DIVERGENCE IN THE G-PROTEIN-COUPLED RECEPTOR MITOGENIC SIGNALING PATHWAY AT THE LEVEL OF RAF KINASE

While activation of tyrosine kinase growth factor receptors is accompa nied by hyperphosphorylation of Raf-1, stimulation of receptors couple d to G-proteins has in some cases been shown to result in activation o f a non-Raf MEKK rather than of Raf itself. Our finding (Weiss, R. H., and Nuccitelli, R. (1992) J. Biol. Chem. 267, 5608-5613) that the thr ombin receptor requires tyrosine phosphorylation for its mitogenic eff ect in vascular smooth muscle cells led us to search for the molecules which are being tyrosine phosphorylated by this receptor. To determin e whether mitogenic signalling of G-protein-coupled growth factor rece ptors results in tyrosine phosphorylation of Raf, we examined activati on of Raf by two such receptors. Both thrombin and angiotensin II are mitogenic in NIH3T3 cells, but only thrombin causes hyperphosphorylati on of Raf-1. Activation of Raf by thrombin is associated with phosphor ylation of Raf-1 on tyrosine residues, whereas activation of Raf by an gio tensin II does not involve significant tyrosine phosphorylation. H owever, She is tyrosine phosphorylated by both thrombin and angiotensi n II. Thus, there exists a divergence in the mitogenic signalling path ways of the G-protein-coupled receptors relative to the Raf signalling cascade. While both thrombin and angiotensin II phosphorylate She and activate Raf catalytic activity, only thrombin phosphorylated Raf-1 o n tyrosine. This signalling through disparate Raf-coupled pathways sug gests one means by which the G-protein coupled receptors may confer sp ecificity in their signalling properties. (C) 1997 Elsevier Science In c.