title: Glycoproteins V and Ib-IX form a noncovalent complex in the platelet membrane.
abstract: Platelet glycoprotein (GP) V is a Mr 82,000 plasma membrane protein of unknown function that is cleaved by the potent platelet agonist, thrombin, to yield a Mr 69,500 fragment (GPVf1). Platelet GPIb, a disulfide-linked alpha beta heterodimer (Mr 160,000) that forms a noncovalent complex with GPIX (Mr 22,000), functions as the platelet adhesion receptor for surface-bound von Willebrand factor. Association between GPV and GPIb-IX has been suggested by the finding that both proteins are deficient in the Bernard-Soulier syndrome, a bleeding disorder characterized by giant platelets and defective interaction with von Willebrand factor. Here we report that GPV and GPIb-IX are coprecipitated by monoclonal antibodies (mAbs) against GPV, GPIb, or GPIX when platelets are solubilized in the mild detergent, digitonin. Treatment of digitonin immunopreciptates with the nonionic detergent, Nonidet P-40, released GPV from anti-GPIb and anti-GPIX mAb precipitates and GPIb-IX from the anti-GPV mAb precipitate. Removal of the Mr 45,000 amino-terminal part of GPIb alpha by treatment with elastase did not abrogate association of GPV with GPIb-IX, showing that the leucine-rich repeat sequences in GPIb alpha are not required for complex formation. Binding studies with 125I-labeled mAbs showed the presence of 24,370 GPIb-IX complexes and 11,170 molecules of GPV/platelet (n = 5). These data show that the leucine-rich glycoproteins GPV and GPIb-IX form a noncovalent complex in the platelet membrane. GPV may play a role in the interaction of platelets with von Willebrand factor.
authors.affiliation: Department of Immunohematology, Central Laboratory of the Netherlands Red Cross Blood Transfusion Service, Amsterdam.
authors.forename: P W
authors.initials: PW
authors.lastname: Modderman
authors.forename: L G
authors.initials: LG
authors.lastname: Admiraal
authors.forename: A
authors.initials: A
authors.lastname: Sonnenberg
authors.forename: A E
authors.initials: AE
authors.lastname: von dem Borne
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003902:Detergents; D004072:Digitonin / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D051379:Mice; D008807:Mice, Inbred BALB C; D017830:Octoxynol; D010974:Platelet Aggregation; D010980:Platelet Membrane Glycoproteins / Q000276:immunology / Q000378:metabolism*; D011092:Polyethylene Glycols; D011233:Precipitin Tests
chemical_list: D000911:Antibodies, Monoclonal; D003902:Detergents; D010980:Platelet Membrane Glycoproteins; D011092:Polyethylene Glycols; D017830:Octoxynol; C010615:Nonidet P-40; D004072:Digitonin