How are proteins typically targeted for degradation by the proteasome?

• A. Through phosphorylation of the protein.
• B. By glycosylation before degradation.
• C. Via ubiquitination of the protein.
• D. By direct binding to the proteasome complex.

Answer: (C)

Proteins are typically targeted for degradation by the proteasome through a process known as ubiquitination. This involves the attachment of a small protein called ubiquitin to the target protein, which serves as a signal marking it for degradation. This tagging process usually occurs through a series of enzymatic reactions that involve E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase) enzymes. Once a protein is polyubiquitinated, it is recognized by the proteasome, which then unfolds and translocates the substrate into its catalytic core for degradation.

This mechanism is crucial for maintaining cellular homeostasis, regulating protein levels, and removing damaged or misfolded proteins. Ubiquitination thus serves as a vital cellular signal, ensuring that only proteins that are no longer needed or are faulty are targeted for destruction, preventing potential cellular dysfunction.