Learning Objective
By the end of this topic, the student should be able to: Explain the structure and function of the proteasome, its role in antigen processing for MHC I presentation, and recognize clinical conditions associated with dysfunction of the ubiquitin-proteasome system.
Overview
The proteasome is a barrel-shaped protein complex responsible for degrading intracellular proteins tagged with polyubiquitin. It plays a vital role in maintaining protein homeostasis, regulating cell cycle progression, and modulating the immune response.
Structure and Function
| Feature | Description |
|---|---|
| Structure | Barrel-shaped core with regulatory caps at both ends |
| Function | Degradation of misfolded, damaged, or short-lived proteins |
| Tagging Mechanism | Proteins are marked with ubiquitin molecules before degradation |
| End Product | Short peptides that can be recycled or used in antigen presentation |
Role in Immune Response
- The proteasome generates peptide fragments that are transported into the endoplasmic reticulum.
- These peptides are loaded onto MHC class I molecules.
- Cytotoxic T cells (CD8⁺) recognize and destroy infected or abnormal cells displaying these peptides.
Clinical Significance
| Condition | Mechanism / Association |
|---|---|
| Neurodegenerative diseases (e.g., Parkinson’s, Alzheimer’s) | Accumulation of misfolded proteins due to proteasome dysfunction |
| Cancer | Dysregulation of protein degradation affecting cell cycle control |
| Multiple myeloma therapy | Proteasome inhibitors (e.g., bortezomib) induce apoptosis in tumor cells |
Key Points
- Proteasome = “Protein shredder” of the cell
- Degrades polyubiquitin-tagged proteins
- Important for antigen presentation (MHC I)
- Defects → Protein aggregation and cellular dysfunction
