Catalytic (repeat 2) domain of the Protein Tyrosine Kinase, Tyrosine kinase 2
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Tyk2 is widely expressed in many tissues. It is involved in signaling via the cytokine receptors IFN-alphabeta, IL-6, IL-10, IL-12, IL-13, and IL-23. It mediates cell surface urokinase receptor (uPAR) signaling and plays a role in modulating vascular smooth muscle cell (VSMC) functional behavior in response to injury. Tyk2 is also important in dendritic cell function and T helper (Th)1 cell differentiation. A homozygous mutation of Tyk2 was found in a patient with hyper-IgE syndrome (HIES), a primary immunodeficiency characterized by recurrent skin abscesses, pneumonia, and elevated serum IgE. This suggests that Tyk2 may play important roles in multiple cytokine signaling involved in innate and adaptive immunity. Tyk2 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 tyr kinase catalytic 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 Tyk2 subfamily 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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