plural: endoplasmic reticula
(cell biology) A membrane-bound organelle that occurs as labyrinthine, interconnected flattened sacs or tubules connected to the nuclear membrane, running through the cytoplasm, and may well extend into the cell membrane
Organelle literally means “little organs”. As the body is composed of various organs, the cell, too, has “little organs” that perform special functions. Some references are strict in their definition of an organelle. An organelle is a structure surrounded by lipid bilayers. In this regard, nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplast (plastid) are regarded as organelles whereas ribosomes and nucleosomes are not.
In the same way, lysosomes and vacuoles, would not qualify as an organelle because they are single-membrane bounded cytoplasmic structures. Other references, though, are less restrictive. An organelle is one that which acts as a specialized subunit inside the cell that performs a specific function. In this case, there are two types of organelles: (1) membrane-bound organelles (included are double-membraned and single-membraned cytoplasmic structures) and (2) non-membrane-bound organelles. Examples of membrane-bound organelles are nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes, and vacuoles. Examples of non-membrane-bound organelles are ribosomes, spliceosome, vault, proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, photosystem I, ATP synthase, nucleosome, centriole, microtubule-organizing center, cytoskeleton, flagellum, nucleolus, stress granule, etc.
Endoplasmic reticulum is one of the most prominent organelles of a eukaryotic cell. The endoplasmic reticulum (ER) is an organelle that occurs as interconnected network of flattened sacs or tubules (called cisternae) in the cytoplasm. The membranes of the ER are connected to the outer nuclear envelope. They may also extend into the cell membrane. The ER is one of the three components of the GERL system, in which the Golgi apparatus and the lysosomes are the other components. It is found in different cell types. However, it is absent in red blood cells and spermatozoa.
There are two kinds of ER: the rER, or the rough endoplasmic reticulum, and the sER, or the smooth endoplasmic reticulum. The rER bears many ribosomes on its outer surface giving it a rough appearance; hence, its name. In contrast, the sER does not have ribosomes attached to its outer surface thus making it relatively smoother. The rER is abundant in hepatocytes whereas the sER is abundant in mammalian liver and gonad cells.
Rough Endoplasmic Reticulum
The rER has a translocon, i.e. a protein complex involved in the translocation of nascent polypeptides, and in this case, from the cytosol to the interior of the rER across the rER membranes. The translocon serves as the binding site where ribosome can dock to the endoplasmic reticulum. The bound ribosomes, though, are attached to the ER transiently. They may come and go. They attach to the endoplasmic reticulum (via the translocon) when a signal peptide is synthesized (i.e. by protein translation at the ribosome) and then recognized by a signal recognition particle.
Smooth Endoplasmic Reticulum
The sER is seen connected to the nuclear envelope, and consists of tubules and vesicles that branch forming a network to increase surface area for the action or storage of key enzymes. It contains the enzyme Glucose-6-phosphatase (which converts glucose-6-phosphate to glucose), a step in gluconeogenesis. A specialized form of sER occurs in muscle cells where calcium ions are stored. It is referred to as sarcoplasmic reticulum. sER is also abundant in hepatocytes to process and detoxify lipophilic drugs.
Other specialized cells with abundant sER are cells of sebaceous glands and gonad cells (e.g. testes and ovaries).
Since rER has ribosomes attached to its surface it is therefore involved in protein synthesis and protein folding, as well as the shuttling of proteins to the Golgi apparatus where the nascent protein undergoes maturation. It synthesizes and secretes serum proteins (such as albumin) in the liver, and hormones (such as insulin) and other substances (such as milk) in the glands. The rER is also involved in the manufacture of lysosomal enzymes (in which a marker, mannose-6-phosphate, is later added in the Golgi apparatus). It is also where certain integral membrane proteins are formed. N-linked glycosylation also occurs here (O-glycosylation occurs in the Golgi).
The sER, on the other hand, does not have ribosomes on its surface. Its functions include synthesis of lipids, metabolism of carbohydrates (i.e. the conversion of glucose-6-phosphate to glucose during glucogenesis via the sER enzyme glucose-6-phosphatase) and calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins. It is also involved in intracellular transport, such as the transport of the products of the rER to other cell parts like Golgi apparatus.
Common biological reactions
Protein synthesis is a process of creating protein molecules. In biological systems, the major steps are amino acid synthesis, transcription and translation. Transcription is a nuclear event wherein mRNA template, encoding the sequence of the protein in the form of a trinucleotide code, is transcribed from DNA to provide a template for translation. Translation is a cytoplasmic process and the site of translation is the ribosomes.
There, the amino acids are added by tRNAs and then are linked together in a specific order as specified in the mRNA transcript. Subsequent to these events are maturation processes, such as proteolysis, post-translational modification, and protein folding. In the early phase of translation, a signal peptide is synthesized (i.e. by protein translation at the ribosome). The signal is an indication that the protein is for further processing in the ER.
When this signal is recognized by a signal recognition particle the ribosome translating the protein docks to the endoplasmic reticulum via the translocon. The ribosome, then, returns back to the translation of the protein. The chain continues to grow as the mRNA transcript is translated through the docked ribosome. The chain eventually makes its way into the ER through the translocon that spans across the ER membranes.
The signal peptide is removed by a signal peptidase in the lumen of the ER. The nascent protein is folded in the ER by the chaperone proteins (e.g. ERp29, protein disulfide isomerase, BiP/Grp78, calnexin, etc.). The properly-folded protein is then packed into a transport vesicle to be shuttled to the Golgi apparatus where it would undergo maturation for transport along the cytoskeleton to other cytoplasmic organelles like lysosomes and peroxisomes or for secretion out of the cell.
Some of the proteins synthesized inside the ER will be retained, such as those that become part of the ER membrane. Those that are retained in the ER have a retention motif, e.g. KDEL (for proteins retained in the ER lumen) and KKXX (for transmembrane proteins in the ER membrane).
An unfolded or misfolded protein triggers an endoplasmic reticulum stress response. This happens when certain disturbances occur, such as disturbances in the redox regulation, calcium regulation, viral infection, and glucose deprivation. A distinctive feature of a misfolded protein is the lack of glucose residues, which are attached via N-linked glycosylation. The initial response is for glycosylation by the enzyme UGGT (UDP-glucose:glycoprotein glucosyltransferase). A heat shock protein glucose regulate protein 78 may bind to the hydrophobic residues of the misfolded protein to prevent its transit. If protein misfolding continues, the protein is headed towards degradation to prevent it from aggregating with other misfolded proteins. By endoplasmic reticulum-association degradation (ERAD), the ERAD chaperone shuttles the misfolded protein to the cytosol for degradation by cytosolic proteasomes (via the ubiquitin-proteasome pathway). If these measures fail to restore the normal function of the cell within a certain period of time, the next response is geared towards apoptosis.
sER is the major site of lipid synthesis, particularly at the membrane contact sites (MCS). MCS are areas where ER membranes make close contact with other cytoplasmic organelles, such as Golgi, mitochondria, lysosomes, peroxisomes, endosomes, chloroplasts, and plasma membrane, and allow the transfer of substances. In particular, contact sites between ER and mitochondria allow the synthesis of phospholipids.
- from the Greek endon, meaning within, plasma, meaning anything formed or moulded, and Latin reticulum, meaning a small net”
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