Pseudokinase (repeat 1) domain of the Protein Tyrosine Kinases, Janus kinases
The Jak subfamily is composed of Jak1, Jak2, Jak3, TYK2, and similar proteins. They 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. The pseudokinase domain shows similarity to tyr kinases but lacks crucial residues for catalytic activity and ATP binding. It modulates the kinase activity of the C-terminal catalytic domain. In the case of Jak2, the presumed pseudokinase (repeat 1) domain exhibits dual-specificity kinase activity, phosphorylating two negative regulatory sites in Jak2: Ser523 and Tyr570. Most Jaks are expressed in a wide variety of tissues, except for Jak3, which is expressed only in hematopoietic cells. 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). Jaks are also involved in regulating the surface expression of some cytokine receptors. The Jak-STAT pathway is involved in many biological processes including hematopoiesis, immunoregulation, host defense, fertility, lactation, growth, and embryogenesis. The Jak subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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