Cloning of human p55 γ, a regulatory subunit of phosphatidylinositol 3-kinase, by a yeast two-hybrid library screen with the insulin-like growth factor-I receptor

We have used the yeast two-hybrid system to identify proteins that interact with the intracellular domain of the insulin-like growth factor-I receptor (IGFIR). In a search of a human fetal brain library we identified a cDNA encoding a protein that is the human homologue of mouse p55 PIK, a regulatory subunit of phosphatidylinositol 3-kinase (hp55 γ). The hp55 γ protein interacts strongly with the activated IGFIR but not with the kinase-negative mutant receptor. hp55 γ also interacts with the insulin receptor (IR) in the yeast two-hybrid system. The putative hp55 γ protein is composed of a unique amino terminal region followed by a proline-rich motif and two Src homology 2 (SH2) domains, which are highly homologous to those in mouse p55 PIK, rat p55 γ, human p85 α and bovine p85 β; it contains no SH3 domain. hp55 γ mRNAs are expressed in most human fetal and adult tissues with particularly high abundance in adult testis. Splice variant(s) of hp55 γ, one of which has a deletion of 36 amino acids at the amino terminus and another which has an insertion of 59 amino acids at position 256 between the SH2 domains, were also identified. A GST–hp55 γ fusion protein interacts in vitro with both the activated IGFIR and IR derived from mammalian cells. Our findings suggest that hp55 γ interacts with the IGFIR and IR and may be involved in PI 3-kinase activation by these receptors.