Research explainer
Follistatin is a glycoprotein that binds and neutralises members of the TGF-β superfamily, most notably activin A and myostatin (GDF-8). FST-344 is not a different molecule from FST-315 — it is the same molecule before processing. The 344 is the precursor including its signal peptide; remove that peptide and you have the 315-residue form that circulates in serum.
That single fact resolves most of the confusion around follistatin, and almost no page selling it explains it. This one covers the isoforms, what the binding actually does, where the well-known muscle data came from, and — importantly — what that data does not transfer to.
Follistatin has a reputation built on one spectacular set of animal experiments. The reputation is deserved; the way it gets applied usually is not. The gap between those two things is the most useful part of this subject, so it gets its own section rather than a footnote.
What follistatin is
Follistatin is a secreted glycoprotein encoded by the FST gene. It was originally isolated from ovarian follicular fluid — hence the name — as a substance that suppressed follicle-stimulating hormone release. Expression is essentially ubiquitous across tissues, with the highest concentrations found in the ovary and then the skin.
Structurally it is built around a series of cysteine-rich follistatin domains. Functionally it works as a binding protein: it does not act on a receptor of its own. It captures certain TGF-β superfamily ligands and prevents them reaching their receptors. Everything follistatin appears to do downstream is a consequence of that sequestration.
The numbers: 344, 317, 315 and 288
This is where most write-ups go wrong, and it is worth getting right, because the numbers describe two different things at once — splicing and processing.
The FST gene produces two primary transcripts by alternative splicing. Each encodes a precursor protein that includes an N-terminal signal peptide. Cleave the signal peptide and you get the mature secreted protein.
| Designation | What it is | Why it matters |
|---|---|---|
| FST-344 | The longer precursor, 344 residues, signal peptide included | Processed to FST-315. This is the form used in the gene-transfer literature. |
| FST-315 | The mature protein produced from FST-344 | The predominant circulating isoform in serum. Its acidic C-terminal tail masks the heparin-binding site, so it stays in solution rather than sticking to tissue. |
| FST-317 | The shorter precursor, 317 residues, signal peptide included | Processed to FST-288. |
| FST-288 | The mature protein produced from FST-317 | Lacks the C-terminal extension, so it binds heparan sulfate proteoglycans strongly and stays bound to cell surfaces and tissue. |
| FST-303 | A proteolytic product of the longer form | Found principally in gonadal tissue. |
So “FST-344 vs FST-315” is not a comparison between two options. They are the same gene product at two stages. The real biological comparison is FST-315 against FST-288 — circulating versus tissue-bound — and it comes down to that C-terminal tail and whether the heparin-binding site is exposed.
What follistatin binds
Follistatin’s characterised ligands sit in the TGF-β superfamily. The ones the literature returns to most often:
- Activin A — the original context. Follistatin binding to activin is the mechanism behind the FSH-suppressing activity that gave the protein its name.
- Myostatin (GDF-8) — a negative regulator of skeletal muscle mass. This is the interaction that generated all the muscle interest.
- Certain bone morphogenetic proteins — with affinities varying considerably between individual BMPs.
The binding is high-affinity and effectively neutralising: a ligand bound by follistatin cannot engage its receptor. Because myostatin restrains muscle growth, removing myostatin from circulation removes a brake. That is the whole mechanistic story, and it is elegant enough that it is easy to stop reading there.
Where the muscle data actually comes from
This is the section that matters most, and it is the one routinely left out.
The results everyone cites — large, durable increases in muscle mass and grip strength, sustained in mice for well over a year, with improvements in dystrophic models and no observed effect on gonadal histology or reproductive capacity — come from a 2008 PNAS study by Haidet and colleagues. The intervention was a single intramuscular injection of an adeno-associated virus vector (AAV1) carrying the FS-344 gene.
