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Mol. Cell. Biol., 04 1997, 1840-1847, Vol 17, No. 4
P van der Geer, M Henkemeyer, T Jacks and T Pawson
The Ras guanine nucleotide-binding protein functions as a molecular switch
in signalling downstream of protein-tyrosine kinases. Ras is activated by
exchange of GDP for GTP and is turned off by hydrolysis of bound GTP to
GDP. Ras itself has a low intrinsic GTPase activity that can be stimulated
by GTPase-activating proteins (GAPs), including p120- Gap and
neurofibromin. These GAPs possess a common catalytic domain but contain
distinct regulatory elements that may couple different external signals to
control of the Ras pathway. p120-Gap, for example, has two N- terminal SH2
domains that directly recognize phosphotyrosine motifs on activated growth
factor receptors and cytoplasmic phosphoproteins. To analyze the role of
p120-Gap in Ras regulation in vivo, we have used fibroblasts derived from
mouse embryos with a null mutation in the gene for p120-Gap (Gap).
Platelet-derived growth factor stimulation of Gap-/- cells led to an
abnormally large increase in the level of Ras-GTP and in the duration of
mitogen-activated protein (MAP) kinase activation compared with wild-type
cells, suggesting that p120-Gap is specifically activated following growth
factor stimulation. Induction of DNA synthesis in response to
platelet-derived growth factor and morphological transformation by the
v-src and EJ-ras oncogenes were not significantly affected by the absence
of p120-Gap. However, we found that normal tyrosine phosphorylation of
p190-rhoGap, a cytoplasmic protein that associates with the p120-Gap SH2
domains, was dependent on the presence of p120-Gap. Our results suggest
that p120-Gap has specific functions in downregulating the Ras/MAP kinase
pathway following growth factor stimulation, and in modulating the
phosphorylation of p190-rhoGap, but is not required for mitogenic
signalling.
Copyright © 1997, American Society for Microbiology
Aberrant Ras regulation and reduced p190 tyrosine phosphorylation in cells lacking p120-Gap
Programme in Molecular Biology and Cancer, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada.
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