Synaptotagmins I and IV promote transmitter release independently of Ca2+ binding in the C2A domain

Robinson, Iain M. and Ranjan, Ravi and Schwarz, Thomas L. (2002) Synaptotagmins I and IV promote transmitter release independently of Ca2+ binding in the C2A domain. Nature, 418 (6895). pp. 336-340. ISSN 0028-0836

Abstract

At nerve terminals, a focal and transient increase in intracellular Ca2+ triggers the fusion of neurotransmitter-filled vesicles with the plasma membrane. The most extensively studied candidate for the Ca2+-sensing trigger is synaptotagmin I, whose Ca2+-dependent interactions with acidic phospholipids and syntaxin1 have largely been ascribed to its C2A domain, although the C2B domain also binds Ca2+ (refs 7, 8). Genetic tests of synaptotagmin I have been equivocal as to whether it is the Ca2+ -sensing trigger of fusion. Synaptotagmin IV, a related isoform that does not bind Ca2+ in the C2A domain, might be an inhibitor of release. We mutated an essential aspartate of the Ca2+-binding site of the synaptotagmin I C2A domain and expressed it in Drosophila lacking synaptotagmin I. Here we show that, despite the disruption of the binding site, the Ca2+-dependent properties of transmission were not altered. Similarly, we found that synaptotagmin IV could substitute for synaptotagmin I. We conclude that the C2A domain of synaptotagmin is not required for Ca2+-dependent synaptic transmission, and that synaptotagmin IV promotes rather than inhibits transmission.