Inhibition of αIIbß3 Ligand Binding by an αIIb Peptide that Clasps the Hybrid Domain to the ßI Domain of ß3.

Agonist-stimulated platelet activation triggers conformational changes of integrin αIIbß3, allowing fibrinogen binding and platelet aggregation. We have previously shown that an octapeptide, p1YMESRADR8, corresponding to amino acids 313-320 of the ß-ribbon extending from the ß-propeller domain of αIIb, acts as a potent inhibitor of platelet aggregation. Here we have performed in silico modelling analysis of the interaction of this peptide with αIIbß3 in its bent and closed (not swing-out) conformation and show that the peptide is able to act as a substitute for the ß-ribbon by forming a clasp restraining the ß3 hybrid and ßI domains in a closed conformation. The involvement of species-specific residues of the ß3 hybrid domain (E356 and K384) and the ß1 domain (E297) as well as an intrapeptide bond (pE315-pR317) were confirmed as important for this interaction by mutagenesis studies of αIIbß3 expressed in CHO cells and native or substituted peptide inhibitory studies on platelet functions. Furthermore, NMR data corroborate the above results. Our findings provide insight into the important functional role of the αIIb ß-ribbon in preventing integrin αIIbß3 head piece opening, and highlight a potential new therapeutic approach to prevent integrin ligand binding.