An Alternate Way to Keep Active

Science's STKE  23 Mar 2004:
Vol. 2004, Issue 225, pp. tw107-TW107
DOI: 10.1126/stke.2252004TW107

Overactivity of the Ras-Raf-MEK [mitogen-activated protein kinase (MAPK) orextracellular signal-regulated kinase (ERK) kinase]-ERK signaling pathway, through which numerous cytokines and growth factors regulate cell growth and proliferation, is associated with many cancers. More than 30 single site mutations of the serine-threonine kinase B-Raf (one of three Raf isoforms) have been identified in human cancers, prompting Wan et al. to investigate the mechanisms whereby these mutations--most of which involve the kinase domain--promote oncogenesis. Eighteen of 22 oncogenic B-Raf mutants analyzed yielded increased MEK phosphorylation in vitro compared with that by wild-type B-Raf (WTB-Raf) and, when expressed in COS cells, stimulated ERK phosphorylation and activity. However, three mutants with reduced in vitro kinase activity compared with that of WTB-Raf also activated ERK in COS cells and Xenopus embryos. These "impaired activity" mutants formed complexes with the C-Raf isoform (as did WTB-Raf and mutants with increased kinase activity) and, unlike WTB-Raf, activated C-Raf. The authors used RNA interference to show that C-Raf was necessary for ERK stimulation by the impaired activity mutants, but not by those with increased kinase activity. Structural analysis indicated that the B-Raf activation segment (within which Raf must be phosphorylated for kinase activity) is maintained in an inactive conformation through association with the nucleotide-binding P loop and that most oncogenic mutations were in these two regions. The authors developed a model in which disrupting the association between the P loop and the activation segment led to oncogenic B-Raf activity; impaired activity mutants (whose mutations also affect kinase function) must signal to ERK indirectly through the activation of C-Raf (see also Preview by Hubbard).

P. T. C. Wan, M. J. Garnett, S. M. Roe, S. Lee, D. Niculescu-Duvaz, V. M. Good, Cancer Genome Project, C. M. Jones, C. J. Marshall, C. J. Springer, D. Barford, R. Marais, Mechanism of activation of the RAF-ERK signaling pathway by oncogenic mutations of B-RAF. Cell 116, 855-867 (2004). [Online Journal]

S. R. Hubbard, Oncogenic mutations in B-Raf: Some losses yield gains. Cell 116, 764-766 (2004). [Online Journal]