title: Epidermal growth factor, but not insulin, stimulates tyrosine phosphorylation of an endogenous protein of Mr 95,000 in triton extracts of human placental syncytiotrophoblast membranes.
abstract: 1. Triton extracts of syncytiotrophoblast membranes were incubated with [gamma-32P]ATP, MgCl2 and MnCl2. Addition of epidermal growth factor (EGF) resulted in increased phosphorylation not only of the EGF receptor and a Mr-35,000 protein as previously described, but also a protein of Mr 95,000 on both tyrosine and serine residues. In addition, a small increase in the phosphorylation of a protein of Mr 105,000 was observed. Spermine had a similar effect on the phosphorylation of the Mr-95,000 protein, without affecting the phosphorylation of the other proteins. In the absence of MnCl2, the effect of spermine on the phosphorylation of Mr-95,000 protein was still evident, whereas that of EGF was greatly diminished. 2. The Mr-95,000 protein bound poorly to wheat-germ-lectin-Sepharose and was not precipitated by antisera specific for insulin and EGF receptors. The protein continued to exhibit serine and tyrosine phosphorylation on addition of [gamma-32P]ATP, MgCl2 and MnCl2 to a glycoprotein-depleted fraction prepared by chromatography on wheat-germ-lectin-Sepharose. The extent of phosphorylation was no longer increased by spermine or EGF, but was inhibited by heparin. 3. It is suggested that the Mr-95,000 protein not only is a possible direct substrate for the EGF-receptor (but not the insulin receptor) tyrosine kinase but is a substrate for other endogenous kinases, including a protein tyrosine kinase which is probably not a glycoprotein, and a protein serine kinase with properties similar to those of casein kinase II.
authors.affiliation: Department of Biochemistry, University of Bristol Medical School, U.K.
authors.forename: J M
authors.initials: JM
authors.lastname: Tavaré
authors.forename: T A
authors.initials: TA
authors.lastname: Diggle
authors.forename: R M
authors.initials: RM
authors.lastname: Denton
mesh_terms: D000596:Amino Acids / Q000032:analysis; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004815:Epidermal Growth Factor / Q000494:pharmacology*; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D010766:Phosphorylation; D011506:Proteins / Q000378:metabolism*; D013096:Spermine / Q000494:pharmacology; D013268:Stimulation, Chemical; D014327:Trophoblasts / Q000187:drug effects / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D007328:Insulin; D011506:Proteins; D013096:Spermine; D014443:Tyrosine; D004815:Epidermal Growth Factor; D006493:Heparin