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Oncogene. 1998 Sep 24;17(12):1539-47.

Oncogenes, growth factors and phorbol esters regulate Raf-1 through common mechanisms.

Author information

1
Department of Medicine, Walther Oncology Center, Indiana University School of Medicine, Indianapolis 46202, USA.

Abstract

We have uniformly examined the regulatory steps required by oncogenic Ras, Src, EGF and phorbol 12-myristate 13-acetate (PMA) to activate Raf-1. Specifically, we determined the role of Ras binding and the phosphorylation of serines 338/339, tyrosines 340/341 and the activation loop (491-508) in response to these stimuli in COS-7 cells. An intact Ras binding domain was found to be essential for Raf-1 kinase activation by each stimulus, including PMA. Brief treatment of COS-7 cells with PMA was found to rapidly promote accumulation of the active, GTP-bound form of Ras. Furthermore, loss of the serine 338/339 and tyrosine 340/341 phosphorylation sites also blocked Raf-1 activation by all stimuli tested. Loss of the serine 497 and serine 499 PKCalpha phosphorylation sites failed to significantly reduce Raf-1 activation by any stimulus including PMA. Alanine substitution of all other potential phosphorylation sites within the Raf-1 activation loop had little or no effect on kinase regulation by Ras[V12] or vSrc although some mutants were less responsive to PMA. These results suggest that in mammalian cells, Raf-1 can be regulated by a variety of different stimuli through a common mechanism involving association with Ras-GTP and multiple phosphorylations of the amino-terminal region of the catalytic domain. Phosphorylation of the activation loop does not appear to be a significant mechanism of Raf-1 kinase regulation in COS-7 cells.

PMID:
9794231
DOI:
10.1038/sj.onc.1202061
[Indexed for MEDLINE]
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