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XB-ART-22070
Mol Cell Biol 1993 Nov 01;1311:7170-9. doi: 10.1128/mcb.13.11.7170-7179.1993.
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Critical tyrosine residues regulate the enzymatic and biological activity of Raf-1 kinase.

Fabian JR , Daar IO , Morrison DK .


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The serine/threonine kinase activity of the Raf-1 proto-oncogene product is stimulated by the activation of many tyrosine kinases, including growth factor receptors and pp60v-src. Recent studies of growth factor signal transduction pathways demonstrate that Raf-1 functions downstream of activated tyrosine kinases and p21ras and upstream of mitogen-activated protein kinase. However, coexpression of both activated tyrosine kinases and p21ras is required for maximal activation of Raf-1 in the baculovirus-Sf9 expression system. In this study, we investigated the role of tyrosine kinases and tyrosine phosphorylation in the regulation of Raf-1 activity. Using the baculovirus-Sf9 expression system, we identified Tyr-340 and Tyr-341 as the major tyrosine phosphorylation sites of Raf-1 when coexpressed with activated tyrosine kinases. Introduction of a negatively charged residue that may mimic the effect of phosphorylation at these sites activated the catalytic activity of Raf-1 and generated proteins that could transform BALB/3T3 cells and induce the meiotic maturation of Xenopus oocytes. In contrast, substitution of noncharged residues that were unable to be phosphorylated produced a protein that could not be enzymatically activated by tyrosine kinases and that could block the meiotic maturation of oocytes induced by components of the receptor tyrosine kinase pathway. These findings demonstrate that maturation of the tyrosine phosphorylation sites can dramatically alter the function of Raf-1. In addition, this is the first report that a transforming Raf-1 protein can be generated by a single amino acid substitution.

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Species referenced: Xenopus laevis
Genes referenced: raf1 src

References [+] :
Beck, The complete coding sequence of the human A-raf-1 oncogene and transforming activity of a human A-raf carrying retrovirus. 1987, Pubmed