Protein synthesis can happen in two separate populations of ribosomes: the membrane-bound ribosomes and the free ribosomes in the cytosol. Though these ribosomes are essentially identical, when a ribosome happens to be making a protein with an ER signal sequence, the ribosome is directed to the ER membrane.
Which of the following is true of the trafficking of proteins from the cytosol to the endoplasmic reticulum?
A. The proteins are first completely synthesized in the cytosol.
B. The signal recognition particle (SRP) does not enter the ER lumen.
C. The polypeptide signal sequence is cleaved by the peptide translocation complex.
D. The SRP receptor faces the ER lumen side of the ER membrane.
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