Identification of putative ligand binding sites within I domain of integrin alpha 2 beta 1 (VLA-2, CD49b/CD29)

Integrin alpha 2 beta 1 is a cell surface adhesion receptor for collagen and echovirus 1. Here we localized the epitopes for anti-alpha 2 monoclonal antibodies using interspecies (human/bovine) alpha 2 chimeras with different lengths of human alpha 2 sequence on the amino-terminal side and site-directed mutagenesis. The antibodies that block the collagen and/or echovirus 1 binding to human alpha 2 beta 1 (6F1, RMAC11, 12F1, and AA10) recognizes a small region (residues 173-259) within the I domain. Asp-160 and Arg-242 are critical for binding of the two other function-inhibiting antibodies, P1H5 and 5E8, respectively. Notably, mutations of Asp-151 and Asp-254 block the binding of alpha 2 beta 1 to collagen. These data suggest that the I domain (residues 140-359) is critically involved in the ligand/receptor interactions, and collagen and echovirus 1 binding sites are adjacent or overlapping within the I domain. The sequence of the residues 173-259 of alpha 2 overlap with the peptide sequences (M11 and M20) that derive from von Willebrand factor A1 and A3 domains (homologous to the alpha 2 I domain) and block von Willebrand factor/collagen interaction, suggesting that the epitope region of alpha 2 (residues 173-259) may really be involved in ligand recognition.