Follistatin-344 is the 344-residue primary translation product of the human FST gene: a secreted activin-binding glycoprotein of roughly 37.8 kDa that neutralises several TGF-β superfamily ligands, including activin A and myostatin (GDF-8), by wrapping around them and blocking receptor contact. It is not a peptide in the working sense of that word — at 344 amino acids it is a folded, disulfide-rich, multi-domain protein produced recombinantly, and it behaves in the laboratory accordingly.
MyPeptide supplies Follistatin-344 as a lyophilized recombinant protein in a 1 mg vial within the IGF and muscle peptides range. Supplied for laboratory research only; not for human or veterinary use.
What is Follistatin-344?
The number in the name is a residue count, and it identifies which isoform is in the vial. The FST gene generates two main products by alternative splicing. FS-344 is the longer transcript's translation product; removal of its N-terminal signal peptide yields the 315-residue form, FS-315, which is the predominant circulating species in serum and carries an acidic C-terminal tail. The alternatively spliced FS-288 lacks that tail, binds heparan sulfate proteoglycans and therefore stays associated with cell surfaces and basement membrane rather than circulating.
That distinction is not academic. The same protein family shows markedly different distribution behaviour depending on which isoform is used, so a study comparing results across sources must confirm the isoform first. Catalogue identifiers here are CAS 80449-31-6 and an approximate molecular weight of 37,800 Da.
Origin and structure
Follistatin was isolated from ovarian follicular fluid in 1987, identified by its ability to suppress pituitary follicle-stimulating hormone secretion — hence the name. Its role as a broad TGF-β ligand trap emerged later, and the muscle literature is later still.
Structurally the protein has an N-terminal domain followed by three cysteine-rich follistatin domains, each containing EGF-like and Kazal-like motifs. The functional unit is a complex: two follistatin molecules encircle one activin dimer, burying the ligand's type I and type II receptor-binding surfaces almost completely. The affinity of that interaction for activin A is in the picomolar range, which is why follistatin functions as an effective neutralising trap rather than a competitive modulator.
Native follistatin is glycosylated. Recombinant material expressed in bacterial systems is not, and glycosylation status affects solubility, clearance and sometimes apparent potency. The expression system is therefore a specification worth checking on the certificate rather than a detail.
How Follistatin-344 is thought to work
The mechanism is ligand sequestration, not receptor antagonism. Follistatin binds free activin A, myostatin (GDF-8), GDF-11 and selected bone morphogenetic proteins, preventing them from engaging the type II activin receptors (ActRIIA and ActRIIB) at the cell surface. Downstream, this reduces receptor-mediated phosphorylation of SMAD2/3 — the signalling arm associated in skeletal muscle with transcriptional suppression of growth — and in muscle models is reported to shift the balance towards Akt/mTOR signalling.
Selectivity is the key mechanistic caveat. Follistatin's binding profile is broad by design: activin A is its highest-affinity partner, and activin signalling is central to reproductive endocrinology, inflammation and haematopoiesis as well as to muscle. A study using follistatin to interrogate the myostatin pathway is not making a clean pathway-specific intervention, and interpreting the result requires acknowledging the activin arm. This is the principal mechanistic difference between follistatin and a receptor-decoy approach such as ACE-031, and between follistatin and a targeted propeptide such as GDF-8 / myostatin propeptide.
Reported circulating persistence for the protein is short — on the order of hours in published pharmacokinetic work — which is one reason the most cited in-vivo studies have used gene delivery rather than protein administration.
What the research has examined
Skeletal muscle and myostatin pathway models
The best-known work is genetic. Transgenic mice overexpressing follistatin in skeletal muscle showed substantially increased muscle mass, and the effect exceeded that seen in myostatin-null animals — evidence that follistatin acts on more than myostatin alone. Follistatin-null and myostatin-null crosses were used to establish that point.
