Fig. 5. Weak activation of MPL-STAT5 signaling by the murine CALR del19 mutant.

a STAT5 transcription activity measured using a luciferase assay. 293T cells were transiently transfected with STAT5-LUC and the CALR WT, CALR del52 mutant, Calr WT, or Calr del19 mutant in the presence of the human thrombopoietin receptor (MPL) or murine Mpl. The y-axis indicates the fold induction of luciferase activity compared with WT CALR. *P < 0.05, **P < 0.01; NS, not significant vs. WT CALR. The average values for relative luciferase activity generated in three independent experiment are shown. Data are presented as means ± SEM. A two-tailed Student’s t test was used. b Binding of human or murine CALR mutant (FLAG-tagged) to human or murine MPL (Myc-tagged) examined by immunoprecipitation and western blot analysis. Binding between the murine CALR del19 mutant and murine MPL was weakest in comparison with the other three combinations