Differential regulation of Raf-1, A-Raf, and B-Raf by oncogenic ras and tyrosine kinases

It has previously been shown that maximal activation of Raf-1 is produced by synergistic signals from oncogenic Ras and activated tyrosine kinases, This synergy arises because Ras-GTP translocates Raf-1 to the plasma membrane where it becomes phosphorylated on tyrosine residues 340 and 341 by membrane bound tyrosine kinases (Marais, R,, Light, Y,, Paterson, H, F,, and Marshall, C. J. (1995) EMBO J. 14, 3136-3145), We have examined whether the other two members of the Raf family, A-Raf and B-Raf, are regulated in a similar way to Raf-1. A-Raf behaves like Raf-1, being weakly activated by oncogenic Ras more strongly activated by oncogenic Src, and these signals synergize to give maximal activation, B-Raf by contrast is strongly activated by oncogenic Ras alone and is not activated by oncogenic Src, These results show that maximal activation of B-Raf merely requires signals that generate Ras-GTP whereas activation of Raf-1 and A-Raf requires Ras GTP together with signals that lead to their tyrosine phosphorylation, B-Raf may therefore be the primary target of oncogenic Ras.