Cell-cell adhesion is paramount in establishing and maintaining the complexity of multicellular organisms. A multitude of cell adhesion molecules has evolved to serve the increasing sophistication and specialization of cell-cell contacts in higher eukaryotes.
Typical epithelial cells form junctional complexes between their apical lateral surfaces that establish cell polarity, ensure epithelial integrity, and limit paracellular diffusion through the combination of three types of junctions: tight junctions (zonula occludens), adherens junctions (zonula adherens), and desmosomes (macula adherens). Cad-herins, a large family of calcium-dependent, homophilic adhesion molecules, have traditionally been regarded as the major adhesive unit at adherens junctions and link the junctional complex to underlying actin belts via catenins. Link
More recently, nectins have been recognized as a novel, calcium-independent adhesion system at cadherin-based adherens junctions. Nectins were originally isolated based on their homology to CD155, the human poliovirus receptor. To date, this emerging family of cell adhesion molecules consists of nectin-1, -2, -3, and -4 as well as CD155 and Tage4. These molecules are glycosylated, type Ia, single-pass transmembrane proteins belonging to the immunoglobulin superfamily with V-C2-C2 ectodomains and relatively short cytoplasmic domains. All nectins form cis-homodimers that interact in a trans-hom-ophilic fashion. In addition, nectin-3 cis-homodimers can trans-interact strongly with nectin-1 or nectin-2 cis-homodimers, and nectin-4 can interact in trans with nectin-1.