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.
This domain is found at the C terminus of proteins belonging to the JAKMIP family (Janus kinase and microtubule-interacting proteins) and is predicted to be a coiled coil. It interacts with the Janus family kinases Tyk2 and Jak1 [, , ].
CUEDC2 is a novel negative regulator of progesterone receptor (PR) and functions to promote the progesterone-induced PR degradation by the ubiquitin-proteasome pathway []. It also acts as the regulator of JAK1/STAT3 signaling through inhibiting cytokine-induced phosphorylation of JAK1 and STAT3 and the subsequent STAT3 transcriptional activity []. This entry represents the CUE domain found in CUEDC2.
Janus kinase and microtubule interacting proteins (JAKMIPs) are predominantly expressed in neural tissues and lymphoid organs. Three family members have been identified, termed JAKMIP1-3. They contain an N-terminal region that targets the protein to microtubule polymers and a C-terminal region that is able to associate with Janus kinase (Jak) family members, such as Tyk2 or Jak1 []. The proteins may have a role in Jak signalling and regulation of microtubule cytoskeleton rearrangements.JAKMIP1 has also been shown to interact with GABA(B) receptor R1 subunits [], and may be involved in microtubule-dependent transport of the GABA-B receptor [].
This is an F2 lobe domain consisting of an acyl-CoA binding protein fold found in FERM region of Jak-family tyrosine kinases []. Multidomain JAK molecules interact with receptors through their FERM and SH2-like domains, triggering a series of phosphorylation events, resulting in the activation of their kinase domains []. Overall, the FERM region maintains the typical three-lobed architecture, with an F1 lobe consisting of a ubiquitin-like fold, an F2 lobe consisting of an acyl-CoA binding protein fold, and an F3 lobe consisting of a pleckstrin-homology (PH) fold. JAK1 FERM-F2 domain has been shown to act as the interaction site for the IFNLR1 box1 motif (PxxLxF) of class II cytokine receptors which is essential for kinase activation [].
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 [].TYK2 was first identified by low-stringency hybridisation screening of ahuman lymphoid cDNA library with the catalytic domain of proto-oncogene c-fms []. Mouse and puffer fish orthlogues have also been identified. In common with JAK1 and JAK2, and by contrast with JAK3, TYK2 appears to be ubiquitously expressed. This entry represents the N-terminal region of TYK2.
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.
