The activation of phosphatidylinositol 3-kinase by Ras.
BACKGROUND
Activation of the mammalian phosphatidylinositol 3-kinase complex can play a critical role in transducing growth factor responses. The lipid kinase complex, which is made up of p85 alpha and p110 alpha regulatory and catalytic subunits, becomes associated with a number of activated receptor protein tyrosine kinases, but the mechanism of its activation has not yet been defined. Recent evidence indicates that Ras can bind to the p85 alpha/p110 alpha complex. We describe here the functional regulation of the mammalian phosphatidylinositol 3-kinase complex by Ras.


RESULTS
Expression of p110 alpha, the catalytic subunit of phosphatidylinositol 3-kinase, in the fission yeast, Schizosaccharomyces pombe, has been used to demonstrate an inhibitory effect of p85 alpha on p110 alpha activity in intact cells; inhibition did not result from a decrease in p110 alpha expression. In this cellular context, we have investigated the effect of a constitutively active mutant of Ras, v-Ras, either on p85 alpha or p110 alpha-alone, or on the p85 alpha/p110 alpha complex. In the presence of the p85 alpha/p110 alpha complex, v-Ras suppressed cell growth, but an effector-domain mutant of v-Ras did not. The growth-suppressive effect of v-Ras was not seen for any other combination of expressed proteins. The phenotype induced by v-Ras was consistent with activation of the p85 alpha/p110 alpha complex: it was sensitive to the phosphatidylinositol 3-kinase inhibitor, wortmannin, and the cells accumulated 3-phosphorylated polyphosphoinositides. Activation of purified p85 alpha/p110 alpha by purified recombinant Ras in vitro was also demonstrated.


CONCLUSIONS
The phosphatidylinositol 3-kinase complex, p85 alpha/p110 alpha, shows a suppressed catalytic function in vivo when compared with free p110 alpha. This complex can, however, be activated by Ras. We suggest that the phosphatidylinositol 3-kinase p85 alpha/p110 alpha complex is a downstream effector of Ras.
Kodaki
T
T
Protein Phosphorylation Laboratory, Imperial Cancer Research Fund, London, UK.
Woscholski
R
R
Hallberg
B
B
Rodriguez-Viciana
P
P
Downward
J
J
Parker
P J
PJ
D000818:Animals; D002417:Cattle; D004789:Enzyme Activation / Q000187:drug effects; D015870:Gene Expression; D066298:In Vitro Techniques; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D012568:Schizosaccharomyces / Q000201:enzymology / Q000235:genetics; D015398:Signal Transduction; D018631:ras Proteins / Q000235:genetics / Q000494:pharmacology*
D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D018631:ras Proteins