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Structural characterization of an activin class ternary receptor complex reveals a third paradigm for receptor specificity

Significance The TGFβ family of ligands plays fundamental roles in controlling diverse signaling events throughout multicellular organisms. The family is divided into 3 classes: activin, BMP, and TGFβ, with each signaling through specific combinations of type I and type II receptors. We demonstrate that activin ligands bind receptors similarly to BMP ligands, but they utilize the ligand fingertip to make specificity-determining contacts with the type I receptors. We also show that different molecular mechanisms have emerged for how GDF11 and TGFβ bind the same type I receptor, whereby GDF11 is more dependent on direct receptor contacts. Thus, this study provides insight at the molecular level into how the activin class ligands bind and assemble their type I and type II receptors. TGFβ family ligands, which include the TGFβs, BMPs, and activins, signal by forming a ternary complex with type I and type II receptors. For TGFβs and BMPs, structures of ternary complexes have revealed differences in receptor assembly. However, structural information for how activins assemble a ternary receptor complex is lacking. We report the structure of an activin class member, GDF11, in complex with the type II receptor ActRIIB and the type I receptor Alk5. The structure reveals that receptor positioning is similar to the BMP class, with no interreceptor contacts; however, the type I receptor interactions are shifted toward the ligand fingertips and away from the dimer interface. Mutational analysis shows that ligand type I specificity is derived from differences in the fingertips of the ligands that interact with an extended loop specific to Alk4 and Alk5. The study also reveals differences for how TGFβ and GDF11 bind to the same type I receptor, Alk5. For GDF11, additional contacts at the fingertip region substitute for the interreceptor interactions that are seen for TGFβ, indicating that Alk5 binding to GDF11 is more dependent on direct contacts. In support, we show that a single residue of Alk5 (Phe84), when mutated, abolishes GDF11 signaling, but has little impact on TGFβ signaling. The structure of GDF11/ActRIIB/Alk5 shows that, across the TGFβ family, different mechanisms regulate type I receptor binding and specificity, providing a molecular explanation for how the activin class accommodates low-affinity type I interactions without the requirement of cooperative receptor interactions.