Uncoupling of membrane ruffling and pinocytosis during Ras signal transduction

Activation of Ras stimulates cell surface membrane ruffling and pinocytosis. Although seen as coupled events, our study demonstrates that membrane ruffling and pinocytosis are regulated by distinct Ras signal transduction pathways. Ras controls membrane ruffling via the small GTPase Rac. In BHK-21 cells, expression of the constitutively active Rac1(G12V) mutant, via a Sindbis virus vector, resulted in a dramatic stimulation of membrane ruffling without affecting the uptake of horseradish peroxidase. Expression of Ha-Ras(G12V), an activated Ras mutant, stimulated both membrane ruffling and horseradish peroxidase uptake. The Ha-Ras(G12V)-stimulated pinocytosis but not membrane ruffling was abolished by either wortmannin or co-expression with a dominant negative mutant of Rab5, Rab5(S34N). Expression of the activated Rab5(Q79L) mutant mimics the stimulatory effect of Ha-Ras(G12V) on pinocytosis but not membrane ruffling. Our data indicate that Ha-Ras(G12V) separately activates Rab5-dependent pinocytosis and Rac1-dependent membrane ruffling.