MUTATION IN THE JAK KINASE JH2 DOMAIN HYPERACTIVATES DROSOPHILA AND MAMMALIAN JAK-STAT PATHWAYS

The Jak (Janus) family of nonreceptor tyrosine kinases plays a critica l role in cytokine signal transduction pathways, In Drosophila melanog aster, the dominant hop(Tum-l) mutation in the Hop Jak kinase causes l eukemia-like and other developmental defects, Previous studies have su ggested that the Hop(Tum-l) protein might be a hyperactive kinase, Her e, we report on the new dominant mutation hop(T42), which causes abnor malities that are similar to but more extreme than those caused by hop (Tum-l). We determined that Hop(T42) contains a glutamic acid-to-lysin e substitution at amino acid residue 695 (E695K), This residue occurs in the JH2 (kinase-like) domain and is conserved among all Jak family members. We determined that Hop(Tum-l) and Hop(T42) both hyperphosphor ylated and hyperactivated D-Stat when overexpressed in Drosophila cell s, Moreover, we found that the hop(T42) phenotype was partially rescue d by a reduction of wild-type D-stat activity, Finally, generation of the corresponding E695K mutation in murine Jak2 resulted in increased autophosphorylation and increased activation of Stat5 in COS cells, Th ese results demonstrate that the mutant Hop proteins do indeed have in creased tyrosine kinase activity, that the mutations hyperactivate the Hop-D-Stat pathway, and that Drosophila is a relevant system for the functional dissection of mammalian Jak-Stat pathways, Finally, we prop ose a model for the role of the Hop-D-Stat pathway in Drosophila hemat opoiesis.