Summary
Ligand binding to integrin αIIbβ3 is a key event of thrombus formation. The propeller
domain of the αIIb subunit has been implicated in ligand binding. Recently, the ligand
binding site of the αV propeller was determined by crystal structure analysis. However,
the structural basis of ligand recognition by the αIIb propeller remains to be determined.
In this study, we conducted site-directed mutagenesis of all residues located in the
loops extending above blades 2 and 4 of the αIIb propeller, which are spatially close
to, but distinct from, the loops that contain the binding site for an RGD ligand in
the crystal structure of the αV propeller. Replacement by alanine of Q111, H112 or
N114 in the loop within the blade 2 (the W2:2-3 loop in the propeller model) abolished
binding of a ligand-mimetic antibody and fibrinogen to αIIbβ3 induced by different
types of integrin activation including activation of αIIbβ3 by β3 cytoplasmic mutation.
CHO cells stably expressing recombinant αIIbβ3 bearing Q111A, H112A or N114A mutation
did not exhibit αIIbβ3mediated adhesion to fibrinogen. According to the crystal structure
of αVβ3, the αV residue corresponding to αIIbN114 is exposed on the integrin surface
and close to the RGD binding site. These results suggest that the Q111, H112 and N114
residues in the loop within blade 2 of the αIIb propeller are critical for ligand
binding, possibly because of direct interaction with ligands or modulation of the
RGD binding pocket.
Keywords
Adhesion receptors/integrins - platelet physiology - protein structure/folding - cell-matrix
interactions