eIF4E binding protein 1 and H-Ras are novel substrates for the protein kinase activity of class-I phosphoinositide 3-kinase.

Class-I phosphoinositide 3-kinases (PI 3-kinases) are dual specificity enzymes that possess both lipid and protein kinase activity. While the best characterized property of this protein kinase is as an autokinase activity, there have also been reports it can phosphorylate exogenous substrates including peptides, IRS-1 and PDE-3B. The identification of two novel potential protein substrates of PI 3-kinase is described here. By employing in vitro kinase assays using recombinant proteins as the substrates, it is shown that the translational regulator 4EBP1 becomes phosphorylated by the p110alpha and p110gamma isoforms of class-I PI 3-kinases. The lipid kinase activity of both these isoforms is increased by allosteric binding of H-Ras or betagamma subunits of heterotrimeric G proteins, but we find this is not the case for the protein kinase activity. Surprisingly though, a site on H-Ras is phosphorylated by p110alpha and p110gamma. This raises the possibility that these proteins could serve as physiological substrates for the protein kinase activity of PI 3-kinase and suggests this activity operates in a physiological context by phosphorylating substrates other than the PI 3-kinase itself. This may be particularly important in regulating the interaction of Ras with PI 3-kinase.