Structure of Human Synaptotagmin 1 C2AB in the Absence of Ca²⁺ Reveals a Novel Domain Association^†,‡

Release of neurotransmitter from synaptic vesicles requires the Ca²⁺/phospholipid-binding protein synaptotagmin 1. There is considerable evidence that cooperation between the tandem C2 domains of synaptotagmin is a requirement of regulated exocytosis; however, high-resolution structural evidence for this interaction has been lacking. The 2.7 Å crystal structure of the cytosolic domains of human synaptotagmin 1 in the absence of Ca²⁺ reveals a novel closed conformation of the protein. The shared interface between C2A and C2B is stabilized by a network of interactions between residues on the C-terminal α-helix of the C2B domain and residues on loops 1−3 of the Ca²⁺-binding region of C2A. These interactions alter the overall shape of the Ca²⁺-binding pocket of C2A, but not that of C2B. Thus, synaptotagmin 1 C2A−C2B may utilize a novel regulatory mechanism whereby one C2 domain could regulate the other until an appropriate triggering event decouples them.