The endoplasmic reticulum (ER) is a widespread network of membranes within eukaryotic cells. The rough endoplasmic reticulum (RER) is a specific part of this network, distinguished by its unique appearance and specialized functions. It modifies, folds, and dispatches proteins to their appropriate cellular locations or for secretion.
Understanding Rough ER Structure
The rough endoplasmic reticulum is named for its “rough” appearance, caused by ribosomes attached to its outer surface. It forms an interconnected system of flattened sacs (cisternae) and tubular structures. Its membranes are continuous with the outer membrane of the cell nucleus, providing a direct link. This network offers an expansive surface area within the cell.
Protein Synthesis and Initial Processing
A primary function of the rough ER is the synthesis and initial processing of proteins destined for specific cellular compartments or secretion. Ribosomes attached to the rough ER membrane translate messenger RNA (mRNA) into polypeptide chains. Proteins meant for the ER, Golgi apparatus, lysosomes, or secretion contain a signal sequence that directs the ribosome to the rough ER. The nascent polypeptide then enters the ER lumen, the space within the ER, through a channel called a translocon.
Inside the ER lumen, proteins undergo modifications. Disulfide bonds, which stabilize protein structure, form between cysteine residues. Many proteins also receive carbohydrate chains through N-linked glycosylation, aiding proper folding and cellular recognition. Chaperone proteins within the ER lumen assist newly synthesized proteins in achieving their correct three-dimensional folded structures. These chaperones prevent misfolding and aggregation.
Quality Control and Cellular Transport
The rough ER maintains a quality control system to ensure that only correctly folded and assembled proteins proceed to their destinations. Proteins that fail to fold properly are identified by this surveillance system. If refolding attempts by chaperone proteins are unsuccessful, these aberrant proteins are targeted for degradation through a pathway known as ER-associated degradation (ERAD). Misfolded proteins are retro-translocated out of the ER into the cytoplasm and degraded by cellular machinery.
Once proteins are correctly folded and modified, they are prepared for transport. Processed proteins are packaged into small, membrane-bound sacs called transport vesicles. These vesicles bud off from the rough ER and travel to the Golgi apparatus, which sorts them. From the Golgi, proteins are then directed to their final destinations, such as the cell membrane, lysosomes, or for secretion.
Rough ER Versus Smooth ER
The endoplasmic reticulum exists in two main forms: rough ER and smooth ER, each with distinct structural characteristics and functions. The most apparent difference is the presence of ribosomes on the surface of the rough ER, which gives it its “rough” appearance. In contrast, the smooth ER lacks these ribosomes, appearing “smooth.”
Functionally, the rough ER is primarily involved in the synthesis, folding, modification, and transport of proteins, particularly those destined for secretion or insertion into membranes. The smooth ER, however, specializes in other cellular processes. These include the synthesis of lipids, phospholipids, and steroids. The smooth ER also plays a part in the detoxification of certain drugs and metabolic byproducts, and it serves as a storage site for calcium ions within the cell. While they perform different roles, both types of ER form a continuous membrane system within the cell.