JAK2
JAK2 (Janus kinase 2) is the cytoplasmic tyrosine kinase that carries signals from hematopoietic cytokine receptors — erythropoietin, thrombopoietin, GM-CSF — into the STAT transcription pathway.
Function
Cytokine receptors of the type I family lack intrinsic kinase domains; they pair constitutively with JAK2, which waits in an autoinhibited state. Ligand binding brings two receptor-associated JAK2 molecules together, they trans-phosphorylate each other, and the activated kinases phosphorylate receptor tails that recruit STAT transcription factors — STAT5 for erythropoietin signaling — which dimerize and enter the nucleus. The same kinase serves IL-6-family gp130 signaling in some contexts. Negative regulation comes from SOCS proteins, themselves STAT targets,
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