Potential inhibitory role of Arg564. (A) JAK2 pseudokinase domain structure16 showing position of Arg564 and a potential interaction with His538. Given the close proximity of these 2 residues, a hydrogen bond (dashed red line) may form between them. (B) Model to show a potential mechanism for an inhibitory role of Arg564. (i) Both K539I and H538L have been described as activating mutations,35 possibly because of His538 and Lys539 being involved in inhibitory interactions with the SH2 domain (pink). The Arg564–His538 H-bond (dashed red line) may position His538 for this inhibitory interaction and anchor the rest of the kinase domain relative to the SH2 domain, keeping JAK2 in an inactive state. (ii) On mutation of Arg to Gln (R564Q), this interaction would be broken, so that His538 is no longer held in place for the inhibitory interaction with the SH2 domain. Further experiments are now required to determine whether Arg564 is indeed involved in this potential inhibitory mechanism.
