Janus kinase 2 (Jak2) has a pivotal role in erythropoietin (Epo) signaling pathway, including erythrocyte differentiation and Stat5 activation. In the course of screening for critical phosphorylation of tyrosine residues in Jak2, we identified tyrosine 913 (Y913) as a novel and functional phosphorylation site, which negatively regulates Jak2. Phosphorylation at Y913 rapidly occurred and was sustained for at least 120 min after Epo stimulation, in contrast to the transient phosphorylation of Y1007/1008 in the activation loop of Jak2. Interestingly, phosphorylation defective mutation of Y913 (Y913F) results in a significant enhancement of Epo-induced Jak2 activation, whereas phosphorylation mimic mutation of Y913 (Y913E) completely abrogated its activation. Furthermore, Jak2 deficient fetal liver cells expressing Y913F mutant generated many mature erythroid BFU-E and CFU-E colonies, while Y913E mutant failed to reconstitute Jak2 deficiency. We also demonstrate, in Jak1, phosphorylation of Y939, a corresponding tyrosine residue with Y913, negatively regulated Jak1 signaling pathway. Accordingly, our results suggest that this tyrosine phosphorylation in JH1 domain may be involved in common negative regulation mechanism for Jak family.
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