Janus kinase

Janus kinase (JAK) is a family of intracellular non-receptor tyrosine kinases that transduce cytokine-mediated signals via the JAK-STAT pathway. They were initially named "just another kinase" 1 & 2 (since they were just two of a large number of discoveries in a PCR-based screen of kinases cite journal | author=Wilks |title=Two putative protein-tyrosine kinases identified by application of the polymerase chain reaction | journal=PNAS| volume=86| issues=5 | date=1989 | pages=1603–7 | doi = 10.1073/pnas.86.5.1603 ] ), but were ultimately published as "Janus kinase". The name is taken from the two-faced Roman god of doorways, Janus, because the JAKs possess two near-identical phosphate-transferring domains. One domain exhibits the kinase activity while the other negatively regulates the kinase activity of the first.

The Janus kinase family

There are four JAK family members:
* Janus kinase 1 (JAK1)
* Janus kinase 2 (JAK2)
* Janus kinase 3 (JAK3)
* Tyrosine kinase 2 (TYK2)

Transgenic mice that do not express JAK1 have defective responses to some cytokines such as interferon-gamma.cite journal |author=Rodig SJ, Meraz MA, White JM, Lampe PA, Riley JK, Arthur CD, King KL, Sheehan KC, Yin L, Pennica D, Johnson EM, Schreiber RD |title=Disruption of the Jak1 gene demonstrates obligatory and nonredundant roles of the Jaks in cytokine-induced biologic responses |journal=Cell |volume=93 |issue=3 |pages=373–83 |year=1998 |pmid=9590172| doi = 10.1016/S0092-8674(00)81166-6 ] JAK1 and JAK2 are involved in type II interferon (interferon-gamma) signalling, whereas JAK1 and TYK2 are involved type I interferon signalling. Mice that do not express TYK2 have defective natural killer cell function.cite journal |author=Stoiber D, Kovacic B, Schuster C, Schellack C, Karaghiosoff M, Kreibich R, Weisz E, Artwohl M, Kleine OC, Muller M, Baumgartner-Parzer S, Ghysdael J, Freissmuth M, Sexl V |title=TYK2 is a key regulator of the surveillance of B lymphoid tumors |journal=J. Clin. Invest. |volume=114 |issue=11 |pages=1650–8 |year=2004 |pmid=15578097 |doi=10.1172/JCI200422315]

General functions of the JAK family

Since members of the type I and type II cytokine receptor families possess no catalytic kinase activity, they rely on the JAK family of tyrosine kinases to phosphorylate and activate downstream proteins involved in their signal transduction pathways. The receptors exist as paired polypeptides thus exhibiting two intracellular signal-transducing domains. JAKs associate with a proline-rich region in each intracellular domain, which is adjacent to the cell membrane and called a box1/box2 region. After the receptor associates with its respective cytokine/ligand it goes through a conformational change, bringing the two JAKs close enough to phosphorylate each other. The JAK autophosphorylation induces a conformational change within itself enabling it to transduce the intracellular signal by further phosphorylating and activating transcription factors called STATs. cite journal | author=Kisseleva et al |title=Signaling through the JAK/STAT pathway, recent advances and future challenges | journal=Gene | volume=285 | issues=1-2 | date=2002 Feb 20 | pages=1–24 | doi=10.1016/S0378-1119(02)00398-0 ] The activated STATs dissociate from the receptor and form dimers before translocating to the cell nucleus where they regulate transcription of selected genes.

Some examples of the molecules that utilize JAK/STAT signaling pathway are colony-stimulating factor, prolactin, growth hormone, and many cytokines.

The structure of JAKs

JAKs range from 120-140 kDa in size and have seven defined regions of homology called Janus homology domain 1–7 (JH1-7). JH1 is the kinase domain important for the enzymatic activity of the JAK and contains typical features of a tyrosine kinase such as conserved tyrosines necessary for JAK activation (e.g. Y1038/Y1039 in JAK1, Y1007/Y1008 in JAK2, Y980/Y981 in JAK3, and Y1054/Y1055 in Tyk2). Phosphorylation of these dual tyrosines leads to the conformational changes in the JAK protein to facilitate binding of substrate. JH2 is a "pseudokinase domain", a domain structurally similar to a tyrosine kinase and is essential for a normal kinase activity yet lacks enzymatic activity. This domain may be involved in regulating the activity of JH1. The JH3-JH4 domain of JAKs shares homology with Src-homology-2 (SH2) domains. The amino terminal (NH₂) end (JH4-JH7) of Jaks is called a FERM domain (short for band 4.1 ezrin, radixin and moesin); this domain is also found in the focal adhesion kinase (FAK) family and is involved in association of JAKs with cytokine receptors and/or other kinases cite journal | author=Kisseleva et al |title=Signaling through the JAK/STAT pathway, recent advances and future challenges | journal=Gene | volume=285 | issues=1-2 | date=2002 Feb 20 | pages=1–24 | doi=10.1016/S0378-1119(02)00398-0 ] .

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