protein expression
Meaning of protein expression: Two cells carrying identical genes can hold wildly different amounts of a protein, and that gap is what expression measures.
Definition of protein expression
Protein expression is the making of a protein from the instructions held in its gene, and the amount of that protein a cell ends up carrying. It covers the whole route: copying the gene into messenger RNA, assembling a chain of amino acids from that copy, folding the chain into shape, and eventually breaking it down. The level reflects production and destruction together.
How does a gene become a protein?
In two steps with several checkpoints. The stretch of DNA making up the gene is first copied into messenger RNA, a short-lived working transcript. Ribosomes then read that transcript three letters at a time and add the matching amino acid to a growing chain. The finished chain folds into a precise shape, and many proteins are trimmed or decorated afterward with sugars, fats or phosphate groups that switch them on or send them to a particular part of the cell. Every one of those steps can be sped up or slowed down, and the cell is simultaneously destroying old copies, so the amount present at any moment is a balance rather than a fixed output.
Why is it not the same as gene expression?
The two are measured differently and do not track each other closely. Gene expression is usually read out as the quantity of messenger RNA, which is quick and cheap to measure across thousands of genes at once. The trouble is that a plentiful transcript may be translated slowly, and a scarce one may yield a protein that lasts for days. Since it is the protein that does the work, RNA levels are a useful proxy and a poor substitute. Some proteins are held in reserve as inactive precursors, adding another layer between the gene and its effect.
How is it measured?
| Method | What it shows |
|---|---|
| Western blot | How much of one protein is in a ground-up sample |
| Immunohistochemistry | Which cells hold it, and where, on a tissue slide |
| Flow cytometry | The amount in each individual cell in a suspension |
| Mass spectrometry | Thousands of proteins at once, without needing antibodies |
These readings guide everyday decisions well beyond research. Staining a tumor for hormone receptors or growth factor receptors is a measurement of protein expression, and the result determines which drugs can work at all. The wider pattern across many proteins at once forms a protein expression profile, which can separate diseases that look alike under the microscope.
Used in a sentence
Immunohistochemistry was used to assess protein expression in the tumor sample.
Good to know
Messenger RNA levels are frequently reported as a stand-in for protein expression, but translation rates and protein lifespans vary so widely that the two measurements can point in different directions.
Word origin
Where “protein expression” comes from
Protein derives from the Greek proteios, meaning of the first rank, chosen to mark these molecules as fundamental to living tissue.