Inhibition Of Alpha Iib Beta 3 Ligand Binding By An Alpha Iib Peptide That Clasps The Hybrid Domain To The Beta I Domain Of Beta 3

Agonist-stimulated platelet activation triggers conformational changes of integrin alpha IIb beta 3, allowing fibrinogen binding and platelet aggregation. We have previously shown that an octapeptide, (p1)YMESRADR(8), corresponding to amino acids 313-320 of the beta-ribbon extending from the beta-propeller domain of alpha IIb, acts as a potent inhibitor of platelet aggregation. Here we have performed in silico modelling analysis of the interaction of this peptide with alpha IIb beta 3 in its bent and closed (not swing-out) conformation and show that the peptide is able to act as a substitute for the beta-ribbon by forming a clasp restraining the beta 3 hybrid and beta I domains in a closed conformation. The involvement of species-specific residues of the beta 3 hybrid domain (E356 and K384) and the beta 1 domain (E297) as well as an intrapeptide bond (pE315-pR317) were confirmed as important for this interaction by mutagenesis studies of alpha IIb beta 3 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 alpha IIb beta-ribbon in preventing integrin alpha IIb beta 3 head piece opening, and highlight a potential new therapeutic approach to prevent integrin ligand binding.