Roles of Rab18 in ER/golgi trafficking

RABS是一个RAS相关蛋白家族，主要通过调节转运囊泡与靶细胞膜的融合、调节转运囊泡与细胞骨架的相互作用以及调节调节转运囊泡膜上脂质的酶来调节膜转运步骤。在人类中有60多种已知的Rab蛋白，其中许多已知具有特定的细胞器定位并调节特定的运输步骤。我们对Rab18特别感兴趣，它存在于内质网和脂滴上。我们发现Rab18的突变或siRNA敲除会导致高尔基体的碎片化和分泌途径的损伤(Dejgaard等人，2008年)。J.细胞科学，121：2768-81)。在另一项单独的研究中，我们发现Rab18的一个激活突变体可以诱导大量的脂滴融合，这依赖于SNARE蛋白Synaxin-5、SNAP23和VAMP4。Synaxin-5对早期分泌途径中的融合事件也是至关重要的，但其调节机制尚不清楚。在目前的应用中，我们建议澄清Rab18在内质网和高尔基体之间的囊泡和货物运动中所扮演的重要角色。我们假设Rab18在早期的分泌途径中发挥作用，部分是作为Synaxin-5的调节因子。我们还探讨了Rab18在早期分泌途径中调节胆固醇动态平衡的可能性，以及它可能调节内质网和高尔基膜之间运输货物的运输中间产物上的马达蛋白。对于这些研究，我们将使用光学显微镜、siRNA和生化技术的组合。

Rabs are a family of Ras-related proteins which typically regulate membrane trafficking steps by regulating fusion of transport vesicles with target membranes, by regulating interaction of these transport vesicles with cytoskeleton and by regulating enzymes that modify lipids on the membrane of transport vesicles. There are more than 60 known Rab proteins in humans and many are known to have a specific organellar localization and to regulate a specific transport step. We have a specific interest in Rab18 which is found on the endoplasmic reticulum and lipid droplets. We have found that mutation or siRNA knockdown of Rab18 causes fragmentation of the Golgi apparatus and impairments in the secretory pathway (Dejgaard et al., 2008. J. Cell Sci., 121:2768-81). In a separate ongong study, we have found that an activated mutant of Rab18 induces profuse fusion of lipid droplets which depends on the SNARE proteins syntaxin-5, SNAP23 and VAMP4. Syntaxin-5 is also critical for fusion events in the early secretory pathway, but its regulation is not well understood. In the current application we propose to clarify the essential roles that Rab18 plays in movement of vesicles and cargo between the endoplasmic reticulum and the Golgi apparatus. We hypothesize that Rab18 is functioning in the early secretory pathway in part as a regulator of Syntaxin-5. We also explore the possibility that Rab18 regulates cholesterol homeostasis in the early secretory pathway and that it could regulate motor proteins on transport intermediates that move cargoes between endoplasmic reticulum and Golgi membranes. For these studies we will use a combination of light microscopy, siRNA and biochemical techniques.