THE CYTOKINE-ACTIVATED TYROSINE KINASE JAK2 ACTIVATES RAF-1 IN A P21(RAS)-DEPENDENT MANNER

JAK2, a member of the Janus kinase superfamily was found to interact f unctionally with Raf-1, a central component of the ras/mitogen-activat ed protein kinase signal transduction pathway. Interferon-gamma and se veral other cytokines that are known to activate JAK2 kinase were also found to stimulate Raf-1 kinase activity toward MEK-1 in mammalian ce lls. In the baculovirus coexpression system, Raf-1 was activated by JA K2 in the presence of p21(ras). Under these conditions, a ternary comp lex of p21(ras), JAK2, and Raf-1 was observed. In contrast, in the abs ence of p21(ras), coexpression of JAK2 and Raf-1 resulted in an overal l decrease in the Raf-1 kinase activity. In addition, JAK2 phosphoryla ted Raf-1 at sites different from those phosphorylated by pp60(v-src). In mammalian cells treated with either erythropoietin or interferon-g amma, a small fraction of Raf-1 coimmunoprecipitated with JAK2 in lysa tes of cells in which JAK2 was activated as judged by its state of tyr osine phosphorylation. Taken together, these data suggest that JAK2 an d p21(ras) cooperate to activate Raf-1.