Binding of the α2 integrin I domain to extracellular matrix ligands:: Structural and mechanistic differences between collagen and laminin binding

The alpha(2)beta(1) integrin functions as a cell surface receptor for collagen on some cells and as both a collagen and laminin receptor on a more restricted subset of cell types including endothelial and epithelial cells. The alpha(2) integrin subunit I domain binds collagen in a divalent cation-dependent manner. In contrast, I domain binding to laminin occurs via both divalent cation-dependent and -independent mechanisms. Saturable binding was observed in the presence of either Mn2+ or EDTA, although the extent of binding in Mn2+ was twice that observed in EDTA. Half-maximal binding occurred at about 22 nM I domain in either case. Whereas laminin binding was significantly enhanced by Mn2+, with half-maximal binding occurring at 1.9 mM Mn2+ Mg2+ was much less effective. Deletion of the N-terminal 35 residues of the I domain, including the DXSXS portion of the MIDAS motif, caused a significant diminution of laminin binding activity. Laminin binding by the I domain was significantly inhibited by the alpha(2)beta(1) function-blocking antibody 6F1 in the presence of either EDTA or Mn2+. The non-function-blocking antibody 12F1 had no effect. In contrast to the binding of the alpha(2) integrin I domain to collagen, the laminin binding activity of the I domain was not enhanced by the addition of the first EF hand motif of the integrin.