JANUS KINASES IN INTERLEUKIN-2-MEDIATED SIGNALING - JAK1 AND JAK3 AREDIFFERENTIALLY REGULATED BY TYROSINE PHOSPHORYLATION

Background: Cytokines mediate a variety of effector cell functions, in cluding cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus ki nases (JAKs) that promote tyrosine phosphorylation of signal transduce rs and activators of transcription (STAT) factors, Although JAK activa tion has been correlated with phosphorylation, the role of this tyrosi ne 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 t argeted 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 contr ast, JAK3 retained catalytic activity when either or both of these act ivation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demon strated that JAK activity was also controlled by intramolecular intera ctions involving the amino-terminal domain of the JAK as well as by th e inherent signaling properties of the kinase domain, Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lack s detectable expression of JAK1 and JAK3. Catalytically active version s of both JAK1 and JAK3 must be present for effective induction of STA T5. Conclusions: JAK1 and JAK3 are differentially regulated by specifi c tyrosines within their respective activation loops. Additionally, th e amino-terminal domain of JAK3 appears to contain regulatory sequence s that modify the function of the kinase domain. Finally, both JAK1 an d JAK3 must retain catalytic function for IL-2-induced STAT5 activatio n. (C) Current Biology Ltd ISSN 0960-9822.