Mutations in the BMP15 prodomain affect synergy with GDF9. COV434 human GCs, transfected with a Smad2/3-responsive luciferase reporter, were stimulated with equal concentrations (1 to 12 ng/ml) of recombinant human GDF9 and FLAG affinity-purified hBMP15 variants. Luciferase activity was measured, and BMP15 variants were separated into those that demonstrated (A) high, (B) moderate, or (C) minimal synergy with GDF9. Data are the mean ± standard deviation of triplicate determinations from representative experiments, relative to an adjusted value of 1.0 for the mean of the control wells. The experiments were repeated 3 times. WT, wild-type.