Controlling synaptotagmin activity by electrostatic screening

Exocytosis of neurosecretory vesicles is mediated by the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins syntaxin-1, synaptobrevin and SNAP-25, with synaptotagmin functioning as the major Ca²⁺ sensor for triggering membrane fusion. Here we show that bovine chromaffin granules readily fuse with large unilamellar liposomes in a SNARE-dependent manner. Fusion is enhanced by Ca²⁺, but only when the target liposomes contain phosphatidylinositol-4,5-bisphosphate and when polyphosphate anions, such as nucleotides or pyrophosphate, are present. Ca ²⁺-dependent enhancement is mediated by endogenous synaptotagmin-1. Polyphosphates operate by an electrostatic mechanism that reverses an inactivating cis association of synaptotagmin-1 with its own membrane without affecting trans binding. Hence, the balancing of trans-and cis-membrane interactions of synaptotagmin-1 could be a crucial element in the pathway of Ca²⁺-dependent exocytosis.