Molecular basis of JAK kinase regulation guiding therapeutic approaches: Evaluating the JAK3 pseudokinase domain as a drug target.

Janus kinases (JAK1-3, TYK2) are critical mediators of cytokine signaling and their role in hematological and inflammatory and autoimmune diseases has sparked widespread interest in their therapeutic targeting. JAKs have unique tandem kinase structure consisting of an active tyrosine kinase domain adjacent to a pseudokinase domain that is a hotspot for pathogenic mutations. The development of JAK inhibitors has focused on the active kinase domain and the developed drugs have demonstrated good clinical efficacy but due to off-target inhibition cause also side-effects and carry a black box warning limiting their use. Our understanding of the regulatory function of the pseudokinase domain in physiological and pathological signaling has improved substantially. The pseudokinase domain maintains the inactive state of JAKs in the absence of cytokine stimulation but it has also a key role in physiological and mutation-driven activation process. Furthermore, the pseudokinase domain has favourable structural characteristics for selective targeting of cytokine signaling, such as unique mode of ATP-binding, and the first pseudokinase targeting inhibitor for TYK2 has been approved for clinical use. Here we describe the recent functional and structural knowledge of JAK signaling and their therapeutic targeting, and present data evaluating the druggability of the JAK3 pseudokinase domain.