Themes in Signaling   Binding of the SH2 domains of p85, the regulatory subunit of PI-3 kinase to tyrosine autophosphorylation sites on activated receptors releases an autoinhibitory constraint that stimulates the catalytic domain (p110). PI-3 kinase catalyzes the phosphorylation of the 3' positions of the inositol ring of PtdIns(4)P and PtdIns(4,5)P2 to generate PtdIns(3,4)P2 and PtdIns(3,4,5)P3 respectively.