Synaptotagmins I and IV promote transmitter release independently of Ca(2+) binding in the C(2)A domain.
At nerve terminals, a focal and transient increase in intracellular Ca(2+) triggers the fusion of neurotransmitter-filled vesicles with the plasma membrane. The most extensively studied candidate for the Ca(2+)-sensing trigger is synaptotagmin I, whose Ca(2+)-dependent interactions with acidic phospholipids and syntaxin have largely been ascribed to its C(2)A domain, although the C(2)B domain also binds Ca(2+) (refs 7, 8). Genetic tests of synaptotagmin I have been equivocal as to whether it is the Ca(2+)-sensing trigger of fusion. Synaptotagmin IV, a related isoform that does not bind Ca(2+) in the C(2)A domain, might be an inhibitor of release. We mutated an essential aspartate of the Ca(2+)-binding site of the synaptotagmin I C(2)A domain and expressed it in Drosophila lacking synaptotagmin I. Here we show that, despite the disruption of the binding site, the Ca(2+)-dependent properties of transmission were not altered. Similarly, we found that synaptotagmin IV could substitute for synaptotagmin I. We conclude that the C(2)A domain of synaptotagmin is not required for Ca(2+)-dependent synaptic transmission, and that synaptotagmin IV promotes rather than inhibits transmission.
Department of Genetics, University of Cambridge, Cambridge CB2 3EH, UK. i.robinson@gen.cam.ac.uk
Iain M
IM
Robinson
Ravi
R
Ranjan
Thomas L
TL
Schwarz
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002135:Calcium-Binding Proteins; D004331:Drosophila melanogaster / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D004594:Electrophysiology; D017353:Gene Deletion; D007814:Larva / Q000235:genetics / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D013569:Synapses / Q000378:metabolism; D009435:Synaptic Transmission; D050863:Synaptotagmin I; D050857:Synaptotagmins
D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D018377:Neurotransmitter Agents; D050863:Synaptotagmin I; D050857:Synaptotagmins; D002118:Calcium