Smith J W, Ruggeri Z M, Kunicki T J, Cheresh D A
Department of Immunology, Scripps Clinic and Research Foundation, La Jolla, California 92037.
J Biol Chem. 1990 Jul 25;265(21):12267-71.
Glycoprotein (GP) IIb-IIIa is the major fibrinogen receptor on platelets and participates in platelet aggregation at the site of a wound. Integrin alpha v beta 3, which contains an identical beta-subunit, is expressed on endothelial cells and also serves as a fibrinogen receptor. Here, we demonstrate by several criteria that purified GPIIb-IIIa and integrin alpha v beta 3 bind to distinct sites on fibrinogen. First, a plasmin-generated fragment of fibrinogen lacking the RGD sequence at residues 572-574 retained the ability to bind GPIIb-IIIa, but failed to bind integrin alpha v beta 3. Second, a monoclonal antibody which exclusively recognizes the RGD sequence at fibrinogen A alpha chain residues 572-574 abolished interaction between integrin alpha v beta 3 and fibrinogen, but had only a minimal effect on fibrinogen binding to GPIIb-IIIa. Finally, we show that the difference in recognition of sites on fibrinogen by these two integrins is probably a consequence of their remarkably different ligand binding properties. Peptides corresponding to fibrinogen gamma chain residues 400-411 effectively blocked RGD sequence and fibrinogen binding by GPIIb-IIIa, but had no effect on the ability of integrin alpha v beta 3 to bind these ligands. We also show that integrin alpha v beta 3 has a higher affinity than GPIIb-IIIa for a synthetic hexapeptide containing the RGD sequence. In fact, this RGD-containing peptide was 150-fold more effective at blocking fibrinogen binding to integrin alpha v beta 3 than to GPIIb-IIIa. Collectively, our results demonstrate that integrins alpha v beta 3 and GPIIb-IIIa display qualitative and quantitative differences in their ligand binding properties, as is evident by their ability to interact with synthetic peptides. The ultimate result of these differences is the recognition of distinct sites on fibrinogen by the two integrins. These observations may have relevance in the processes of hemostasis and wound healing.
糖蛋白（GP）IIb-IIIa是血小板上主要的纤维蛋白原受体，参与伤口部位的血小板聚集。整合素αvβ3含有相同的β亚基，在内皮细胞上表达，也作为纤维蛋白原受体。在此，我们通过多项标准证明，纯化的GPIIb-IIIa和整合素αvβ3与纤维蛋白原上不同的位点结合。首先，纤溶酶生成的纤维蛋白原片段在572-574位残基处缺乏RGD序列，保留了与GPIIb-IIIa结合的能力，但无法与整合素αvβ3结合。其次，一种单克隆抗体专门识别纤维蛋白原Aα链572-574位残基处的RGD序列，它消除了整合素αvβ3与纤维蛋白原之间的相互作用，但对纤维蛋白原与GPIIb-IIIa的结合只有极小的影响。最后，我们表明这两种整合素对纤维蛋白原位点识别的差异可能是由于它们显著不同的配体结合特性。与纤维蛋白原γ链400-411位残基对应的肽有效地阻断了GPIIb-IIIa对RGD序列和纤维蛋白原的结合，但对整合素αvβ3结合这些配体的能力没有影响。我们还表明，整合素αvβ3对含有RGD序列的合成六肽的亲和力高于GPIIb-IIIa。事实上，这种含RGD的肽在阻断纤维蛋白原与整合素αvβ3的结合方面比与GPIIb-IIIa的结合有效150倍。总体而言，我们的结果表明，整合素αvβ3和GPIIb-IIIa在配体结合特性上表现出质和量的差异，这从它们与合成肽相互作用的能力中可以明显看出。这些差异最终导致两种整合素识别纤维蛋白原上不同的位点。这些观察结果可能与止血和伤口愈合过程有关。
