A SMALL RAB GTPASE IS DISTRIBUTED IN CYTOPLASMIC VESICLES IN NONPOLARIZED CELLS BUT COLOCALIZES WITH THE TIGHT JUNCTION MARKER ZO-1 IN POLARIZED EPITHELIAL-CELLS

Small rab/Ypt1/Sec4 GTPase family have been involved in the regulation of membrane traffic along the biosynthetic and endocytic pathways in eucaryotic cells. Polarized epithelial cells have morphologically and functionally distinct apical and basolateral surfaces separated by tig ht junctions. The establishment and maintenance of these structures re quire delivery of membrane proteins and lipids to these domains. In th is work, we have isolated a cDNA clone from a human intestinal cDNA li brary encoding a small GTPase, rab13, closely related to the yeast Sec 4 protein. Confocal microscopy analysis on polarized Caco-2 cells show s that rab13 protein colocalized with the tight junction marker ZO-1. Cryostat sections of tissues confirm that rab13 localized to the junct ional complex region of a variety of epithelia, including intestine, k idney, liver, and of endothelial cells. This localization requires ass embly and integrity of the tight junctions. Disruption of tight juncti ons by incubation in low Ca2+ media induces the redistribution of rab1 3. In cells devoid of tight junctions, rab13 was found associated with vesicles dispersed throughout the cytoplasm. Cell-cell contacts initi ated by E-cadherin in transfected L cells do not recruit rab13 to the resulting adherens-like junction complexes. The participation of rab13 in polarized transport, in the assembly and/or the activity of tight junctions is discussed.