Synaptotagmin 1 oligomers clamp and regulate different modes of neurotransmitter release.

Synaptotagmin 1 (Syt1) synchronizes neurotransmitter release to action potentials (APs) acting as the fast Ca release sensor and as the inhibitor (clamp) of spontaneous and delayed asynchronous release. While the Syt1 Ca activation mechanism has been well-characterized, how Syt1 clamps transmitter release remains enigmatic. Here we show that C2B domain-dependent oligomerization provides the molecular basis for the Syt1 clamping function. This follows from the investigation of a designed mutation (F349A), which selectively destabilizes Syt1 oligomerization. Using a combination of fluorescence imaging and electrophysiology in neocortical synapses, we show that Syt1 is more efficient than wild-type Syt1 (Syt1) in triggering synchronous transmitter release but fails to clamp spontaneous and synaptotagmin 7 (Syt7)-mediated asynchronous release components both in rescue (Syt1 knockout background) and dominant-interference (Syt1 background) conditions. Thus, we conclude that Ca-sensitive Syt1 oligomers, acting as an exocytosis clamp, are critical for maintaining the balance among the different modes of neurotransmitter release.