Phosphorylation of Jak2 on Ser523 inhibits Jak2-dependent leptin receptor signaling

The leptin receptor, LRb, and other cytokine receptors are devoid of intrinsic enzymatic activity and rely upon the activity of constitutively associated Jak family tyrosine kinases to mediate intracellular signaling. In order to clarify mechanisms by which Jak2, the cognate LRb-associated Jak kinase, is regulated and mediates downstream signaling, we employed tandem mass spectroscopic analysis to identify phosphorylation sites on Jak2. We identified Ser(523) as the first-described site of Jak2 serine phosphorylation and demonstrated that this site is phosphorylated on Jak2 from intact cells and mouse spleen. Ser(523) was highly phosphorylated in HEK293 cells independently of LRb-Jak2 activation, suggesting a potential role for the phosphorylation of Ser(523) in the regulation of LRb by other pathways. Indeed, mutation of Ser(523) sensitized and prolonged signaling by Jak2 following activation by the intracellular domain of LRb. The effect of Ser(523) on Jak2 function was independent of Tyr(570)-mediated inhibition. Thus, the phosphorylation of Jak2 on Ser(523) inhibits Jak2 activity and represents a novel mechanism for the regulation of Jak2-dependent cytokine signaling.