Journal of Cell Signaling
Open Access

General Endoplasmic Reticulum and its Applications

Andrea Tales^{* *}Correspondence: Andrea Tales, Department of Cell Biology, University of Bristol, Bristol, United Kingdom, Email:

Author info »

Description

The transportation system of the eukaryotic cell is primarily the Endoplasmic Reticulum (ER), which also carries out various other crucial processes like protein folding. This specific type of organelle is made up of two subunits called the Rough Endoplasmic Reticulum (RER) and the Smooth Endoplasmic Reticulum (SER).Most eukaryotic cells have the endoplasmic reticulum, which organises itself into a network of tubular structures in the SER and flattened, membrane-enclosed sacs known as cisternae in the RER.

Both ER types participate in the creation of various lipids including cholesterol, as well as other shared functions and proteins. Depending on the activity of the cell, various types of cells have varied ratios of the two types of ER. RER and SER are mostly located in relation to the cell's nucleus and cell membrane, respectively.

The SER doesn't have ribosomes and performs the processes of lipid synthesis, but not metabolism, steroid hormone production, or detoxification. The SER is particularly prevalent in the liver and gonad cells of mammals.

The rough endoplasmic reticulum, also known as granular endoplasmic reticulum and frequently abbreviated RER or rough ER, is covered in protein-producing ribosomes, giving it a "rough" look (hence its name). However, because they are constantly linked to and released from the membrane, ribosomes are not a stable component of this organelle's structure. Only after a certain protein-nucleic acid combination occurs in the cytoplasm does a ribosome bind to the RER. When a free ribosome starts translating the mRNA of a protein intended for the secretory route, a unique complex emerges.

A signal recognition particle recognises and binds a signal peptide, which is made up of the first 5 to 30 polymerized amino acids (SRP). The ribosome complex binds to the RER translocon, where translation resumes and the developing (new) protein forms into the RER lumen or membrane as translation is halted. A signal peptidase enzyme breaks down the protein in the ER lumen, removing the signal peptide. At this moment, ribosomes might be discharged back into the cytosol, but it is also known that non-translating ribosomes stick with translocons.

Large double-membrane sheets that are contiguous with and close to the nuclear envelope are formed by the rough endoplasmic reticulum membrane. The "Terasaki ramps," or several right- or left-handed helical ramps, are used to stack and link the double membrane sheets, creating a structure that resembles a parking garage. Despite the absence of a continuous membrane between the endoplasmic reticulum and the Golgi apparatus, proteins are shuttled between these two organelles via membrane-bound transport vesicles. COPI and COPII, two coating proteins, enclose vesicles. Vesicles are directed by COPII to the Golgi apparatus, where they are marked by COPI to be returned to the rough endoplasmic reticulum.

To direct new proteins to their correct locations, the Golgi complex collaborates with the rough endoplasmic reticulum. The membranes of the endoplasmic reticulum and other organelles are kept tightly together in regions known as membrane contact sites, enabling the transfer of lipids and other small molecules out of the endoplasmic reticulum.

The smooth endoplasmic reticulum, often known as SER, is generally insufficient in cells. Instead, there are regions of the ER that are alternately smooth and rough; this region is known as the transitional ER. Because it has ER departure sites, the transitional ER is thus named. These are the locations where the ER-produced lipid and protein-containing transport vesicles separate from the ER and begin their journey to the Golgi apparatus. Smooth endoplasmic reticulum can be abundant in specialised cells, and in these cells, smooth ER serves a variety of purposes.

Steroids, phospholipids, and lipids are all synthesised by it. The sebaceous glands, ovaries, and testes are only a few examples of the cells that secrete these substances. These cells also have a lot of smooth endoplasmic reticulum. Additionally, it is responsible for the detoxification of natural metabolic products, alcohol, and pharmaceuticals, as well as the metabolism of steroids and carbohydrates. It controls the concentration of calcium ions in muscle cells.

Animal and plant cells both have smooth endoplasmic reticulum, which has diverse roles in each kind of cell.

The enzyme glucose-6-phosphatase, which converts glucose-6- phosphate to glucose as a stage in gluconeogenesis, is also present in the smooth endoplasmic reticulum. It consists of tubules that are close to the cell periphery and is related to the nuclear envelope. These tubes can sometimes branch, creating a reticular-looking network. There are dilated regions, similar to the rough endoplasmic reticulum sacs, in some cells. A larger surface area can be devoted to the action or storage of important enzymes and their byproducts thanks to the smooth endoplasmic reticulum network.

Numerous general activities of the endoplasmic reticulum include transporting produced proteins in vesicles to the Golgi apparatus and folding protein molecules into sacs termed cisternae. The rough endoplasmic reticulum contributes to the production of proteins. Several endoplasmic reticulum chaperone proteins, such as Protein Di-sulfide Isomerase (PDI), ERp29, the Hsp70 family member BiP/Grp78, calnexin, calreticulin, and the peptidylprolyl isomerase family, enable the correct folding of newly made proteins.

Conclusion

Unfolded proteins trigger the unfolded protein response, which is the ER's stress response, and are not transferred from the rough ER to the Golgi apparatus. Endoplasmic reticulum stress response (ER stress), a condition in which the folding of proteins slows and a rise in unfolded proteins results, can be brought on by disturbances in redox regulation, calcium regulation, glucose deprivation, viral infection, or overexpression of proteins. In illnesses such as hypoxia/ischemia, insulin resistance, and others, this stress is now being recognized as a potential source of harm. The endoplasmic reticulum is associated with the Unfolded Protein Response (UPR), a cellular stress response. When there is an accumulation of unfolded or improperly folded proteins in the endoplasmic reticulum lumen, the UPR is triggered. By stopping protein translation, breaking down misfolded proteins, and activating signalling pathways that result in an increase in the synthesis of molecular chaperones involved in protein folding, the UPR helps the cell resume its normal function.