Phosphoinositide-3-Kinase Regulatory Subunit 3 (PIK3R3)

The predicted 402-amino acid PIK3R3 protein shares 96% amino acid identity with mouse p55(Pik) and 95% identity with rat p55-gamma; it also shares homology with human p85-alpha (PIK3R1). Like other regulatory subunits of PI 3-kinase, PIK3R3 contains 2 Src homology-2 (SH2) domains, an inter-SH2/p110-binding site, and a proline-rich motif. The N-terminal region also contains a YXXM motif, a potential SH2-domain binding site.The alternatively spliced regions were not necessary for interaction with IGFIR or insulin receptor (INSR) in yeast 2-hybrid assays. Northern blot analysis detected a 5.8-kb PIK3R3 transcript in all fetal and adult tissues examined except peripheral blood leukocytes. Expression was hardly detectable in fetal liver, low in adult liver, and most abundant in adult testis.