Activation loop sequences confer substrate specificity to phosphoinositide 3-kinase α (PI3Kα) -: Functions of lipid kinase-deficient PI3Kα in signaling

Phosphoinositide 3-kinases (PI3Ks) are dual specificity lipid and protein kinases. While the lipid-dependent PI3K downstream signaling is well characterized, little is known about PI3K protein kinase signaling and structural determinants of lipid substrate specificity across the various PI3K classes. Here we show that sequences C-terminal to the PI3K ATP-binding site determine the lipid substrate specificity of the class IA PI3K alpha (p85/p110 alpha). Transfer of such activation loop sequences from class II PI3Ks, class III PI3Ks, and a related mammalian target of rapamycin (FRaP) into p110 alpha turns the lipid substrate specificity of the resulting hybrid protein into that of the donor protein, while leaving the protein kinase activity unaffected. All resulting hybrids lacked the ability to produce phosphatidylinositol 3,4,5-trisphosphate in intact cells. Amino acid substitutions and structure modeling showed that two conserved positively charged (Lys and Arg) residues in the activation loop are crucial for the functionality of class I PI3Ks as phosphatidylinositol 4,5-bisphosphate kinases. By transient transfecion of 293 cells, we show that p110 alpha hybrids, although unable to support lipid-dependent PI3K signaling, such as activation of protein kinase B/Akt and p70(S6k), retain the capability to associate with and phosphorylate insulin receptor substrate-1, with the same specificity and higher efficacy than wild type PI3K alpha. Our data lay the basis for the understanding of the class I PI3K substrate selectivity and for the use of PI3K alpha hybrids to dissect PI3K alpha function as lipid and protein kinase.