What's the smallest protein?

The shortest defensible answer is: 11 amino acids for a natural functional translated product and 10 amino acids for a designed molecule that folds like a protein. The distinction matters because very short chains sit at the boundary between peptides and proteins, so the answer changes depending on whether you require a natural biological function, a stable folded structure, or a conventional mature protein.

The shortest natural functional translated product is 11 amino acids

The best-documented natural example is the TAL/pri peptide, an 11-amino-acid translated product required for Drosophila development.[1]Peptides encoded by short ORFs control development and define a new eukaryotic gene familyPLoS Biology · 2007View source

The 2007 PLoS Biology paper reported that the tarsal-less (tal) gene acts through several 33-nucleotide open reading frames, each translated into 11-amino-acid peptides. The authors showed that these peptides control gene expression and tissue folding in Drosophila, making TAL/pri a strong answer when the question means "smallest natural translated product with a documented biological function."[1]Peptides encoded by short ORFs control development and define a new eukaryotic gene familyPLoS Biology · 2007View source

The caveat is terminology. Many biologists would call an 11-amino-acid molecule a peptide, not a conventional protein. For search intent, it is still the most useful headline answer because it is translated, functional, and much smaller than textbook proteins.

The shortest designed folding miniprotein is 10 amino acids

Chignolin is a 10-amino-acid designed peptide that folds into a defined beta-hairpin structure in water.[2]10 residue folded peptide designed by segment statisticsStructure · 2004View source

Honda and colleagues reported the sequence GYDPETGTWG in 2004. Their abstract describes chignolin as folding into a unique structure in water and showing a cooperative thermal transition, both presented as hallmarks of protein-like behavior.[2]10 residue folded peptide designed by segment statisticsStructure · 2004View source

RCSB PDB entry 1UAO records chignolin as a 10-residue de novo protein structure solved by solution NMR, with a reported structure weight of 1.08 kDa.[3]1UAO: NMR structure of designed protein, chignolin, consisting of only ten amino acidsRCSB Protein Data Bank · May 2, 2026View source That makes chignolin the cleanest answer when the question means "shortest designed molecule with experimentally determined protein-like folding."

Protein versus peptide definitions change the answer

There is no single universal cutoff where a peptide becomes a protein.

One influential review defines small proteins as proteins of 50 amino acids or less when they acquire that size directly from translation rather than from processing of a larger precursor.[6]Small Proteins Can No Longer Be IgnoredAnnual Review of Biochemistry · 2014View source Another review says polypeptides containing 100 amino acids or fewer are generally considered small proteins, while noting that no strict definition exists.[7]Small proteins: untapped area of potential biological importanceFrontiers in Genetics · 2013View source

That definition problem is why "smallest protein" should usually be answered with categories rather than one absolute record.

Source context: TAL/pri length and function come from Galindo et al.; chignolin length and folding behavior come from Honda et al. and RCSB PDB 1UAO; Trp-cage data come from RCSB PDB 1L2Y; insulin precursor and chain information come from UniProt P01308.[1]Peptides encoded by short ORFs control development and define a new eukaryotic gene familyPLoS Biology · 2007View source[2]10 residue folded peptide designed by segment statisticsStructure · 2004View source[3]1UAO: NMR structure of designed protein, chignolin, consisting of only ten amino acidsRCSB Protein Data Bank · May 2, 2026View source[4]1L2Y: NMR structure of Trp-cage miniprotein construct TC5bRCSB Protein Data Bank · May 2, 2026View source[5]Insulin UniProtKB entry P01308UniProt · May 2, 2026View source

Trp-cage is 20 amino acids and remains a key folding benchmark

The Trp-cage TC5b construct is a 20-amino-acid protein-like miniprotein model, not the absolute shortest folding example.[4]1L2Y: NMR structure of Trp-cage miniprotein construct TC5bRCSB Protein Data Bank · May 2, 2026View source

RCSB PDB entry 1L2Y records TC5b as a 20-residue de novo protein structure with a reported structure weight of 2.17 kDa.[4]1L2Y: NMR structure of Trp-cage miniprotein construct TC5bRCSB Protein Data Bank · May 2, 2026View source The primary citation abstract describes 20-residue constructs that are more than 95% folded in water at physiological pH, which is why Trp-cage became a standard benchmark for folding experiments and simulations.[4]1L2Y: NMR structure of Trp-cage miniprotein construct TC5bRCSB Protein Data Bank · May 2, 2026View source

Compared with chignolin, Trp-cage is larger but more globular. That makes it less useful as a "smallest" answer, but more useful as an example of how little sequence can be needed for a compact protein-like fold.

