Direct binding of collagen to the I domain of integrin α2β1 (VLA-2, CD49b/CD29) in a divalent cation-independent manner

Integrin α2β1 is a major divalent cation-dependent receptor for collagen. Here, we show that the recombinant inserted/interactive domain (I domain) of α2 specifically interacts with collagen, indicating the I domain contains all the components necessary for collagen binding. Evidence was obtained that divalent cations are not required for collagen binding to the I domain fragment, indicating that divalent cations are not involved in the actual binding to collagen but probably in the regulation of the binding. We identified Thr-221 within the previously identified putative ligand binding region as a residue critical for collagen binding to both α2β1 and the I domain fragment. Thr-221 may be involved in the actual collagen binding and recognition.