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 the question means natural function, stable folded structure, a PDB entry, or a familiar human protein.

The shortest natural functional example is TAL/pri. The shortest solved folding example is chignolin, with the sequence GYDPETGTWG.

Shortest protein records

The smallest natural functional translated product with strong experimental support is the TAL/pri peptide, an 11-amino-acid product required for Drosophila development.^[1]

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]

Chignolin is a 10-amino-acid designed peptide that folds into a defined beta-hairpin structure in water.^[2]

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]

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] That makes chignolin the cleanest answer when the question means "shortest designed molecule with experimentally determined protein-like folding."

The caveat is terminology. Many biologists would call a 10- or 11-amino-acid molecule a peptide, not a conventional folded protein. That is why the smallest-protein answer is best presented as a set of records rather than one universal name.

Human examples

There is no single universally accepted "smallest protein in the human body" because human databases include tiny peptides, processed chains, immune-region segments, and conventional folded proteins under related protein records.

In reviewed UniProtKB human entries sorted by length, the shortest records include TRDD1 at 2 amino acids, tuftsin at 4 amino acids, and other short immune or peptide entries. Those are real curated human entries, but they are not ordinary folded proteins in the way many readers mean the word "protein."^[9]

For a biologically familiar short human translated product, MOTS-c is a 16-amino-acid mitochondrial-derived peptide with the sequence MRWQEMGYIFYPRKLR.^[10] For a familiar mature human hormone, insulin totals 51 amino acids across its A and B chains.^[5]

Source context: the human-length examples come from reviewed UniProtKB human records sorted by sequence length, while MOTS-c and insulin chain details come from their specific UniProt entries.^[9][10][5]

Insulin is small, but it is not the smallest known protein and it is not the shortest human protein entry. Mature human insulin has 51 amino acids across two chains, so it is much larger than TAL/pri, chignolin, Trp-cage, MOTS-c, and many short peptide records.^[5]

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] Albumin is not a small-protein candidate in this context: human serum albumin is a 609-amino-acid precursor with a mature albumin chain mapped to residues 25-609.^[11]

Protein and peptide definitions

The smallest unit of a protein chain is an amino acid residue; in classroom language, the building blocks or monomers of proteins are amino acids.

NCBI Bookshelf's primary protein structure chapter describes proteins as one or more polypeptides, meaning linear chains of amino acids linked by peptide bonds. It also defines primary structure as the linear amino acid sequence of a protein.^[12]

The wording matters:

So the smallest protein is not the same thing as the smallest unit of a protein. A protein is a chain; an amino acid residue is one unit in that chain.

There is also no universal minimum number of amino acids that turns a chain into a protein.

Short chains are often called peptides or oligopeptides, while longer or structurally complex chains are more often called proteins. NCBI Bookshelf summarizes the common classroom distinction by noting that shorter chains, often fewer than 40 amino acids, are usually classified as peptides or oligopeptides, while average proteins imply enough size or structural complexity to behave as proteins.^[12]

That is why "minimum amino acids in a protein" has to be answered by definition:

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] Another review says polypeptides containing 100 amino acids or fewer are generally considered small proteins, while noting that no strict definition exists.^[7]

Examples and sequences

Small proteins range from 10-amino-acid designed folding peptides to 51-amino-acid mature insulin and much larger proteins that are still small compared with average globular proteins.

Source context: chignolin length and folding behavior come from Honda et al. and RCSB PDB 1UAO; TAL/pri length and function come from Galindo et al.; MOTS-c and insulin details come from UniProt; Trp-cage data come from RCSB PDB 1L2Y.^{[2][3][1][10][5][4]}

The shortest named sequence in this article is chignolin, a 10-amino-acid designed folding peptide with the sequence GYDPETGTWG.

For natural function, the better answer is TAL/pri at 11 amino acids, but the exact TAL/pri peptide sequence depends on which short open reading frame and species context is being discussed. For human examples, MOTS-c has the named 16-amino-acid sequence MRWQEMGYIFYPRKLR.^[10]

Source context: chignolin sequence and PDB code come from Honda et al. and RCSB 1UAO; TAL/pri length comes from Galindo et al.; MOTS-c sequence comes from UniProt; Trp-cage structure data come from RCSB 1L2Y.^{[2][3][1][10][4]}

The PDB codes are useful for structure searches. 1UAO is the RCSB Protein Data Bank entry for chignolin, the 10-residue designed peptide often cited as the shortest protein-like molecule with an experimentally determined fold.^[3] The entry describes an NMR structure of a designed protein consisting of ten amino acids and reports a structure weight of 1.08 kDa.^[3]

1L2Y is the RCSB PDB entry for the 20-amino-acid Trp-cage TC5b miniprotein construct, a classic benchmark for compact protein folding.^[4] RCSB PDB entry 1L2Y records TC5b as a 20-residue de novo protein structure with a reported structure weight of 2.17 kDa.^[4]

The primary citation abstract describes 20-residue Trp-cage 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] 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.

