Critical residues for ligand binding in an I domain-like structure of the integrin β1 subunit

Several integrin α subunits have an inserted sequence of about 200 residues (the I or A domain) that is critical for ligand interactions. The presence of an I domain-like structure within the integrin β subunit has been proposed based on the similarity of the hydropathy profiles and the homology of sequences between the α and β subunits. This study was designed to determine whether the region of the β1 subunit that includes residues 101335 has the characteristics of an I domain. We found novel critical residues for ligand binding (Ser-132, Asn-224, Asp-226, Glu-229, Asp-233, Asp-267, and Asp-295, in addition to the previously reported Asp-130) using site-directed mutagenesis. The critical residues for ligand binding are located in several of loop structures of the region (or in a potential loop between an α helix and a β strand), which have been predicted using multiple secondary structure prediction methods. The data suggest that the β subunit has multiple disrupted critical oxygenated residues for ligand binding similar to those found in the α I domain.