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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

Selective activation of MEK1 but not MEK2 by A-Raf from epidermal growth factor-stimulated Hela cells.

Activation of the mitogen-activated protein kinase cascade is a critical event in mitogenic growth factor signal transduction. Mitogen-activated protein kinase is directly activated by a dual specific kinase, MEK, which itself is activated by serine phosphorylation. The c-Raf kinase has been implicated in mediating the signal transduction from mitogenic growth factor receptors to MEK activation. Recently, the B-Raf kinase was shown to be capable of phosphorylating and activating MEK as a result of growth factor stimulation. In this report, we used the yeast two-hybrid screening to isolate MEK interacting proteins. All three members of the Raf family kinases were identified as positive clones when the mutant MEK1S218/222A, in which the two phosphorylation serine residues were substituted by alanines, was used as a bait, whereas no positive clones were isolated when the wild type MEK1 was used as a bait in a similar screening. These results suggest that elimination of the phosphorylation sites of a target protein ( MEK1 in our study) may stabilize the interaction between the kinase (Raf) and its substrate ( MEK1), possibly due the formation of a nonproductive complex. These observations seem to suggest a general strategy using mutants to identify the upstream kinase of a phosphoprotein or the downstream targets of a kinase. Although c-Raf and B-Raf have been implicated in growth factor- induced MEK activation, little is known about A-Raf. We observed that stimulation of Hela cells with epidermal growth factor resulted in a rapid and transient activation of A-Raf, which is then capable of phosphorylating and activating MEK1. Interestingly, A-Raf does not activate MEK2, although c-Raf can activate both MEK1 and MEK2. Our data demonstrated that A-Raf is, indeed, a MEK1 activator and may play a role in growth factor signaling.[1]


  1. Selective activation of MEK1 but not MEK2 by A-Raf from epidermal growth factor-stimulated Hela cells. Wu, X., Noh, S.J., Zhou, G., Dixon, J.E., Guan, K.L. J. Biol. Chem. (1996) [Pubmed]
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