Phosphatidylinositol 3-kinase (PI 3-kinase) is composed of p85 and p110 subunits. p85 lacks PI 3-kinase activity and acts as an adapter, coupling p110 to activated protein tyrosine kinase. Two forms of p85 have been described (p85(alpha) and p85(beta)), each possessing one SH3 and two SH2 domains. Various p110 isoforms have been identified. p110(alpha) and p110(beta) interact with p85(alpha), and p110(alpha) has also been shown to interact with p85(beta) in vitro. p110(delta) expression is restricted to white blood cells. It has been shown to bind p85(alpha) and p85(beta), but it apparently does not phosphorylate these subunits. p110(delta) seems to have the capacity to autophosphorylate. p110(gamma) does not interact with the p85 subunits. It has been shown to be activated by (alpha) and (beta)(gamma) heterotrimeric G proteins.