Follistatin is a glycoprotein which occurs in nature and has been widely studied in the fields of molecular biology, endocrinology and research into cell signalling. It is especially well known for its ability to bind members of the transforming growth factor-beta (TGF-β) superfamily, such as activins and myostatin.
Although follistatin is frequently associated with whole distinct types of biological molecules, its chemical structure and signaling mechanism clearly place it in the field of protein biology rather than steroid chemistry. When examining the experimental literature on follistatin and its several molecular forms, it is crucial to remember this distinction.
Is Follistatin a Steroid?
Follistatin isn't a steroid; it's a glycosylated protein.
Steroids are defined by a four-ring system of carbon atoms which comes from sterol chemistry, while follistatin is made up of a polypeptide chain of amino acids and its biological properties are due to its three-dimensional protein structure, its cysteine-rich domains, and its interactions with other proteins.
Early structural studies described follistatin as a single-chain, glycosylated polypeptide and found several homologous, cysteine-containing domains in the molecule.
It is chemically incorrect to refer to follistatin as a steroid.
Why Is Follistatin Studied in TGF-β Signaling?
A key aspect of follistatin research is the study of its interactions with proteins from the TGF-β superfamily.
The researchers at first discovered follistatin as a result of their studies into the regulation of follicle-stimulating hormone (FSH), but later research demonstrated that the molecule's activity is closely associated with high-affinity binding to activin.
Researchers have since investigated follistatin interactions with several TGF-β-related ligands, including:
- activin A
- myostatin
- selected bone morphogenetic proteins (BMPs)
They serve as experimental models for the study of extracellular regulation of growth-factor signalling rather than that of steroid-receptor signaling.
How Does Follistatin Interact With Activin?
Follistatin binds activin with high affinity, forming a protein complex that blocks activin receptor-binding sites and thereby regulates activin-mediated signalling in experimental systems.
Structural studies have given a detailed account of how follistatin–activin complexes are formed.
Follistatin binds to activin and physically hinders the areas that are needed for interaction with the activin receptor. Research using crystallography has shown that two molecules of follistatin can surround an activin dimer, blocking the receptor-binding surfaces and as a result altering the availability of signalling.
It is therefore useful to use follistatin in experimental research concerned with how extracellular binding proteins regulate the availability of ligands and receptor-mediated signalling.
Research has likewise demonstrated that the molecular arrangement of follistatin is important for this interaction, the N-terminal region and the follistatin domains contributing differently to the recognition and binding of activin.
Why Are Follistatin Domains Important?
The protein follistatin has three distinct cysteine-rich follistatin domains, usually referred to as FSD1, FSD2 and FSD3.
Experiments involving the introduction of mutations and the deletion of segments show that these domains are not functionally interchangeable. Specifically, studies have found the N-terminal area together with FSD1 and FSD2 to be important elements of the molecular structure concerned with activin binding.
This is the reason why researchers take into account both the conformation of the protein and its amino-acid composition when they are studying follistatin.
Altering individual domains or residues can change the ligand-binding properties, which is why structural variants are useful as experimental tools for the study of protein–protein interactions.
What Is the Relationship Between Follistatin and Myostatin Research?
Myostatin is also a signaling protein belonging to the TGF-β superfamily which has been studied in connection with follistatin.
Research into structure has shown that complexes can be formed between follistatin and myostatin, and a crystallographic study of myostatin when bound to follistatin has enabled scientists to determine the molecular surfaces taking part in ligand recognition and in blocking the receptor site.
The diverse domains of follistatin-type proteins contribute differently to their interactions with activin A and myostatin, according to studies comparing these proteins.
That is why the follistatin–myostatin relationship is of particular importance to the fields of protein-binding, structural biology, and TGF-β signaling research, rather than providing evidence that follistatin belongs to the steroid class.
Why Does Follistatin Classification Matter?
Helping researchers to interpret the results of their experiments and to compare follistatin with suitable molecular systems is the advantage of correct classification.
Steroids and proteins are very different in their chemical structure, in the way they are synthesised, in the way they interact at the molecular level and in their analytical properties. It would therefore be misleading to describe follistatin as a steroid.
Follistatin is better discussed in the context of glycoproteins, extracellular ligand-binding proteins and the regulation of TGF-β superfamily signalling.
This difference also enables a more solid basis to be established for explaining why structural studies concentrate on protein domains, ligand-binding interfaces and molecular complexes when looking at follistatin.
Frequently Asked Questions about Follistatin Classification
Follistatin is a glycosylated protein made up of amino acids and has a structure different from that of steroid compounds.
Follistatin is usually regarded as a protein or glycoprotein and not as a small peptide since it is made up of a relatively long chain of amino acids which are arranged into several structural domains.
Follistatin has been widely studied for its interactions with various members of the TGF-β superfamily, especially with activins and myostatin.
The cysteine-rich structural sections of follistatin are responsible for giving the protein its shape and for enabling its interactions with other signaling proteins.
Activin is a well-established binding partner of follistatin, which means that the follistatin–activin complex constitutes a recognized system for the study of the extracellular regulation of TGF-β-family signalling.
The way in which the N-terminal area of follistatin is structured and the individual domains of follistatin affect ligand recognition and binding means that changes to its structure can cause alterations in the protein interactions that are measured in experiments.
Scientific references
- 1 Esch FS, Shimasaki S, Mercado M, Cooksey K, Ling N, Ying S, Ueno N, Guillemin R. Structural characterization of follistatin: a novel follicle-stimulating hormone release-inhibiting polypeptide from the gonad. Mol Endocrinol. 1987 Nov;1(11):849-55. doi: 10.1210/mend-1-11-849. PMID: 3153465. https://pubmed.ncbi.nlm.nih.gov/3153465/
- 2 Phillips DJ, de Kretser DM. Follistatin: a multifunctional regulatory protein. Front Neuroendocrinol. 1998 Oct;19(4):287-322. doi: 10.1006/frne.1998.0169. PMID: 9799587. https://pubmed.ncbi.nlm.nih.gov/9799587/
- 3 Sidis Y, Schneyer AL, Sluss PM, Johnson LN, Keutmann HT. Follistatin: essential role for the N-terminal domain in activin binding and neutralization. J Biol Chem. 2001 May 25;276(21):17718-26. doi: 10.1074/jbc.M100736200. Epub 2001 Mar 14. PMID: 11279126. https://pubmed.ncbi.nlm.nih.gov/11279126/
- 4 Thompson TB, Lerch TF, Cook RW, Woodruff TK, Jardetzky TS. The structure of the follistatin:activin complex reveals antagonism of both type I and type II receptor binding. Dev Cell. 2005 Oct;9(4):535-43. doi: 10.1016/j.devcel.2005.09.008. PMID: 16198295. https://pubmed.ncbi.nlm.nih.gov/16198295/
- 5 Harrington AE, Morris-Triggs SA, Ruotolo BT, Robinson CV, Ohnuma S, Hyvönen M. Structural basis for the inhibition of activin signalling by follistatin. EMBO J. 2006 Mar 8;25(5):1035-45. doi: 10.1038/sj.emboj.7601000. Epub 2006 Feb 16. PMID: 16482217; PMCID: PMC1409725. https://pubmed.ncbi.nlm.nih.gov/16482217/
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