Scale and detection limits

The smallest protein-like examples are only 10-11 amino acids, while the largest known protein, PKZILLA-1, contains 45,212 amino acids.^[13]

That means the largest known protein is more than 4,000 times longer than chignolin by residue count, linking the shortest peptide-like examples to the other end of the protein size scale.

This size spread is one reason "small protein" is a relative phrase. A 100-amino-acid protein is small next to most enzymes, but it is still about ten times longer than chignolin.

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] The chignolin case shows that 10 amino acids can be enough for a designed beta-hairpin with protein-like folding behavior.^[2] 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] A 2024 Nature Communications study expanded the protein-sequence scale dramatically by constructing a global microbiome catalog containing 964,970,496 smORFs, including 43,642,695 high-quality predictions.^[8]

Methodology

This article separates four 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]
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][3]
3. Human protein or peptide examples use reviewed UniProtKB human entries and specific entries for MOTS-c, insulin, and albumin. These examples are not collapsed into one record because databases include short peptides, processed chains, immune-region segments, and conventional proteins under related labels.^{[9][10][5][11]}
4. Smallest unit of protein is a different question from smallest protein. The unit is an amino acid residue in a chain, not a whole protein.^[12]

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

How to cite this page

This guide is best treated as a dated source because protein-size records depend on definitions and database conventions.

ProteinIQ. "Smallest protein." Updated June 29, 2026. Accessed [your access date]. https://proteiniq.io/guides/smallest-protein

Sources▼

1. Peptides encoded by short ORFs control development and define a new eukaryotic gene family PLoS Biology · 2007. https://pubmed.ncbi.nlm.nih.gov/17439302/
2. 10 residue folded peptide designed by segment statistics Structure · 2004. https://pubmed.ncbi.nlm.nih.gov/15296744/
3. 1UAO: NMR structure of designed protein, chignolin, consisting of only ten amino acids RCSB Protein Data Bank · June 29, 2026. https://www.rcsb.org/structure/1UAO
4. 1L2Y: NMR structure of Trp-cage miniprotein construct TC5b RCSB Protein Data Bank · June 29, 2026. https://www.rcsb.org/structure/1L2Y
5. Insulin UniProtKB entry P01308 UniProt · June 29, 2026. https://rest.uniprot.org/uniprotkb/P01308.txt
6. Small Proteins Can No Longer Be Ignored Annual Review of Biochemistry · 2014. https://pmc.ncbi.nlm.nih.gov/articles/PMC4166647/
7. Small proteins: untapped area of potential biological importance Frontiers in Genetics · 2013. https://pmc.ncbi.nlm.nih.gov/articles/PMC3864261/
8. A catalog of small proteins from the global microbiome Nature Communications · 2024. https://www.nature.com/articles/s41467-024-51894-6
9. Reviewed human UniProtKB proteins sorted by sequence length UniProt · June 29, 2026. https://rest.uniprot.org/uniprotkb/search?query=(organism_id:9606)%20AND%20(reviewed:true)&fields=accession,protein_name,gene_names,organism_name,length,ft_chain&format=tsv&sort=length%20asc&size=25
10. MOTS-c UniProtKB entry A0A0C5B5G6 UniProt · June 29, 2026. https://rest.uniprot.org/uniprotkb/A0A0C5B5G6.tsv?fields=accession,protein_name,gene_names,organism_name,length,sequence,ft_chain&format=tsv
11. Albumin UniProtKB entry P02768 UniProt · June 29, 2026. https://rest.uniprot.org/uniprotkb/P02768.tsv?fields=accession,protein_name,gene_names,organism_name,length,ft_chain&format=tsv
12. Biochemistry, Primary Protein Structure StatPearls, NCBI Bookshelf · June 29, 2026. https://www.ncbi.nlm.nih.gov/books/NBK564343/
13. Giant polyketide synthase enzymes in haptophyte algae Science · 2024. https://www.science.org/doi/10.1126/science.adp7199