements that probe single C2 domain-membrane interactions, we show that conserved polybasic regions in both domains contribute to membrane binding cooperatively, and both are required for evoked release, likely by increasing the overall affinity of Synaptotagmin-1 for negatively charged membranes.