Fischer von Mollard G, Südhof T C, Jahn R
Department of Neurochemistry, Max-Planck Institute for Psychiatry, Martinsried, Germany.
Nature. 1991 Jan 3;349(6304):79-81. doi: 10.1038/349079a0.
Low-molecular-weight GTP-binding proteins are strong candidates for regulators of membrane traffic. In yeast, mutations in the sec4 or ypt1 genes encoding small GTP-binding proteins inhibit constitutive membrane flow at the plasma membrane or Golgi complex, respectively. It has been suggested that membrane fusion-fission events are regulated by cycling of small GTP-binding proteins between a membrane-bound and free state, but although most of these small proteins are found in both soluble and tightly membrane-bound forms, there is no direct evidence to support such cycling. In rat brain a small GTP-binding protein, rab3A, is exclusively associated with synaptic vesicles, the secretory organelles of nerve terminals. Here we use isolated nerve terminals to study the fate of rab3A during synaptic vesicle exocytosis. We find that rab3A dissociates quantitatively from the vesicle membrane after Ca2(+)-dependent exocytosis and that this dissociation is partially reversible during recovery after stimulation. These results are direct evidence for an association-dissociation cycle of a small GTP-binding protein during traffic of its host membrane.
低分子量GTP结合蛋白是膜转运调节因子的有力候选者。在酵母中，编码小GTP结合蛋白的sec4或ypt1基因发生突变，分别抑制质膜或高尔基体复合体处的组成型膜流。有人提出，膜融合-裂变事件受小GTP结合蛋白在膜结合态和游离态之间循环的调节，但是，尽管这些小蛋白大多以可溶性和紧密膜结合形式存在，却没有直接证据支持这种循环。在大鼠脑中，一种小GTP结合蛋白rab3A专门与突触小泡（神经末梢的分泌细胞器）相关联。在此，我们利用分离出的神经末梢来研究rab3A在突触小泡胞吐过程中的命运。我们发现，rab3A在Ca2+依赖的胞吐作用后定量地从囊泡膜上解离，并且这种解离在刺激后的恢复过程中部分是可逆的。这些结果是小GTP结合蛋白在其宿主膜运输过程中存在缔合-解离循环的直接证据。
