RAF-1 AND P21(V-RAS) COOPERATE IN THE ACTIVATION OF MITOGEN-ACTIVATEDPROTEIN-KINASE

Mitogen-activated protein (MAP) kinases Raf-1, pp60src, and p21ras all play important roles in the transfer of signals from the cell surface to the nucleus. We have used the baculovirus/Sf9 insect cell system t o elucidate the regulatory relationships between pp60v-src, p21v-ras, MAP kinase (p44erk1/mapk), and Raf-1. In Sf9 cells, p44erk1/mapk is ac tivated by coexpression with either v-Raf or a constitutively activate d form of Raf-1 (Raf 22W). In contrast, p44erk1/mapk is activated to o nly a limited extent by coexpression with either Raf-1 or p21v-ras alo ne. This activation of p44erk1/mapk is greatly enhanced by coexpressio n with both p21v-ras and Raf-1. Since we have previously shown that p2 1v-ras stimulates Raf-1 activity, the activation of p44erk1/mapk by p2 1v-ras may occur exclusively via a Raf-1-dependent pathway. However, a dominant-inhibitory mutant of Raf-1 (Raf301) does not block the activ ation of p44erk1/mapk by p21v-ras. Further, pp60v-src, which activates Raf-1 at least as effectively as p21v-ras, fails to enhance p44erk1/m apk activity greatly when coexpressed with Raf-1. These data suggest t hat activation of p44erk1/mapk by p21v-ras may occur via both Raf-1-de pendent and Raf-1-independent pathways.