The identification and characterization of an epidermal growth factor-stimulated phosphorylation of a specific low molecular weight GTP-binding protein in a reconstituted phospholipid vesicle system

The abilities of different GTP-binding proteins to serve as phosphosubstrates for the epidermal growth factor (EGF) receptor/tyrosine kinase have been examined in reconstituted phospholipid vesicle systems. During the course of these studies we discovered that a low molecular mass, high affinity GTP-binding protein from bovine brain (designated as the 22-kDa protein) served as an excellent phosphosubstrate for the tyrosine-agarose-purified human placental EGF receptor. The EGF-stimulated phosphorylation of the purified 22-kDa protein occurs on tyrosine residues, with stoichiometries approaching 2 mol of ³²P(i) incorporated/mol of [³⁵S]guanosine 5'-O-(3-thiotriphosphate) (GTPγS)-binding sites. The EGF-stimulated phosphorylation of the brain 22-kDa protein requires its reconstitution into phospholipid vesicles. No phosphorylation of this GTP-binding protein is detected if it is simply mixed with the purified EGF receptor in detergent solution or if detergent is added back to lipid vesicles containing the EGF receptor and the 22-kDa protein. The EGF-stimulated phosphorylation of this GTP-binding protein is also markedly attenuated by guanine nucleotides, i.e. GTP, GTPγS, or GDP, suggesting that maximal phosphorylation occurs when the GTP-binding protein is in a guanine nucleotide-depleted state. Purified preparations of the 22-kDa phosphosubstrate do not cross-react with antibodies against the ras proteins. However, they do cross-react against two different peptide antibodies generated against specific sequences of the human platelet (and placental) GTP-binding protein originally designated Gp (Evans, T., Brown, M.L., Fraser, E.D., and Northrup, J.K. (1986) J. Biol. Chem. 261, 7052-7059) and more recently named G25K (Polakis, P.G., Synderman, R., and Evans, T. (1989) Biochem. Biophys. Res. Commun. 160, 25-32). When highly purified preparations of the human platelet Gp (G25K) protein are reconstituted with the purified EGF receptor into phospholipid vesicles, an EGF-stimulated phosphorylation of the platelet GTP-binding protein occurs with a stoichiometry approaching 2 mol of ³²P(i) incorporated/mol of [³⁵S]GTPγS-binding sites. As is the case for the brain 22-kDa protein, the EGF-stimulated phosphorylation of the platelet GTP-binding protein is attenuated by guanine necleotides. Overall, these results suggest that the brain 22-kDa phosphosubstrate for the EGF receptor is very similar, if not identical, to the Gp (G25K) protein. Although guanine nucleotide binding to the brain 22-kDa protein or to the platelet GTP-binding protein inhibits phosphorylation, the phosphorylated GTP-binding proteins appear to bind [³⁵S]GTPγS slightly better than their nonphosphorylated counterparts.