Peptide Basics
Signal Peptides: The Address Label at the Start of a Protein
A signal peptide is a short run of amino acids, 18 to 30 in the human proteins on this page, at the start of a new protein that sends it into the cell’s secretory pathway and is then cut off. This page explains how the cell reads that signal while the protein is still being made, what removes it, how it differs from a propeptide, which other sorting signals exist, and why the hormones people search for, insulin, GLP-1, oxytocin, growth hormone, each begin life with one. Every length is taken from UniProt, and every definition from the U.S. National Library of Medicine.
What a signal peptide is
A cell makes thousands of proteins in the same place, on ribosomes, and has to send each one to the right destination. It does this with sorting signals written into the protein’s own sequence. MeSH defines protein sorting signals as amino acid sequences found in transported proteins that selectively guide them to specific compartments of the cell.
The signal peptide is the best known of them. It sits at the very start of the chain, the N-terminus, and marks a protein for the endoplasmic reticulum, the network of membranes from which proteins go on to be secreted from the cell, built into its membrane or delivered to the compartments in between. Once it has done that job it is cut away, so the finished hormone or enzyme no longer carries it.
How the cell reads it, step by step
- The signal emerges. The ribosome makes a protein from its N-terminus onwards, so the signal peptide is the first part to appear.
- A particle catches it and pauses the ribosome. MeSH describes the signal recognition particle as a complex of one RNA (7S RNA) and at least six proteins, which induces elongation arrest of new secretory and membrane proteins until the ribosome becomes attached to the rough endoplasmic reticulum. UniProt’s record for its subunit SRP54 describes the same targeting happening while the protein is still being made.
- The protein threads into the endoplasmic reticulum, where the signal peptidase complex cuts the signal off. UniProt’s record for its catalytic subunit, SEC11A, states that it cleaves N-terminal signal sequences as new proteins are moved into the endoplasmic reticulum, and that it acts on signal peptides whose water-repelling core is shorter than about 18 to 20 amino acids.
- The rest travels on. MeSH describes the secretory pathway as the sequence of membrane-bound compartments through which new proteins are modified, packaged and stored or sent to the cell surface; the Golgi apparatus sorts them into secretory vesicles, lysosomes or the cell membrane.
Sources: NLM MeSH descriptors D021382, D018271, D021381, D055571 and D006056; UniProt P67812 (SEC11A, citing PubMed 34388369) and P61011 (SRP54). Read 1 November 2026.
Signal peptides in familiar proteins
Human proteins, ordered by the length of their signal. The full length is the precursor as first made, signal included.
| Protein | UniProt | Full length (amino acids) | Signal peptide (amino acids) |
|---|---|---|---|
| Albumin | P02768 | 609 | 18 (then a 6-amino-acid propeptide is also removed) |
| Oxytocin precursor (oxytocin-neurophysin 1) | P01178 | 125 | 19 |
| Proglucagon (source of GLP-1 and glucagon) | P01275 | 180 | 20 |
| Beta-2-microglobulin | P61769 | 119 | 20 |
| Alpha-2-macroglobulin | P01023 | 1,474 | 23 |
| Insulin precursor (preproinsulin) | P01308 | 110 | 24 |
| Growth hormone (somatotropin) | P01241 | 217 | 26 |
| POMC (source of ACTH, alpha-MSH, beta-endorphin) | P01189 | 267 | 26 |
| Cathelicidin (source of LL-37) | P49913 | 170 | 30 |
Source: UniProt “Signal” features of each entry listed. Read 1 November 2026.
Signal peptide, propeptide, mature peptide
Many peptide hormones lose two kinds of extra sequence on the way to their final form. Insulin is the textbook case: UniProt lists its 110-amino-acid precursor with a 24-amino-acid signal peptide at the start and a 31-amino-acid C peptide in the middle. The signal goes first, as the chain enters the endoplasmic reticulum, turning preproinsulin into proinsulin; the C peptide is removed later, leaving the two chains of insulin. “Pre” names the signal, “pro” the propeptide.
The same pattern runs through the peptides on this site. Proglucagon starts with a 20-amino-acid signal before it is cut into glucagon and GLP-1; cathelicidin carries a 30-amino-acid signal and a 101-amino-acid cathelin-like domain, and LL-37 is the last 37 amino acids of the chain. Albumin, a large protein rather than a peptide hormone, loses an 18-amino-acid signal and then a short 6-amino-acid propeptide.
Sources: UniProt P01308, P01275, P49913 and P02768. Read 1 November 2026.
Other sorting signals
The signal peptide is one address among several. Proteins bound for other parts of the cell carry different signals:
- Mitochondrial transit peptides. A protein imported into mitochondria from the rest of the cell can begin with one. UniProt lists a 32-amino-acid transit peptide at the start of ornithine transcarbamylase, a mitochondrial enzyme of 354 amino acids.
- Nuclear localization signals. MeSH describes them as short, predominantly basic amino acid sequences that act as import signals into the nucleus, thought to interact with receptors at the nuclear pore.
