Janus kinases (JAKs) are tyrosine kinases that function in membrane-proximal signalling events initiated by a variety of extracellular factors binding to cell surface receptors []. Many type I and II cytokine receptors lack a protein tyrosine kinase domain and rely on JAKs to initiate the cytoplasmic signal transduction cascade. Ligand binding induces oligomerisation of the receptors, which then activates the cytoplasmic receptor-associated JAKs. These subsequently phosphorylate tyrosine residues along the receptor chains with which they are associated. The phosphotyrosine residues are a target for a variety of SH2 domain-containing transducer proteins. Amongst these are the signal transducers and activators of transcription (STAT) proteins, which, after binding to the receptor chains, are phosphorylated by the JAK proteins. Phosphorylation enables the STAT proteins to dimerise and translocate into the nucleus, where they alter the expression of cytokine-regulated genes. This system is known as the JAK-STAT pathway.Four mammalian JAK family members have been identified: JAK1, JAK2, JAK3, and TYK2. They are relatively large kinases of approximately 1150 amino acids, with molecular weights of ~120-130kDa. Their amino acid sequences are characterised by the presence of 7 highly conserved domains, termed JAK homology (JH) domains. The C-terminal domain (JH1) is responsible for the tyrosine kinase function. The next domain in the sequence (JH2) is known as the tyrosine kinase-like domain, as its sequence shows high similarity to functional kinases but does not possess any catalytic activity. Although the function of this domain is not well established, there is some evidence for a regulatory role on the JH1 domain, thus modulating catalytic activity. The N-terminal portion of the JAKs (spanning JH7 to JH3) is important for receptor association and non-catalytic activity, and consists of JH3-JH4, which is homologous to the SH2 domain, and lastly JH5-JH7, which is a FERM domain.This entry represents the non-receptor tyrosine kinase JAK2 []. JAK2 was initially cloned using a PCR-based strategy utilising primers corresponding to conserved motifs within the catalytic domain of protein-tyrosine kinases []. In common with JAK1 and TYK2, and by contrast with JAK3, JAK2 appears to be ubiquitously expressed.
Jak2 is widely expressed in many tissues. It is essential for the signaling of hormone-like cytokines such as growth hormone, erythropoietin, thrombopoietin, and prolactin, as well as some IFNs and cytokines that signal through the IL-3 and gp130 receptors []. Disruption of Jak2 in mice results in an embryonic lethal phenotype with multiple defects including erythropoietic and cardiac abnormalities []. It is the only Jak gene that results in a lethal phenotype when disrupted in mice. A mutation in the pseudokinase domain of Jak2, V617F, is present in many myeloproliferative diseases, including almost all patients with polycythemia vera, and 50% of patients with essential thrombocytosis and myelofibrosis [, , , ].Jak2 is a cytoplasmic (or nonreceptor) PTK containing an N-terminal FERM domain, followed by a Src homology 2 (SH2) domain, a pseudokinase domain, and a C-terminal tyr kinase domain. The pseudokinase domain shows similarity to tyr kinases but lacks crucial residues for catalytic activity and ATP binding. Despite this, the presumed pseudokinase (repeat 1) domain of Jak2 exhibits dual-specificity kinase activity, phosphorylating two negative regulatory sites in Jak2: Ser523 and Tyr570. Inactivation of the repeat 1 domain increased Jak2 basal activity, suggesting that it modulates the kinase activity of the C-terminal catalytic (repeat 2) domain [].This entry represents the pseudokinase domain of Jak2.
Jak2 is widely expressed in many tissues. It is essential for the signaling of hormone-like cytokines such as growth hormone, erythropoietin, thrombopoietin, and prolactin, as well as some IFNs and cytokines that signal through the IL-3 and gp130 receptors []. Disruption of Jak2 in mice results in an embryonic lethal phenotype with multiple defects including erythropoietic and cardiac abnormalities []. It is the only Jak gene that results in a lethal phenotype when disrupted in mice. A mutation in the pseudokinase domain of Jak2, V617F, is present in many myeloproliferative diseases, including almost all patients with polycythemia vera, and 50% of patients with essential thrombocytosis and myelofibrosis [, , , ].Jak2 is a cytoplasmic (or nonreceptor) PTK containing an N-terminal FERM domain, followed by a Src homology 2 (SH2) domain, a pseudokinase domain, and a C-terminal tyr kinase domain. The pseudokinase domain shows similarity to tyr kinases but lacks crucial residues for catalytic activity and ATP binding. Despite this, the presumed pseudokinase (repeat 1) domain of Jak2 exhibits dual-specificity kinase activity, phosphorylating two negative regulatory sites in Jak2: Ser523 and Tyr570. Inactivation of the repeat 1 domain increased Jak2 basal activity, suggesting that it modulates the kinase activity of the C-terminal catalytic (repeat 2) domain [].This entry represents the C-terminal catalytic domain.
Jak2 is a protein tyrosine kinase involved in a specific subset of cytokine receptor signaling pathways. It has been found to be constitutively associated with the prolactin receptor []and is required for responses to gamma interferon []. Mice that do not express an active protein for this gene exhibit embryonic lethality associated with the absence of definitive erythropoiesis []. 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 [, , ].
