Activity and analysis of the latent GDF8 prodomain complex. (A) HEK293 (CAGA)12 cells treated with latent GDF8 prodomain complex (GDF8L; black), acid-activated (GDF8AA; orange), and free mature (GDF8apo; green) ligand. Experiments were performed at least twice with each data point measured in triplicate. Shown is a representative experiment. Data were fit by nonlinear regression to a variable slope to determine the EC50. (B) Activity measurement following 1-h incubation of purified GDF8L (40 nM) at the indicated pH, followed by neutralization before exogenous administration to HEK293 (CAGA)12 cells. Data are shown as the mean ± SEM. (C) SEC analysis of GDF8L, GDF8AA, and reformed (GDF8R) prodomain:ligand complexes. (Inset) The protein composition of the peak visualized by SDS/PAGE under nonreducing conditions.