Follistatin 315 Research, Specifications & Scientific Information

Introduction

Follistatin 315 is the form of follistatin that actually exists in blood, and that is the cleanest way to distinguish it from the number it is usually confused with.

The human FST gene yields two precursors by alternative splicing, of 344 and 317 amino acids ^[1]. Each begins with a 29-residue signal peptide that is removed during secretion. So the 344 precursor produces a mature chain of 315 residues, and the 317 precursor produces one of 288. Those two mature chains — FST-315 and FST-288 — are the proteins. The precursor numbers describe coding sequences.

What makes the 315 form worth its own entry rather than a footnote to the precursor entry is that it has a distinct and rather unexpected human literature. Not a trial literature: nobody has administered it. An endocrine one. Over the last decade the circulating pool of this protein has been shown to come substantially from the liver and to be governed by the ratio of glucagon to insulin, which places a protein discovered in ovarian follicular fluid squarely in the middle of metabolic physiology ^{[8, 9]}.

This page is a reference record. It describes research. It contains no guidance of any kind on handling the material.

What Is Follistatin 315?

Follistatin 315 is a secreted, N-glycosylated protein of 315 amino acids, built from a sequence of alternating follistatin-like and Kazal-like domains encoded by precisely separated exons — an architecture the 1988 authors compared with human epidermal growth factor and human pancreatic secretory trypsin inhibitor and read as evidence of exon shuffling ^[1].

Functionally it is a trap. It binds members of the transforming growth factor beta superfamily — activin A and B, myostatin, growth and differentiation factor 11, and bone morphogenetic proteins 6 and 7 — and prevents them reaching their receptors. Bone morphogenetic proteins 2 and 4 are not bound ^[3].

Structurally, the one feature that separates it from its shorter sibling sits at the C-terminus: a markedly acidic extension, visible in the sequence as a run of glutamate and aspartate residues. The 288-residue isoform lacks it. That extension is not a passive tail; it is the reason the two isoforms distribute differently, and the reason they can produce opposite results in the same assay.

It is not an approved medicine anywhere, and no clinical trial has administered it.

Follistatin 315 Specifications

The 315-residue chain above is the sequence recorded in the FDA/NCATS Global Substance Registration System under UNII 506IY26H2I, where the substance is named follistatin, FST and activin-binding protein, with a calculated average mass of 31600 g/mol and a cross-reference to UniProtKB accession P19883. It corresponds to residues 30 to 344 of the 344-residue precursor: the mature chain that remains after the 29-residue signal peptide is removed during secretion. That is why this substance and the one named follistatin 344 are not alternatives to each other but successive stages of the same molecule, and why 315 is the number that describes a protein found in blood. The C-terminus of this chain carries a markedly acidic stretch, visible above as the run of glutamate and aspartate residues near the end; the shorter 288-residue isoform, derived from the alternative 317-residue precursor, lacks that extension and binds cell-surface proteoglycans far more strongly as a result. The register carries no CAS registry number and no molecular formula for the substance, and no PubChem compound identifier resolves for the name, so those fields are shown as unknown. The calculated mass excludes glycosylation; the protein is N-glycosylated in vivo and a measured mass will differ.

Values that a public register does not carry are shown as not publicly characterised rather than estimated. Identifiers are reference values; the certificate of analysis supplied with a laboratory order is the record for a given lot.

How Does Follistatin 315 Work?

By sequestration, with a twist that is specific to this isoform.

The straightforward part: follistatin binds activin and myostatin with high affinity and neutralises them. A protein that removes a negative regulator of growth in skeletal musculature has obvious consequences, and the animal literature — described under the precursor entry and summarised below — bears that out.

The twist is that neutralisation is not simply a function of binding affinity. Where the trap sits matters as much as how tightly it grips. Follistatin isoforms bind cell-surface heparan sulphate proteoglycans to very different degrees, and a trap anchored at the cell surface intercepts ligands before they reach receptors on that same surface, while a freely circulating trap does not. This isoform is the weakly anchored one.

