Weisel J W, Nagaswami C, Vilaire G, Bennett J S
Department of Anatomy, University of Pennsylvania School of Medicine, Philadelphia 19104.
J Biol Chem. 1992 Aug 15;267(23):16637-43.
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.
血小板整合素糖蛋白IIb-IIIa（GPIIb-IIIa）是一种钙依赖性异二聚体，在血小板激活后可结合纤维蛋白原、血管性血友病因子和纤连蛋白。在用铂/钨进行旋转阴影投射后，我们通过电子显微镜检查了单独的GPIIb-IIIa以及与这些配体结合的GPIIb-IIIa。正如之前所观察到的，我们发现，在存在去污剂和2 mM Ca2+的情况下，GPIIb-IIIa由一个8×12纳米的球状头部和从一侧延伸出的两条18纳米的柔性尾部组成。我们还发现，在存在乙二胺四乙酸（EDTA）的情况下，GPIIb-IIIa会解离成两个相似的逗号形亚基，每个亚基包含一部分球状头部和一条单一的尾部。使用针对GPIIb、GPIIIa和GPIIb-IIIa异二聚体的单克隆抗体，我们发现尾部包含每个亚基的羧基末端，而结节状头部由两个亚基的氨基末端片段组成。与纤维蛋白原结合的GPIIb-IIIa的电子显微镜观察显示，GPIIb-IIIa的结节状头部与三结节纤维蛋白原分子的远端以及与纤维蛋白原长轴成约98度角横向延伸的GPIIb-IIIa的尾部之间存在高度特异性相互作用。当一个GPIIb-IIIa与单个纤维蛋白原的两端结合时，尾部朝向纤维蛋白原的相反两侧，使纤维蛋白原能够桥接两个相邻的血小板。与纤连蛋白结合的GPIIb-IIIa的电子显微镜观察显示，GPIIb/IIIa结合位点位于纤连蛋白分子两端氨基末端距离的约三分之二处，而发现GPIIb-IIIa沿着分子中央结节附近的杆状区域与血管性血友病因子原结合。
