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PI3-kinase phosphorylates the D-3 position of the inositol ring of phosphatidylinositol (PtdIns), PtdIns(4)P and PtdIns(4,5)P2 to produce the respective PI3-phosphorylated derivatives. PI3-kinase exists as a heterodimer of 85 kDa (p85) and 110 kDa (p110) subunits. The p85 subunit contains two SH2 domains and an SH3 domain. It associates with and serves as a substrate for activated growth factor receptor tyrosine kinases. p85 may serve as regulator of the catalytic subunit, p110, by acting as the link between PI3-kinase and the ligand-activated receptor. Two distinct forms of the p85 subunit have been described: 1) p85 α; , which binds tightly to the catalytic subunit, and 2) p85ß, a protein whose function is presently unknown. Both isoforms bind to activated receptors and serve as tyrosine kinase substrates.
