Integrin ligands at a glance

Abstract

Integrins are one of the major families of cell adhesion receptors (Humphries, 2000; Hynes, 2002). All integrins are non-covalently linked, heterodimeric molecules containing an α and a β subunit. Both subunits are type I transmembrane proteins, containing large extracellular domains and mostly short cytoplasmic domains (Springer and Wang, 2004; Arnaout et al., 2005). Mammalian genomes contain 18 α subunit and 8 β subunit genes, and to date 24 different α-β combinations have been identified at the protein level. Although some subunits appear only in a single heterodimer, 12 integrins contain the β1 subunit, and five contain αV.Integrin function has been determined through a combination of cell biological and genetic analyses. On the cytoplasmic face of the plasma membrane, integrin occupancy coordinates the assembly of cytoskeletal polymers and signalling complexes; on the extracellular face, integrins engage either extracellular matrix macromolecules or counter-receptors on adjacent cell surfaces. These bidirectional linkages impose spatial restrictions on signalling and extracellular matrix assembly, and thereby integrate cells with their microenvironment. In turn, membrane-proximal interactions initiate more distal functions such as tissue patterning (extracellularly) and cell fate determination (intracellularly). Genetic analyses of engineered or natural mutations have confirmed key roles for integrins in tissue integrity, cell trafficking, and differentiation (Bouvard et al., 2001; Bokel and Brown, 2002).A characteristic feature of most integrin receptors is their ability to bind a wide variety of ligands. Moreover, many extracellular matrix and cell surface adhesion proteins bind to multiple integrin receptors (Humphries, 1990; Plow et al., 2000; van der Flier and Sonnenberg, 2001). In recent years, structure-function analyses of both integrins and their ligands have revealed a similar mode of molecular interaction that explains this promiscuity. Nonetheless, the integrin literature is replete with studies describing different integrin-ligand pairs, and the major aim of this article is to provide a clarification of this picture.The poster shows the major integrin-ligand combinations, using hypothetical cell surfaces. We have not attempted a comprehensive cataloguing, but instead we have consulted with a number of colleagues and reached a consensus view on the best-validated integrin ligands. There are many other ligands for different integrins, the inclusion of which would overly complicate the poster. By citing the best studied receptor-ligand combinations, we are aware that some reports and low-affinity interactions (which are nonetheless functionally relevant) may be discriminated against, and for this we apologise. Some of the interactions that are supported by convincing data are nonetheless included below.Historically, most integrin-ligand pairs have been identified either by affinity chromatography or through the ability of subunit-specific monoclonal antibodies to block adhesion of cells to specific ligands. In some cases, direct protein-protein binding assays have been used to support biochemical or cell biological data. Despite their wide variety, it is possible to cluster integrin-ligand combinations into four main classes, reflecting the structural basis of the molecular interaction. These classes do not necessarily reflect evolutionary relationships.All five αV integrins, two β1 integrins (α5, α8) and αIIbβ3 share the ability to recognise ligands containing an RGD tripeptide active site. Crystal structures of αVβ3 and αIIbβ3 complexed with RGD ligands have revealed an identical atomic basis for this interaction (Xiong et al., 2002; Xiao et al., 2004). RGD binds at an interface between the α and β subunits, the R residue fitting into a cleft in a β-propeller module in the α subunit, and the D coordinating a cation bound in a von Willebrand factor A-domain in the β subunit. The RGD-binding integrins are among the most promiscuous in the family, with β3 integrins in particular binding to a large number of extracellular matrix and soluble vascular ligands. Although many ligands are shared by this subset of integrins, the rank order of ligand affinity varies, presumably reflecting the preciseness of the fit of the ligand RGD conformation with the specific α-β active site pockets.α4β1, α4β7, α9β1, the four members of the β2 subfamily and αEβ7 recognise related sequences in their ligands. α4β1, α4β7 and α9β1 bind to an acidic motif, termed `LDV', that is functionally related to RGD. Fibronectin contains the prototype LDV ligand in its type III connecting segment region, but other ligands (such as VCAM-1 and MAdCAM-1) employ related sequences. Although definitive structural information is lacking, it is highly likely that LDV peptides bind similarly to RGD at the junction between the α and β subunits. Osteopontin also interacts with α4β1, α4β7 and α9β1, but this apparently involves a different pe