A hormone signal reaches a cell and causes the cell to produce a large quantity of Protein X. After some time, the hormone signal disappears and the cell no longer needs a large quantity of Protein X. How will the cell remove the excess protein?

• A. The repressor protein for the Protein X gene will stop the transcription of the gene.
• B. The excess Protein X will be tagged with ubiquitin proteins and degraded over time.
• C. The Protein X mRNA will be bound by a microRNA blocking its translation.
• D. Over time the excess Protein X will diffuse out of the cell.

Answer: (B)

Verified step by step guidance

1. Understand the role of ubiquitin in protein degradation: Ubiquitin is a small protein that tags other proteins for degradation. When a protein is no longer needed, it is marked with ubiquitin molecules, signaling it for destruction by the proteasome, a complex that breaks down proteins.
2. Identify the process of ubiquitination: The cell uses enzymes called ubiquitin ligases to attach ubiquitin to the target protein. This process involves multiple steps where ubiquitin molecules are added sequentially to form a polyubiquitin chain.
3. Recognize the function of the proteasome: Once a protein is tagged with ubiquitin, it is recognized by the proteasome. The proteasome is a large protein complex that degrades and recycles proteins into amino acids, which can be reused by the cell.
4. Consider alternative mechanisms for reducing protein levels: Besides degradation, cells can regulate protein levels by controlling mRNA translation. MicroRNAs can bind to mRNA molecules, preventing their translation into proteins, thus reducing protein production.
5. Evaluate the diffusion of proteins: While diffusion is a passive process where molecules move from areas of high concentration to low concentration, it is not typically how cells manage excess proteins. Proteins are generally too large to diffuse freely across cell membranes without specific transport mechanisms.