Question

Which of the following statements regarding ubiquitin-mediated proteolysis is FALSE?

A. Ubiquitin serves as a molecular tag for targeted protein degradation.
B. The 26S proteasome is responsible for the degradation of ubiquitinated proteins.
C. Polyubiquitin chains are typically added to lysine residues of target proteins.
D. Ubiquitin is a large protein that directly hydrolyzes target proteins.

Answer 1

The false statement about ubiquitin-mediated proteolysis is that ubiquitin is a large protein directly involved in hydrolyzing target proteins. Ubiquitin is actually a small protein that tags proteins for degradation by the 26S proteasome.

Step-by-step explanation:

The statement regarding ubiquitin-mediated proteolysis that is FALSE is: 'Ubiquitin is a large protein that directly hydrolyzes target proteins.' In ubiquitin-mediated proteolysis, ubiquitin is in fact a small protein of about 8.5 kDa that acts as a molecular tag. This tag signals for the protein to be degraded by the 26S proteasome, a complex that does not directly hydrolyze the proteins but rather unfolds them, and subsequently they are degraded into short peptides within its core.

As a molecular tag, ubiquitin does not have proteolytic activity. Instead, it is the proteasome that is responsible for the degradation of ubiquitinated proteins. The polyubiquitin chains are typically added to lysine residues of target proteins, which then direct the tagged proteins to the proteasome, where they are recognized and processed for degradation.