RGD-containing Peptides Inhibit Fibrinogen Binding to Platelet α IIbβ3 Inducing an Allosteric Change in the Amino-terminal Portion of αIIb

Abstract

To determine the molecular basis for the insensitivity of rat α IIbβ3 to inhibition by RGD-containing peptides, hybrids of human and rat αIIbβ3, and chimeras of αIIbβ3 in which αIIb was composed of portions of human and rat αIIb were expressed in Chinese hamster ovary cells and B lymphocytes, and the ability of the tetrapeptide RGDS to inhibit fibrinogen binding to the various forms of αIIbβ3 was measured. These measurements indicated that sequences regulating the sensitivity of α IIbβ3 to RGDS are located in the seven amino-terminal repeats of αIIb. Moreover, replacing the first three or four (but not the first two) repeats of rat αIIb with the corresponding human sequences enhanced sensitivity to RGDS, whereas replacing the first two or three repeats of human αIIb with the corresponding rat sequences had little or no effect. Nevertheless, RGDS bound to Chinese hamster ovary cells expressing αIIbβ 3 regardless whether the αIIb in the heterodimers was human, rat, or a rat-human chimera. These results indicate that the sequences determining the sensitivity of αIIbβ 3 to RGD-containing peptides are located in the third and fourth amino-terminal repeats of αIIb. Because RGDS binds to both human and rat αIIbβ3, the results suggest that differences in RGDS sensitivity result from differences in the allosteric changes induced in these repeats following RGDS binding.