The juxtamembrane linker of synaptotagmin 1 regulates Ca 2+ binding via liquid-liquid phase separation.

Synaptotagmin (syt) 1, a Ca sensor for synaptic vesicle exocytosis, is thought to act as a multimer. It senses Ca via tandem C2-domains that are connected to a single transmembrane domain via a juxtamembrane linker. Here, we show that this linker segment harbors a lysine-rich, intrinsically disordered region that is necessary and sufficient to mediate liquid-liquid phase separation (LLPS). Unexpectedly, these interactions negatively regulate the Ca -sensitivity of the tandem C2-domains of syt1. Ca and anionic phospholipids promote the observed phase separation. This potentially creates a feedback loop that might serve to finetune the ability of syt1 to trigger release, via alterations in Ca binding activity and potentially through the impact of LLPS on membrane curvature during fusion reactions. In summary, the juxtamembrane linker of syt1 emerges as a regulator of syt1 function by driving self-association via LLPS.