Roles of Rab18 in ER/golgi trafficking

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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.