Src homology 2 (SH2) domain in the Janus kinase (Jak) family
The Janus kinases (Jak) are a family of 4 non-receptor tyrosine kinases (Jak1, Jak2, Jak3, Tyk2) which respond to cytokine or growth factor receptor activation. To transduce cytokine signaling, a series of conformational changes occur in the receptor-Jak complex upon extracellular ligand binding. This results in trans-activation of the receptor-associated Jaks followed by phosphorylation of receptor tail tyrosine sites. The Signal Transducers and Activators of Transcription (STAT) are then recruited to the receptor tail, become phosphorylated and translocate to the nucleus to regulate transcription. Jaks have four domains: the pseudokinase domain, the catalytic tyrosine kinase domain, the FERM (band four-point-one, ezrin, radixin, and moesin) domain, and the SH2 (Src Homology-2) domain. The Jak kinases are regulated by several enzymatic and non-enzymatic mechanisms. First, the Jak kinase domain is regulated by phosphorylation of the activation loop which is associated with the catalytically competent kinase conformation and is distinct from the inactive kinase conformation. Second, the pseudokinase domain directly modulates Jak catalytic activity with the FERM domain maintaining an active state. Third, the suppressor of cytokine signaling (SOCS) family and tyrosine phosphatases directly regulate Jak activity. Dysregulation of Jak activity can manifest as either a reduction or an increase in kinase activity resulting in immunodeficiency, inflammatory diseases, hematological defects, autoimmune and myeloproliferative disorders, and susceptibility to infection. Altered Jak regulation occurs by many mechanisms, including: gene translocations, somatic or inherited point mutations, receptor mutations, and alterations in the activity of Jak regulators such as SOCS or phosphatases. In general SH2 domains are involved in signal transduction. They typically bind pTyr-containing ligands via two surface pockets, a pTyr and hydrophobic binding pocket, allowing proteins with SH2 domains to localize to tyrosine phosphorylated sites.
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