RGD-containing peptides inhibit fibrinogen binding to platelet αIIbβ3 by inducing an allosteric change in the amino-terminal portion of αIIb

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