Phosphatidylinositol 3-kinase regulates Raf1 through Pak phosphorylation of serine 338.
We have previously shown that inhibition of phosphatidylinositol (PI) 3-kinase severely attenuates the activation of extracellular signal-regulated kinase (Erk) following engagement of integrin/fibronectin receptors and that Raf is the critical target of PI 3-kinase regulation [1]. To investigate how PI 3-kinase regulates Raf, we examined sites on Raf1 required for regulation by PI 3-kinase and explored the mechanisms involved in this regulation. Serine 338 (Ser338), which was critical for fibronectin stimulation of Raf1, was phosphorylated in a PI 3-kinase-dependent manner following engagement of fibronectin receptors. In addition, fibronectin activation of a Raf1 mutant containing a phospho-mimic mutation (S338D) was independent of PI 3-kinase. Furthermore, integrin-induced activation of the serine/threonine kinase Pak-1, which has been shown to phosphorylate Raf1 Ser338, was also dependent on PI 3-kinase activity and expression of a kinase-inactive Pak-1 mutant blocked phosphorylation of Raf1 Ser338. These results indicate that PI 3-kinase regulates phosphorylation of Raf1 Ser338 through the serine/threonine kinase Pak. Thus, phosphorylation of Raf1 Ser338 through PI 3-kinase and Pak provides a co-stimulatory signal which together with Ras leads to strong activation of Raf1 kinase activity by integrins.
Chaudhary
A
A
Department of Cell Biology, Harvard Medical School, Boston, 02115, USA.
King
W G
WG
Mattaliano
M D
MD
Frost
J A
JA
Diaz
B
B
Morrison
D K
DK
Cobb
M H
MH
Marshall
M S
MS
Brugge
J S
JS
D000818:Animals; D019556:COS Cells; D016023:Integrins / Q000378:metabolism; D016296:Mutagenesis; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics / Q000378:metabolism*; D012694:Serine / Q000235:genetics / Q000378:metabolism*; D054462:p21-Activated Kinases
D016023:Integrins; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D012694:Serine; D054462:p21-Activated Kinases