JAK2 domain organization and structural data for JH2 pseudokinase domain. (A) Domain organization of JAK2 shown to linear scale (human, 1132 residues). Select residues mapped as positive- and negative-regulatory phosphorylation sites are labeled in green and red, respectively. The position of the most prevalent MPN mutation, V617F, is also indicated. (B) Crystal structure of JAK2 JH2, wild-type and V617F.³³  For wild-type JH2, the N lobe is colored light gray and the C lobe is dark gray. The C helix (αC) is colored blue, the activation loop is colored green, and the catalytic loop is colored orange. The structure of JH2 V617F is superimposed on the wild-type structure and is colored pink throughout. For αC and the preceding linker (β3-αC linker), where the differences between the 2 structures are found, Cα traces are shown. The positions of Val617 (dark gray) and Phe617 (magenta) are indicated. N, N-terminus (residue 536). The C-terminus is not in view. The inset shows a magnified area of the region near Phe617, in particular, the π-stacking interactions between Phe617, Phe594 (αC), and Phe595 (αC) in V617F. The side chains of Phe594 and Phe595 in wild-type JH2 are colored blue. (C) The autoinhibitory pose of JAK2 JH2-JH1 that was derived from MD simulations.³⁹  JH2 is colored orange, JH1 is colored cyan, the SH2-JH2 linker is colored green, and the JH2-JH1 linker is colored gray. Select MPN or leukemia mutations are shown in stick-and-sphere representation (side chains) and colored pink (carbon atoms). Phosphorylated Ser523 and Tyr570 are shown in stick representation. Red, oxygen atoms; blue, nitrogen atoms; yellow, sulfur atoms; black, phosphorus atoms. N, N-terminus (residue 520); C, C-terminus (residue 1132). The inset shows a magnified area of the region near Arg683 (JH2) and Asp873 (JH1). Salt bridges are indicated by dashed black lines. FERM, N-terminal band 4.1, ezrin, radixin, moesin; PK, pseudokinase; TK, tyrosine kinase.