Where are proteins destined for secretion or the plasma membrane typically synthesized?

• A. Rough endoplasmic reticulum (rough ER)
• B. Smooth endoplasmic reticulum
• C. Golgi apparatus
• D. Peroxisomes

Answer: (A)

Proteins destined for secretion or the plasma membrane are made by ribosomes bound to the rough endoplasmic reticulum. A signal peptide on the growing polypeptide directs the ribosome to the ER, where translation continues and the new protein is threaded into the ER lumen through a channel. Inside the ER, proteins fold with the help of chaperones and often receive initial modifications; membrane proteins acquire their transmembrane segments and become anchored in the ER membrane. This set the proteins on a path through the secretory system, moving next to the Golgi for processing and sorting, and then on to the plasma membrane or for secretion outside the cell.

The smooth endoplasmic reticulum lacks ribosomes, so it’s not where secreted or membrane proteins are synthesized. The Golgi apparatus is where proteins are modified and sorted after synthesis in the ER, not the site of initial synthesis. Peroxisomes import proteins that are made in the cytosol and then directed to the peroxisome, rather than synthesizing them themselves.