Any of a family of receptor-associated tyrosine kinases that play a crucial role in many intracellular signalling pathways, particularly ones involving cytokines. Binding of ligand to the associated receptors brings two JAKs together, enabling them to activate each other and phosphorylate tyrosine residues in the intracellular regions of the receptors. This recruits cytosolic proteins called STATs (signal transducers and activators of transcription), which are phosphorylated by the JAKs, associate in pairs (dimerize), and migrate to the nucleus, where they bind to various gene control sites to alter levels of gene transcription. The name ‘Janus’ reflects the symmetrical arrangement of the kinase domains in the paired JAK proteins, after the Roman god and gatekeeper of heaven who had two faces, one in front and one behind.
Subjects: Chemistry — Biological Sciences.