CV2 preferentially binds BMP-9. (A) Recombinant BMP-9 (50 ng), CV2 (100 ng), BMP-4 (50 ng), and carrier were combined as indicated at the top, and the presence of the respective protein was confirmed by immunoblotting (IB; top 3 blots). Interactions between the proteins were analyzed by immunoprecipitation (IP) followed by immunoblotting using the indicated Abs. (B) BMP-9 (50 ng), CV2 (100 ng), and BMP-4 (50 ng) were combined as indicated at the top, chemically cross-linked, and analyzed by immunoblotting using the indicated Abs. (C) CV2 (100 ng), BMP-4 (50, 250 ng, 500 ng, or 1000 ng), and BMP-9 (50 ng) were combined as indicated at the top, immunoprecipitated by anti-CV2 Abs and analyzed by immunoblotting using anti–BMP-4 and anti–BMP-9 Abs. (D) CV2 (100 ng), biotin-BMP-9 (100 ng), and ALK1-Fc (100 ng) were combined as indicated at the top, immunoprecipitated by anti-ALK1 Abs and analyzed by immunoblotting using antibiotin Abs.