JAK2 is mutated and constitutively activated in CHRF-288-11 cells. (A) Homozygous JAK2C2624A mutation encoding JAK2T875N in CHRF-288-11 cells. (B) Constitutive phosphorylation of JAK2 in CHRF-288-11 and HEL (positive control expressing JAK2V617F) cells but not in CMK cells. (C) Ribbon diagram representations of the JAK2 kinase domain crystal structure solved by Levine et al.³²  β-Strands are shown as arrows, α-helices as coils. The activation loop is colored orange with the phosphorylated tyrosines shown in stick representation. The N-lobe, C-lobe, C-helix, glycine-rich P-loop, and catalytic cleft are all labeled. This crystal structure was solved in complex with a JAK-specific inhibitor, 2-tert-butyl-9-fluoro-3,6-dihydro-7H-benzo[h]imidazo-[4,5-f]isoquinoline-7-one, which is shown as space-filling spheres with carbon atoms colored yellow. The side-chain of threonine-875 is shown as red space-filling spheres and falls in the loop region between strands β2 and β3. (D) Ribbon diagram as in panel C but with the view rotated through 90 degrees. (E) Close up of the β2-β3 loop. Side-chains of residues comprising this loop are shown in stick format. Threonine-875 is additionally indicated with space-filling spheres and is labeled. (F) Same view as panel E, but illustrating the T875N point mutation with a model of the substitution. Modeling of the mutation of threonine-875 to asparagine suggests a change in the surface properties of this loop, which may disrupt potential JH1-JH2 interactions.