Crystal structure of an integrin-binding fragment of vascular cell adhesion molecule-1 at 1.8 Å resolution

THE cell-surface glycoprotein vascular cell adhesion molecule-1 (VCAM-1; ref. 1) mediates intercellular adhesion 2 by specific binding to the integrin very-late antigen-4 (VLA-4, α 4 β 1 ; ref. 3). VCAM-1, with the intercellular adhesion molecules ICAM-1, ICAM-2, ICAM-3 and the mucosal vascular addressin MAd-CAM-1, forms an integrin-binding subgroup of the immuno-globulin superfamily. In addition to their clinical relevance in inflammation, these molecules act as cellular receptors for viral and parasitic agents 2 . The predominant form of VCAM-1 in vivo has an ammo-terminal extracellular region comprising seven immunoglobulin-like domains. Functional studies 4-7 have identified a conserved integrin-binding motif in domains 1 and 4, variants of which are present in the N-terminal domain of all members of the immunoglobulin superfamily subgroup. We report here the crystal structure of a VLA-4-binding fragment composed of the first two domains of VCAM-1. The integrin-binding motif (Q38IDSPL) is highly exposed and forms the N-terminal region of the loop between β-strands C and D of domain 1. This motif exhibits a distinctive conformation which we predict will be common to all the integrin-binding IgSF molecules. These, and additional data, map VLA-4 binding to the face of the CFG β-sheet, the surface previously identified 8 as the site for intercellular adhesive interactions between members of the immunoglobulin superfamily.