Allosteric regulation of alpha(IIb)beta(3) by beta(3) 95-105

The linear recognition sequences of an anti-beta(3) antibody that blocked platelet aggregation were identified using beta(3) tryptic peptides. Two of these recognition sequence-containing peptides were mapped to beta(3) 92-105, and antibodies affinity purified using these peptides blocked platelet aggregation. Examining the structure of alpha(IIb)beta(3) identified beta(3) 95-105 as the solvent accessible sequence within beta(3) 92-105. A peptide corresponding to beta(3) 95-105 was synthesized and used to affinity purify the beta(3) antibody. Anti-beta(3) 95-105 completely blocked platelet aggregation and agonist-induced fibrinogen binding to platelets, but had no effect on cyclic-RGD binding. Binding of anti-beta(3) 95-105 to alpha(IIb)beta(3) also did not alter the structure of the alpha(IIb) cap subdomain, as measured by anti-alpha(IIb) 201-217 binding. beta(3) 95-105 and peptides spanning two adjacent sequences in the structure of beta(3) did not bind fibrinogen and were ineffectual in blocking agonist-induced platelet aggregation. Structure analysis revealed that beta(3) 95-105 is adjacent to one of the two hinges in beta(3) that allows for the outward swing of the hybrid and PSI domains which is central to the conversion of alpha(IIb)beta(3) from a low into a high affinity state. Thus, the binding of an antibody to beta(3) 95-105 could serve as a fulcrum for allosteric regulation of alpha(IIb)beta(3) by regulating the movement of the hybrid-PSI domain.