ROLE OF THE C2A DOMAIN OF SYNAPTOTAGMIN IN TRANSMITTER RELEASE AS DETERMINED BY SPECIFIC ANTIBODY INJECTION INTO THE SQUID GIANT SYNAPSE PRETERMINAL

Squid synaptotagmin (Syt) cDNA, including its open reading frame, was cloned and polyclonal antibodies were obtained in rabbits immunized wi th glutathione S-transferase (GST)-Syt-C2A. Binding assays indicated t hat the antibody, anti-Syt-C2A, recognized squid Syt and inhibited the Ca2+-dependent phospholipid binding to the C2A domain, This antibody, when injected into the preterminal at the squid giant synapse, blocke d transmitter release in a manner similar to that previously reported for the presynaptic injection of members of the inositol high-polyphos phate series. The block was not accompanied by any change in the presy naptic action potential or the amplitude or voltage dependence of the presynaptic Ca2+ current, The postsynaptic potential was rather insens itive to repetitive presynaptic stimulation, indicating a direct effec t of the antibody on the transmitter release system. Following block o f transmitter release, confocal microscopical analysis of the pretermi nal junction injected with rhodamine-conjugated anti-Syt-C2A demonstra ted fluorescent spots at the inner surface of the presynaptic plasmale mma next to the active zones. Structural analysis of the same preparat ions demonstrated an accumulation of synaptic vesicles corresponding i n size and distribution to the fluorescent spots demonstrated confocal ly, Together with the finding that such antibody prevents Ca2+ binding to a specific receptor in the C2A domain, these results indicate that Ca2+ triggers transmitter release by activating the C2A domain of Syt . We conclude that the C2A domain is directly related to the fusion of synaptic vesicles that results in transmitter release.