Related Research Articles
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.
Protein targeting or protein sorting is the biological mechanism by which proteins are transported to their appropriate destinations within or outside the cell. Proteins can be targeted to the inner space of an organelle, different intracellular membranes, the plasma membrane, or to the exterior of the cell via secretion. Information contained in the protein itself directs this delivery process. Correct sorting is crucial for the cell; errors or dysfunction in sorting have been linked to multiple diseases.
A signal peptide is a short peptide present at the N-terminus of most newly synthesized proteins that are destined toward the secretory pathway. These proteins include those that reside either inside certain organelles, secreted from the cell, or inserted into most cellular membranes. Although most type I membrane-bound proteins have signal peptides, the majority of type II and multi-spanning membrane-bound proteins are targeted to the secretory pathway by their first transmembrane domain, which biochemically resembles a signal sequence except that it is not cleaved. They are a kind of target peptide.
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.
The C-terminus is the end of an amino acid chain, terminated by a free carboxyl group (-COOH). When the protein is translated from messenger RNA, it is created from N-terminus to C-terminus. The convention for writing peptide sequences is to put the C-terminal end on the right and write the sequence from N- to C-terminus.
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amine group is bonded to the carboxylic group of another amino acid, making it a chain. That leaves a free carboxylic group at one end of the peptide, called the C-terminus, and a free amine group on the other end called the N-terminus. By convention, peptide sequences are written N-terminus to C-terminus, left to right (in LTR writing systems). This correlates the translation direction to the text direction, because when a protein is translated from messenger RNA, it is created from the N-terminus to the C-terminus, as amino acids are added to the carboxyl end of the protein.
A secretory protein is any protein, whether it be endocrine or exocrine, which is secreted by a cell. Secretory proteins include many hormones, enzymes, toxins, and antimicrobial peptides. Secretory proteins are synthesized in the endoplasmic reticulum.
P24 protein family is a group of transmembrane proteins that are major components of COPI and COPII-coated vesicles. The family is also known as EMP24/GP25L/p24 family and TMP21-like proteins. The latter naming was after transmembrane emp24 domain-containing protein 10 that was found in the human brain. It was claimed to block the beta-amyloid peptide, which is implicated in the pathogenesis of Alzheimer's disease.
The coatomer is a protein complex that coats membrane-bound transport vesicles. Two types of coatomers are known:
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.
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.
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.
KDEL (Lys-Asp-Glu-Leu) endoplasmic reticulum protein retention receptors (KDELR) are the members of a group of receptor proteins:
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.
Protein FAM214A, also known as protein family with sequence similarity 214, A (FAM214A) is a protein that, in humans, is encoded by the FAM214A gene. FAM214A is a gene with unknown function found at the q21.2-q21.3 locus on Chromosome 15 (human). The protein product of this gene has two conserved domains, one of unknown function (DUF4210) and another one called Chromosome_Seg. Although the function of the FAM214A protein is uncharacterized, both DUF4210 and Chromosome_Seg have been predicted to play a role in chromosome segregation during meiosis.
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.
HDEL is a target peptide sequence in plants and yeasts located on the C-terminal end of the amino acid structure of a protein. The HDEL sequence prevents a protein from being secreted from the endoplasmic reticulum (ER) and facilitates its return if it is accidentally exported.
References
- ↑ Mariano Stornaiuolo; Lavinia V. Lotti; Nica Borgese; Maria-Rosaria Torrisi; Giovanna Mottola; Gianluca Martire; Stefano Bonatti (March 2003). "KDEL and KKXX Retrieval Signals Appended to the Same Reporter Protein Determine Different Trafficking between Endoplasmic Reticulum, Intermediate Compartment, and Golgi Complex". Molecular Biology of the Cell. 14 (3): 889–902. doi:10.1091/mbc.E02-08-0468. PMC 151567 . PMID 12631711.
 | This molecular biology article is a stub. You can help Wikipedia by expanding it. |