Localization of the sites responsible for the responsiveness of αIIbβ3 to RGDS to the C-terminal halves of the αIIb W3:4-1 and W4:4-1 loops. (A) Amino acid sequences of the human, rat, and mouse αIIb W3:4-1 and W4:4-1 loops; “:” indicates sequence identity. Numbering is for human αIIb. (B) Effect of increasing concentration of RGDS on FITC-labeled fibrinogen binding to CHO cells expressing recombinant αIIbβ3 containing WT HαIIb, RαIIb, and HαIIb in which the N- or C-terminal halves of the human W3:4-1 and W4:4-1 loops have been replaced by the corresponding rat sequence. (C) Percent inhibition of fibrinogen binding to the human/rat chimeras shown in panel B by 1 mM RGDS. Data shown are the mean plus or minus 1 SE of 3 experiments.