EFFECT OF RAF FAMILY PROTEIN KINASES ON CELL PHYSIOLOGY

点击翻译按钮获取中文摘要

We have examined the function of c-raf serine/threonine-specific protein kinase in fibroblastic and lymphoid cells. Structure-function analysis suggests a protein structure model for the unstimulated enzyme in which the active site is buried within the protein. Reversible activation in fibroblastic and lymphoid cells can be achieved by many growth factors or intracellular mitogens which use at least two independent pathways--one involving protein kinase C and another which involves direct tyrosine phosphorylation of c-raf by transmembrane or intracellular tyrosine kinases. Receptor-mediated activation of c-raf protein kinase is followed by translocation of the normally cytosolic enzyme to the perinuclear area and the nucleus. Transmembrane tyrosine kinase proteins which recruit raf kinase activity in a ligand-dependent manner include receptors for platelet derived growth factor (PDGF), colony stimulating factor-1, epidermal growth factor, insulin, and fibroblast growth factor. Thus, members from all four structural classes of transmembrane tyrosine kinase receptors share this coupling mechanism for signal transduction to the nucleus. Moreover, growth factors that use non-kinase receptors such as interleukin-2, interleukin-3, granulocyte-macrophage colony stimulating factor, as well as the T-cell receptor, and Thy-1 also activate raf kinase activity. Interestingly, those receptors that apparently couple via intracellular tyrosine kinases activate raf concordant with high-stoichiometry tyrosine phosphorylation (about 10- to 50-fold higher than platelet-derived growth factor-R). Expression of activated raf stimulates the activity of PEAI, a member of the c-jun gene family. Both v-raf and activated forms of either c-raf-I or A-raf stimulate PEAI, suggesting that phosphorylation is involved in this activation. We conclude that c-raf acts as a shuttle enzyme which connects mitogen-initiated events at the plasma membrane to events in the nucleus, presumably by activity-modulation of transcription factors via phosphorylation.