LIGAND-BINDING TO INTEGRIN ALPHA(IIB)BETA(3) IS DEPENDENT ON A MIDAS-LIKE DOMAIN IN THE BETA(3) SUBUNIT

Substitution of beta(3) residue Asp(119), Ser(121), or Ser(123) result s in a loss of the ligand binding function of integrin alpha(IIb)beta( 3). Homologous residues in other integrin beta subunits are similarly critical for ligand binding function. This DXSXS motif is also present in the I domain of certain integrin iv subunits, where it constitutes a portion of the unique metal ion-dependent adhesion site (MIDAS). In this report, we have utilized the crystal structure of the recombinan t alpha(M) I domain to produce a three-dimensional model of the homolo gous region in the integrin beta(3) subunit. We performed mutagenesis of candidate amino acid residues predicted from this model to be invol ved in cation coordination and ligand binding. We report the identific ation of Asp(217) and Glu(220) as residues essential for the ligand bi nding function of alpha(IIb)beta(3). Alanine substitution of these res idues did not affect receptor expression but abolished the binding of activation-dependent (PAC1) and -independent (OPG2) ligand mimetic ant ibodies. In our proposed model, beta(3) Asp(217) is analogous to a met al-coordinating residue in the alpha(M) MIDAS domain, while Glu(220) d oes not correspond to a functional MIDAS domain residue. Substitution of the highly conserved beta(3) residue Thr(197) corresponding to a cr itical MIDAS metal-coordinating Thr residue did not affect ligand bind ing function, suggesting that this region of beta(3) adopts a structur e that is very similar to but not identical to that of the MIDAS domai n. These data support a functional linkage between these two sequences and further define a common feature of ligand binding to integrins.