Figure 6.

Type I and type II receptors of the transforming growth factor β (TGF-β) family. Structures of type I (A–D) and II (E–H) receptor ectodomains are shown as ribbon diagrams, with disulfide bonds depicted as sticks with yellow sulfur atoms. Disulfide bond letters and β-strand numbers correspond to those in Fig. 5. A loop without reliable electron density in ALK3 is shown as a dashed line. Structures are for ALK6 (Kotzsch et al. 2009), ALK3 (Kirsch et al. 2000), ALK1 (Townson et al. 2012), ALK5 (Groppe et al. 2008), ActRII (Greenwald et al. 1999), ActRIIB (Thompson et al. 2003), BMPRII (Mace et al. 2006), and TβRII (Boesen et al. 2002).