# ACE-031 (ActRIIB-Fc): Decoy Receptor Design, Ligand Promiscuity and Research Context > A dimeric Fc fusion that captures myostatin before it reaches the membrane, what else it captures, and why that breadth ended its clinical programme. Web page: https://peptidemedixeu.com/en/learn/what-is-ace-031/ · Peptide Medix EU · 6 min read · Updated: 2026-05-05 [ACE-031](https://peptidemedixeu.com/en/products/ace-031/) is a decoy receptor in soluble form: the ligand-binding extracellular portion of human activin receptor type IIB joined to the Fc region of human IgG1 and produced as a disulfide-bonded homodimer. It captures myostatin and other TGF-β superfamily ligands out in the extracellular space, so they never reach the receptor embedded in the membrane. Acceleron Pharma developed it, and it is also catalogued as ActRIIB-Fc or ramatercept; it progressed to Phase 2 trials before being discontinued in 2013, a history that shapes how the molecule should be interpreted in the literature. Peptide Medix EU supplies ACE-031 as a lyophilized [recombinant protein](https://peptidemedixeu.com/en/glossary/recombinant/) in a 1 mg vial in the IGF and muscle peptides range, for laboratory research only. ## Defining ACE-031 ACE-031 is a fusion protein rather than a peptide, assembled from two parts. One is the extracellular domain of ActRIIB, one of the two type II activin receptors that sit at the head of SMAD2/3 signalling in skeletal muscle. The other is the Fc fragment of human IgG1, a familiar protein-engineering component that contributes two things simultaneously: it forces dimerisation, so the construct carries two ligand-binding arms, and it exploits neonatal Fc receptor recycling, lengthening persistence in circulation from hours to days. What emerges has a receptor's binding specificity paired with an antibody's pharmacokinetics and no signalling ability of its own, the textbook ligand-trap architecture. Being a dimeric glycoprotein, it migrates well above 70 kDa on SDS-PAGE, which doubles as a handy identity check. ## Origin and design The concept comes straight from myostatin pathway genetics. Myostatin (GDF-8) holds skeletal muscle mass in check, and both animals and people carrying loss-of-function variants display marked muscle overgrowth. Since myostatin signals via ActRIIB, flooding the extracellular space with a soluble copy of that receptor draws the ligand away from the membrane-bound version. The critical engineering issue is the rest of ActRIIB's ligand repertoire. It naturally binds activin A and activin B, GDF-11 (a near [relative of myostatin](https://peptidemedixeu.com/en/glossary/myostatin-gdf-8/) with developmental roles) and several bone morphogenetic proteins, BMP9 and BMP10 among them. A soluble ActRIIB trap therefore takes far more than myostatin out of circulation, and subsequent engineering in this class has concentrated on tightening that specificity. ## Proposed mechanism It acts by competitive ligand capture upstream of the receptor. Once ligand is held by the trap, ActRIIB on the myocyte surface goes unoccupied, the type I receptor (ALK4/ALK5) is never recruited or phosphorylated, and SMAD2/3 signalling to the nucleus declines. Muscle models describe this as lifting transcriptional suppression of growth programmes and tipping the balance towards Akt/mTOR-linked protein synthesis signalling. Its broad binding profile has two mechanistic consequences. Preclinical work reported bone effects alongside muscle effects, which fits the role of activins and BMPs in bone remodelling. More importantly, BMP9 and BMP10 signalling via ALK1 in vascular endothelium is an established regulator of vascular integrity, and the literature generally attributes the vascular findings that ended ACE-031's clinical development to that arm rather than to the muscle pathway. For researchers this means treating ACE-031 as a broad ActRIIB ligand trap, not a myostatin-specific reagent, and saying so explicitly in any interpretation. Set against [Follistatin-344](https://peptidemedixeu.com/en/learn/what-is-follistatin-344/), which binds the ligands directly, and against GDF-8 / myostatin propeptide, the ligand's own natural inhibitor and the most selective of the three, ACE-031 interrupts the axis at the receptor interface. Picking among them is a genuine experimental choice about where in the pathway to intervene. ## Research to date ### Muscle and bone models Rodent experiments using soluble ActRIIB-Fc constructs reported large gains in [lean mass](https://peptidemedixeu.com/en/learn/peptides-for-muscle-growth-research-overview/) and fibre size, including in dystrophic and cachexia models, while other studies reported higher bone mineral density. These results prompted clinical development. ### Phase 1 pharmacology A Phase 1 trial in healthy postmenopausal volunteers reported increased lean mass and changes in markers of bone formation and fat mass after a single administration, with pharmacokinetics matching the Fc-extended half-life. This is the clearest published demonstration that the mechanism engages its target in humans. ### Duchenne studies and the halt A Phase 2 programme in boys with Duchenne muscular dystrophy stopped in 2013, and the stated reason was not an absence of muscle effect but findings outside muscle: nosebleeds, bleeding gums and dilated surface blood vessels, consistent with disrupted BMP9/BMP10-ALK1 vascular signalling. Work on ACE-031 was never restarted, and any research use of the molecule should be framed accordingly. ### Later compounds The class continued without it. More selective ActRII-directed agents followed, including ligand traps with narrowed binding and monoclonal [antibodies](https://peptidemedixeu.com/en/learn/research-catalog-type-antibodies/) raised against the receptor, several of which were approved in haematology and pulmonary indications. ACE-031 is best viewed as the