Peptide-specific Antibodies as Pro\bes of the Topography of the Ca²⁺-binding Motifs in α_IIbβ₃ ^[*]

Summary

Antibodies against synthetic peptides corresponding to four Ca²⁺- binding motifs of the α_IIb subunit have been obtained and used as molecular probes to analyze the topography of the α_IIbβ₃ complex. The specificity of the antibodies has been characterized by ELISA and Western immunoblotting in terms of binding capacity and affinity to the isolated α_IIbβ₃ and its α_IIb subunit. Our data suggest that: (a) all four Ca²⁺-binding motifs of the α_IIb are partially exposed on the surface of the intact molecule and accessible to antipeptide antibodies. However, they are not in close vicinity to the ligand recognition domain since the antibodies do not produce complete inhibition of platelet aggregation, (b) The conformation of amino acid stretches which form the second Ca²⁺-binding motif of α_IIb is particularly dependent upon the presence of cation, and this region undergoes significant conformational alterations upon Ca²⁺ expulsion.

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