Study of mechanisms for secretory granule exocytosis from salivary gland

小gtp结合蛋白Rab27参与调节不同类型的膜运输，包括黑素细胞中的黑素小体运输和各种分泌细胞的调节分泌事件。我们之前已经证明Rab27参与控制异丙肾上腺素（IPR）诱导的大鼠腮腺腺泡细胞的淀粉酶释放。虽然Rab27在静息条件下主要定位在分泌颗粒上，但β刺激后其细胞内定位的变化从未被阐明。本研究研究了IPR诱导的Rab27B在腮腺腺泡细胞中的重分布，发现IPR处理5分钟后，Rab27B从分泌颗粒转移到根尖下区域，然后在IPR处理30分钟后扩散到细胞质中。Rab27B从根尖质膜上的解离可能是通过细胞质溶胶中的rabgdp解离抑制剂（GDI）介导的，该抑制剂从膜上提取GDP结合的rabb蛋白，因为在IPR刺激后30分钟，细胞质溶胶中Rab27B-GDI复合物的数量急剧增加。这些结果表明，在腮腺腺泡细胞中，由于暴露于IPR， Rab27B以一种时间依赖性的方式从分泌颗粒转移到顶质膜，然后通过与GDI结合进入细胞质。

Small GTP-binding protein, Rab27, has been implicated in the regulation of different types of membrane trafficking, including melanosome transport in melanocytes and regulated secretion events in a wide variety of secretory cells. We have previously shown that Rab27 is involved in the control of isoproterenol (IPR)-induced amylase release from rat parotid acinar cells. Although Rab27 is predominantly localized on secretory granules under resting conditions, changes to its intracellular localization after β-stimulation have never been elucidated. The present study investigated IPR-induced redistribution of Rab27B in the parotid acinar cells, revealing translocation from secretory granules to the subapical region after 5 min of IPR treatment and then diffusion into the cytosol after 30 min of IPR treatment. Dissociation of Rab27B from the apical plasma membrane is probably mediated through the Rab GDP dissociation inhibitor (GDI) in the cytosol extracting GDP-bound Rab protein from membranes, as a dramatic increase in the amount of the Rab27B-GDI complex in the cytosol was observed 30 min after stimulation with IPR. These results indicate that, in parotid acinar cells, Rab27B is translocated, in a time-dependent manner, from secretory granules into the apical plasma membrane as a result of exposure to IPR, and then into the cytosol through binding with the GDI.