Janus kinases (Jaks), members of the non-receptor protein tyrosine kinase family, are key components of signaling pathways in cells of the immune system and in hematopoietic cells. Jaks are associated with the cytoplasmic domains of cytokine receptors and, upon cytokine-mediated receptor dimerization, undergo trans- autophosphorylation on tyrosine residues, which stimulates their tyrosine kinase activity. Activated Jaks phosphorylate STATs (signal transducers and activators of transcription), which translocate to the nucleus and serve as transcriptional activators. There are four mammalian members of the Jak family (Jak1-3 and Tyk2) which possess four domains in common: an N-terminal FERM domain, an SH2-like domain, a pseudokinase domain, and a tyrosine kinase domain. Extensive biochemical data, as well as gain-of-function mutations that cause myeloproliferative diseases/cancers, have implicated the pseudokinase domain of Jaks as crucial for maintaining a low basal level of tyrosine kinase activity. The goal of this proposal is to understand the structural/molecular mechanisms by which the pseudokinase domain negatively regulates the tyrosine kinase activity of Jak2. To achieve this goal, x-ray crystallography will be employed to determine the three-dimensional structures of the pseudokinase domain and the tandem pseudokinase and tyrosine kinase domains.
Gain-of-function mutations in Jaks, predominantly in the pseudokinase domain, are causative for various myeloproliferative diseases/cancers in humans, such as polycythemia vera, primary myelofibrosis, and acute lymphoblastic leukemia. The goal of this proposal is to determine by x-ray crystallography the three- dimensional structure of the Jak2 pseudokinase and tyrosine kinase domains, to understand the molecular bases for these myeloproliferative diseases and to facilitate the development of novel small-molecule inhibitors for therapeutic intervention.
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