A mutation in the integrin α_IIb subunit that selectively inhibits α_IIbβ₃ receptor function

The α_IIb and α_v integrins have been shown to play a significant role in a variety of disease processes. α_IIbβ₃ is a platelet-specific fibrinogen receptor that is critical for thrombosis and hemostasis. Determination of the basis of ligand recognition by α_IIbβ₃ is essential for modulation of platelet function. To identify α_IIb residues involved in α_IIbβ₃ ligand binding function, cells expressing a constitutively active variant of α_IIbβ₃ were randomly mutagenized and selected for loss of α_IIbβ₃ ligand binding function. One mutant isolated in this manner contained a single amino acid substitution at position 96 in α_IIb (Ser96→Leu). Cells expressing this α_IIb mutant did not bind the ligand mimetic antibody PACI or adhere to fibrinogen. In addition, the mutant receptor did not bind to an RGD affinity matrix. Substitution of conserved serine residues at position I in β strand A of all seven repeats of α_IIb similarly inhibited ligand binding to α_IIbβ₃. α_IIbS96 maps to the central cavity of the β-propeller fold of the α_IIb subunit immediately adjacent to a structurally important sequence at the center of the α and β subunit interface. In contrast, substitution of the analogous residues in α_v or α₄ did not disrupt the ligand binding function of α_vβ₃ or α₄β₁. These data support a potential unique structural or mechanistic role for this residue in α_IIbβ₃ receptor function.