USA Biology Olympiad (USABO) Practice Exam

Which protein typically tags a protein for degradation by the proteasome?

1. Ubiquitin

2. Actin

3. Myosin

4. Histone

The correct answer is: Ubiquitin

Ubiquitin is a small protein that plays a crucial role in the cellular process of tagging proteins for degradation via the proteasome, which is a large protein complex responsible for breaking down unneeded or damaged proteins. The process of ubiquitination involves the attachment of ubiquitin molecules to a target protein, effectively signaling that it should be directed towards degradation. This tagging mechanism is essential for maintaining protein homeostasis within the cell and regulating various cellular processes, including the cell cycle, signaling pathways, and responses to oxidative stress. In contrast, actin and myosin are proteins primarily involved in muscle contraction and maintaining cell structure. They do not have a role in tagging proteins for degradation. Histones, on the other hand, are proteins that package and order DNA into structural units called nucleosomes, which also do not participate in the degradation process. Therefore, ubiquitin is the specific protein that marks other proteins for destruction by the proteasome, making it the correct choice.