Role of the Rab family small G proteins in cell polarity and adhesion

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Tight junctions (TJs) are points of adhesion between epithelial cells that ensure the development and maintenance of the apical and basolateral plasma membrane (PM) domains. Proteins constituting TJs include the transmembrane proteins mediating cell-cell adhesion and the cytosolic plaque proteins linking to the cytoskeleton and participating in the intracellular signaling. Occludin and claudins constitute the transmembrane proteins in TJs. The Rab family small G proteins, which consist of more than 60 family members in mammalian cells, play a crucial role in etermining the specificity of vesicular transport pathways. Two Rab family members, Rab3B and Rab13, localize to TJs in polarized epithelial cells and cytoplasmic vesicular structures in non-polarized fibroblasts, but their functions are poorly understood. In the present study, we have examined the role of Rab3B and Rab13 in regulating the vesicular transport of TJ transmembrane proteins. In the exocytotic pathways, we found that Rab3B and Rab13 direct th cell-surface transport of a basolateral transmembrane protein, low-density lipoprotein receptor, and claudin-1, respectively. We also found that a pool of occludin was continuously endocytosed and recycled back to the cell-surface in epithelial cells. Our results indicated that Rabl3, but not Rab3B, specifically regulated the endocytic recycling of occludin. Biochemical endocytosis and recycling assays demonstrated that Rab13 directed the recycling of endocytosed occludin, but not its endocytosis. Furthermore, we found that Rab13 did not affect the recycling of transferrin receptor, that was recycled to the basolateral PM domain. We isolated a protein that was specifically interacted with GTP-bound form of Rab13, but not its GDP-bound form. Our results suggested that Rab13 was a crucial regulator for the transport of occludin and claudins to TJs.