In an experiment, ribosomes detach from the rough endoplasmic reticulum. This would most directly impair the synthesis of which type of proteins?

• A. Synthesis of steroid hormones
• B. Endocytosis
• C. Synthesis of secretory proteins
• D. Synthesis of cytosolic proteins

Answer: (C)

Ribosome location determines where a protein is synthesized and how it is processed. Ribosomes bound to the rough endoplasmic reticulum specifically make proteins destined for secretion, insertion into membranes, or delivery to lysosomes; these proteins are translocated into the ER lumen as they are being synthesized and then pass through the Golgi for secretion.

If ribosomes detach from the rough ER, the direct consequence is impaired production of secretory proteins, because their synthesis relies on the ER-bound ribosomes. In contrast, many cytosolic proteins are synthesized by free ribosomes in the cytosol, so their production would be less directly affected. Steroid hormones are produced mainly in mitochondria and by pathways in the smooth ER, not by rough ER-bound ribosomes, and endocytosis is a cellular process rather than a protein type. Thus, the synthesis most directly impaired would be secretory proteins.