title: Negative regulation of the serine/threonine kinase B-Raf by Akt.
abstract: B-Raf contains multiple Akt consensus sites located within its amino-terminal regulatory domain. One site, Ser(364), is conserved with c-Raf but two additional sites, Ser(428) and Thr(439), are unique to B-Raf. We have investigated the role of both the conserved and unique phosphorylation sites in the regulation of B-Raf activity in vitro and in vivo. We show that phosphorylation of B-Raf by Akt occurs at multiple residues within its amino-terminal regulatory domain, at both the conserved and unique phosphorylation sites. The alteration of the serine residues within the Akt consensus sites to alanines results in a progressive increase in enzymatic activity in vitro and in vivo. Furthermore, expression of Akt inhibits epidermal growth factor-induced B-Raf activity and inhibition of Akt with LY294002 up-regulates B-Raf activity, suggesting that Akt negatively regulates B-Raf in vivo. Our results demonstrate that B-Raf activity can be negatively regulated by Akt through phosphorylation in the amino-terminal regulatory domain of B-Raf. This cross-talk between the B-Raf and Akt serine/threonine kinases is likely to play an important role in modulating the signaling specificity of the Ras/Raf pathway and in promoting biological outcome.
authors.affiliation: Department of Biological Chemistry and the Institute of Gerontology, University of Michigan, Ann Arbor, Michigan 48109, USA.
authors.forename: K L
authors.initials: KL
authors.lastname: Guan
authors.forename: C
authors.initials: C
authors.lastname: Figueroa
authors.forename: T R
authors.initials: TR
authors.lastname: Brtva
authors.forename: T
authors.initials: T
authors.lastname: Zhu
authors.forename: J
authors.initials: J
authors.lastname: Taylor
authors.forename: T D
authors.initials: TD
authors.lastname: Barber
authors.forename: A B
authors.initials: AB
authors.lastname: Vojtek
mesh_terms: D000595:Amino Acid Sequence; D002460:Cell Line; D016384:Consensus Sequence; D004789:Enzyme Activation; D006801:Humans; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D011233:Precipitin Tests; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics / Q000378:metabolism*
chemical_list: D011518:Proto-Oncogene Proteins; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019908:Proto-Oncogene Proteins c-raf