RAB5A IS A COMMON COMPONENT OF THE APICAL AND BASOLATERAL ENDOCYTIC MACHINERY IN POLARIZED EPITHELIAL-CELLS

In nonpolarized cells, the small GTPase Rab5a is localized to the plas ma membrane, clathrin-coated vesicles, and early endosomes. Rab5a is r equired for early endosome fusion in vitro and regulates transport bet ween the plasma membrane and early endosomes, in vivo. In polarized ep ithelial cells endocytosis occurs from separate apical and basolateral plasma membrane domains. Internalized molecules are initially deliver ed to distinct apical or basolateral early endosomes. In vitro, apical early endosomes can readily fuse with one another but not with the ba solateral endosomes and vice versa, thereby indicating that the apical and basolateral early endocytic pathways are controlled by distinct m achineries. Here, we have investigated the localization and function o f Rab5a in polarized epithelial cells. Confocal immunofluorescence mic roscopy on mouse kidney sections revealed association of the protein w ith the apical and basolateral plasma membrane domains and underlying structures. In polarized Madin-Darby canine kidney I cells, endogenous and overexpressed Rab5a have the same distribution. Moreover, overexp ression of the protein causes a 2-fold increase in fluid-phase uptake from both domains of the cell, thus showing that Rab5a functions in ap ical and basolateral endocytosis. Our data indicate that the apical an d basolateral endocytic machineries of epithelial cells share common r egulatory components and that Rab5a per se is not sufficient to target endocytic vesicles to apical or basolateral early endosomes.