title: Identification of Raf-1 S471 as a novel phosphorylation site critical for Raf-1 and B-Raf kinase activities and for MEK binding.
abstract: The Ras-Raf-MAPK cascade is a key growth-signaling pathway and its uncontrolled activation results in cell transformation. Although the general features of the signal transmission along the cascade are reasonably defined, the mechanisms underlying Raf activation remain incompletely understood. Here, we show that Raf-1 dephosphorylation, primarily at epidermal growth factor (EGF)-induced sites, abolishes Raf-1 kinase activity. Using mass spectrometry, we identified five novel in vivo Raf-1 phosphorylation sites, one of which, S471, is located in subdomain VIB of Raf-1 kinase domain. Mutational analyses demonstrated that Raf-1 S471 is critical for Raf-1 kinase activity and for its interaction with mitogen-activated protein kinase kinase (MEK). Similarly, mutation of the corresponding B-Raf site, S578, resulted in an inactive kinase, suggesting that the same Raf-1 and B-Raf phosphorylation is needed for Raf kinase activation. Importantly, the naturally occurring, cancer-associated B-Raf activating mutation V599E suppressed the S578A mutation, suggesting that introducing a charged residue at this region eliminates the need for an activating phosphorylation. Our results demonstrate an essential role of specific EGF-induced Raf-1 phosphorylation sites in Raf-1 activation, identify Raf-1 S471 as a novel phosphorylation site critical for Raf-1 and B-Raf kinase activities, and point to the possibility that the V599E mutation activates B-Raf by mimicking a phosphorylation at the S578 site.
authors.affiliation: Cardiovascular Research Institute, The Texas A&M University Health Science Center, Temple, TX 76504, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhu
authors.forename: Vitaly
authors.initials: V
authors.lastname: Balan
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Bronisz
authors.forename: Karina
authors.initials: K
authors.lastname: Balan
authors.forename: Hengrui
authors.initials: H
authors.lastname: Sun
authors.forename: Deborah T
authors.initials: DT
authors.lastname: Leicht
authors.forename: Zhijun
authors.initials: Z
authors.lastname: Luo
authors.forename: Jun
authors.initials: J
authors.lastname: Qin
authors.forename: Joseph
authors.initials: J
authors.lastname: Avruch
authors.forename: Guri
authors.initials: G
authors.lastname: Tzivion
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular; D004815:Epidermal Growth Factor / Q000502:physiology; D050883:HSC70 Heat-Shock Proteins / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D013058:Mass Spectrometry; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D010766:Phosphorylation; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics / Q000378:metabolism*; D012694:Serine / Q000378:metabolism
chemical_list: D050883:HSC70 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D012694:Serine; D004815:Epidermal Growth Factor; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf; D020929:Mitogen-Activated Protein Kinase Kinases