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The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae, and tubular structures in the SER. The membranes of the ER are continuous with the outer nuclear membrane. The endoplasmic reticulum is not found in red blood cells, or spermatozoa.
The endomembrane system is composed of the different membranes (endomembranes) that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane among others. The system is defined more accurately as the set of membranes that forms a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport. Importantly, the endomembrane system does not include the membranes of plastids or mitochondria, but might have evolved partially from the actions of the latter.
The translocon is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transports nascent polypeptides with a targeting signal sequence into the interior space of the endoplasmic reticulum (ER) from the cytosol. This translocation process requires the protein to cross a hydrophobic lipid bilayer. The same complex is also used to integrate nascent proteins into the membrane itself. In prokaryotes, a similar protein complex transports polypeptides across the (inner) plasma membrane or integrates membrane proteins. In either case, the protein complex are formed from Sec proteins, with the heterotrimeric Sec61 being the channel. In prokaryotes, the homologous channel complex is known as SecYEG.
COPI is a coatomer, a protein complex that coats vesicles transporting proteins from the cis end of the Golgi complex back to the rough endoplasmic reticulum (ER), where they were originally synthesized, and between Golgi compartments. This type of transport is retrograde transport, in contrast to the anterograde transport associated with the COPII protein. The name "COPI" refers to the specific coat protein complex that initiates the budding process on the cis-Golgi membrane. The coat consists of large protein subcomplexes that are made of seven different protein subunits, namely α, β, β', γ, δ, ε and ζ.
ER retention refers to proteins that are retained in the endoplasmic reticulum, or ER, after folding; these are known as ER resident proteins.
Oligosaccharyltransferase or OST (EC 2.4.1.119) is a membrane protein complex that transfers a 14-sugar oligosaccharide from dolichol to nascent protein. It is a type of glycosyltransferase. The sugar Glc3Man9GlcNAc2 (where Glc=Glucose, Man=Mannose, and GlcNAc=N-acetylglucosamine) is attached to an asparagine (Asn) residue in the sequence Asn-X-Ser or Asn-X-Thr where X is any amino acid except proline. This sequence is called a glycosylation sequon. The reaction catalyzed by OST is the central step in the N-linked glycosylation pathway.
Aminopeptidases are enzymes that catalyze the cleavage of amino acids from the amino terminus (N-terminus) of proteins or peptides (exopeptidases). They are widely distributed throughout the animal and plant kingdoms and are found in many subcellular organelles, in cytosol, and as membrane components. Aminopeptidases are used in essential cellular functions. Many, but not all, of these peptidases are zinc metalloenzymes.
The coatomer is a protein complex that coats membrane-bound transport vesicles. Two types of coatomers are known:
TAP-associated glycoprotein, also known as tapasin or TAPBP, is a protein that in humans is encoded by the TAPBP gene.
Nuclear pore glycoprotein-210 (gp210) is an essential trafficking regulator in the eukaryotic nuclear pore complex. Gp-210 anchors the pore complex to the nuclear membrane. and protein tagging reveals its primarily located on the luminal side of double layer membrane at the pore. A single polypeptide motif of gp210 is responsible for sorting to nuclear membrane, and indicate the carboxyl tail of the protein is oriented toward the cytoplasmic side of the membrane.
Ribophorins are dome shaped transmembrane glycoproteins which are located in the membrane of the rough endoplasmic reticulum, but are absent in the membrane of the smooth endoplasmic reticulum. There are two types of ribophorines: ribophorin I and II. These act in the protein complex oligosaccharyltransferase (OST) as two different subunits of the named complex. Ribophorin I and II are only present in eukaryote cells.
Minor histocompatibility antigen H13 is a protein that in humans is encoded by the HM13 gene.
KDEL (Lys-Asp-Glu-Leu) endoplasmic reticulum protein retention receptor 1, also known as KDELR1, is a protein which in humans is encoded by the KDELR1 gene.
Protein transport protein Sec61 subunit alpha isoform 1 is a protein that in humans is encoded by the SEC61A1 gene.
ER lumen protein retaining receptor 2 is a protein that in humans is encoded by the KDELR2 gene.
ER lumen protein retaining receptor 3 is a protein that in humans is encoded by the KDELR3 gene.
KDEL is a target peptide sequence in mammals and plants located on the C-terminal end of the amino acid structure of a protein. The KDEL sequence prevents a protein from being secreted from the endoplasmic reticulum (ER) and facilitates its return if it is accidentally exported.
A target peptide is a short peptide chain that directs the transport of a protein to a specific region in the cell, including the nucleus, mitochondria, endoplasmic reticulum (ER), chloroplast, apoplast, peroxisome and plasma membrane. Some target peptides are cleaved from the protein by signal peptidases after the proteins are transported.
KKXX and for some proteins XKXX is a target peptide motif located in the C terminus in the amino acid structure of a protein responsible for retrieval of endoplasmic reticulum (ER) membrane proteins to and from the Golgi apparatus. These ER membrane proteins are transmembrane proteins that are then embedded into the ER membrane after transport from the Golgi. This motif is exclusively cytoplasmic and interacts with the COPI protein complex to target the ER from the cis end of the Golgi apparatus by retrograde transport.
Sir Hugh Reginald Brentnall Pelham, is a cell biologist who has contributed to our understanding of the body's response to rises in temperature through the synthesis of heat shock proteins. He served as director of the Medical Research Council (MRC) Laboratory of Molecular Biology (LMB) between 2006 and 2018.
References
- ↑ Denecke J.; De Rycke R.; Botterman J. (Jun 1992). "Plant and mammalian sorting signals for protein retention in the endoplasmic reticulum contain a conserved epitope". EMBO Journal. 11 (6): 2345–2355. doi:10.1002/j.1460-2075.1992.tb05294.x. PMC 556702 . PMID 1376250.
- ↑ Dean N.; Pelham HR. (Aug 1990). "Recycling of proteins from the Golgi compartment to the ER in yeast". The Journal of Cell Biology. 111 (2): 369–377. doi:10.1083/jcb.111.2.369. PMC 2116185 . PMID 2199456.
- ↑ Napier R.M.; Fowke L.C.; Hawes C.; Lewis M.; Pelham H.R. (Jun 1992). "Immunological evidence that plants use both HDEL and KDEL for targeting proteins to the endoplasmic reticulum" . Journal of Cell Science. 102 (2): 261–271. doi:10.1242/jcs.102.2.261. PMID 1383243.
- ↑ Napier R.M.; Fowke L.C.; Hawes C.; Lewis M.; Pelham H.R. (Jun 1992). "Immunological evidence that plants use both HDEL and KDEL for targeting proteins to the endoplasmic reticulum" . Journal of Cell Science. 102 (2): 261–271. doi:10.1242/jcs.102.2.261. PMID 1383243.
- ↑ Denecke J.; De Rycke R.; Botterman J. (Jun 1992). "Plant and mammalian sorting signals for protein retention in the endoplasmic reticulum contain a conserved epitope". EMBO Journal. 11 (6): 2345–2355. doi:10.1002/j.1460-2075.1992.tb05294.x. PMC 556702 . PMID 1376250.