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 Rad (Rac1V12) accumulate a large central aggregate of membranes beneath the ap ical membrane that contains filamentous actin, Rac1V12, rab11, and the resi dent apical membrane protein GP-135. To examine the roles of Rad in membran e traffic and the formation of this aggregate, we analyzed endocytic and bi osynthetic trafficking pathways in MDCK cells expressing Rac1V12 and domina nt inactive Rad (Rac1N17). Rac1V12 expression decreased the rates of apical and basolateral endocytosis, whereas Rac1N17 expression increased those ra tes from both membrane domains. Basolateral-to-apical transcytosis of immun oglobulin 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 Ra c1V12. These effects of Rac1V12 on trafficking pathways to the apical membr ane were the result of the delivery and trapping of these proteins in the c entral aggregate. Ln contrast to abnormalities in apical trafficking events , basolateral recycling of transferrin, degradation of EGF internalized fro m the basolateral membrane, and delivery of newly synthesized pIgR from the Golgi to the basolateral membrane were all relatively unaffected by Rac1V1 2 expression. Rac1N17 expression had little or no effect on these postendoc ytic or biosynthetic trafficking pathways. These results show that in polar ized MDCK cells activated Rad may regulate the rate of endocytosis from bot h membrane domains and that expression of dominant active Rac1V12 specifica lly alters postendocytic and biosynthetic membrane traffic directed to the apical, but not the basolateral, membrane.