Negative regulation of Jak2 by its auto-phosphorylation at tyrosine 913 via the Epo signaling pathway

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 Y^1007/1008 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.