Selective alterations in biosynthetic and endocytic protein traffic in Madin-Darby canine kidney epithelial cells expressing mutants of the small GTPase Rac1.

Madin-Darby canine kidney (MDCK) cells expressing constitutively active Rac1 (Rac1V12) accumulate a large central aggregate of membranes beneath the apical membrane that contains filamentous actin, Rac1V12, rab11, and the resident apical membrane protein GP-135. To examine the roles of Rac1 in membrane traffic and the formation of this aggregate, we analyzed endocytic and biosynthetic trafficking pathways in MDCK cells expressing Rac1V12 and dominant inactive Rac1 (Rac1N17). Rac1V12 expression decreased the rates of apical and basolateral endocytosis, whereas Rac1N17 expression increased those rates from both membrane domains. Basolateral-to-apical transcytosis of immunoglobulin A (IgA) (a ligand for the polymeric immunoglobulin receptor [pIgR]), apical recycling of pIgR-IgA, and accumulation of newly synthesized GP-135 at the apical plasma membrane were all decreased in cells expressing Rac1V12. These effects of Rac1V12 on trafficking pathways to the apical membrane were the result of the delivery and trapping of these proteins in the central aggregate. In contrast to abnormalities in apical trafficking events, basolateral recycling of transferrin, degradation of EGF internalized from the basolateral membrane, and delivery of newly synthesized pIgR from the Golgi to the basolateral membrane were all relatively unaffected by Rac1V12 expression. Rac1N17 expression had little or no effect on these postendocytic or biosynthetic trafficking pathways. These results show that in polarized MDCK cells activated Rac1 may regulate the rate of endocytosis from both membrane domains and that expression of dominant active Rac1V12 specifically alters postendocytic and biosynthetic membrane traffic directed to the apical, but not the basolateral, membrane.