Cloning of human junctional adhesion molecule 3 (JAM3) and its identification as the JAM2 counter-receptor

We have identified a third member of the junctional adhesion molecule (JAM) family. At the protein level JAM3 displays 36 and 32% identity to JAM2 and JAM1, respectively. The coding region is distributed over 9 exons and maps to chromosome 11q25. The gene shows widespread tissue expression with high er levels apparent in the kidney, brain, and placenta. At the cellular leve l we show expression of JAM3 transcript within endothelial cells. Our major finding is that JAM3 and JAM2 are binding partners. Thus, JAM3 ectodomain binds firmly to JAM2-Fc. This heterotypic interaction is maintained when JA M3-Fc is used to capture Chinese hamster ovary cells expressing full-length JAM2. In static adhesion assays we show that JAM3 is unable to bind to leu kocyte cell lines. This is consistent with the lack of JAM2 expression. How ever, using JAM2-Fc pulldown experiments in combination with polyclonal ant i-JAM3 serum, we demonstrate that JAM3 is the previously uncharacterized 43 -kDa counter-receptor that mediates JAM2 adhesion to T cells. Most signific antly we demonstrate up-regulation of JAM3 protein on peripheral blood lymp hocytes following activation. Finally we show the utility of JAM3 ectodomai n as an inhibitor of JAM2 adhesion.