What happens to proteins synthesized by ribosomes in the rough endoplasmic reticulum?

Proteins synthesized by ribosomes in the rough endoplasmic reticulum (RER) typically undergo a specific process that often leads to their secretion outside the cell. The rough endoplasmic reticulum is studded with ribosomes where protein synthesis occurs, particularly for proteins that are destined for secretion or for use in the cell membrane or organelles.

Once synthesized, these proteins enter the lumen of the RER, where they undergo folding and post-translational modifications. From there, they are packaged into vesicles that bud off from the ER and transport them to the Golgi apparatus. In the Golgi, the proteins can be further modified, sorted, and then either secreted from the cell by exocytosis or sent to specific locations within the cell.

This path effectively links the rough endoplasmic reticulum to the secretory pathway, explaining the mechanism behind protein secretion. Therefore, the correct answer highlights the primary fate of proteins synthesized in the RER, emphasizing their role in the export process that is vital for cellular communication and function.