Fig. 4. Conserved region 1 and ALK1-specificity determinants.

a ALK1-binding residues are mapped onto the BMP10 surface (grey), with those conserved across all BMPs in Fig. 3a coloured in red, those from the BMP9 and BMP10 conserved region 1 in cyan, and other variable residues in yellow. b ALK1 (magenta cartoon) binding to BMP10 (surface), with residues interacting with BMP10 shown in sticks. c–f Detailed interactions between BMP10 and ALK1. g Sequence alignment of four BMP-binding type I receptors, ALK1, ALK2, ALK3 and ALK6, with the four specificity-determining residues in ALK1 highlighted in yellow. Loop 4 and loop 5 are the two loops surrounding the 3₁₀ helix (Supplementary Fig. 4). h Overlaid structures of BMP type I receptors. The structure of ALK1 in the BMP10:ALK1 complex (magenta) is overlaid onto ALK1 in BMP9:ALK1 complex (PBD:4FAO, orange), ALK3 (PDB:2GOO, light green) and ALK6 (PDB:3EVS, light grey). ALK1 residues highlighted in g are shown in sticks.