title: Tyrosine phosphorylation of P-selectin in intact platelets and in a disulphide-linked complex with immunoprecipitated pp60c-src.
abstract: P-selectin is a 140 kDa membrane glycoprotein found in secretory granules of platelets and endothelial cells where it is rapidly translocated to the plasma membrane upon cell activation. It then functions as a receptor for various types of leucocytes. Metabolic labelling of resting platelets with 32Pi showed that P-selectin is primarily phosphorylated on serine residues, although some tyrosine phosphorylation was observed as well. However, tyrosine phosphorylation of P-selectin was greatly stimulated by treatment with the permeating phosphatase inhibitor, pervanadate. When P-selectin immunoprecipitates were incubated with [gamma-32P]ATP (in vitro kinase assay), a fraction of P-selectin was phosphorylated on its tyrosine residues by a co-precipitated kinase. P-selectin phosphorylated in vitro co-migrated with 140 kDa surface-labelled 125I-P-selectin during SDS/PAGE under reducing conditions. Under non-reducing conditions, however, phosphorylated P-selectin was disulphide-linked to unknown protein(s) in a 205 kDa complex. In vitro kinase assays of the most abundant platelet tyrosine kinase, pp60c-src, demonstrated the presence of similar 140 and 205 kDa phosphorylated proteins in SDS/PAGE under reducing and non-reducing conditions respectively. Extraction and reprecipitation studies with proteins phosphorylated in vitro indicated that P-selectin and pp60c-src form a 205 kDa 1:1 disulphide-linked complex. In the complex, pp60c-src autophosphorylation is inhibited and P-selectin is phosphorylated on tyrosine residues. As protein disulphides in the cytoplasm of intact cells are extremely rare, our results suggest that P-selectin and pp60c-src, which co-localize in platelet dense granules, may be non-covalently associated and spontaneously form disulphide bridges during lysis. In addition, the observed tyrosine phosphorylation of P-selectin in intact platelets suggests that its function might be regulated by phosphorylation by pp60c-src.
authors.affiliation: Department of Immunohaematology, The Netherlands Red Cross Blood Transfusion Service, Amsterdam.
authors.forename: P W
authors.initials: PW
authors.lastname: Modderman
authors.forename: A E
authors.initials: AE
authors.lastname: von dem Borne
authors.forename: A
authors.initials: A
authors.lastname: Sonnenberg
mesh_terms: D001792:Blood Platelets / Q000378:metabolism*; D004220:Disulfides / Q000737:chemistry*; D006801:Humans; D066298:In Vitro Techniques; D019007:P-Selectin; D010766:Phosphorylation; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism*; D011233:Precipitin Tests; D017027:Protein Tyrosine Phosphatases / Q000037:antagonists & inhibitors; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D016392:Proto-Oncogene Proteins pp60(c-src) / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism*; D014638:Vanadates / Q000494:pharmacology
chemical_list: D004220:Disulfides; D019007:P-Selectin; D010980:Platelet Membrane Glycoproteins; C076715:pervanadate; D014638:Vanadates; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; D016392:Proto-Oncogene Proteins pp60(c-src); D017027:Protein Tyrosine Phosphatases