Robinson Iain M, Ranjan Ravi, Schwarz Thomas L
Department of Genetics, University of Cambridge, Cambridge CB2 3EH, UK.
Nature. 2002 Jul 18;418(6895):336-40. doi: 10.1038/nature00915. Epub 2002 Jul 7.
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.
在神经末梢，细胞内钙离子（Ca(2+)）的局部短暂增加会触发充满神经递质的囊泡与质膜的融合。对于钙离子传感触发机制，研究最广泛的候选蛋白是突触结合蛋白I，其与酸性磷脂和 syntaxin 的钙离子依赖性相互作用很大程度上归因于其C(2)A结构域，尽管C(2)B结构域也能结合钙离子（参考文献7、8）。关于突触结合蛋白I是否是融合的钙离子传感触发因素，基因测试结果并不明确。突触结合蛋白IV是一种相关的异构体，其C(2)A结构域不结合钙离子，可能是一种释放抑制剂。我们对突触结合蛋白I的C(2)A结构域钙离子结合位点的一个关键天冬氨酸进行了突变，并在缺乏突触结合蛋白I的果蝇中进行表达。在此我们表明，尽管结合位点被破坏，但传递的钙离子依赖性特性并未改变。同样，我们发现突触结合蛋白IV可以替代突触结合蛋白I。我们得出结论，突触结合蛋白的C(2)A结构域对于钙离子依赖性突触传递不是必需的，并且突触结合蛋白IV促进而非抑制传递。
