Integrin α2 I-domain is a binding site for collagens

ABSTRACT Integrins α1β1 and α2β1 are major cellular receptors for collagens. The α1 and α2 subunits contain a ∼200 amino acid inserted domain (I-domain) in their N-terminal region and, because of the homology between the I-domains and the collagen-binding A-domains of von Willebrand factor, it has been suggested that the I-domains might mediate the collagen-binding functions of α1β1 and α2β1. In order to fully investigate this hypothesis, we have generated recombinant human α2 I-domain (rα2I) by reverse transcriptase-polymerase chain reaction/bacterial expression and tested its ability to mediate the collagen-binding functions of α2β1. Rα2I binds specifically to type I collagen in a concentration-dependent manner: binding is cation dependent and, like the complete receptor, is supported by magnesium and manganese ions but not by calcium ions. Rα2I is recognised by anti-functional anti-α2 monoclonal antibodies 6F1, 5E8 and P1E6 in capture ELISAs, and anti-functional antibodies inhibited rα2I-collagen binding. In addition, rα2I inhibits cell spreading on collagen. Rα2I is therefore a collagen-binding domain and can account for many of the collagen-binding functions of integrin α2β1. We have also determined the collagen specificity of rα2I and found that it binds types I, II and XI collagen.