Gene delivery studies
The FS-344 isoform is the one used in adeno-associated virus gene-transfer work. Early-phase clinical studies delivering AAV1-FS344 to muscle in Becker muscular dystrophy and sporadic inclusion body myositis were reported in the 2010s, with muscle biopsy and walking-distance endpoints. These studies delivered the gene, not the recombinant protein, and were small early-phase investigations; they are not evidence about protein administration.
Reproductive and endocrine biology
The original and still largest literature concerns follistatin's regulation of FSH secretion and its role in ovarian follicular development, where activin binding is the mechanism of interest. Much of the assay work in this field uses recombinant follistatin as a neutralising reagent.
Fibrosis and inflammation models
Because activin A participates in fibrotic and inflammatory signalling, follistatin has been used as a pathway-blocking tool in rodent liver, lung and kidney fibrosis models. Findings are preclinical.
Forms and sizes we supply
| Attribute | Specification |
|---|---|
| Product | Follistatin-344 |
| Form | Lyophilized recombinant protein, sealed glass vial |
| Available size | 1 mg |
| Purity | ≥98% by HPLC, lot-matched COA available |
| CAS number | 80449-31-6 |
| Molecular weight | Approximately 37,800 Da (37.8 kDa) |
| Residue count | 344 amino acids |
| Protein class | Activin-binding glycoprotein, TGF-β superfamily antagonist |
| Principal binding partners | Activin A, myostatin (GDF-8), GDF-11, selected BMPs |
| Isoform relationship | Precursor to circulating FS-315; distinct from cell-associated FS-288 |
| Intended use | Laboratory research only |
The 1 mg vial is a large quantity in molar terms for a 37.8 kDa protein used at nanomolar working concentrations, so most laboratories aliquot the full vial on first reconstitution rather than returning to it. Sourcing checks are covered in our Follistatin-344 buying guide.
Reconstitution and storage in a lab context
Follistatin is handled as a recombinant protein, which means gentler technique than a small synthetic peptide tolerates. Sterile or bacteriostatic water is added slowly down the vial wall and the vial left to stand rather than vortexed; a disulfide-rich multi-domain protein is far more susceptible to shear denaturation than a 24-mer. Visible cloudiness after reconstitution indicates aggregation and is a reason to stop, not to shake harder.
The arithmetic is straightforward — 1 mg made up to 1 mL gives 1 mg/mL, and 0.1 mL contains 100 mcg — but working concentrations are typically several dilutions below that. At those concentrations adsorption to plastic becomes the dominant loss route, so a carrier protein such as 0.1% bovine serum albumin is standard in dilute stocks. Method detail is in our reconstitution guide.
Lyophilized vials are stored at −20 °C, protected from light and moisture. Reconstituted material is used promptly at 2–8 °C or aliquoted and held at −20 °C or below; repeated freezing and thawing is the single most common cause of lost activity in proteins of this class. Wider guidance is in how to store peptides.
Purity, COA and how to read it
For a protein of this size, HPLC purity alone is not sufficient. The entries that carry information are the SDS-PAGE lane, which should show a single band at the expected mass under reducing conditions and reveals aggregates and clipped species that a reversed-phase trace can miss; the expression system, since bacterial expression yields non-glycosylated protein while mammalian or insect systems do not; and endotoxin content, which matters for any cell-culture application. A bioactivity statement — typically activin A neutralisation in a reporter assay — is the most meaningful specification of all, because a correctly sized but misfolded protein will pass a purity test and fail an experiment. Our guide to reading a certificate of analysis covers the general structure of these documents.
Regulatory status
Follistatin-344 has no marketing authorisation as a medicine anywhere. The gene-therapy construct studied in early-phase muscular dystrophy trials is an investigational product and is not the same thing as recombinant protein supplied as a reagent. Material on this site is a research-use-only reagent for qualified laboratory personnel — not a supplement, not a medicine, and not for human or veterinary administration. Myostatin-pathway agents are prohibited in sport under anti-doping rules.
Related molecules and further reading
Within the same pathway, ACE-031 is the receptor-decoy approach and GDF-8 / myostatin propeptide the ligand-specific one; the three block the axis at different points and are not interchangeable tools. Broader orientation is in our peptides for muscle growth research overview.