Janus kinase

Overview
Janus kinase (JAK) is a family of intracellular non-receptor tyrosine kinases, ranging from 120-140 kDa in size, that transduce cytokine-mediated signals via the JAK-STAT pathway. Initially discovered in a PCR-based screen of kinases and named ironically as "just another kinase" 1 & 2 (since they were two of a large number of new kinases discovered in the screen) JAK1 and JAK2 were ultimately published as Janus kinase 1 and Janus kinase 2. The name is taken from Janus, the Thessalian gatekeeper of heaven worshipped by the Romans; he is described as having two faces while the JAKs possess two near-identical phosphate-transferring domains. One domain exhibits the kinase activity while the other stabilises the JAK's conformational structure enabling it express that activity.

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. The activated STATs dissociate from the receptor and form dimers before translocating to the cell nucleus where they regulate transcription of selected genes.

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. 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.

The structure of JAKs
JAKs have seven defined regions of homology; these are 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.