Proteins synthesized by fixed ribosomes on the Rough ER are typically destined for which locations?

• A. Secretion, lysosomes, or incorporation into membranes
• B. Mitochondrial matrix
• C. Nuclear lumen
• D. Cytosol

Answer: (A)

Proteins made by fixed ribosomes on the Rough ER are directed toward the secretory pathway. A signal sequence on the growing polypeptide directs the ribosome to the ER membrane, where the protein is translocated into the ER lumen or integrated into the membrane. From there, it moves through the Golgi apparatus and vesicles to its final destinations, which include secretion outside the cell, lysosomes (as soluble enzymes that are sent through the Golgi with targeting to lysosomes), or incorporation into cellular membranes. This pathway distinguishes these proteins from those synthesized in the cytosol for mitochondria or nuclear compartments, which use different targeting signals and import mechanisms.