Cation-dependent changes in the binding specificity of the platelet receptor GPIIb/IIIa

The presence of manganese (Mn2+) significantly increases the binding of the platelet surface receptor GPIIb/IIIa to two synthetic peptides Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) and Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val (L10) that contain the recognition sequences RGD and KQAGDV, respectively. This results in an increase in the amount of GPIIb/IIIa adsorbed by GRGDSPK- and L10-Sepharose by 12-20-fold. Additionally, Mn2+ eliminates contaminating platelet vitronectin receptor, alpha v beta 3, which copurifies with GPIIb/IIIa on the peptide affinity columns in the absence of Mn2+. In contrast to this increased peptide binding of GPIIb/IIIa, Mn2+ reduces the binding of GPIIb/IIIa to its macromolecular RGD-containing ligands fibrinogen, fibronectin, and vitronectin. These results could mean that Mn2+ changes the structure of the binding site on GPIIb/IIIa such that it is now better suited to accommodate conformations available to the RGD sequence within short, linear synthetic peptides but not available to the RGD sequences within the natural ligands. To support this hypothesis we tested a conformationally restricted cyclic peptide, cyclic 2,10-GPenGHRGDLRCA, which in competition assays, preferentially inhibits the binding of GPIIb/IIIa to fibrinogen but does not inhibit well the binding of other RGD-dependent integrins, alpha v beta 3 and alpha 5 beta 1 to their respective ligands. In such assays, the presence of Mn2+ dramatically changed the binding specificity of GPIIb/IIIa by shifting the preference of the receptor away from the selective peptide, cyclic 2,10-GPen-GHRGDLRCA toward the nonselective GRGDSP peptide. This shift parallels the Mn2(+)-dependent change of the binding of GPIIb/IIIa to its natural protein ligands.