Read the original: Haidet et al., “Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors”, PNAS 2008The authors chose the FS-344 construct deliberately, because it “exclusively generates the serum circulating FS-315 isoform” once the peptide is cleaved — they wanted the soluble form rather than the tissue-binding one. Note what that means: the transfected muscle became a factory, manufacturing and secreting follistatin continuously for the life of the animal.
That is a gene therapy experiment, not a protein administration experiment. The dependent variable was sustained endogenous expression from a viral vector. A lyophilised recombinant protein in a vial is a fundamentally different intervention with a different exposure profile, and the 2008 results do not transfer to it by analogy. Any page that cites those mouse figures next to a product listing is inviting you to make a substitution the study does not support.
What the literature does not establish
Being specific about the edges of the evidence is more useful than another summary of the highlights.
- It does not establish that injected recombinant follistatin protein reproduces the outcomes seen with AAV-delivered gene expression.
- It does not provide a human pharmacokinetic profile for administered follistatin protein — serum half-life, clearance and bioavailability are not characterised the way they are for approved biologics.
- It does not settle what long-term systemic activin blockade does outside skeletal muscle. Activin signalling is involved in reproductive endocrinology, wound repair, immune regulation and inflammation, and follistatin is not selective for the myostatin branch.
- It does not support “follistatin 315 versus 344” as a purchasing decision, because those are the same molecule before and after signal peptide cleavage.
- There is no approved follistatin product for human or veterinary use in any jurisdiction, and no established protocol for one.
None of that makes follistatin uninteresting. It makes it an open research question rather than a settled one, which is a more honest and frankly more interesting place for it to sit.
Handling in a laboratory context
Follistatin is a glycoprotein of roughly 35–40 kDa depending on isoform and glycosylation — considerably larger and more fragile than the short synthetic peptides most laboratories handle routinely. Proteins of this size are sensitive to freeze-thaw cycling, to surface adsorption at low concentrations, and to mechanical shear.
The general handling principles that apply to any lyophilised protein preparation apply here: reconstitute with an appropriate diluent introduced gently down the vial wall rather than directly onto the powder, avoid vortexing, aliquot before freezing so the stock is thawed once, and record lot numbers against every analytical result. If you work with lyophilised material regularly, the relevant consumables are ordinary — buy BAC water online, buy a peptide reconstitution kit, and a buy a digital milligram scale accurate enough that your mass figures mean something. Our full range of lab supplies for sale covers the rest.
These are laboratory handling notes for laboratory materials. They are not instructions for use in a person, and nothing on this page should be read that way.
Where this sits in our catalogue
Strate Labs does not stock follistatin, in any isoform. We would rather say that plainly than leave you clicking through a catalogue looking for it. If you came here to buy FST-344, this is not the shop, and we are not going to pretend an adjacent product is the same thing.
What Strate Labs does carry is a range of characterised research peptides with better-defined literature behind them. You can buy research peptides online across roughly fifty compounds, and if you want to know how we handle purity documentation before you buy anything, our FAQs set out what we provide and what we do not.
Frequently asked questions
What is follistatin?
Follistatin is a secreted glycoprotein encoded by the FST gene. It has no receptor of its own; it works by binding and neutralising certain TGF-β superfamily ligands, principally activin A and myostatin (GDF-8), so those ligands cannot reach their receptors. It was first isolated from ovarian follicular fluid as a suppressor of follicle-stimulating hormone release, which is where the name comes from.
What is follistatin 344?
FST-344 is the 344-residue precursor form of follistatin, including its N-terminal signal peptide. Once that signal peptide is cleaved, the result is FST-315, the mature protein that circulates in serum. The FS-344 construct is the one used in the well-known AAV gene-transfer studies, specifically because it yields the soluble circulating isoform rather than the tissue-bound one.
What is the difference between follistatin 315 and 344?
They are the same gene product at different stages. FST-344 is the precursor with the signal peptide attached; FST-315 is the mature protein after that peptide is removed. This is not a choice between two variants. The genuine isoform comparison is FST-315 against FST-288: FST-315 retains an acidic C-terminal tail that masks its heparin-binding site and keeps it circulating, while FST-288 lacks that tail, binds heparan sulfate proteoglycans strongly, and remains associated with cell surfaces and tissue.
