Phosphorylation of Jak2 on Ser 523 InhibitsJak2-Dependent Leptin ReceptorSignaling

Abstract

ABSTRACT 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.