The consequence is that follistatin 315 can act as a reservoir rather than an inhibitor in some systems — holding ligand, but holding it where it can still be released near a receptor. That is not a theoretical possibility; it has been measured, and it reversed the direction of an effect.

Follistatin 315 Mechanism of Action

In vitro research

Affinity is equal; localisation is not

Recombinant follistatin isoforms and follistatin-like 3 were produced and compared head to head. Activin-binding affinities and kinetics were comparable across all of them. Cell-surface binding differed markedly, ranking FST288 above FST303 above FST315 above follistatin-like 3 ^[3].

Inhibition of endogenous activin activity tracked that surface-binding ranking rather than the affinity measurements. Neutralisation of exogenous activin, when the follistatin was also supplied exogenously, tracked the affinities instead — which is exactly what the localisation account predicts, since neither species is anchored in that configuration ^[3].

The demonstration that settles it is a sign reversal. In an in vitro bioassay, FST288 suppressed activin-dependent TT cell proliferation while FST315 enhanced it ^[3]. Two proteins with equivalent activin-binding affinity, differing only in a C-terminal extension, moved the same readout in opposite directions.

The converse experiment was also run: follistatin-like 3, which does not associate with cell membranes at all, had its endogenous activin inhibitory activity increase dramatically when it was expressed as a membrane-anchored protein ^[3]. Anchoring a trap makes it a better inhibitor; releasing one makes it a worse inhibitor, or something else entirely.

Which domain binds which ligand

Point mutants and domain-swapping constructs established that activin binding and neutralisation depend primarily on the second follistatin domain, whereas myostatin binding depends more on the first. Deleting the second domain, or substituting an extra copy of the first for it, produced constructs with strong myostatin antagonism and greatly reduced activin antagonism. Those same constructs still bound growth and differentiation factor 11 ^[5].

What the structure shows

The crystal structure of myostatin bound to the 288-residue isoform showed the antagonist's N-terminal domain rearranging conformationally to bind its ligand, and the complex generating a continuous electropositive surface with substantially increased heparin affinity. That increase translated into stronger interaction with the cell surface and enhanced myostatin degradation in the presence of either the 288 or the 315 form ^[6].

What Is Follistatin 315 Being Researched For?

Three lines, none of which involves administering the protein to a person.

• Circulating follistatin as an endocrine signal. The largest and most active human line: what sets the plasma concentration, and what it tracks ^{[8, 9, 10]}.
• Isoform and domain biochemistry. Why the isoforms differ, which domains bind which ligands, and what the complexes look like ^{[3, 5, 6]}.
• Myostatin-pathway manipulation in animals. Almost entirely by genetic overexpression rather than protein administration ^[4].

A fourth line exists and belongs to a different entry: gene transfer of the FST344 coding sequence by adeno-associated virus, which has reached completed early-phase human trials ^[7]. The protein produced in those participants is this 315-residue chain, made locally by transduced cells. That is a real connection and it is also the limit of the connection: a vector injected once into the quadriceps is not a supplied protein, and nothing about those trials describes what an administered protein would do.

Human Physiology Data on Follistatin 315

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

The human studies below measure endogenous follistatin in plasma. They do not administer it. Assays for circulating follistatin do not generally resolve the isoforms, but the 315 chain is generally regarded as the circulating species because the 288 chain is retained on cell surfaces ^[3].

The older view, and the evidence against an ovarian endocrine role

Follistatin was discovered as a gonadal protein that suppresses follicle-stimulating hormone release, which made an ovarian endocrine function the obvious hypothesis. Serum measurements in women were used to test it, and the conclusion drawn was against that hypothesis — the circulating pool did not behave as an ovarian endocrine signal would ^[2]. The question of where circulating follistatin comes from was left open for two decades.

Exercise, the glucagon-to-insulin ratio, and a pancreatic clamp

Population and design. Young healthy men performed a two-hour bicycle exercise bout followed by five hours of supine rest, with and without a pancreatic clamp that prevented the exercise-induced rise in the glucagon-to-insulin ratio. Separately, patients with type 2 diabetes and healthy controls performed one hour of bicycle exercise followed by three hours of rest ^[8].