Insulin is small, but it is not the smallest protein

Mature human insulin has 51 amino acids across two chains, so it is small by protein standards but much larger than TAL/pri, chignolin, or Trp-cage.[5]Insulin UniProtKB entry P01308UniProt · May 2, 2026View source

UniProt entry P01308 lists human insulin as a 110-amino-acid precursor. The mature hormone contains a B chain and an A chain; UniProt maps many insulin structures as residues 25-54 and 90-110, corresponding to a 30-amino-acid B chain and a 21-amino-acid A chain.[5]Insulin UniProtKB entry P01308UniProt · May 2, 2026View source

Insulin is therefore a good example of a small conventional human protein, especially because it has a stable disulfide-linked structure and a clear physiological role. It is not the smallest known functional translated product.

How small can functional proteins be?

Functional translated products can be as short as 11 amino acids, but database-scale discovery becomes harder as sequences get shorter.

The TAL/pri case shows that an 11-amino-acid translated product can have a developmental function.[1]Peptides encoded by short ORFs control development and define a new eukaryotic gene familyPLoS Biology · 2007View source The chignolin case shows that 10 amino acids can be enough for a designed beta-hairpin with protein-like folding behavior.[2]10 residue folded peptide designed by segment statisticsStructure · 2004View source Those examples should not be generalized into a universal lower bound for all functions, because binding, signaling, folding, membrane insertion, and catalysis impose different physical requirements.

Small proteins are also undercounted. The 2014 Annual Review of Biochemistry review emphasized that small proteins have traditionally been overlooked because annotation and biochemical detection methods often miss them.[6]Small Proteins Can No Longer Be IgnoredAnnual Review of Biochemistry · 2014View source A 2024 Nature Communications study expanded the scale dramatically by constructing a global microbiome catalog containing 964,970,496 smORFs, including 43,642,695 high-quality predictions.[8]A catalog of small proteins from the global microbiomeNature Communications · 2024View source

Methodology

This article separates three different meanings of "smallest protein."

1. Natural functional translated product refers to molecules that are translated from open reading frames and have experimental evidence of biological function. TAL/pri is placed here because Galindo et al. reported functional 11-amino-acid products from 33-nucleotide ORFs.[1]Peptides encoded by short ORFs control development and define a new eukaryotic gene familyPLoS Biology · 2007View source
2. Designed folding miniprotein refers to a synthetic or designed sequence with evidence of a defined folded structure. Chignolin is placed here because Honda et al. reported a 10-amino-acid sequence that folds in water, and RCSB PDB 1UAO records a 10-residue NMR structure.[2]10 residue folded peptide designed by segment statisticsStructure · 2004View source[3]1UAO: NMR structure of designed protein, chignolin, consisting of only ten amino acidsRCSB Protein Data Bank · May 2, 2026View source
3. Conventional mature protein refers to familiar folded proteins such as insulin. Insulin is not used as the record holder because its mature A and B chains total 51 amino acids, while its UniProt precursor is 110 amino acids.[5]Insulin UniProtKB entry P01308UniProt · May 2, 2026View source

The comparison table does not merge these categories into one record because the sources use different concepts: peptide function, protein-like folding, PDB structure length, and mature-protein processing.

Matic Broz

Founder & CEO, ProteinIQ

Matic founded ProteinIQ to make computational biology accessible to every researcher. He builds code-free bioinformatics tools used by thousands of scientists worldwide for protein analysis, molecular docking, and drug discovery.