Study of mechanisms for secretory granule exocytosis from salivary gland

小GTP结合蛋白Rab 27参与调节不同类型的膜运输，包括黑素细胞中的黑素体运输和多种分泌细胞中的调节分泌事件。我们以前已经表明，Rab 27参与控制异丙肾上腺素（IPR）诱导的大鼠腮腺腺泡细胞淀粉酶释放。尽管Rab 27在静息状态下主要定位于分泌颗粒上，但β刺激后其细胞内定位的变化从未得到阐明。本研究调查了IPR诱导的Rab 27 B在腮腺腺泡细胞中的再分布，揭示了IPR处理5分钟后从分泌颗粒到根尖下区域的易位，然后在IPR处理30分钟后扩散到胞质溶胶中。Rab 27 B从顶端质膜的解离可能是通过Rab GDP解离抑制剂（GDI）介导的胞质溶胶中提取GDP结合的Rab蛋白从膜，作为一个显着增加的Rab 27 B-GDI复合物的量在胞质溶胶中观察到30分钟后刺激IPR。这些结果表明，在腮腺腺泡细胞，Rab 27 B易位，在一个时间依赖性的方式，从分泌颗粒到顶端质膜作为暴露于IPR的结果，然后进入细胞质通过与GDI结合。

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.