Requirement of Ras-GTP-Raf Complexes for Activation of Raf-1 by Protein Kinase C

Abstract

Receptor tyrosine kinase–mediated activation of the Raf-1 protein kinase is coupled to the small guanosine triphosphate (GTP)–binding protein Ras. By contrast, protein kinase C (PKC)–mediated activation of Raf-1 is thought to be Ras independent. Nevertheless, stimulation of PKC in COS cells led to activation of Ras and formation of Ras–Raf-1 complexes containing active Raf-1. Raf-1 mutations that prevent its association with Ras blocked activation of Raf-1 by PKC. However, the activation of Raf-1 by PKC was not blocked by dominant negative Ras, indicating that PKC activates Ras by a mechanism distinct from that initiated by activation of receptor tyrosine kinases.