The family of junctional adhesion molecules (JAM), comprised of at least three members, are type I transmembrane receptors belonging to the immunoglobulin (Ig) superfamily (1, 2). These proteins are localized in the tight junctions between endothelial cells or epithelial cells. Some family members are also found on blood leukocytes and platelets. Mouse JAM-C cDNA predicts a 310 amino acid (aa) residue precursor protein with a putative 31 aa signal peptide, a 210 aa extracellular region containing two Ig domains, an 18 aa transmembrane domain and a 51 aa cytoplasmic domain containing a PDZ-binding motif and a PKC phosphorylation site (3). Mouse JAM-C shares 86% aa sequence identity with its human homologue. It also shares approximately 31% and 35% aa sequence homology with mouse JAM-A and JAM-B, respectively (2). Mouse JAM-C is highly expressed during embryogenesis. In adult tissues, mouse JAM-C is restricted to endothelial cells, lymph endothelial cells in the kidney, lymph node and Peyer’s patches where the protein can be localized to the high endothelial venules (3). Although human JAM-C is expressed on human platelets and a subset of leukocytes, mouse JAM-C expression was not detected on any mouse lymphocytes (4). In contrast to human JAM-C which show weak homotypic interactions, mouse JAM-C was reported to exhibit homotypic interactions (3). Mouse JAM-C has also been shown to have heterotypic interaction with JAM-B. It is likely that mouse JAM-C may play a role in lymphocyte transendothelial migration (4).
The nomenclature used for the JAM family proteins is confusing. VE-JAM has been referred to in the literature variously as JAM-B or JAM-C. Until further clarification, R&D Systems has adopted the nomenclature where both mouse and human VE-JAM are referred to as JAM-B.
