POST-DOCKING ROLE FOR SYNAPTOBREVIN IN SYNAPTIC VESICLE FUSION

We have used the squid giant synapse to determine the role of synaptob revin, integral membrane proteins of small synaptic vesicles, in neuro transmitter release. The sequence of squid synaptobrevin, deduced by c DNA cloning, is 65%-68% identical to mammalian isoforms and includes t he conserved cleavage site for tetanus and botulinum B toxins. Injecti on of either toxin into squid nerve terminals caused a slow, irreversi ble inhibition of release without affecting the Ca2+ signal which trig gers release. Microinjection of a recombinant protein corresponding to the cytoplasmic domain of synaptobrevin produced a more rapid and rev ersible inhibition of release, whereas two smaller peptide fragments w ere without effect. Electron microscopy of tetanus-injected terminals revealed an increased number of both docked and undocked synaptic vesi cles. These data indicate that synaptobrevin participates in neurotran smitter release at a step between vesicle docking and fusion.