Amino Acids & Proteins MCAT Practice Question
A 6-year-old boy with cystic fibrosis presents with recurrent pulmonary infections and pancreatic insufficiency. Genetic testing reveals a nonsense mutation in the CFTR gene resulting in a truncated protein of 493 amino acids instead of the normal 1,480 residues. The truncated protein is rapidly degraded by the proteasome despite retaining some secondary structure in its N-terminal domain. Which of the following best explains why this partially structured protein is targeted for proteasomal degradation?
Answer choices
- AThe truncated protein lacks sufficient molecular weight to be recognized as a functional protein by cellular ribosomes
- BIncomplete protein domains expose hydrophobic patches and irregular backbone conformations that are recognized as misfolded by ER-associated quality control machineryCorrect answer
- CThe absence of the C-terminal PDZ-binding domain prevents interaction with stabilizing cytoskeletal anchoring proteins
- DPremature termination generates a free carboxyl group that recruits ubiquitin ligases to the protein surface
- EThe N-terminal domain lacking its normal C-terminal partner cannot oligomerize, making the protein too unstable to function
- FTruncated proteins are constitutively recognized as foreign by pattern recognition receptors in the endoplasmic reticulum
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