15. Secretory proteins synthesized by ER-associated ribosomes traverse through
(a) Mitochondria (b) Peroxisomes
(c) The Golgi apparatus (d) The nucleus

Secretory proteins synthesized by ER-associated ribosomes primarily traverse the Golgi apparatus for processing and sorting. This multiple-choice question tests understanding of the classical secretory pathway in eukaryotic cells. The correct answer is (c) The Golgi apparatus.​

Correct Answer

Proteins destined for secretion begin synthesis on ribosomes bound to the rough endoplasmic reticulum (ER), entering the ER lumen co-translationally via a signal peptide. These proteins move from the ER to the Golgi apparatus in COPII-coated vesicles for further modification, glycosylation, and sorting into secretory vesicles. This ER-to-Golgi transport defines the constitutive secretory pathway essential for hormone release, antibody production, and extracellular matrix formation.​

Option Explanations

• (a) Mitochondria: Mitochondria handle energy production via oxidative phosphorylation and import most nuclear-encoded proteins post-translationally from the cytosol, not secretory proteins from ER ribosomes.​

• (b) Peroxisomes: Peroxisomes manage lipid metabolism and reactive oxygen detoxification; their proteins enter post-translationally via peroxisomal targeting signals, bypassing the ER-Golgi route.​

• (c) The Golgi apparatus: Correct—secretory proteins transit here for processing before exocytosis, as confirmed by pulse-chase experiments tracking radiolabeled proteins from rough ER to Golgi.​

• (d) The nucleus: The nucleus stores genetic material and imports proteins with nuclear localization signals directly from the cytosol; it plays no role in secretory protein trafficking.