Autophosphorylation of serine 608 in the p85 regulatory subunit of wild type or cancer-associated mutants of phosphoinositide 3-kinase does not affect its lipid kinase activity

ABSTRACT: BACKGROUND: The alpha-isoform of the Type 1A Phosphoinositide 3-kinases (PI3Kalpha) has protein kinase activity as well as phosphoinositide lipid kinase activity. The best described substrate for its protein kinase activity is its regulatory subunit, p85alpha, which becomes phosphorylated on Serine 608. Phosphorylation of Serine 608 has been reported to down-regulate its lipid kinase activity. RESULTS: We have assessed whether oncogenic mutants of PI3Kalpha, which have up-regulated lipid kinase activity, have altered levels of Serine 608 phosphorylation compared to wild type PI3Kalpha, and whether differential phosphorylation of Serine 608 contributes to increased activity of oncogenic forms of PI3Kalpha with point mutations in the helical or the kinase domains. Despite markedly increased lipid kinase activity, protein kinase activity was not altered in oncogenic compared to wild type forms of PI3Kalpha. By manipulating levels of phosphorylation of Serine 608 in vitro, we found no evidence that the protein kinase activity of PI3Kalpha affects its phosphoinositide lipid kinase activity in either wild-type or oncogenic mutants of PI3Kalpha. CONCLUSIONS: Phosphorylation of p85alpha S608 is not a significant regulator of wild-type or oncogenic PI3Kalpha lipid kinase activity.