Position of synaptotagmin I at the membrane interface: Cooperative interactions of tandem C2 domains

Synaptotagmin I is a synaptic vesicle associated membrane protein that appears to regulate Ca²⁺-mediated exocytosis. Here, the Ca ²⁺-dependent membrane interactions of a water soluble fragment of synaptotagmin I (C2AB) that contains its two C2 domains (C2A and C2B) were determined using site-directed spin labeling. Membrane depth parameters were obtained for 19 spin-labeled mutants of C2AB when bound to phosphatidylcholine and phosphatidylserine membranes, and these distance constraints were used in combination with the high-resolution structures of C2A and C2B to generate a model for the membrane orientation and position of synaptotagmin at the bilayer interface. Both C2A and C2B bind to the membrane interface with their first and third Ca2+ binding loops penetrating the membrane interface. The polybasic face of C2B does not interact with the membrane lipid but is available for electrostatic interaction with other components of the fusion machinery. When compared to positions determined previously for the isolated domains, both C2A and C2B have similar orientations; however, the two domains are positioned deeper into the bilayer interior when present in the tandem construct. These data indicate that C2A and C2B do not act independently but influence their mutual membrane penetration. This may explain the occurrence of multiple C2 domains in proteins that function in membrane trafficking and repair.