Identification of putative ligand-binding sites of the integrin α4β1 (VLA-4, CD49d/CD29)

Integrin α4β1 recognizes both fibronectin (CS-1 sequence) and vascular cell adhesion molecule-1 (VCAM-1). To localize the ligand-binding sites of α4, we located the epitopes for function-blocking anti-α4 monoclonal antibodies (mAbs), including those that recognize previously described (but not yet physically localized) functional epitopes (A, B1, B2 and C) using interspecies α4 chimeras expressed in mammalian cells. Epitopes B1 and B2 were associated with ligand binding, and epitopes A and B2 with homotypic cellular aggregation, mAbs P4C2 (epitope B2), 20E4 and PS/2 were mapped within residues 108-182; mAbs HP2/1 (epitope B1), SG/73 and R1-2 within residues 195-268; mAbs HP1/3 (epitope A) and P4G9 within residues 1-38; and B5G10 (epitope C) within residues 269-548. The data suggest that residues 108-268, which do not include bivalent-cation-binding motifs, are related to VCAM-1 and CS-1 binding, and more N-terminal portions of α4 (residues 1-38 and 108-182) to homotypic aggregation. Since mAbs PS/2 and HP2/1 block α4β7 binding to mucosal addressin cell adhesion molecule-1 (MAdCAM-1), the MAdCAM-1-binding site is close to, or overlapping with, VCAM-1- and CS-1-binding sites. The role of Asp-130 of β1 in the binding to VCAM-1 and CS-1 peptide was examined. Chinese hamster ovary (CHO) cells expressing β1(D130A) (Asp-130 to Ala mutant of β1) and α4 showed much less binding to both ligands than CHO cells expressing wild-type β1 and α4 [a dominant negative effect of β1(D130A)], suggesting that Asp-130 of β1 is critical for binding to both ligands and that the two ligands share common binding mechanisms.