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 signal recognition particle (SRP) is an abundant, cytosolic, universally conserved ribonucleoprotein that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes.
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 hetrotrimeric Sec61 being the channel. In prokaryotes, the homologous channel complex is known as SecYEG.
Calnexin (CNX) is a 67kDa integral protein of the endoplasmic reticulum (ER). It consists of a large N-terminal calcium-binding lumenal domain, a single transmembrane helix and a short, acidic cytoplasmic tail.
Signal recognition particle (SRP) receptor, also called the docking protein, is a dimer composed of 2 different subunits that are associated exclusively with the rough ER in mammalian cells. Its main function is to identify the SRP units. SRP is a molecule that helps the ribosome-mRNA-polypeptide complexes to settle down on the membrane of the endoplasmic reticulum.
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.
Glucosidase 2 subunit beta is an enzyme that in humans is encoded by the PRKCSH gene.
Homocysteine-responsive endoplasmic reticulum-resident ubiquitin-like domain member 1 protein is a protein that in humans is encoded by the HERPUD1 gene.
Protein transport protein Sec61 subunit beta is a protein that in humans is encoded by the SEC61B gene.
Selenoprotein S, also known as SELS, is a human gene.
Dolichyl-diphosphooligosaccharide—protein glycosyltransferase subunit 1 is an enzyme that in humans is encoded by the RPN1 gene.
Translocon-associated protein subunit alpha is a protein that in humans is encoded by the SSR1 gene.
Ribosome-binding protein 1, also referred to as p180, is a protein that in humans is encoded by the RRBP1 gene.
Protein transport protein Sec61 subunit alpha isoform 1 is a protein that in humans is encoded by the SEC61A1 gene.
Endoplasmic reticulum resident protein 44 (ERp44) also known as thioredoxin domain-containing protein 4 (TXNDC4) is a protein that in humans is encoded by the ERP44 gene.
Translocon-associated protein subunit delta is a protein that in humans is encoded by the SSR4 gene.
Signal recognition particle receptor subunit beta is a protein that in humans is encoded by the SRPRB gene.
Vesicular integral-membrane protein VIP36 is a protein that in humans is encoded by the LMAN2 gene.
SRP receptor alpha subunit is a protein that in humans is encoded by the SRPRA gene.
