What is the primary function of the endoplasmic reticulum in protein synthesis?

The endoplasmic reticulum (ER) plays a crucial role in the synthesis of proteins, specifically through its rough region, which is studded with ribosomes. These ribosomes are the sites where polypeptides are synthesized. As ribosomes translate mRNA into protein, the growing polypeptide chains are threaded into the lumen of the rough ER, where they undergo several processes.

In the rough ER, polypeptides are co-translationally imported and begin folding into their functional conformations. Additionally, the ER is involved in initial modifications of these proteins, such as glycosylation (the addition of sugar moieties), which is essential for the proper functioning and stability of many proteins. Thus, while the rough ER does not complete the synthesis of all types of polypeptides itself, it is integral to the process of synthesizing and initiating the processing of polypeptides destined for export or for cellular membranes.

Other options present different processes that do not directly pertain to the primary function of the ER in protein synthesis. The degradation of misfolded proteins mainly occurs in the proteasome, while protein storage is not a function primarily associated with the ER, and RNA processing is primarily conducted in the nucleus. Therefore, the