Janus kinase

Janus kinase (JAK) is a family of intracellular, nonreceptor tyrosine kinases that transduce cytokine-mediated signals via the JAK-STAT pathway. They were initially named "just another kinase" 1 and 2 (since they were just two of a large number of discoveries in a PCR-based screen of kinases,) but were ultimately published as "Janus kinase". The name is taken from the two-faced Roman god of beginnings and endings, 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.

Family

The four JAK family members are:

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 in type I interferon signalling. Mice that do not express TYK2 have defective natural killer cell function.

Janus kinase 2

Janus kinase 2 (commonly called JAK2) is a non-receptor tyrosine kinase. It is a member of the Janus kinase family and has been implicated in signaling by members of the type II cytokine receptor family (e.g. interferon receptors), the GM-CSF receptor family (IL-3R, IL-5R and GM-CSF-R), the gp130 receptor family (e.g., IL-6R), and the single chain receptors (e.g. Epo-R, Tpo-R, GH-R, PRL-R). JAK2 signaling is activated downstream from the prolactin receptor.

The distinguishing feature between janus kinase 2 and other JAK kinases is the lack of Src homology binding domains (SH2/SH3) and the presence of up to seven JAK homology domains (JH1-JH7). Nonetheless the terminal JH domains retain a high level of homology to tyrosine kinase domains. An interesting note is that only one of these carboxy-terminal JH domains retains its function whilst the other, due to lacking important amino acids for kinase functionality, does not work; it is therefore termed as the pseudokinase domain.

Janus kinase 3

Tyrosine-protein kinase JAK3 is an enzyme that in humans is encoded by the JAK3 gene.

Janus kinases

Janus kinase 3 is a tyrosine kinase that belongs to the janus family of kinases. Other members of the Janus family include JAK1, JAK2 and TYK2. Janus kinases (JAKs) are relatively large kinases of approximately 1150 amino acids with apparent molecular weights of 120-130 kDa. They are cytosolic tyrosine kinases that are specifically associated with cytokine receptors. Since cytokine receptor proteins lack enzymatic activity, they are dependent upon JAKs to initiate signaling upon binding of their ligands (e.g. cytokines). The cytokine receptors can be divided into five major subgroups based on their different domains and activation motifs. JAK3 is required for signaling of the type I receptors that use the common gamma chain (γc).

Function

In contrast to the relatively ubiquitous expression of JAK1, JAK2 and Tyk2, JAK3 is predominantly expressed in hematopoietic lineage such as NK cells, T cells and B cells and intestinal epithelial cells. JAK3 functions in signal transduction and interacts with members of the STAT (signal transduction and activators of transcription) family. JAK3 is predominantly expressed in immune cells and transduces a signal in response to its activation via tyrosine phosphorylation by interleukin receptors. Mutations that abrogate Janus kinase 3 function cause an autosomal SCID (severe combined immunodeficiency disease).