Crystal structures of the Jak2 pseudokinase domain and the pathogenic mutant V617F

The protein tyrosine kinase Jak2 mediates signaling through numerous cytokine receptors. Jak2 possesses a pseudokinase domain (JH2) and a tyrosine kinase domain (JH1). Through unknown mechanisms, JH2 regulates the catalytic activity of JH1, and hyperactivating mutations in the JH2 region of human Jak2 are causative for myeloproliferative neoplasms (MPNs). We showed previously that Jak2 JH2 is in fact catalytically active. Here, we present crystal structures of human Jak2 JH2, both wild-type and the most prevalent MPN mutant, V617F. The structures reveal that JH2 adopts the fold of a prototypical protein kinase but binds Mg-ATP non-canonically. The structural and biochemical data indicate that the V617F mutation rigidifies α-helix C in the N lobe of JH2, which facilitates trans-phosphorylation of JH1. The crystal structures of JH2 afford new opportunities for the design of novel Jak2 therapeutics targeting MPNs.