first-generation molecule whose shortcomings shaped those later designs. ## Format and size For a protein used at nanomolar working concentrations, 1 mg is a substantial molar quantity, so [aliquoting](https://peptidemedixeu.com/en/learn/aliquoting-peptides/) the entire vial at first reconstitution is standard practice. Sourcing points are covered in our ACE-031 buying guide. ## Reconstitution and storage in the lab Fc-fusion proteins are the most delicate items in a peptide catalogue and need corresponding care. Add sterile or bacteriostatic water slowly against the vial wall and let the vial rest; vortexing or shaking a large glycoprotein creates shear-driven aggregates that no further mixing will undo. Haze that persists after dissolution signals aggregation and should be treated as a quality failure rather than incomplete mixing. The concentration maths is trivial (1 mg brought to 1 mL is 1 mg/mL, and 0.1 mL holds 100 mcg), but working concentrations lie far below that, where adsorption onto plastic and glass becomes the main source of loss. Adding a carrier protein such as 0.1% bovine serum albumin to dilute stocks is routine, and low-binding tubes are preferable. The general method is in our reconstitution guide. Store lyophilized vials sealed at −20 °C, shielded from light and moisture. Use reconstituted material at 2-8 °C for immediate experiments or aliquot and freeze it; repeatedly freezing and thawing an Fc-fusion protein dependably produces aggregates and costs activity. Wider guidance is in [how to store](https://peptidemedixeu.com/en/learn/how-to-store-peptides/) peptides. ## Purity and reading the COA A fusion protein certificate contains different information from a peptide one, and the useful parts are not the HPLC sections. SDS-PAGE run under both reducing and non-reducing conditions should reveal monomer and disulfide-linked dimer at the expected sizes; a non-reducing lane dominated by monomer points to a construct that failed to assemble. Where size-exclusion chromatography is supplied, it measures high-molecular-weight aggregate, the failure most often behind an inactive vial. The expression system and glycosylation status should be declared, and [endotoxin levels](https://peptidemedixeu.com/en/glossary/endotoxin/) matter for any cell-based assay. A bioactivity value from a binding or reporter assay tells you more than any purity figure. Our guide to reading a certificate of analysis explains how these sections fit together. ## Regulatory position ACE-031 holds no marketing authorisation in any jurisdiction. Clinical development ended in 2013 and never resumed, so no reference-listed product and no approved labelling exist for the molecule. Agents acting on the myostatin pathway are banned in sport under anti-doping regulations. ## Related molecules and further reading Our catalogue offers two other ways to interrupt this axis: Follistatin-344, a wide-spectrum ligand trap, and GDF-8 / myostatin propeptide, the most ligand-selective of the set. For how the muscle research families relate to one another, see our peptides for muscle growth research overview. ## Frequently asked questions ### What type of molecule is ACE-031? It is a fusion protein, not a peptide: the extracellular ligand-binding domain of human activin receptor type IIB linked to the Fc region of human IgG1, forming a disulfide-bonded homodimer with two binding arms. The Fc part both enforces dimerisation and prolongs circulation through neonatal Fc receptor recycling. ### In what way does ACE-031 interrupt myostatin signalling? Through competitive capture ahead of the receptor. Soluble ActRIIB in the extracellular space binds the ligand so that membrane ActRIIB stays unoccupied, the type I receptor is never recruited, and SMAD2/3 signalling into the nucleus drops. ### Does ACE-031 only bind myostatin? No. ActRIIB also binds activin A and B, GDF-11 and several bone morphogenetic proteins including BMP9 and BMP10, so a soluble trap clears far more than myostatin. That lack of selectivity is the molecule's key interpretive limitation and motivated the narrower agents that followed. ### Why did development of ACE-031 stop? The Phase 2 programme in Duchenne muscular dystrophy was halted in 2013 following findings outside muscle: nosebleeds, bleeding gums and dilated surface vessels, consistent with disturbed BMP9/BMP10-ALK1 vascular signalling. The programme was never restarted. ### How does it compare with Follistatin-344? Both interrupt the same axis but at different points. Follistatin binds the ligands themselves and has its strongest affinity for activin A, whereas ACE-031 offers a decoy copy of the receptor those ligands would normally engage. Binding profiles, molecular size and handling needs all differ. ### What should an ACE-031 certificate of analysis include? SDS-PAGE under reducing and non-reducing conditions showing both monomer and disulfide-linked dimer, size-exclusion data on high-molecular-weight aggregate, the expression system and glycosylation status, endotoxin content, and preferably a bioactivity result from a binding or reporter assay. For a fusion protein a purity percentage on its own means little. ### What is the right way to reconstitute ACE-031? Slowly and gently: run sterile or bacteriostatic water down the vial wall and then leave the vial to stand. Shaking or vortexing a large glycoprotein drives irreversible shear aggregation. Haze remaining after dissolution means aggregates have formed, not that mixing is incomplete. ## Related products - [ACE-031](https://peptidemedixeu.com/en/products/ace-031/): from €191 --- For laboratory research use only. This page is provided for scientific and educational information. Materials referenced here are sold strictly for in-vitro laboratory research by qualified professionals — not for human or veterinary use, and nothing on this page is medical advice.