Follistatin is a single chain glycoprotein that binds activin A and myostatin with high affinity and neutralises them by occluding both receptor binding surfaces. It is not a peptide in the sense that most compounds in this library are; it is a folded, disulfide-rich protein of roughly 300 residues expressed as two principal isoforms. Research interest concentrates on its use as a selective ligand trap for the TGF-beta family members that restrain skeletal muscle growth.
Key facts
| Type | Secreted glycoprotein, activin and myostatin binding protein |
| Gene | FST, chromosome 5q11.2 in humans |
| Principal isoforms | FST-288 and FST-315, from alternative splicing of a 344 residue precursor |
| Mature length | 288 or 315 residues after cleavage of the 29 residue signal peptide |
| Approximate mass | 31 to 39 kDa depending on isoform and glycosylation |
| Domain structure | N-terminal domain plus three cysteine-rich follistatin domains |
| Primary ligands | Activin A, activin B, myostatin, GDF-11, BMP-2 and BMP-7 |
| Synonyms | FST, FS, follistatin-344, activin binding protein |
Structure and chemistry
The FST gene encodes a 344 residue precursor. Removal of the 29 residue signal peptide during secretion yields FST-315, the long isoform that circulates in plasma. Alternative splicing removes the acidic C-terminal tail and produces FST-288, which stays bound to cell surfaces. The designation follistatin-344 that appears in supplier catalogues refers to the cDNA construct and not to a distinct mature protein, and material sold under that name is FST-315 after processing.
The fold is modular. An N-terminal domain is followed by three follistatin domains, each a cysteine-rich EGF-like and Kazal-like pair stabilised by internal disulfides. Ten disulfide bonds across the mature chain make correct oxidative folding the limiting step in recombinant expression, which is why the protein is produced in mammalian or insect cell systems and not in bacteria without refolding. Two N-linked glycosylation sites contribute to the mass heterogeneity seen on SDS-PAGE.
The functional distinction between the isoforms is electrostatic. FST-288 carries an exposed basic patch in the first follistatin domain that binds heparan sulfate proteoglycans, holding the protein at the cell surface where it is available for endocytosis with its bound ligand. The acidic tail of FST-315 folds back over that patch and masks it, so the long isoform stays soluble. Heparin affinity chromatography separates the two on that basis and is the standard analytical check on isoform identity.
Mechanism of action
Activin A and myostatin signal through type II receptors, ActRIIA and ActRIIB, which recruit and phosphorylate the type I receptor ALK4 or ALK5. The activated complex phosphorylates SMAD2 and SMAD3, which partner with SMAD4 and enter the nucleus. In skeletal muscle that pathway raises atrogene transcription and restrains myoblast differentiation. Follistatin binds the mature ligand dimer in a two to one stoichiometry, wrapping around it so that both the type I and type II receptor epitopes are buried, and no signalling complex can assemble.
Because the block is at the ligand and not at the receptor, follistatin removes several inputs at once. Activin A, activin B, myostatin and GDF-11 are all neutralised, with lower affinity for BMP-2 and BMP-7. That breadth is what separates it from receptor-directed approaches such as soluble ActRIIB constructs, which intercept a different but overlapping ligand set. In culture, the readout is loss of SMAD2 and SMAD3 phosphorylation, falling atrogene transcript and higher fusion index in differentiating myoblasts.
Research applications
- Myoblast differentiation assays: fusion index and myosin heavy chain expression in C2C12 culture with myostatin neutralisation.
- SMAD2 and SMAD3 phosphorylation time courses used to quantify ligand trap potency against activin A.
- Surface plasmon resonance and biolayer interferometry measuring binding kinetics to activin A, myostatin and GDF-11.
- Heparin affinity chromatography separating FST-288 from FST-315 and confirming isoform identity in a preparation.
- Granulosa and gonadal cell culture examining activin dependent follicle stimulating hormone regulation.
- Comparative ligand trap studies run against soluble activin receptor constructs to map overlapping and distinct specificity.
Compounds studied around muscle and connective tissue signalling sit in NuVion’s Tissue Repair Signalling category.
Handling in the laboratory
Recombinant follistatin is supplied lyophilised, frequently with a carrier protein or a sugar to stabilise the fold. Reconstitution is with sterile or bacteriostatic water added slowly down the wall of the vial and swirled. Folded proteins are less tolerant of agitation than short peptides, and vortexing a ten disulfide protein at an air interface is a reliable way to lose activity without changing the appearance of the solution. The reconstitution calculator converts vial content and diluent volume into a stock concentration.
Carrier-free preparations adsorb to plastic at low concentration, so dilute working solutions are prepared in buffer containing bovine serum albumin unless the assay forbids it. Reducing agents open the disulfide network and destroy the ligand binding surface, so buffers are checked for dithiothreitol and tris(2-carboxyethyl)phosphine before use. Reconstituted material is aliquoted and held frozen, since repeated freeze and thaw cycles aggregate the protein and the resulting loss of potency is not visible by eye.
Testing and supply from NuVion
NuVion supplies a tested range across tissue repair signalling, including BPC-157, TB-500, GHK-Cu and KPV, together with the KLOW blend. That range is manufactured at a GMP-audited facility and independently tested by Janoshik Analytical, with purity determined by RP-HPLC and identity confirmed by mass spectrometry, and the certificates are published in the Certificate of Analysis library. Follistatin is not part of the current catalogue and this page is a reference profile.
Related compounds
Signalling downstream of the growth hormone axis is covered in IGF-1 LR3 and HGH. Compounds studied in muscle and connective tissue repair include BPC-157, TB-500 and thymosin beta-4. For matrix and collagen work, GHK-Cu covers the copper peptide arm.
Frequently asked questions
What is the difference between follistatin-288 and follistatin-315?
Both come from the same gene by alternative splicing. FST-315 keeps an acidic C-terminal tail that folds over the heparin binding patch, so the protein stays soluble and circulates. FST-288 lacks that tail, leaves the basic patch exposed and binds heparan sulfate on cell surfaces. Ligand affinity is similar; the distribution is what differs, and heparin affinity chromatography is how the two are told apart.
Is follistatin-344 a separate isoform?
No. 344 is the residue count of the unprocessed precursor including its signal peptide. Material catalogued as follistatin-344 is expressed from that full length construct and is secreted as FST-315 once the signal peptide is cleaved, so the number describes the expression construct and not the protein in the vial.
Why is follistatin produced in mammalian cells instead of bacteria?
The mature chain carries ten disulfide bonds and two N-linked glycosylation sites. Bacterial cytoplasm is a reducing environment and does not glycosylate, so bacterial expression yields inclusion bodies that need refolding with low recovery. Mammalian and insect cell systems fold and glycosylate the protein directly, which is why recombinant preparations cost what they do.
Is follistatin listed in the Australian Register of Therapeutic Goods?
No. Follistatin is not included in the Australian Register of Therapeutic Goods and has not been assessed by the Therapeutic Goods Administration for quality, safety or efficacy. Recombinant material of this kind is supplied in Australia as a laboratory reagent for in vitro research.
Research use only. The compound described on this page is discussed as a laboratory chemical used in in vitro research. It is not included in the Australian Register of Therapeutic Goods and has not been assessed by the Therapeutic Goods Administration for quality, safety or efficacy. It is not for human or veterinary use, and nothing on this page is a representation about therapeutic use.
