Explain how proteins that are produced in a cell are correctly directed to the cytosol, the endomembrane system, and
the mitochondria.
What will be an ideal response?
ANSWER: Proteins destined for the cytosol are synthesized on free ribosomes, so the proteins are simply
released into the cytosol after termination of translation. Proteins targeted for the
endomembrane system are also made on free ribosomes. However, they have a short signal
sequence of amino acids near their N-terminal ends that allows the proteins to be delivered to
the RER, where translation is completed. Once protein synthesis is complete, the proteins
may remain in the RER, or they may be directed to another location if they contain a specific
tag directing their transport elsewhere. Proteins that need to be delivered to the mitochondria
are also made on free ribosomes, but contain a targeting signal found at the ends of the
proteins or within the proteins. The targeting signal directs them for transport to the
mitochondria.
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