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 (p85alpha and p85beta), each possessing one SH3 and two SH2 domains. Various p110 isoforms have been identified. p110alpha and p110beta interact with p85alpha, and p110alpha has also been shown to interact with p85beta in vitro. p110delta expression is restricted to white blood cells. It has been shown to bind p85alpha and p85beta, but it apparently does not phosphorylate these subunits. p110delta seems to have the capacity to autophosphorylate. p110gamma does not interact with the p85 subunits. It has been shown to be activated by alpha and betagamma heterotrimeric G proteins.