Final answer:
The presence or absence of a signal sequence on a protein determines whether it will be translated on the ER or in the cytosol.
Step-by-step explanation:
The location where a protein is translated, whether on the endoplasmic reticulum (ER) or in the cytosol, is determined by the presence or absence of a signal sequence on the protein. Proteins that are destined to be translated on the ER have a signal sequence at their N-terminus that directs them to the ER. As soon as these amino acids are translated, a signal recognition particle (SRP) binds to the signal sequence and stops translation while the mRNA-ribosome complex is shuttled to the ER. Once they arrive at the ER, the SRP is removed and translation resumes. In contrast, proteins without a signal sequence are translated in the cytosol.