Q11 — Proteasome vs. Lysosome Degradation and Protein Half-Life

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

Endomembrane System & Protein Trafficking

Proteasomes, cytoplasmic multiprotein complexes, and lysosomes are major proteolysis (protein degradation) sites in eukaryotic cells.

In Figure 1-a, researchers studied the abundance of proteins A, B, and C over time in cultured human cells using cycloheximide, an inhibitor of translation elongation in eukaryotic 80S ribosomes. Note that cycloheximide does not affect the abundance of protein A, whose half-life is shorter than that of protein B.

In Figure 1-b, they observed how MG132, a proteasome inhibitor, and chloroquine, a lysosome inhibitor, affect the abundance of proteins B and C over time under cycloheximide treatment.

Top: a line graph of protein A, B, and C abundance over time, before and after adding cycloheximide — A stays flat, B and C decline. Bottom: two bar charts (for proteins B and C) comparing abundance over time under control, MG132, and chloroquine conditions. Figure 1. 1-a Y: Protein abundance, X: Time (hours), I: Before adding cycloheximide, II: After adding cycloheximide, A: protein A, B: protein B, C: protein C. 1-b Y: Protein abundance, X: Time after adding cycloheximide (hours), B: protein B, C: protein C, N: control, M: MG132, Q: Chloroquine.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. Cycloheximide treatment alone provides direct information on the rate of protein synthesis.

B. The half-life of protein B is shorter than the half-life of protein C.

C. Protein A is more likely to be mitochondrial than proteins B and C.

D. Protein B is more likely to be an extracellular protein than protein C.

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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF