Phosphorylation of human Jak3 at tyrosines 904 and 939 positively regulates its activity

Department of Integrative Biology and Pharmacology, University of Texas Health Science Center at Houston Medical School, Houston, TX 77030, USA; Department of Biological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA

	Abstract

Janus tyrosine kinase 3 (Jak3) is essential for signaling by IL-2 family cytokines and proper immune function. Dysfunctional regulation of Jak3 may result in certain disease states. However, the molecular mechanisms governing Jak3 activation are not fully understood. In this study we used a functional proteomics approach to identify two novel tyrosine phosphorylation sites within Jak3, Y904 and Y939, which are conserved among Jak family proteins. Using phosphospecific antibodies, both residues were observed to be rapidly induced by stimulation of cells with IL-2 or other c-cytokines. Mechanistic studies indicated that Y904 and Y939 regulate Jak3 activities. Phenylalanine substitution of either site greatly reduced Jak3 kinase activity in vitro and its ability to phosphorylate Signal transducer and activator of transcription 5 (Stat5) in vivo, suggesting that phosphorylation of these previously unrecognized residues positively regulate Jak3 activity. Y904 and Y939 were required for optimal ATP usage by Jak3, while phosphorylation of Y939 preferentially promoted Stat5 activity in intact cells. Together, these findings demonstrate positive functional roles for two novel Jak3 phosphoregulatory sites which may be similarly important for other Jak family members. Identification of these sites also provides new therapeutic opportunities to modulate Jak3 function.