摘要：

THE kinase Raf-1 can be activated by treatment of cells with mitogens and by the protein kinase C (PKC) activator 12--tetradecanoyl-phorbol-13-acetate (TPA) (reviewed in refs 1,2). Activated Raf-1 triggers a protein kinase cascade by direct phosphorylation of MAP kinase kinase, resulting in phosphorylation of ternary complex factorand Jimby MAP kinase. Here we investigate the molecular mechanism and biological consequences of PKCα-mediated Raf-1 activation in NIH3Tfibroblasts. PKCα directly phosphorylates and activates Raf-1 bothand. PKCα induces Raf-1 phosphorylation at several sites, including a serine residue at position 499. Mutation of serine at this position or at residue 259 does not abrogate Raf-1 stimulation by a combination of Ras plus thetyrosine kinase Lck, but severely impedes Raf-1 activation by PKCα. Consistent with such a direct interaction is the observation that Raf-1 and PKCα cooperate in the transformation of NIH3T3 cells. The Ser499 phosphorylation site is necessary for this synergism.

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