Identification of a Binding Site on the Type II Activin Receptor for Activin and Inhibin

Abstract

Type II activin receptors (ActRII and ActRIIB) are single-transmembrane domain serine/threonine kinase receptors that bind activin to initiate the signaling and cellular responses triggered by this hormone. Inhibin also binds type II activin receptors and antagonizes many activin effects. Here we describe alanine scanning mutagenesis of the ActRII extracellular domain. We identify a cluster of three hydrophobic residues (Phe 42 , Trp 60 , and Phe 83 ) that, when individually mutated to alanine in the context of the full-length receptor, cause the disruption of activin and inhibin binding to ActRII. Each of the alanine-substituted ActRII mutants retaining activin binding maintains the ability to form crosslinked complexes with activin and supports activin cross-linking to the type I activin receptor ALK4. Unlike wild-type ActRII, the three mutants unable to bind activin do not cause an increase in activin signaling when transiently expressed in a corticotroph cell line. Together, our results implicate these residues in forming a critical binding surface on ActRII required for functional interactions with both activin and inhibin. This first identification of a transforming growth factor-␤ family member binding site may provide a general basis for characterizing binding sites for other members of the superfamily.Activins and inhibins (1) belong to the transforming growth factor-␤ (TGF-␤) 1 family of growth and differentiation factors (2) and were originally identified based on their role in regulating reproductive function. Activin was first identified and isolated based on its ability to stimulate the release of folliclestimulating hormone from gonadotrophs in the anterior pituitary (3), whereas inhibin was identified based on its ability to inhibit this process (4). Activins (ϳ28,000 Da) and inhibins (ϳ32,000 Da) are disulfide-linked dimers of related polypeptides, with activins consisting of two ␤ chains (activin A, ␤A-␤A; activin AB, ␤A-␤B; and activin B, ␤B-␤B) and inhibins possessing an ␣ chain disulfide-linked to a ␤ chain (inhibin A, ␣-␤A; and inhibin B, ␣-␤B) (1). The structure of these factors is determined by several conserved cysteine residues that form disulfide bonds in the tightly folded "cystine-knot" motif shared by TGF-␤ superfamily members (6). Although inhibins frequently counteract the effects of activins (7), there are cell types in which inhibin is unable to attenuate the activin response (8, 9), making it unlikely that inhibin blocks activin effects exclusively by competing with activin for receptor binding as has been previously suggested (10).Type II activin receptors (ActRII and the closely related ActRIIB) are single-transmembrane domain serine/threonine kinase receptors that bind activin with high affinity and thereby initiate the cellular responses triggered by activin (11)(12)(13)(14). Like TGF-␤ receptors (15), activin receptors are thought to exist as homodimers, a finding consistent with their proposed role in binding dimeric ligand. Inhibin also binds ActRII and ActRIIB, although...

Identification of a Functional Binding Site for Activin on the Type I Receptor ALK4

“…The complex structure of BMP-7 dimer bound to the ActRII-ECD shows that the ActRII-ECD makes contact with only one of the dimer subunits (23). The binding interface revealed by the x-ray structure agrees with the binding affinities available for mutants of ActRII (24), activin-A (25,26), and BMP-2 (27). The structure of BMP-2 in complex with BMP receptor IA (ALK3, BR1A) was also solved recently (28), revealing a type I receptor ECD fold similar to that of the ActRII-ECD.…”

“…To incorporate the amino-terminal FLAG tag (following glycine 28) and to generate mutations in the ECD of ALK4-trunc or full-length ALK4, we utilized an overlapping PCR strategy (24). Primers were constructed to incorporate a 5Ј-HindIII site and a 3Ј-EcoRI site, and PCR products were gel-purified and digested with both enzymes and then subcloned into HindIII/EcoRI-digested pcDNA3 vector to yield mutant receptor constructs.…”

“…For Western blotting, cells were solubilized in 200 l of 1% Triton X-100, and protein concentrations were determined using the BCA method according to the manufacturer's instructions. SDS-PAGE and electrotransfer to nitrocellulose were carried out using NuPAGE gels and a NOVEX X-cell II apparatus as described previously (24). To detect FLAG-tagged ALK4 constructs expressed at the cell surface, intact cells were fixed in paraformaldehyde, incubated with anti-FLAG antibody, washed, and treated with peroxidase-conjugated anti-mouse Ig.…”

“…To detect FLAG-tagged ALK4 constructs expressed at the cell surface, intact cells were fixed in paraformaldehyde, incubated with anti-FLAG antibody, washed, and treated with peroxidase-conjugated anti-mouse Ig. Specific antibody staining was measured using 3,3Ј,5,5Ј-tetramethyl-benzidine peroxidase substrate as described previously (24).…”

“…Covalent Cross-linking-Covalent cross-linking was performed by incubating transfected HEK293T cells (4 ϫ 10 6 ) with 125 I-activin-A (10 6 cpm/well) for 2 h at room temperature in binding buffer (24) with gentle rocking. Cells were washed, resuspended in 1 ml of Hepes buffer (HDB; 12.5 mM Hepes, pH 7.4, 140 mM NaCl, and 5 mM KCl), brought to 0.5 mM disuccinimidyl suberate, and incubated for 30 min on ice.…”