CV2 limits complex formation involving ALK2, ALK1, BMP-4, and BMP-9. (A) To compare interactions between BMP-4, BMP-9, and their receptors in the presence of different antagonists, HAECs were treated for 8 hours with nonspecific IgG (300 ng/mL), anti–BMP-9 Abs (300 ng/mL), CV2 (100 ng/mL), Noggin (100 ng/mL), or anti–BMP-4 Abs (300 ng/mL). Cell lysates were then immunoprecipitated (IP) with nonspecific IgG Abs, or Abs to BMP-9, BMP-4, ALK1, or ALK2. The precipitates were analyzed by immunoblotting using the Ab combinations indicated in the figure. (B) HAECs were treated for 8 hours with nonspecific control IgG (control; 300 ng/mL), BMP-4 (40 ng/mL), BMP-9 (10 ng/mL), BMP-4 and BMP-9 together, CV2 (100 ng/mL), or BMP-4 and BMP-9 together with CV2. The cells were costained for ALK2 (green fluorescence), ALK1 (red fluorescence). DAPI was used to visualize the nuclei.