Post-Translational Modifications
Post-translational modifications are covalent chemical changes made to a protein after it has been synthesised. By adding chemical groups (such as phosphate or sugar chains), attaching other proteins (such as ubiquitin), or cleaving the chain, the cell can change a protein's activity, stability, location, and interactions. These modifications greatly expand the functional diversity of the proteome beyond what the genome encodes directly.
Definition
Post-translational modifications are enzyme-catalysed covalent alterations of a protein after its synthesis, including the addition of small chemical groups, attachment of sugars or other proteins, and proteolytic cleavage, that modulate the protein's activity, localisation, stability, and interactions.
Scope
This topic covers the main classes of post-translational modification, including phosphorylation, glycosylation, ubiquitination, acetylation, methylation, lipidation, and proteolytic processing, and how they regulate protein function. It is a molecular reference and does not give clinical advice.
Core questions
- How can a fixed set of gene products produce a much larger range of protein functions?
- Which chemical groups are added to proteins, and to which amino acids?
- How do reversible modifications act as molecular switches?
- How does a modification change a protein's fate within the cell?
Key concepts
- Proteome diversification
- Phosphorylation (kinases and phosphatases)
- Glycosylation (N-linked and O-linked)
- Ubiquitination
- Acetylation and methylation
- Lipidation
- Proteolytic processing
- Reversible modification as a molecular switch
Mechanisms
Dedicated enzymes attach or remove chemical groups at specific amino-acid side chains. Phosphorylation by kinases (and removal by phosphatases) provides a rapidly reversible switch that toggles activity and signalling. Glycosylation adds sugar chains, predominantly in the endoplasmic reticulum and Golgi, shaping folding, stability, and recognition (Varki, 1993). Ubiquitination attaches the small protein ubiquitin to mark substrates for fates including degradation (Hershko & Ciechanover, 1998). Acetylation, methylation, lipidation, and proteolytic processing further tune activity, localisation, and interactions, so that a single gene product can exist in many functionally distinct forms (Walsh et al., 2005).
Clinical relevance
Because modifications such as phosphorylation and ubiquitination regulate signalling, growth, and protein turnover, their dysregulation is studied in many diseases, and modification-detecting assays are used in research and diagnostics. This entry describes mechanisms and their general significance and is not a guide to individual diagnosis or treatment.
History
The discovery of reversible protein phosphorylation in the mid-twentieth century, recognised by the 1992 Nobel Prize in Physiology or Medicine, established modification as a control mechanism. The characterisation of glycosylation and of the ubiquitin system (Hershko & Ciechanover, 1998) broadened the picture, and systematic surveys later framed the full chemical diversity of modifications as a major source of proteome complexity (Walsh et al., 2005).
Key figures
- Christopher Walsh
- Edmond Fischer
- Edwin Krebs
- Aaron Ciechanover
- Avram Hershko
Related topics
Seminal works
- walsh-2005
- hershko-1998
Frequently asked questions
- Why are post-translational modifications important if the gene already specifies the protein?
- The gene specifies the amino-acid sequence, but modifications change what that protein does, where it goes, and how long it lasts. They let one gene product take many functional forms and respond rapidly to signals.
- Are post-translational modifications permanent?
- Many are reversible. Phosphorylation, for example, can be added and removed, acting as a switch, while others such as proteolytic cleavage are not reversed.