Which cellular structure is responsible for the folding of newly synthesized proteins?

The endoplasmic reticulum (ER) plays a crucial role in the folding of newly synthesized proteins. Once proteins are synthesized by ribosomes, they are often translocated into the lumen of the rough endoplasmic reticulum, where they undergo various modifications and proper folding. The ER provides an ideal environment, with the help of molecular chaperones, for proteins to reach their functional conformations.

The rough ER, in particular, is associated with ribosomes on its cytoplasmic surface, allowing for the co-translational translocation of nascent polypeptides into the ER. Within the ER, proteins can then undergo post-translational modifications, such as glycosylation, which are also important for their proper folding and functionality. This ensures that proteins are correctly assembled, preventing misfolding and aggregation, which could lead to cellular stress or disease. Therefore, the endoplasmic reticulum is essential for the proper maturation and quality control of newly synthesized proteins.