Fig. 4. The ActRIIB:activin A interface. (A) Surfaces of interaction between activin A and ActRIIB. Molecular surfaces showing contact residues at the complex interface for the activin A monomer (right) and ActRIIB (left). Positively and negatively charged residues are colored blue and red, respectively, while polar and hydrophobic residues are colored purple and green, respectively. (B) Hydrophobic interactions at the ActRIIB:activin A interface. (C) Hydrophilic interactions at the ActRIIB:activin A interface.