Integrins

The integrin family of adhesion molecules is composed of structurally related proteins that combine to form noncovalent a|3 heterodimers. Currently 16 a subunits and eight 3 subunits have been cloned, resulting in a minimum of 22 different combinations. The VLA (very late antigen) integrins, leukocyte integrins, and cytoadhesins represent integrin subfamilies based on the pairing of a subunits with the (3,, p2 or p3 subunits, respectively. However, the identification of five other |3 subunits, together with the ability of certain a subunits to associate with more than one subunit, blurs the distinctions between these integrin subfamilies. Integrin ligands include extracellular matrix (ECM) proteins, as well as cell surface proteins. Many of the integrin cell surface counter-receptors are members of the immunoglobulin (Ig) superfamily (see below). In addition, some bacteria and viruses use integrins to bind to host rissue. Ligand binding specificity is contributed by both a and subunits. The ligand binding regions of integrins are invariably located at cation-binding sites, and consequently integrin-mediated adhesion is divalent cation dependent.

Table 1 Adhesion molecules in immunology

Receptor

Alternate name

Cell type

Extracellular matrix ligands

Cell surface/ other ligands

aiibßs

ICAM-2 ICAM-3 MAdCAM-1

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