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Mol. Cell. Biol., Feb 1997, 732-741, Vol 17, No. 2
H Cai, U Smola, V Wixler, I Eisenmann-Tappe, MT Diaz-Meco, J Moscat, U Rapp and GM Cooper
The Raf protein kinases function downstream of Ras guanine nucleotide-
binding proteins to transduce intracellular signals from growth factor
receptors. Interaction with Ras recruits Raf to the plasma membrane, but
the subsequent mechanism of Raf activation has not been established.
Previous studies implicated hydrolysis of phosphatidylcholine (PC) in Raf
activation; therefore, we investigated the role of the epsilon isotype of
protein kinase C (PKC), which is stimulated by PC-derived diacylglycerol,
as a Raf activator. A dominant negative mutant of PKC epsilon inhibited
both proliferation of NIH 3T3 cells and activation of Raf in COS cells.
Conversely, overexpression of active PKC epsilon stimulated Raf kinase
activity in COS cells and overcame the inhibitory effects of dominant
negative Ras in NIH 3T3 cells. PKC epsilon also stimulated Raf kinase in
baculovirus-infected Spodoptera frugiperda Sf9 cells and was able to
directly activate Raf in vitro. Consistent with its previously reported
activity as a Raf activator in vitro, PKC alpha functioned similarly to PKC
epsilon in both NIH 3T3 and COS cell assays. In addition, constitutively
active mutants of both PKC alpha and PKC epsilon overcame the inhibitory
effects of dominant negative mutants of the other PKC isotype, indicating
that these diacylglycerol-regulated PKCs function as redundant activators
of Raf-1 in vivo.
Copyright © 1997, American Society for Microbiology
Role of diacylglycerol-regulated protein kinase C isotypes in growth factor activation of the Raf-1 protein kinase
Division of Molecular Genetics, Dana-Farber Cancer Institute, and Department of Pathology, Harvard Medical School, Boston, Massachusetts 02115, USA.
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