Amino Acids & Proteins MCAT Practice Question
A 34-year-old woman with a history of recurrent respiratory infections undergoes genetic testing and is found to have a mutation in the CFTR gene that results in a 27-amino acid hydrophobic signal sequence remaining attached to the N-terminus of the cystic fibrosis transmembrane conductance regulator protein. During translation of this mutant mRNA, the ribosome begins synthesizing the protein and the signal recognition particle (SRP) binds to the emerging hydrophobic signal sequence. The ribosome docks at the translocon in the endoplasmic reticulum membrane, and the growing polypeptide begins entering the ER lumen. Which of the following best explains why a hydrophobic signal sequence is essential for directing nascent proteins into the endoplasmic reticulum?
Answer choices
- AHydrophobic amino acids form disulfide bonds with ER chaperone proteins, creating an affinity tag that recruits the translocon complex to the ribosome
- BThe hydrophobic sequence interacts favorably with the nonpolar fatty acid chains of the ER membrane lipid bilayer, allowing the translocon channel to recognize and accept the growing polypeptide chainCorrect answer
- CHydrophobic residues are phosphorylated by kinases in the cytoplasm, creating a binding motif for SRP receptor proteins on the ER surface
- DThe signal sequence forms stable alpha-helical and beta-sheet structures that are too rigid to fold in the aqueous cytoplasmic environment, triggering immediate ribosomal stalling and ER translocation
- EHydrophobic amino acids are cleaved by signal peptidase I while the protein is still being synthesized, releasing the mature protein directly into the ER lumen before translation is complete
- FHydrophobic sequences recruit cytoplasmic chaperones that inhibit ribosomal translation until the ribosome physically contacts the translocon protein channel
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