title: Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation.
abstract: BACKGROUND
  Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood.
  
  
  RESULTS
  We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5.
  
  
  CONCLUSIONS
  JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.
authors.affiliation: Gladstone Institute of Virology and Immunology, San Francisco, California 94141-9100, USA.
authors.forename: K D
authors.initials: KD
authors.lastname: Liu
authors.forename: S L
authors.initials: SL
authors.lastname: Gaffen
authors.forename: M A
authors.initials: MA
authors.lastname: Goldsmith
authors.forename: W C
authors.initials: WC
authors.lastname: Greene
mesh_terms: D000818:Animals; D019556:COS Cells; D002384:Catalysis; D004789:Enzyme Activation; D005354:Fibrosarcoma; D006801:Humans; D007376:Interleukin-2 / Q000378:metabolism*; D053613:Janus Kinase 1; D053616:Janus Kinase 3; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011505:Protein-Tyrosine Kinases / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D015375:Receptors, Interleukin-2 / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000276:immunology*; D013379:Substrate Specificity; D014407:Tumor Cells, Cultured; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D007376:Interleukin-2; D015375:Receptors, Interleukin-2; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; C507904:JAK1 protein, human; C507933:JAK3 protein, human; D053613:Janus Kinase 1; D053616:Janus Kinase 3; D017346:Protein Serine-Threonine Kinases