Protein Folding, Targeting, and Post-Translational Modification
From Polypeptide to Functional Protein
Folding Principles
A protein's amino acid sequence determines its likely fold, but folding often requires chaperones to avoid aggregation and misfolding. Thermodynamics favors the native state when it is the lowest free-energy conformation under cellular conditions.
Misfolding Matters
Protein misfolding can cause loss of function, toxic aggregation, and disease when quality-control systems fail.
Targeting and Sorting
Signal peptides and localization sequences direct proteins to the nucleus, mitochondria, endoplasmic reticulum, peroxisomes, or secretion pathway. These signals are interpreted by receptor proteins and translocation machinery.
Common Post-Translational Modifications
| Modification | Typical Effect |
|---|---|
| Phosphorylation | Alters activity or signaling |
| Glycosylation | Affects folding, stability, recognition |
| Ubiquitination | Targets proteins for degradation or signaling |
| Acetylation | Can influence chromatin and protein function |
What is one major role of molecular chaperones?
Chaperones assist folding and prevent aggregation.
Correct answer: They help proteins fold correctly
Why can phosphorylation rapidly change protein activity?
Adding a phosphate group changes charge and can switch regulatory states on or off.
Correct answer: It can alter protein conformation, interactions, or localization.
Folding vs Modification
Folding
- Creates 3D structure
- Often occurs during or after translation
Modification
- Refines activity or lifespan
- Can regulate signaling and trafficking
Protein Maturation Pathway
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1
Step 1: The nascent polypeptide emerges from the ribosome.
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2
Step 2: Chaperones assist folding and prevent aggregation.
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3
Step 3: Targeting signals direct the protein to a destination.
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4
Step 4: Post-translational modifications tune function.
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5
Step 5: Quality control removes damaged or misfolded proteins.
Which modification commonly marks proteins for degradation by the proteasome?
Polyubiquitin chains often target proteins for proteasomal degradation.
Correct answer: Ubiquitination
How does a signal peptide help route a protein to the secretory pathway?
Signal recognition directs co-translational or post-translational import into the pathway.
Correct answer: It is recognized by targeting machinery that delivers the ribosome or protein to the endoplasmic reticulum.