Final answer:
Mono-ubiquitination does not typically signal a protein for degradation by the proteasome; instead, poly-ubiquitination is the usual process that marks proteins for destruction. The ubiquitin-proteasome pathway is essential for maintaining cellular function by regulating protein levels and activity.
Step-by-step explanation:
The statement that mono-ubiquitination signals a protein for degradation by the proteasome is not entirely accurate. While ubiquitination does indeed play a role in protein degradation, it is usually poly-ubiquitination, not mono-ubiquitination, that targets proteins for destruction in the proteasome. The process of protein degradation is critical for cellular function and involves a series of steps:
- A protein is marked for degradation by the addition of an ubiquitin group, serving as a tag that indicates the end of the protein's lifespan.
- Several more ubiquitins are usually attached, forming a poly-ubiquitin chain.
- The poly-ubiquitinated protein is recognized by the proteasome, particularly by one of its 19S 'CAP' structures.
- Ubiquitins are released and recycled, and ATP hydrolysis unfolds the target protein.
- The protein passes into the core of the proteasome where it is broken down into short peptide fragments and further degraded to amino acids.
This pathway is essential for regulating the cell cycle, gene expression, and the response to stressors among other cellular processes.