Junctional adhesion molecule (JAM) is phosphorylated by protein kinase C upon platelet activation

Junctional adhesion molecule (JAM) is a member of the immunoglobulin superfamily (IgSF) expressed in tight junctions of epithelial cells and endothelial cells, and implicated in transendothelial migration of leukocytes. Recently, JAM is reported to be constitutively expressed on circulating monocytes, neutrophils, lymphocytes subsets, and platelets. However, the role of JAM is not known. Here, we examined how phosphorylaton of JAM is regulated upon platelet activation. Phosphorylation of JAM was induced by thrombin, collagen, but not by ADP. The phosphorylated amino acids were shown to be serine residues by phosphoamino acid analysis. Inhibition of JAM's phosphorylation by PKC inhibitors and Ca++ chelator suggests the involvement of conventional types of PKCs. By in vitro kinase assays, we demonstrated that JAM could be directly phosphorylated by cPKCs. We also demonstrated phosphorylation of Ser 284, a putative PKC phosphorylation site, by immunoblotting with anti-phosphoserine-JAM antibody in thrombin-stimulated platelets. In addition to the phosphorylation, JAM seemed to form clusters at several sites of cell-cell contact in aggregated platelets by immunoelectron microscopic study. We speculate that JAM may be directly phosphorylated by cPKC(s) upon platelet activation and that the phosphorylation might be involved in platelet activation. (C) 2000 Academic Press.