The integrin alpha 1 A-domain is a ligand binding site for collagens and laminin

The integrin alpha 1 beta 1 is a cell surface receptor for collagens and laminin. The alpha 1 subunit contains an A-domain, and the A-domains of other integrins are known to mediate ligand binding. To determine the role of the alpha 1 A-domain in ligand binding and the extent to which it reproduced the ligand binding activity and specificity of the parent molecule, we produced recombinant alpha 1 A-domain and tested its ability to bind collagens and laminin. In solid phase assays, the A-domain from alpha 1 was found to bind to collagen I, collagen IV, and laminin in a largely cation-dependent manner. The alpha 2 A-domain, from the alpha 2 beta 1 integrin, also bound to these ligands, but the binding hierarchy differed from that seen for alpha 1. This is the first demonstration of laminin binding by A-domains. Specificity of A-domain-ligand binding was further investigated using the triple-helical proteolytic fragment of collagen IV, CB3, and its subfragments, F1 and F4. alpha 1 A-domain bound to all three fragments, while the alpha 2 A-domain bound CB3 less well and exhibited little binding to F1 and no binding to F4. These differences mirror previous reports of distinct integrin binding sites in collagen IV and for the first time identify a limited proteolytic fragment of a ligand that contains integrin A-domain binding activity. To gain insight into the contribution that the A-domain makes to ligand binding within the whole integrin heterodimer, we measured binding constants for A-domain-collagen interactions using surface plasmon resonance biosensor technology. The values obtained were similar to those reported for intact integrin binding, suggesting that the A-domain is the major collagen binding site in the alpha 1 beta 1 and alpha 2 beta 1 integrins.