Result. In healthy participants the glucagon-to-insulin ratio rose approximately tenfold during exercise (p < 0.002), and the clamp abolished that rise. Under the clamp, the exercise-induced increase in fibroblast growth factor 21 was completely blunted (p = 0.007), while the increase in follistatin was reduced by approximately 50% (p = 0.04). In patients with type 2 diabetes the exercise-induced rise in fibroblast growth factor 21 was completely absent and the follistatin rise was impaired ^[8].

Why the 50% matters. The authors drew the obvious inference from the incompleteness: if clamping the ratio removes only half the follistatin response, an additional regulatory mechanism must exist. A result that only partly confirms a hypothesis is more informative than one that confirms it entirely, and the authors treated it that way.

The reinterpretation

A narrative review of this line of work set out the change in view directly: substantial evidence indicates that the liver contributes significantly to circulating follistatin and that the circulating pool is tightly regulated by the glucagon-to-insulin ratio, displacing the earlier assumption that plasma concentrations are spill-over from local autocrine and paracrine actions in various tissues. The picture the authors describe is that conditions with elevated circulating follistatin share a metabolic denominator of reduced insulin sensitivity, raised glucagon, or both ^[9].

Associations with metabolic and anthropometric variables

Circulating activin A, activin B, follistatin and follistatin-like 3 were measured across participants grouped by age and fitness status, before and after graded exercise to exhaustion, and examined against anthropometric and metabolic variables ^[10]. The stated motivation was that inhibitors of the myostatin pathway were entering clinical evaluation while the physiology of the endogenous regulators of that pathway remained incompletely described in humans.

Limitations across all of the above. These are observational and physiological studies in modest numbers of participants, measuring an endogenous protein under controlled conditions. None of them establishes what administering follistatin would do, in humans or otherwise.

Preclinical Research on Follistatin 315

Animal research

The animal literature on follistatin is largely genetic rather than pharmacological, and the most informative single result is one that constrains what follistatin can be said to inhibit.

Mice lacking myostatin carry roughly twice the mass of skeletal musculature of wild-type animals, an effect reproduced by naturally occurring loss-of-function mutations in cattle, sheep, dogs and a described human. A follistatin transgene added on top of that myostatin-null background produced animals with about four times wild-type mass ^[4].

That arithmetic is the point. If follistatin acted solely by inhibiting myostatin, adding it to an animal with no myostatin should have changed nothing. It doubled again, which establishes that other ligands with similar activity are also being trapped — consistent with the binding data showing activin and growth and differentiation factor 11 among follistatin's partners ^{[3, 5]}.

Findings described in this section were observed in animals, and nothing in them establishes anything about humans. Note also that the protein in that experiment was produced by the animal's own cells from a transgene, not administered.

Current Research Status

Status as of September 22, 2026. This block is rendered from maintained fields, not from prose, so it cannot go stale in one place and stay current in another.

Follistatin 315 vs Related Compounds

The comparisons below are structural and evidential. No comparison of effect is made on this page.

Follistatin 344 :: The precursor of this protein, counted with its 29-residue signal peptide. Same molecule, earlier stage. The gene therapy trials in this field used the 344 coding sequence, and what transduced tissue then secretes is this 315-residue chain.

FST-288

The mature chain from the alternative 317-residue precursor, lacking this one's acidic C-terminal extension. Equivalent activin affinity, far stronger cell-surface binding, and in at least one assay the opposite effect ^[3].

Follistatin-like 3

A related protein that does not associate with cell membranes at all, and whose inhibitory activity rises sharply when it is artificially anchored ^[3]. It is the control that makes the localisation argument work.

ACE-031 :: A different construction aimed at the same pathway — a soluble activin receptor type IIB extracellular domain fused to an antibody Fc region, which traps ligands at the receptor end rather than at the binding-protein end. It reached phase 2 and was halted; that record belongs to its own entry.

Chemical & Molecular Characteristics

The sequence is a register record. The 315-residue chain is carried in the FDA/NCATS Global Substance Registration System under UNII 506IY26H2I, named follistatin, FST and activin-binding protein, with a calculated average mass of 31600 g/mol and a cross-reference to UniProtKB accession P19883. Against that accession it corresponds to residues 30 to 344 of the precursor.