What is the difference between follistatin 288 and 344?
They come from different splice variants of the same gene. FST-288 is the mature product of the shorter FST-317 precursor and is tissue-bound because its heparin-binding site is exposed. FST-344 is the longer precursor whose mature product, FST-315, stays in circulation because an acidic C-terminal extension masks that site. In short: one form is designed by its structure to stick to tissue and the other to travel.
How does follistatin inhibit myostatin?
By direct high-affinity binding. Myostatin, also called GDF-8, is a TGF-β superfamily ligand that acts as a negative regulator of skeletal muscle mass. Follistatin binds it and prevents it engaging its receptor, so the restraining signal is not transmitted. Follistatin is not selective for myostatin, though — it also binds activin A and several bone morphogenetic proteins, which is why systemic effects are difficult to predict from the muscle literature alone.
What did the follistatin muscle studies actually show?
The most cited work is Haidet and colleagues, published in PNAS in 2008. Using a single intramuscular injection of an AAV1 viral vector carrying the FS-344 gene, they reported sustained increases in muscle mass and grip strength in normal mice over two years, dose-dependent improvements in dystrophic mdx mice, and no observed change in reproductive capacity or gonadal histology. The critical detail is the method: this was sustained gene expression from a viral vector, not administration of a protein.
Does injecting follistatin protein produce the same results as the gene therapy studies?
That has not been established, and the two are not equivalent interventions. AAV gene transfer turns transfected tissue into a continuous source of the protein for the life of the animal; a recombinant protein preparation produces a transient exposure that has not been characterised in the same way. Citing the 2008 mouse figures alongside a vial of protein asks you to assume an equivalence the study does not demonstrate.
What are the reported side effects of follistatin?
There is no adverse event profile in the sense that term is used for an approved medicine, because there is no approved follistatin product and no controlled human safety programme behind one. What the biology suggests is worth noting: activin signalling participates in reproductive endocrinology, wound repair, immune regulation and inflammation, and follistatin does not act only on the myostatin branch. Broad, sustained activin blockade therefore has consequences that the skeletal muscle literature does not describe.
Is follistatin approved for human use?
No. There is no follistatin product approved by the FDA or any comparable regulator for human or veterinary use, it is not a dietary supplement, and there is no established protocol for administration to a person. Material sold in this sector is supplied for laboratory research use only.
Does Strate Labs sell follistatin 344?
No. We do not stock follistatin in any isoform and we would rather tell you directly than have you search the catalogue for it. We carry a range of characterised research peptides, ancillaries and laboratory supplies, and if follistatin is specifically what you need, another supplier is the answer.
How should lyophilised protein preparations be stored?
In a laboratory setting, larger glycoproteins are generally handled more conservatively than short synthetic peptides. Lyophilised material is typically kept frozen and protected from light and moisture, reconstituted gently with diluent run down the vial wall rather than directly onto the powder, not vortexed, and aliquoted before freezing so a stock is thawed once rather than repeatedly. Lot numbers should be recorded against every analytical result so a figure can always be traced back to the material that produced it.
Why do purity figures matter more for a protein than for a small peptide?
Because there is more that can go wrong and more of it is invisible to a single number. A glycoprotein can be the correct molecule at the correct purity by HPLC while being misfolded, aggregated or incorrectly glycosylated. That is why the useful document is a lot-specific Certificate of Analysis from a named laboratory, including the chromatogram and the method used, rather than a purity percentage printed on a product page.
Everything discussed here is supplied for laboratory research use only. These compounds are not approved by the FDA for human or veterinary use, they are not dietary supplements, and they are not for consumption.
Strate Labs would rather lose a sale than make a claim a study does not support, which is why this page reads the way it does. If something here does not match what you have read elsewhere, the PNAS paper is linked above and open access — read it and see which account matches it. If you have a question we have not answered, email support@strateslab.com.
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