Phosphatidylinositol 3-kinase (PI 3-kinase) is composed of 85 kDa (p85) and
110 kDa (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 (p85α and p85β), each possessing one SH3 and
two SH2 domains. Various p110 isoforms have been identified. p110α and
p110β interact with p85α, and p110α has also been shown to interact with
p85β in vitro. p110δ expression is restricted to white blood cells. It has been
shown to bind p85α and β, but it apparently does not phosphorylate these
subunits. p110δ seems to have the capacity to autophosphorylate. p110γ
does not interact with the p85 subunits. It has been shown to be activated
by α and βγ heterotrimeric G proteins.