The acidic tail is visible in the sequence. The run of glutamate and aspartate residues near the C-terminus — SEDTEEEEEDEDQDYSFPISSILEW — is the extension that the 288-residue isoform lacks, and it is the structural basis of the difference in cell-surface binding described above.

The calculated mass excludes glycosylation. Follistatin is N-glycosylated, and the register's figure is computed from the amino acid chain alone. A mass measured on real material will exceed it, by an amount that depends on the expression system.

Three fields are blank on purpose. No CAS registry number and no molecular formula are published for the substance by the register, and no PubChem compound identifier resolves for the name. All three are shown as unknown rather than estimated.

Two proteins share the bare name. Material sold simply as "follistatin" may be the 315 or the 288 chain, and the two behave differently in the assays described above. The certificate of analysis supplied with a laboratory order is the record of which one a given lot contains.

Frequently Asked Questions

Scientific References

1. Shimasaki S, Koga M, Esch F, et al.. Primary structure of the human follistatin precursor and its genomic organization Proceedings of the National Academy of Sciences of the United States of America; 1988. PMID 3380788 doi:10.1073/pnas.85.12.4218
2. Khoury RH, Wang QF, Crowley WF Jr, et al.. Serum follistatin levels in women: evidence against an endocrine function of ovarian follistatin The Journal of clinical endocrinology and metabolism; 1995. PMID 7714112 doi:10.1210/jcem.80.4.7714112
3. Sidis Y, Mukherjee A, Keutmann H, et al.. Biological activity of follistatin isoforms and follistatin-like-3 is dependent on differential cell surface binding and specificity for activin, myostatin, and bone morphogenetic proteins Endocrinology; 2006. PMID 16627583 doi:10.1210/en.2006-0089
4. Lee SJ. Quadrupling muscle mass in mice by targeting TGF-beta signaling pathways PloS one; 2007. PMID 17726519 doi:10.1371/journal.pone.0000789
5. Schneyer AL, Sidis Y, Gulati A, et al.. Differential antagonism of activin, myostatin and growth and differentiation factor 11 by wild-type and mutant follistatin Endocrinology; 2008. PMID 18535106 doi:10.1210/en.2008-0259
6. Cash JN, Rejon CA, McPherron AC, et al.. The structure of myostatin:follistatin 288: insights into receptor utilization and heparin binding The EMBO journal; 2009. PMID 19644449 doi:10.1038/emboj.2009.205
7. Mendell JR, Sahenk Z, Malik V, et al.. A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy Molecular therapy : the journal of the American Society of Gene Therapy; 2015. PMID 25322757 doi:10.1038/mt.2014.200
8. Hansen JS, Pedersen BK, Xu G, et al.. Exercise-Induced Secretion of FGF21 and Follistatin Are Blocked by Pancreatic Clamp and Impaired in Type 2 Diabetes The Journal of clinical endocrinology and metabolism; 2016. PMID 27163358 doi:10.1210/jc.2016-1681
9. Hansen JS, Plomgaard P. Circulating follistatin in relation to energy metabolism Molecular and cellular endocrinology; 2016. PMID 27264073 doi:10.1016/j.mce.2016.06.002
10. Perakakis N, Mougios V, Fatouros I, et al.. Physiology of Activins/Follistatins: Associations With Metabolic and Anthropometric Variables and Response to Exercise The Journal of clinical endocrinology and metabolism; 2018. PMID 30085147 doi:10.1210/jc.2018-01056

Every identifier above is resolved against PubMed, Crossref or ClinicalTrials.gov at build time, and the title returned by the register is compared with the title stored here. A page does not publish if a reference fails to resolve.

Research-Use Information

For in vitro research use only. This material is a laboratory reagent. It is not a drug, food, dietary supplement, or cosmetic and is not for human or veterinary use, including ingestion, injection, or any other administration. No information on this page describes or implies any effect in humans or animals. Sold only to researchers under our Terms of Sale.