Genentech, Inc. South San Francisco, CA 94080, USA
The Janus kinases, also known as JAKs, are a family of multidomain, non-receptor tyrosine kinases essential for cytokine and interferon signaling through type I and type II cytokine receptors. JAKs are constitutively bound to their cognate transmembrane signaling receptors, and ligand-mediated dimerization of two JAK-bound receptors facilitates activation of the kinase domain leading ultimately to activation of STAT family transcription factors and transcription of target genes. JAK signaling drives processes as diverse as adaptive immune functions, hematopoiesis, metabolism, and cellular growth, and abberant JAK activation can trigger several myeloproliferative and auto-immune disorders. Pharmacological inhibition of JAK kinase activity has been shown to effectively treat several of these indications, and given the promise of these anti-JAK therapies, a complete understanding of the mechanisms of JAK activation has the potential to reveal novel means for therapeutic inhibition. The four JAK family members, JAK1, JAK2, JAK3, and TYK2, share a conserved domain architecture consisting of an N-terminal FERM domain, followed by a SH2-like domain, a pseudokinase domain, and a kinase domain. Through their FERM and SH2-like domains, JAKs constitutively associate with specific peptide motifs (called "box1" and "box2") present on the intracellular domain of cytokine receptors. We have recently determined crystal structures of the human JAK1 and TYK2 FERM/SH2 domains in complex with box1- and box2-containing peptides, respectively, shedding light on how these kinases interact with their receptors and where these interactions may have evolved from. The structures have also raised some unanticipated questions about how the receptors may influence kinase-kinase interactions inside the cell. JAK kinase regulation has also been an active area of research in our group. For many years it has been understood that the pseudokinase domain plays a role in regulating kinase activity. This work includes the discovery of a large set of myeloproliferative neoplasm (MPN)-associated JAK2 mutations, such as V617F allele, that localize to the pseudokinase domain and result in constitutive kinase activation. We have recently determined the structure of the two domain pseudokinase-kinase fragment of human TYK2, which revealed that the pseudokinase and kinase assume a dimeric conformation. Furthermore, we found that >90% of MPN-associated mutations lie in or near the dimeric interface between the pseudokinase and kinase. This analysis suggests that the pseudokinase-kinase dimer is an autoinhibited conformation common across JAK family members, and reveals a likely cause for the constitutive activity found in MPN-associated mutant JAK alleles.