Cell Biology MCAT Practice Question
A molecular biologist is studying protein synthesis in a cell-free system. When mRNA encoding a secretory protein is translated in vitro without microsomes, the resulting polypeptide is 25 amino acids longer than the mature secreted protein found in vivo. The extra residues are located at the N-terminus and are highly hydrophobic. Addition of microsomes to the in vitro system produces a protein of the expected size. Which of the following is most likely explaining this observation?
Answer choices
- AThe signal peptide is not being recognized by the ribosome
- BThe proteasome is cleaving the signal peptide in vivo
- CPost-translational ubiquitination is removing the signal sequence
- DSignal Recognition Particle (SRP) normally directs ribosome-nascent chain to the ER for co-translational cleavageCorrect answer
- EMethionine aminopeptidase is removing N-terminal residues
- FPeptidyl transferase activity of the ribosome is cleaving the hydrophobic N-terminal sequence during translation
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