Agonist-dependent traffic of Raft-associated Ras and Raf-1 is required foractivation of the mitogen-activated protein kinase cascade

Stimulation of HIRcB fibroblasts with insulin leads to accumulation of acti ve components of the mitogen-activated protein kinase cascade in endocytic compartments. However, the factors that regulate the mobilization of these components through the endocytic pathway and the relevance of this event to cellular signaling remain unclear. Here we report that Ras proteins are as sociated with lipid rafts in resting HIRcB fibroblasts. Ras is rapidly inte rnalized into the endocytic compartment following stimulation with insulin. The redistribution of Ras is independent of its activation. Attachment of the C-terminal 20 amino acids of Ha-Ras to green fluorescent protein was su fficient to target this construct to the same loci as the endogenous Ras pr otein, indicating that Ras distribution is a consequence of the association of its lipid modified C terminus with membranes. Depletion of plasma membr ane cholesterol delocalized Ras and blocked insulin-dependent Ras traffic. Cholesterol depletion also blocked insulin-dependent phosphorylation of MEK and mitogen-activated protein kinase (MAPK) but had no effects on the tran slocation and activation of Raf-1. A second inhibitor of endocytosis, cytoc halasin D, also blocked insulin-dependent MAPK phosphorylation. Taken toget her, these results suggest that mobilization of active Raf-1 through the en docytic compartment is required for completion of the MAPK cascade.