Intermolecular Interactions of the p85a Regulatory Subunit of Phosphatidylinositol 3-Kinase*

The regulatory subunit of phosphatidylinositol 3-ki- nase, p85, contains a number of well defined domains involved in protein-protein interactions, including an SH3 domain and two SH2 domains. In order to investi- gate in detail the nature of the interactions of these domains with each other and with other binding part- ners, a series of deletion and point mutants was con- structed, and their binding characteristics and appar- ent molecular masses under native conditions were analyzed. The SH3 domain and the first proline-rich motif bound each other, and variants of p85 containing the SH3 and BH domains and the first proline-rich motif were dimeric. Analysis of the apparent molecular mass of the deletion mutants indicated that each of these domains contributed residues to the dimerization inter- face, and competition experiments revealed that there were intermolecular SH3 domain-proline-rich motif in- teractions and BH-BH domain interactions mediating dimerization of p85a both in vitro and in vivo. Binding of SH2 domain ligands did not affect the dimeric state of p85a. Recently, roles for the p85 subunit have been pos- tulated that do not involve the catalytic subunit, and if p85 exists on its own we propose that it would be dimeric.