Cloning and Characterization of the Human Homolog of Mouse Jak2

Members of the Jak family play a critical role in signal transduction mediated by cytokine and hormone receptors. In this study, we report the cloning and characterization of human Jak2. The predicted amino acid sequence shows 91% homology to the described murine Jak2, but with a significant difference in the extreme C-terminal sequence. Using the human cDNA as a probe, we localized the gene for human Jak2 to chromosome 9p23-24. Human Jak2 mRNA is highly expressed in the spleen, lymph nodes, and peripheral blood lymphocytes (PBLs). A polyclonal antibody raised against the unique C-terminus of human Jak2 was used to characterize Jak2 protein. Levels of Jak2 protein expression increased significantly in mitogen- and anti-IgM–stimulated B cells and to a lesser degree in activated T cells. In addition, high levels of Jak2 protein were detected in pre-B leukemia cells.