Where are proteins intended for export from the cell synthesized?

Proteins intended for export from the cell are synthesized by ribosomes that are located on the rough endoplasmic reticulum (RER). The rough endoplasmic reticulum is characterized by its studded appearance due to ribosomes attached to its cytoplasmic surface. These ribosomes translate messenger RNA (mRNA) into polypeptide chains, which then enter the lumen of the RER, where they undergo folding and post-translational modifications.

This process is critical for the synthesis of secretory proteins, which are then packaged into vesicles and sent to the Golgi apparatus for further processing and eventual export from the cell. The structure of the RER facilitates the proper folding and processing of these proteins, making it an essential site for the synthesis of proteins destined for secretion.

In contrast, the Golgi apparatus plays a role in modifying, sorting, and packaging proteins after they have been synthesized in the RER, but it is not the site of their initial synthesis. The smooth endoplasmic reticulum is involved in lipid synthesis and detoxification but does not play a direct role in synthesizing proteins for export. The nucleus is where mRNA transcription occurs, but it is not the site of protein synthesis itself. Thus, the rib