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

Abstract

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.

Activin A forms a non-signaling complex with ACVR1 and type II Activin/BMP receptors via its finger 2 tip loop

“…This observation supports a key role for D406 in binding ACVR1. Our model is further supported by the fact that there is a conserved hydrophobic residue (M79 in ACVR1) that rests in a hydrophobic pocket formed by the Activin A dimer, forming a ‘knob-in-hole’ motif, which is also present in the BMP2:BMPR1A ( Allendorph et al, 2006 ) and GDF11•TGFBR1 complexes (PDB:2GOO, 6MAC) ( Goebel et al, 2019 ).…”

“…Substitution of the region encompassed by the pre-helix and post-helix region (amino acids 355 to 391 of mature Activin A) with the corresponding region from BMP2, switched type I (but not type II) receptor utilization ( Korupolu et al, 2008 ), whereas swapping the pre-helix region of Activin A with that of MSTN switched the preference of Activin A from ACVR1B to TGFBR1 ( Cash et al, 2009 ). Moreover, the recently solved structure of GDF11 (an Activin family ligand) in complex with TGFBR1 and ACVR2B (PDB:6MAC), shows that the finger two region is particularly important for determining type I receptor specificity for the Activin class ( Goebel et al, 2019 ). Superimposition of Activin A into this ternary complex predicts that Activin A’s finger two tip loop (F2TL; amino acids 406–409) also participates in binding to type I receptors ( Figure 3—figure supplement 1 ).…”

Activins as Dual Specificity TGF-β Family Molecules: SMAD-Activation via Activin- and BMP-Type 1 Receptors

“…Similarly, activin A is a weak activator of SMAD1/5, whereas the heterodimer activin AB is approximately as potent as activin B. Type I receptors have been shown to bind at the interface between the two protomers in the dimeric ligand [43,44]. Thus, our observations raise the possibility that only one of the ligand protomers is needed to activate ALK2 signaling.…”

“…Sequences both in the receptors and the ligand determine binding specificity and strength. Recent studies have suggested that the finger 2 tip loops in activin A and GDF11 are important for their type I receptor binding specificities [44,45]. Moreover, in some cell types, only mutated ALK2, such as the variant encoded by ACVR1 (R206H), has been shown to relay a signal to SMAD1/5 after activin binding [11,12].…”

Human and mouse activin genes: Divergent expression of activin A protein variants and identification of a novel heparan sulfate-binding domain in activin B

“…Activins exert their biological activity by interacting with cell surface tetrameric complexes consisting of 2 type I receptors (usually ALK4 or ALK7) and 2 type II receptors (usually ACVR2A or ACVR2B) [6]. As currently understood, the mature activin dimers initially bind to type II receptors with high affinity, and this results in recruitment of type I receptors and phosphorylation by the constitutively active type II kinase which activates the serine/thr kinase on the type I receptor [10]. The activated receptor complex subsequently phosphorylates SMAD2/3 proteins that combine with SMAD4 and translocate to the nucleus where they activate down-stream target genes.…”