We have previously reported the purification and characterization of a 32 kDa platelet surface glycoprotein that is recognized by the stimulatory monoclonal antibody, F11. The cDNA has been cloned and found to encode the human homolog of the murine junctional adhesion molecule, JAM; we therefore named this human homolog JAM-1. Northern blot analysis indicated that JAM-1 mRNA is expressed as multiple species, the predominant transcript being approximately 4.0 kb in size. Genetic mapping analysis using fluorescence in situ hybridization (FISH) showed that it is localized to chromosome 1q21.1-21.3. Recombinant JAM-1, when expressed in Chinese hamster ovary (CHO) cells, localized to the cell membrane with intense staining where two adjacent cells actually made contact with each other, suggesting that, similar to murine JAM, human JAM-1 may also localize at the cell-cell junction. In well-spread cells, JAM-1 co-localized with F-actin at the cell-cell contacts and at the membrane ruffles, but not at the stress fibers. Interestingly, JAM-1 localizes only to the cell-cell junctions formed by two transfected cells and not to the cell-cell junctions formed by a transfected cell with an untransfected cell, suggesting that JAM-1 may facilitate cell adhesion through homophilic binding. In addition, human platelets specifically bind to a monolayer of CHO cells expressing human JAM-1, further supporting homophilic interactions. The results presented here indicate that JAM-1, a receptor for a platelet-activating antibody, is the human homolog of the junctional adhesion molecule. JAM-1 is a single copy gene, which is constitutively expressed on various tissues and cells, and may be involved in cell to cell adhesion through homophilic interaction.
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