First, RER-directed proteins must be recruited via a specific signal peptide which is recognized by the ribonucleoprotein complex SRP (Signal Recognition Particle). The RER’s involvement in post-translational modifications includes glycosylation (N-glycosylation), sulfurization, and correct protein folding. In particular, this ER region is in contact with the nuclear envelope and allows the mRNA to enter from the nucleus to the RER lumen. Īs mentioned, RER is specialized to accommodate both membrane and secreted protein synthesis, as well as post-translational modifications. Thus, proteins are not evenly distributed along with the ER but associated in clusters. Īdditionally, there is a specific region within SER and RER called nanodomains, where molecules and proteins are grouped, harmonically working together to perform a precise function. Further, since SER participates in lipid metabolism, its presence is required in adipocytes as well. As a result, SER is more developed in cells with a high detoxification power, such as hepatocytes, or in both smooth and skeletal muscle cells (where SER is called sarcoplasmic reticulum). In contrast, SER has additional functions related to calcium storage, detoxification, lipid metabolism, steroidal hormones, and bile acid production. Therefore, in cell types that hold a high rate of protein synthesis, such as hepatocytes, it is essential that the RER is further developed. This indicates that RER is more associated with protein synthesis than SER and newly synthesized proteins in RER ribosomes could enter the RER lumen to achieve their final conformation. Specifically, RER is formed of flattened sheets and contains a large quantity of ribosomes, whereas SER has a more irregular construct, consisting of a tubular structure, and is lacking in ribosomes. These structures have a unique architecture that is specialized for different cellular mechanisms. In addition, two very well differentiated structures can be found within the ER: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The ER membrane is a lipid bilayer comprising two compartments: a cytosolic region in contact with the cytoplasm and a luminal region, which is the space between the two ER membranes. For this reason, ER mass or area can fluctuate depending on cellular state and conditions. The ER is often in a state of constant change, shifting its structure to promote cell adaptation to environmental changes. Its wide and diverse functionality transforms the ER into a key organelle in cellular stress, signaling, vesicle transport, and lipid homeostasis. The endoplasmic reticulum (ER) is a dynamic organelle largely responsible for essential cellular functions. Nowadays, a plethora of pathologies like non-alcoholic steatohepatitis (NASH), cancer, and neurological alterations have been associated with ER malfunctions. Due to the importance of the ER in a variety of biological processes, alterations in its functionality have relevant implications for multiple diseases. This small cytosolic gap plays a key role in several crucial mechanisms from autophagosome synthesis to phospholipid transfer. Additionally, there is a specific region between the ER-mitochondria interface called Mitochondria-Associated Membranes (MAMs). Similarly, lipid droplets are vital structures in lipid homeostasis that are formed from the ER membrane. Peroxisomes are synthesized from a specific ER section, and they are related to very-long-chain fatty acid metabolism. Furthermore, the ER is in contact with most cellular organelles, such as mitochondria, peroxisomes, Golgi apparatus, lipid droplets, plasma membrane, etc. Moreover, the ER is implicated in cholesterol, plasmalogen, phospholipid, and sphingomyelin biosynthesis. Endoplasmic Reticulum (ER) is the largest and one of the most complex cellular structures, indicating its widespread importance and variety of functions, including synthesis of membrane and secreted proteins, protein folding, calcium storage, and membrane lipid biogenesis.
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