Rough endoplasmic reticulum

Rough Endoplasmic Reticulum

The rough endoplasmic reticulum (RER) is a type of endoplasmic reticulum (ER) characterized by the presence of ribosomes on its cytoplasmic surface, giving it a "rough" appearance under a microscope. It plays a crucial role in the synthesis and processing of proteins destined for secretion, incorporation into the cell membrane, or for use in lysosomes.

Structure

The rough endoplasmic reticulum is a network of membranous tubules and flattened sacs called cisternae. The outer surface of the RER is studded with ribosomes, which are the sites of protein synthesis. These ribosomes are attached to the RER membrane via a complex of proteins that anchor them in place.

The RER is continuous with the nuclear envelope, allowing for the direct transfer of newly synthesized proteins into the ER lumen. This structural continuity is essential for the coordination of protein synthesis and processing.

Function

The primary function of the rough endoplasmic reticulum is the synthesis of proteins. Ribosomes on the RER translate mRNA into polypeptide chains, which are then translocated into the lumen of the RER. Once inside, these polypeptides undergo folding and post-translational modifications, such as glycosylation.

The RER is also involved in the quality control of proteins. Misfolded proteins are identified and targeted for degradation through a process known as ER-associated degradation (ERAD).

Protein Synthesis and Processing

1. Translation Initiation: Ribosomes bind to the mRNA and begin translating the genetic code into a polypeptide chain. 2. Translocation: The nascent polypeptide is translocated into the RER lumen through a protein-conducting channel called the translocon. 3. Folding and Modification: Inside the RER, proteins fold into their functional three-dimensional structures with the help of chaperone proteins. Enzymes in the RER also add carbohydrate groups to form glycoproteins. 4. Quality Control: Properly folded proteins are packaged into vesicles and transported to the Golgi apparatus for further processing. Misfolded proteins are retained in the ER and eventually degraded.

Clinical Significance

Dysfunction of the rough endoplasmic reticulum can lead to a variety of diseases, collectively known as ER stress-related diseases. These include neurodegenerative disorders like Alzheimer's disease, diabetes, and certain types of cancer. The accumulation of misfolded proteins can trigger the unfolded protein response (UPR), which attempts to restore normal function but can lead to cell death if homeostasis is not achieved.

Also see

• Smooth endoplasmic reticulum
• Ribosome
• Protein synthesis
• Golgi apparatus
• Endoplasmic reticulum stress