Regulation of the Raf kinase by phosphorylation

The Raf serine/threonine kinase plays an essential role to relay intracellular signals from the protooncogene Ras to activation of the mitogen-activated protein kinase (MAPK) cascade. The Raf kinase family consists of C-Raf (Raf-1), B-Raf, and A-Raf. Extensive efforts have been made in the last decade to study Raf regulation; however, precise molecular mechanism for Raf activation is still not fully understood. In this report, we discuss the current model of Raf regulation. Here we also report our recent findings that phosphorylation of Thr598 and Ser601, which lie between kinase subdomains VII and VIII, is essential for B-Raf activation by Ras. Substitution of these residues to alanine (B-RafAA) abolished Ras-induced B-Raf activation, without altering the association of B-Raf with other signaling proteins. Phosphopeptide mapping and immunoblotting with phosphospecific antibodies, which selectively recognize Thr598 and Ser601, phosphorylated B-Raf, confirmed that Thr598 and Ser601 are in vivo phosphorylation sites induced by Ras. Further, replacement of these two sites with acidic residues (B-RafED) renders B-Raf constitutively active. Consistent with these data, B-RafAA and B-RafED exhibited diminished and enhanced ability, respectively, to stimulate extracellular signal-regulated kinase (ERK) and Elk-dependent transcription. Moreover, functional studies revealed that B-RafED was able to promote NIH3T3 cell transformation and PC12 cell differentiation. Because Thr598 and Ser601 are conserved in all Raf family members, from Caenorhabditis elegans to mammals, we propose that phosphorylation of these two residues may be a general mechanism for Raf activation.