How does the ubiquitin-proteasome system regulate protein levels?

• A. It adds lysine residues to proteins to increase activity.
• B. It inhibits translation.
• C. Proteins tagged with ubiquitin are targeted to the proteasome for degradation.
• D. It exports proteins out of the cell.

Answer: (C)

The main idea is that protein levels are controlled by selective degradation through tagging with ubiquitin and delivery to the proteasome. In this system, enzymes attach ubiquitin molecules to target proteins—often forming a chain on a lysine residue. The proteasome recognizes these ubiquitin tags, uses energy from ATP to unfold the marked protein, and then degrades it into smaller peptides, lowering the protein’s cellular abundance. This tagging is highly specific, guided by E3 ligases that pick out particular substrates, so the cell can finely tune which proteins are removed and when. This mechanism explains how protein amounts are actively reduced rather than simply produced or exported. The other ideas—adding lysine residues to boost activity, broadly inhibiting translation, or exporting proteins out of the cell—do not describe how the ubiquitin-proteasome system regulates protein levels.