title: Examination of the platelet membrane glycoprotein IIb-IIIa complex and its interaction with fibrinogen and other ligands by electron microscopy.
abstract: The platelet integrin, glycoprotein IIb-IIIa (GPIIb-IIIa), is a calcium-dependent heterodimer that binds fibrinogen, von Willebrand factor, and fibronectin after platelet activation. We examined GPIIb-IIIa alone and bound to these ligands by electron microscopy after rotary shadowing with platinum/tungsten. We found, as observed previously, that in the presence of detergent and 2 mM Ca2+, GPIIb-IIIa consists of an 8 x 12-nm globular head with two 18-nm flexible tails extending from one side. We also found that in the presence of EDTA, GPIIb-IIIa dissociates into two similar comma-shaped subunits, each containing a portion of the globular head and a single tail. Using monoclonal antibodies to GPIIb, GPIIIa, and the GPIIb-IIIa heterodimer, we found that the tails contained the carboxyl termini of each subunit, while the nodular head was composed of amino-terminal segments of both subunits. Electron microscopy of GPIIb-IIIa bound to fibrinogen revealed a highly specific interaction of the nodular head of GPIIb-IIIa with the distal end of the trinodular fibrinogen molecule and with the tails of GPIIb-IIIa extended laterally at an angle of approximately 98 degrees with respect to the long axis of fibrinogen. When a GPIIb-IIIa was bound to each end of a single fibrinogen, the tails were oriented to opposite sides of fibrinogen, enabling fibrinogen to bridge two adjacent platelets. Electron microscopy of GPIIb-IIIa bound to fibronectin revealed GPIIb/IIIa-binding sites approximately two-thirds of the distance from the amino terminus of each end of the fibronectin molecule, while GPIIb-IIIa was found to bind to von Willebrand factor protomers along a rod-like region near the central nodule of the molecule.
authors.affiliation: Department of Anatomy, University of Pennsylvania School of Medicine, Philadelphia 19104.
authors.forename: J W
authors.initials: JW
authors.lastname: Weisel
authors.forename: C
authors.initials: C
authors.lastname: Nagaswami
authors.forename: G
authors.initials: G
authors.lastname: Vilaire
authors.forename: J S
authors.initials: JS
authors.lastname: Bennett
mesh_terms: D003902:Detergents / Q000494:pharmacology; D004492:Edetic Acid / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D005340:Fibrinogen / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D008024:Ligands; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008958:Models, Molecular; D008970:Molecular Weight; D010980:Platelet Membrane Glycoproteins / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D011485:Protein Binding; D011487:Protein Conformation; D014841:von Willebrand Factor / Q000378:metabolism / Q000648:ultrastructure
chemical_list: D003902:Detergents; D008024:Ligands; D046911:Macromolecular Substances; D010980:Platelet Membrane Glycoproteins; D014841:von Willebrand Factor; D005340:Fibrinogen; D004492:Edetic Acid