CLONING AND SUBCELLULAR-LOCALIZATION OF NOVEL RAB PROTEINS REVEALS POLARIZED AND CELL-TYPE-SPECIFIC EXPRESSION

Small GTPases of the rab subfamily are specific regulators of vesicula r transport, The intracellular localization of these proteins has been mostly investigated in cultured cells where they have been found asso ciated with distinct compartments of the exocytic and endocytic pathwa ys, Using a PCR-based cloning approach we have recently identified sev eral novel rab proteins, extending the total number of this family to more than 30 members, Here, we have investigated the mRNA expression i n different tissues and the intracellular localization in organ cryose ctions of two rab proteins, rab18 and rab20. Both northern blot analys is and confocal immunofluorescence microscopy demonstrated that these proteins are expressed in a tissue- and cell type-dependent manner, De spite their presence in non-polarized cells and polarized cells, both proteins are highly expressed on the apical side of kidney tubule epit helial cells, Electron microscopic studies revealed that rab18 and rab 20 are located in apical dense tubules, endocytic structures underlyin g the apical plasma membrane, suggesting that they play a role in apic al endocytosis/recycling. In intestinal epithelial cells as well, both proteins were localized apically, but, in addition, rab18 was found a ssociated with the basolateral domain, suggesting that this protein is not restricted to the apical transport machinery of polarized epithel ial cells, The results demonstrate that, depending on the epithelial c ell type, rab proteins that are also expressed in non-polarized cells may be enriched in one or both surface domains, Together with the obse rved tissue- and cell type-dependent variation in the expression of th e rab proteins, this suggests that the large number of mammalian rab p roteins might reflect the specific requirements in the organization of membrane traffic encountered by different cell types.