[Campbell Biology P.403] A researcher discovers a mutation in a protein that is normally sec... | Practice Question

A researcher discovers a mutation in a protein that is normally secreted from the cell. This mutation prevents the protein's signal peptide from being recognized by the signal-recognition particle SRP . What would be the most likely fate of this mutated protein?

  • A: It would be secreted outside the cell, but at a reduced rate.
  • B: It would be integrated into the ER membrane.
  • C: It would complete its synthesis on free ribosomes and remain in the cytosol.
  • D: It would be immediately degraded by proteasomes.

Explanation

The text states that polypeptide synthesis always begins in the cytosol on a free ribosome. If the signal peptide is not recognized by the SRP, the ribosome will not be escorted to the ER. Therefore, the synthesis will continue to completion on the free ribosome in the cytosol, and the protein will remain there instead of being secreted.