Movie S1.

A feasible pathway for conformational change of pro-BMP9 between open-armed and cross-armed conformations. A cross-armed model of pro-BMP9 was constructed as described in Discussion. The movie, if looped, will display reversible conformational change from cross-armed to open-armed and open-armed to cross-armed. Structures are shown in cartoon with yellow disulfide bonds. BMP9 is shown with magenta and orange monomers. Prodomain monomers are shown in blue and green. Different shades are used for different elements. Thus for the prodomain monomer on the left, the arm domain and α2-helix are light blue, the α1-helix plus latency loop are marine, and the α5-helix is dark blue. The cross-armed pro-BMP9 model accommodates the longer α5-helix of pro-BMP9 in the same position as the shorter α5-helix of pro-TGF-β1. The open-armed pro-BMP9 model adds the α1-helix and the straight jacket elements to the pro-BMP9 crystal structure in positions where they do not clash. Using the open-armed and cross-armed models as starting and ending states, 88 morphing states in-between were calculated with adiabatic mapping using the script morph_dist from the Morph Server (1).