The major classes of adhesion receptors, shown embedded in a putative plasma membrane
Cadherins typically mediate a calcium-dependent, homophilic adhesion between cells. The crystal structure of the domains of these receptors suggests that adjacent molecules form dimers and interact with dimers on the opposite cell. N-cadherin is one of the adhesion molecules mediating the outgrowth of neurites. The second major class, the immunoglobulin superfamily, may also mediate homophilic interactions. The binding sites of these receptors are characteristically, but not always, in the two most distal domains. They mediate cell–cell adhesion that is not dependent on divalent cations. Platelet–endothelial-cell adhesion molecule 1 (PECAM-1), expressed also on leukocytes, appears to have a role in the migration of leukocytes across endothelium. Other members of the immunoglobulin superfamily mediate heterophilic interactions with integrins. For example, vascular-cell adhesion molecule 1 (VCAM-1) found on stimulated endothelial cells binds the α4β1 or α4β7 integrin on lymphocytes. Integrins, the third major class, are heterodimers whose two chains contribute to ligand binding, which requires the presence of divalent cations. Many integrins bind to proteins in the extracellular matrix, as shown with α5β1, which is a receptor for fibronectin and can support the assembly of the fibronectin matrix. Integrins also bind soluble adhesion molecules, such as fibrinogen, which forms cross-links with the platelet integrin αIIbβ3 during platelet aggregation. A short motif in the amino acid sequence, such as arginine–glycine–aspartic acid (RGD), is often the primary site of recognition by the integrin receptor. Finally, the selectins, which have a distal calcium-dependent lectin domain, interact with carbohydrate groups on highly glycosylated protein ligands. P-selectin is found on activated platelets and endothelial cells and mediates adhesion to specific mucins present on many leukocytes.
Ref: Frenette, PS, Wagner, DD. Adhesion molecules--Part 1. N Engl J Med. 334:1526-1529. 1996