H-ras but not K-ras traffics to the plasma membrane through the exocytic pathway

Ras proteins must be localized to the inner surface of the plasma membrane to be biologically active. The motifs that effect Ras plasma membrane targe ting consist of a C-terminal CAAX motif plus a second signal comprising pal mitoylation of adjacent cysteine residues or the presence of a polybasic do main. In this study, we examined how Ras proteins access the cell surface a fter processing of the CAAX motif is completed in the endoplasmic reticulum (ER). We show that palmitoylated CAAX proteins, in addition to being local ized at the plasma membrane, are found throughout the exocytic pathway and accumulate in the Golgi region when cells are incubated at 15 degrees C. In contrast, polybasic CAAX proteins are found only at the cell surface and n ot in the exocytic pathway. CAAX proteins which lack a second signal for pl asma membrane targeting accumulate in the ER and Golgi. Brefeldin A (BFA) s ignificantly inhibits the plasma membrane accumulation of newly synthesized , palmitoylated CAAX proteins without inhibiting their palmitoylation. BFA has no effect on the trafficking of polybasic CAAX proteins. We conclude th at H-ras and K-ras traffic to the cell surface through different routes and that the polybasic domain is a sorting signal diverting K-Ras out of the c lassical exocytic pathway proximal to the Golgi. Farnesylated Ras proteins that lack a polybasic domain reach the Golgi but require palmitoylation in order to traffic further to the cell surface. These data also indicate that a Ras palmitoyltransferase is present in an early compartment of the exocy tic pathway.