Calcium-sensitive synaptotagmin 11-lipid interaction modulates exo-endocytosis

Synaptotagmins (Syts) are the primary Ca²⁺-sensors for synaptic vesicle exocytosis, while most mammalian Syts are non-Ca²⁺-affinitive and play critical roles in neurotransmission and synaptic plasticity with unclear mechanisms. Here, we show that high-alkaline non-Ca²⁺-binding Syt11 exhibits higher affinity for acidic phospholipids and Ca²⁺-inhibited liposome-binding, thereby competing with the Ca²⁺-binding Syt1. Physiological levels of Ca²⁺ eliminate this competition by promoting Ca²⁺-dependent membrane insertion of Syt1 while suppressing Syt11's binding through electrostatic shielding of the membrane surface. Site-directed mutagenesis reveals a dual-regional lipid-binding mode (a lysine-rich motif for Ca²⁺-independent binding and Ca²⁺-binding loops for Ca²⁺-facilitation) for Syt1, and a redundant multi-point lipid-binding interface for Syt11. Consistent with the Ca²⁺-dependent competition, Syt11 inhibits both the early stages of exocytosis and endocytosis in neurons, while the maximal rate of exocytosis remains intact. This Ca²⁺-sensitivity of Syt11 proposes Syt1-Syt11 inter-switching in membrane-occupancy as a critical step precisely controlling exocytosis and endocytosis during synaptic transmission.