RAB17, A NOVEL SMALL GTPASE, IS SPECIFIC FOR EPITHELIAL-CELLS AND IS INDUCED DURING CELL POLARIZATION

The rab subfamily of small GTPases has been demonstrated to play an im portant role in the regulation of membrane traffic in eukaryotic cells . Compared with nonpolarized cells, epithelial cells have distinct api cal and basolateral transport pathways which need to be separately reg ulated. This raises the question whether epithelial cells require spec ific rab proteins. However, all rab proteins identified so far were fo und to be equally expressed in polarized and nonpolarized cells. Here we report the identification of rab17, the first epithelial cell-speci fic small GTPase. Northern blot analysis on various mouse organs revea led that the rab17 mRNA is present in kidney, liver, and intestine but not in organs lacking epithelial cells nor in fibroblasts. To determi ne whether rab17 is specific for epithelial cells we studied its expre ssion in the developing kidney. We found that rab17 is absent from the mesenchymal precursors but is induced upon their differentiation into epithelial cells. In situ hybridization studies on the embryonic kidn ey and intestine revealed that rab17 is restricted to epithelial cells . By immunofluorescence and immunoelectron microscopy on kidney sectio ns, rab17 was localized to the basolateral plasma membrane and to apic al tubules. Rab proteins associated with two distinct compartments hav e been found to regulate transport between them. Therefore, our data s uggest that rab17 might be involved in transcellular transport.