Follistatin - FST(30-344) / FS-315 (merged record)

1 Identity

Recombinant multi-domain, cysteine-rich, disulfide-bonded glycoprotein. Not a peptide and not a synthetic article. UniProt P19883. "Follistatin 344" and "Follistatin 315" are the same protein under two numbering conventions and are not two products.

Sequence

No defined synthetic sequence. Human follistatin is the product of the FST gene; the canonical transcript encodes a 344-residue precursor whose 29-residue signal peptide is removed on secretion, leaving the 315-residue mature chain FST(30-344). "Follistatin 344" and "Follistatin 315" name the same secreted protein under precursor-inclusive and mature numbering conventions. A second splice variant encodes a 317-residue precursor giving the 288-residue heparin-binding form FS-288; that is a genuinely different species and a different article. Roughly 35 to 40 kDa as expressed in mammalian cells, with two reported N-glycosylation sites. Exact residue numbering, cysteine count, disulfide connectivity and glycosylation positions are fixed against UniProt P19883 and against nothing else — not a supplier datasheet, and not this page, which deliberately does not restate them from memory. Produced by recombinant expression and refolding, not by chemical synthesis; any supplier describing it as synthetic is describing something else.

Molecular formula

Not established, and not establishable as a single formula. A glycoprotein is a population of glycoforms, and a formula is a property of one molecule. Any supplier printing a molecular formula for this article is printing a backbone calculation and calling it a formula.

Average mass

Not established, and no single figure exists. The CHO-expressed Gly30-Trp344 article from one institutional supplier, the closest thing to a characterized reference on the market, states a PREDICTED molecular weight of 34.7 kDa against an observed 40 to 50 kDa by reducing SDS-page. That 5 to 15 kDa gap is the glycan, and it is a property of the cell line and the culture process rather than of the sequence. The consequence, stated plainly: two lots from two production sites are not the same article even at identical sequence. The best-characterized supplier in this space reports the mass as a 10 kDa range rather than as a number, and this catalog will not improve on that with a computation.

Monoisotopic mass

Not applicable and not measurable. A monoisotopic mass requires an isotopically resolved envelope, which is not obtainable on a 40 kDa heterogeneously glycosylated protein by any routine method. Intact-mass work on this article is deconvoluted average mass across a glycoform distribution, reported as a profile. Any monoisotopic figure offered for follistatin is fabricated.

Salt / variant note

Multi-molecule name at kilodalton scale, so the arithmetic is done on a gel and on an intact-mass deconvolution rather than to four decimal places. (1) FS-315 / FST(30-344): the 315-residue mature secreted chain, predicted backbone 34.7 kDa, observed 40 to 50 kDa glycosylated; this is the CHO-expressed Gly30-Trp344 article described above. (2) FS-288: the 288-residue heparin-binding splice form from the 317-residue precursor, a genuinely different gene product variant at roughly 31 to 32 kDa backbone, separated from FS-315 by about 3 kDa and by peptide mapping across the missing C-terminal domain. (3) FS-300: a truncated form derived from FS-315 by removal of 15 C-terminal residues and sold as a distinct catalog item — Sf21 baculovirus-expressed, Gly30-Asp329 plus an N-terminal Met, 301 residues, predicted 31 kDa, observed 38 to 43 kDa. A third number in a family already carrying two. (4) the uncleaved 344-residue precursor at roughly 38 kDa, which is not a premium article but a processing failure. (5) expression system as the decisive variant: CHO, HEK, Sf21 baculovirus and E. Coli refold products all ship under the same name; the E. Coli article carries no glycan and unverifiable disulfide fidelity, and no peptide assay distinguishes a correctly folded lot from a scrambled one. Of those four routes, only the E. Coli refold is consistent with what the research channel actually charges per milligram, which sits far below any plausible cost base for characterized mammalian expression — and that is evidence about what is in those vials, not merely a commercial benchmark. (6) Fc-fusion and tagged constructs, which change the mass by tens of kilodaltons and are sold under the same trade name.

2 Class & testing panel

Form

Single article

Testing panel

P4 — panel definition

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

No study summary is carried for this article, and no statement is made about what the protein does in any body. What is recorded is the documentary position. Identity reference: UniProt P19883, a curated sequence database entry rather than a study. Manufacturer characterization: the datasheets for the CHO-expressed Gly30-Trp344 article and for the Sf21-expressed truncated article, which are supplier documents. Sport-governance instrument: the WADA Prohibited List, which names follistatin within the myostatin-pathway class. No compendial monograph exists in USP or NF. No reference standard is obtainable from any source located.

4 Storage & specification

Storage

Lyophilized, excipient-stabilized presentation held at -80 degrees C, with a documented freeze-thaw limit and a documented reconstitution-stability window on the lot record, as a correctly folded, glycosylated 315-residue protein requires.

Shelf life

Each lot is dated from its date of manufacture and carries a retest interval set on that lot's own stability data rather than a catalog-wide figure. The stability-indicating attributes for this molecule are aggregation and disulfide scrambling, measured on the P4 panel, and they are what limits the interval: a date issued without measuring those pathways is a date rather than a specification.