This entry represents the SH3 domain of STAM1 (signal transducing adapter molecule 1). STAM1 is part of the endosomal sorting complex required for transport (ESCRT-0) and is involved in sorting ubiquitinated cargo proteins from the endosome []. It may also be involved in the regulation of IL2 and GM-CSF mediated signaling, and has been implicated in neural cell survival [, ]. STAMs were discovered as proteins that are highly phosphorylated following cytokine and growth factor stimulation []. They function in cytokine signaling and surface receptor degradation, as well as regulate Golgi morphology. They associate with many proteins including Jak2 and Jak3 tyrosine kinases [], Hrs, AMSH, and UBPY. STAM adaptor proteins contain VHS (Vps27, Hrs, STAM homology), ubiquitin interacting (UIM), and SH3 domains [].
This entry represents the SH3 domain of STAM2 (signal transducing adapter molecule 2). STAM2, a subunit of the ESCRT-0 complex, is an endosomal protein acting as a regulator of receptor signaling and trafficking. It contains an N-terminal VHS domain (Vps-27/Hrs/Stam), UIM (ubiquitin-interacting motif), SH3 (Src homology 3), CC ("coiled coil"), SSM (STAM-specific motif) and C-terminal ITAM (immunoreceptor tyrosine-based activation motif) domain. It may play a regulatory role in the endosomal sorting of ubiquitinated membrane proteins []. STAMs were discovered as proteins that are highly phosphorylated following cytokine and growth factor stimulation []. They function in cytokine signaling and surface receptor degradation, as well as regulate Golgi morphology. They associate with many proteins including Jak2 and Jak3 tyrosine kinases [], Hrs, AMSH, and UBPY. STAM adaptor proteins contain VHS (Vps27, Hrs, STAM homology), ubiquitin interacting (UIM), and SH3 domains [].
Protein phosphorylation, which plays a key role in most cellular activities, is a reversible process mediated by protein kinases and phosphoprotein phosphatases. Protein kinases catalyse the transfer of the gamma phosphate from nucleotide triphosphates (often ATP) to one or more amino acid residues in a protein substrate side chain, resulting in a conformational change affecting protein function. Phosphoprotein phosphatases catalyse the reverse process. Protein kinases fall into three broad classes, characterised with respect to substrate specificity []:Serine/threonine-protein kinasesTyrosine-protein kinasesDual specificity protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)Protein kinase function is evolutionarily conserved from Escherichia coli to human []. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation []. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins. The catalytic subunits of protein kinases are highly conserved, and several structures have been solved [], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [].Tyrosine-protein kinases can transfer a phosphate group from ATP to a tyrosine residue in a protein. These enzymes can be divided into two main groups []:Receptor tyrosine kinases (RTK), which are transmembrane proteins involved in signal transduction; they play key roles in growth, differentiation, metabolism, adhesion, motility, death and oncogenesis []. RTKs are composed of 3 domains: an extracellular domain (binds ligand), a transmembrane (TM) domain, and an intracellular catalytic domain (phosphorylates substrate). The TM domain plays an important role in the dimerisation process necessary for signal transduction []. Cytoplasmic / non-receptor tyrosine kinases, which act as regulatory proteins, playing key roles in cell differentiation, motility, proliferation, and survival. For example, the Src-family of protein-tyrosine kinases [].Janus kinases (JAKs) are tyrosine kinases that function in membrane-proximal signalling events initiated by a variety of extracellular factors binding to cell surface receptors []. Many type I and II cytokine receptors lack a protein tyrosine kinase domain and rely on JAKs to initiate the cytoplasmic signal transduction cascade. Ligand binding induces oligomerisation of the receptors, which then activates the cytoplasmic receptor-associated JAKs. These subsequently phosphorylate tyrosine residues along the receptor chains with which they are associated. The phosphotyrosine residues are a target for a variety of SH2 domain-containing transducer proteins. Amongst these are the signal transducers and activators of transcription (STAT) proteins, which, after binding to the receptor chains, are phosphorylated by the JAK proteins. Phosphorylation enables the STAT proteins to dimerise and translocate into the nucleus, where they alter the expression of cytokine-regulated genes. This system is known as the JAK-STAT pathway.Four mammalian JAK family members have been identified: JAK1, JAK2, JAK3, and TYK2. They are relatively large kinases of approximately 1150 amino acids, with molecular weights of ~120-130kDa. Their amino acid sequences are characterised by the presence of 7 highly conserved domains, termed JAK homology (JH) domains. The C-terminal domain (JH1) is responsible for the tyrosine kinase function. The next domain in the sequence (JH2) is known as the tyrosine kinase-like domain, as its sequence shows high similarity to functional kinases but does not possess any catalytic activity. Although the function of this domain is not well established, there is some evidence for a regulatory role on the JH1 domain, thus modulating catalytic activity. The N-terminal portion of the JAKs (spanning JH7 to JH3) is important for receptor association and non-catalytic activity, and consists of JH3-JH4, which is homologous to the SH2 domain, and lastly JH5-JH7, which is a FERM domain.This represents the non-receptor tyrosine kinase JAK1, which is involved in the IFN-alpha/beta/gamma signal pathway. Jak1 acts as the kinase partner for the interleukin (IL)-2 receptor []and interleukin (IL)-10 receptor []. It directly phosphorylates STAT but also activates STAT signalling through the transactivation of other JAK kinases associated with signalling receptors [, ].JAK1 was initially cloned using a PCR-based strategy utilising degenerateprimers corresponding to conserved motifs within the catalytic domain of protein-tyrosine kinases []. In common with JAK2 and TYK2, and by contrastwith JAK3, JAK1 appears to be ubiquitously expressed.
