Regulatory subunit of phosphoinositide-3-kinase (PI3K), a kinase that phosphorylates PtdIns(4,5)P2 (Phosphatidylinositol 4,5-bisphosphate) to generate phosphatidylinositol 3,4,5-trisphosphate (PIP3). PIP3 plays a key role by recruiting PH domain-containing proteins to the membrane, including AKT1 and PDPK1, activating signaling cascades involved in cell growth, survival, proliferation, motility and morphology. Binds to activated (phosphorylated) protein-tyrosine kinases, through its SH2 domain, and acts as an adapter, mediating the association of the p110 catalytic unit to the plasma membrane. Indirectly regulates autophagy. Promotes nuclear translocation of XBP1 in a ER stress- and/or insulin-dependent manner during metabolic overloading in the liver and hence plays a role in glucose tolerance improvement (By similarity). Heterodimer of a regulatory subunit PIK3R2 and a p110 catalytic subunit (PIK3CA, PIK3CB or PIK3CD). Interacts with AXL. Interacts with FLT1 (tyrosine-phosphorylated) and FLT4 (tyrosine-phosphorylated). Interacts with NYAP1, NYAP2 and MYO16. Interacts with FBXL2; PIK3R2 is a substrate of the SCF(FBXL2) complex. Interacts with PTPN13; dephosphorylates PIK3R2. Interacts with XBP1; the interaction is direct and induces translocation of XBP1 into the nucleus in a ER stress- and/or insulin-dependent but PI3K-independent manner. Interacts with PIK3R1; the interaction is dissociated in an insulin-dependent manner (By similarity). Interacts with SRC (By similarity). The SH2 2 domain is required for interaction with FBXL2 and PTPN13. Phosphorylated in response to signaling from activated receptor-type protein kinases. Dephosphorylated by PTPRJ. Dephosphorylated at Tyr-649 by PTPN13. Phosphorylation of Tyr-649 impairs while its dephosphorylation promotes interaction with FBXL2 and SCF(FBXL2)-mediated polyubiquitination. Ubiquitinated. Polyubiquitination by the SCF(FBXL2) complex probably promotes proteasomal degradation of PIK3R2. Belongs to the PI3K p85 subunit family.