The idea that proteins carry their own address was recognised with the Nobel Prize in Physiology or Medicine 1999, awarded to Günter Blobel “for the discovery that proteins have intrinsic signals that govern their transport and localization in the cell”.
Sources: UniProt P00480; NLM MeSH descriptors D019913 and D024101; nobelprize.org, Physiology or Medicine 1999. Read 1 November 2026.
Signal peptides and research peptides
The peptides described in our guide library are short, finished molecules, often copies or analogues of the mature form of a natural peptide. The signal peptide belongs to the precursor a cell makes, not to the finished hormone, so it is not part of them. A peptide made by chemical synthesis does not need one at all: the signal exists to route a protein inside a living cell. Our page on how peptides are made sets out the difference between biological and chemical production.
Skincare marketing uses “signal peptides” for something else: synthetic peptides in cosmetics said to signal skin cells. Our page on types of peptides separates those cosmetic labels from the biology.
Medibact does not manufacture or sell peptides. We supply USP-grade bacteriostatic water, made in the USA, for research use only, and publish reference material like this page.
Where this sits in the rest of the reference
FAQ
What is a signal peptide?
A signal peptide, also called a signal sequence, is a short stretch of amino acids at the start of a newly made protein that tells the cell where to send it. The U.S. National Library of Medicine’s MeSH vocabulary files these under protein sorting signals: amino acid sequences in transported proteins that selectively guide them to specific compartments of the cell. The classic signal peptide sends a protein into the endoplasmic reticulum, the first step towards secretion or the cell membrane, and is then cut off.
How long is a signal peptide?
In the human proteins listed on this page, between 18 and 30 amino acids: 18 in albumin, 24 in the insulin precursor, 26 in growth hormone and 30 in cathelicidin, according to UniProt. The signal is a small fraction of the protein it leads; albumin’s is 18 of 609 amino acids.
What does the signal peptide do?
It is an address label read while the protein is still being made. As the signal emerges from the ribosome, a particle called the signal recognition particle binds it and pauses the protein’s growth until the ribosome docks on the endoplasmic reticulum, according to MeSH. The protein then threads into the endoplasmic reticulum, the signal is cut off by the signal peptidase complex, and the rest of the protein travels on through the secretory pathway.
What removes the signal peptide?
The signal peptidase complex in the membrane of the endoplasmic reticulum. UniProt’s record for its catalytic subunit, SEC11A, states that it cleaves N-terminal signal sequences from new proteins as they are moved into the endoplasmic reticulum, and that it specifically cleaves signal peptides whose water-repelling core is shorter than about 18 to 20 amino acids.
Is a signal peptide the same as a propeptide?
No. Both are cut away, but at different steps and for different reasons. The signal peptide is removed first, as the protein enters the endoplasmic reticulum. A propeptide is removed later, during maturation: UniProt’s insulin record lists a 24-amino-acid signal and a 31-amino-acid C peptide, which is removed afterwards to leave the two insulin chains. That is why a precursor with both is called pre-pro-insulin.
Do all proteins have a signal peptide?
No. It is the mark of proteins headed for secretion, the cell membrane or the compartments of the secretory pathway. Other destinations use other signals: proteins imported into mitochondria can carry a transit peptide, 32 amino acids in ornithine transcarbamylase according to UniProt, and proteins sent into the nucleus carry nuclear localization signals, which MeSH describes as short, mostly basic sequences.
Who discovered signal peptides?
The Nobel Prize in Physiology or Medicine 1999 was awarded to Günter Blobel “for the discovery that proteins have intrinsic signals that govern their transport and localization in the cell”, according to the Nobel Foundation.
Are “signal peptides” in skincare the same thing?
No. Cosmetic marketing uses “signal peptide” for short synthetic peptides in creams that are said to signal skin cells, which is a different idea from the biological signal sequence on this page. Our page on types of peptides separates the cosmetic labels from the biological ones.
Is this medical advice?
No. This page explains a piece of cell biology from public scientific records. It gives no dose, schedule, protocol or recommendation, and peptides sold for research are labelled research use only, not for human use. Medibact supplies USP-grade bacteriostatic water, made in the USA, and does not manufacture or sell peptides.
References: NLM MeSH descriptors D021382, D018271, D021381, D055571, D006056, D019913 and D024101; UniProt P67812, P61011, P02768, P01178, P01275, P61769, P01023, P01308, P01241, P01189, P49913 and P00480; nobelprize.org, Physiology or Medicine 1999. Read 1 November 2026.
Educational use only — not medical advice. This guide summarizes information reported in published research and community practice for educational purposes. It is not medical advice and not a recommendation to use any compound. Any doses, schedules, or combinations shown are examples of what has been reported, not instructions for you. Many peptides described here are research compounds that are not FDA-approved for the uses discussed and may be investigational or restricted. Effects, risks, and legal status vary; individual needs and results vary. Consult a qualified, licensed healthcare professional before making any decision. Do not use this content to diagnose, treat, or dose yourself.