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

Abstract

The cell-surface glycoprotein vascular cell adhesion molecule-1 (VCAM-1; ref. 1) mediates intercellular adhesion by specific binding to the integrin very-late antigen-4 (VLA-4, alpha 4 beta 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 immunoglobulin superfamily. In addition to their clinical relevance in inflammation, these molecules act as cellular receptors for viral and parasitic agents. The predominant form of VCAM-1 in vivo has an amino-terminal extracellular region comprising seven immunoglobulin-like domains. Functional studies 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 beta-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 beta-sheet, the surface previously identified as the site for intercellular adhesive interactions between members of the immunoglobulin superfamily.

Contrasting Roles for Domain 4 of VCAM-1 in the Regulation of Cell Adhesion and Soluble VCAM-1 Binding to Integrin α4β1

“…By introducing interdomain disulfide bonds at the interface between fibronectin type III repeats 9 and 10, Altroff et al (43) have recently shown that maintaining a 28°angle of tilt between these domains results in higher-affinity interactions with integrin ␣ 5 ␤ 1 . The crystal structures of VCAM-1 domains 1 and 2 demonstrate that the relative orientation of the two domains is not fixed (40,41) and that there is a high degree of flexibility (ranging from 7.3-39.9°from five different crystal monomers) in the two-dimensional angle of tilt between the two domains (44). Perhaps removal of Ig-like domains COOH-terminal to domain 3 or merely substituting Asp 328 with Ala results in a subtle conformational change that maintains the angle between domains 1 and 2 such that the domain 1 C-D loop and domain 2 CЈ-E loop-E strand synergy site present an optimal integrin ␣ 4 ␤ 1 binding topology.…”

“…Also, removal of Ig-like domains COOH-terminal to domain 3 or simply substituting Asp 328 with Ala increases the relative affinity of 7D VCAM-1 for ␣ 4 ␤ 1 . Previous functional and structural studies of the two NH 2 -terminal domains of VCAM-1 (domains 1 and 2) have demonstrated that the primary ␣ 4 ␤ 1 binding site in domain 1 resides in a solvent-exposed loop (the C-D loop, which contains the essential residue Asp 40 ) (25,27,29,40,41). A secondary "synergy" site, the CЈ-E loop-E strand, is located in the upper face of domain 2 (30), spatially close to the C-D loop of domain 1 (40,41).…”

Domain 1 of Mucosal Addressin Cell Adhesion Molecule Has an I1-set Fold and a Flexible Integrin-binding Loop

“…These include VCAM, ICAM-1, ICAM-2, ICAM-3, and ICAM-5, and crystal structures are available for all (5)(6)(7)(8)(9)(10)(11)(12). All have two to nine IgSF domains, with N-terminal domains 1 and 2 (compared below) sufficient for integrin binding.…”

Expression, crystallization, and preliminary X-ray analysis of a sialic acid-binding fragment of sialoadhesin in the presence and absence of ligand

“…Structural studies on both rat and human CD2 (Jones et al, 1992;Bodian et al, 1994) have provided information on the adhesive interactions between IgSF domains; studies on VCAM-I (Jones et al, 1995) gave insight into IgSF/integrin adhesion. It is hoped that the structural analysis of this fragment of Sn will lead to information on the details of IgSF/carbohydrate interactions and their role in intercellular adhesion, and provide a template for studying other members of the recently characterized sialoadhesin family (Kelm et al, 1994a;Freeman et al, 1995).…”