FIGURE 4.

Site I and II interactions at the ALK1/BMP9 interface. A, close-up of the interaction surfaces, with the color scheme retained from Fig. 3. ALK1 is shown in schematic, with key residues drawn as sticks; BMP9 is shown in surface representation. For clarity the overall complex is shown to the side. B, detailed close-up of the interfaces, showing specific polar contacts (dashed lines) between residues along the F2 and F3 loops of ALK1. Key residues are highlighted with asterisks. C, sequence alignment of type I receptors (ALK1–7), with contact residues highlighted in gold. D, sequence alignment of BMP9 and BMP10 with select TGF-β ligands, showing key contact residues from both BMP9 protomers (red and blue). Asterisks coincide with panel B.