Janus kinase

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Janus kinase (JAK, or "Just another kinase") 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[1]), 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.

Contents

The Janus kinase family

There are four JAK family members:

Transgenic mice that do not express JAK1 have defective responses to some cytokines such as interferon-gamma.[2] 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.[3]

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

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