Raf-1 activates MAP kinase-kinase

Abstract

THE normal cellular homologue of the acutely transforming oncogene v-ra/is c-raf-l, which encodes a serine/threonine protein kinase that is activated by many extracellular stimuli¹. The physiological substrates of the protein c-Raf-1 are unknown. The mitogen-activated protein (MAP) kinases ErkI and 2 are also activated by mitogens through phosphorylation of Erk tyrosine and threonine residues catalysed by a protein kinase of relative molecular mass 50,000, MAP kinase-kinase (MAPK-K)^2–7. Here we report that MAPK-K as well as Erkl and 2 are constitutively active in v-raf-transformed cells. MAPK-K partially purified from v-raf-transformed cells or from mitogen-treated cells³ can be deactivated by phosphatase 2A. c-Raf-1 purified after mitogen stimulation can reactivate the phosphatase 2A-inactivated MAPK-K over 30-fold in vitro. c-Raf-1 reactivation of MAPK-K coincides with the selective phosphorylation at serine/threonine residues of a polypeptide with M_r 50,000 which coelutes precisely on cation-exchange chromatography with the MAPK-K activatable by c-Raf-1. These results indicate that c-Raf-1 is an immediate upstream activator of MAPK-K in vivo. To our knowledge, MAPK-K is the first physiological substrate of the c-raf-l protooncogene product to be identified.