Close membrane-membrane proximity induced by Ca²⁺-dependent multivalent binding of synaptotagmin-1 to phospholipids

Synaptotagmin acts as a Ca²⁺ sensor in neurotransmitter release through its two C₂ domains. Ca²⁺-dependent phospholipid binding is key for synaptotagmin function, but it is unclear how this activity cooperates with the SNARE complex involved in release or why Ca²⁺ binding to the C₂B domain is more crucial for release than Ca ²⁺ binding to the C₂A domain. Here we show that Ca ²⁺ induces high-affinity simultaneous binding of synaptotagmin to two membranes, bringing them into close proximity. The synaptotagmin C₂B domain is sufficient for this ability, which arises from the abundance of basic residues around its surface. We propose a model wherein synaptotagmin cooperates with the SNAREs in bringing the synaptic vesicle and plasma membranes together and accelerates membrane fusion through the highly positive electrostatic potential of its C₂B domain.