Catalytic (repeat 2) domain of the Protein Tyrosine Kinase, Janus kinase 3
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Jak3 is expressed only in hematopoietic cells. It binds the shared receptor subunit common gamma chain and thus, is essential in the signaling of cytokines that use it such as IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Jak3 is important in lymphoid development and myeloid cell differentiation. Inactivating mutations in Jak3 have been reported in humans with severe combined immunodeficiency (SCID). Jak3 is a member of the Janus kinase (Jak) subfamily of proteins, which are cytoplasmic (or nonreceptor) PTKs containing an N-terminal FERM domain, followed by a Src homology 2 (SH2) domain, a pseudokinase domain, and a C-terminal catalytic tyr kinase domain. Jaks are crucial for cytokine receptor signaling. They are activated by autophosphorylation upon cytokine-induced receptor aggregation, and subsequently trigger downstream signaling events such as the phosphorylation of signal transducers and activators of transcription (STATs). The PTKc family is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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