Endoplasmic Reticulum

General presentation

The endoplasmic reticulum is a structure which one meets only at the Eucaryotes; it is always absent at the Procaryotes (archeobacteries and eubacteries).

RE is a under-compartmentation of the cell. It is composed of a membrane (of composition different from the plasmic Membrane) and of a light. Its functions is explained by its capacity to delimit a volume of the remainder of the Cytoplasme.

It constitutes an essential component of the membrane network interns cells Eucaryotes, in continuity with the nuclear Enveloppe and in relation to the other compartments, in particular the blisters of the Appareil of Golgi.

Structure

RE consists of a wide membranous network. The membrane separates the light from the reticulum of the Cytosol. Parts of the membrane of the reticulum are in continuity with the external membrane of the core, and the light of RE is in continuity with space intermembranaire core.

Part of RE is covered of Ribosome S which assembles the amino-acid in proteinic chains according to the information come from the core. The rough appearance of these parts to the Electron microscope is worth the qualification of granulous RE to them (REG. or the RER). The parts without ribosomes are called RE smoothes (REL). Ribosomes on the REG. insert directly synthesized protein in the light of RE, where they acquire their configuration before gaining the Appareil of Golgi.

Ribosomes can be separate from/to each other, or be assembled in clusters and connected by a filament of ARN. In this last case they form the Polysome S or Polyribosome S.

The REG. and REL have different functions.

Granulous RE

The REG. assembles and transports the Protéine S intended for the membranes and secretion. A few minutes after the synthesis of proteins, the majority gain the Appareil of Golgi in blisters golgiennes. Within the REG. the proteins can be modified, folded up and their “controlled” quality. The REG. is the site of the translation and folding up at the time of the Synthèse of the proteins.

The REG. carries on its surface a multitude of small balls. Each one of these balls is a Ribosome in full proteinic synthesis, fixed at the external surface of the reticulum. The protein which it synthesizes is injected into the light of the reticulum by a special pore. In the light, the protein will undergo a maturation then it will be sent to the apparatus of golgi which will integrate it into a blister of Exocytose. The granulous endoplasmic reticulum synthesizes membrane proteins even straightforwardly excreted in the extracellular medium. The external nuclear membrane belongs to this system.

Smooth RE

REL intervenes in several metabolic processes. It takes part in the synthesis of Lipides (Phospholipide S membrane, fatty-acids, Stéroïde S…) and plays a role for the beginning of the glycosilation, the detoxification of the cells and the storage of the Calcium. In certain cellular types the REL specialized for the storage of calcium is called Calciosome S or, in the muscular cells, Réticulum sarcoplasmic.

Its functions are very diversified. It can be a question first of all of a zone of granulous reticulum but where for the moment no synthesis is exerted. It is also a zone of transit between the areas of proteinic synthesis and those where they are excreted. But of other places ensure the synthesis of the membrane lipids. These lipids will be integrated into blisters of exocytose which will provide their lipids to the membrane while amalgamating with it.

Another very important function of the endoplasmic reticulum smoothes is of regulator of intracellular calcium. Calcium is indeed a poison for the majority of the metabolic processes, the cell thus contains the minimum of it. However this calcium is used as signal by some of the membrane receivers. As there is very little in the cell of it, it is enough to few ions to increase the concentration in high proportions. It allows inter alia starting the muscular Contraction, the potential of action or the Exocytose and even the fusion of the pronuclei at the time of the Fécondation. The cell must thus maintain a concentration intracellular of calcium very low, while being ensured that there is enough for the calcic signal, and then evacuate the calcium of the signal as quickly as possible to allow the arrival of a new signal, while preventing that the concentration increases too much and reaches the lethal threshold. Certain zones of the smooth reticulum take part in this regulation by constituting a calcium reserve for the signal and by recovering cytoplasmic calcium, then by evacuating the calcium excess about the external middle.

Functions

RE ensures of multiple functions.

Storage and concentration of molecules. It does it by Endocytose, Pinocytose, or starting from substances worked out by the cell. One notes for example accumulation in the Vacuole S of the RE of the Plasmocyte S, of Immunoglobuline S.

Role of detoxification, with the transformation of toxic molecules into atoxic molecules, partly thanks to the P450 cytochrome. That with especially place in the Kidney and the Liver.

Role in the metabolism of calcium. Calcium is stored in RE. The regulation of calcium with Inositol-sorting-phosphate (IP3) in particular, plays a part in the control of the cellular proliferation, the Apoptose, and the cellular Métabolisme.

Secretion of H+ and Cl by the cells bordering of the stomach and of these bowels.

Production of biomembrane: the REG. produces blisters (known as of “transition”), which generate the Appareil of Golgi, this last will produce Vésicules of secretion, at the origin of the Exocytose. The membrane of these blisters will in the final analysis be incorporated in the plasmic Membrane, thus regenerated permanently.

Facilitation of the folding up of plant proteins

The correct folding up of the proteinic chains is ensured by following proteins of RE:

Protein disulfide isomerase
Family of the Hsp70 and Hsp60
Calnexine
Calreticuline
Peptidylpropyl isomerase

Only the correctly folded up proteins are transported REG. towards Golgi.

Transport of proteins

The cytoplasmic proteins intended to be synthesized on polysomes related to the reticulum are marked by a polypeptide sequence called Séquence signal. This sequence is removed when polypeptide arrives at destination. The proteins are transported in blisters which circulate along the Cytosquelette.

As regards proteins of excretion, they will be sent initially towards the CIREG (Intermediate Compartment of the Granular Endoplasmic Reticulum) by blisters COP II. Then, they will be exported in Golgi by blisters COP I. the remainder of work will be carried out in the Appareil of Golgi.

Glycosylation of proteins

The Glycosylation of a protein is the whole of the phenomena which ensure the transformation of a protein into a glycoprotéine.

The synthesized proteins in a traditional way by the cytoribosomes are not glycosylées. This phenomenon relates to only proteins synthesized on the level of RE. There exist two types of Glycosylation: O-glycosylation and N-glycosylation. The NR is most frequent and the Asparagine is the amino-acid of the protein which will be glycosylée. This type of glycosylation begins in RE to finish in Golgi.

One starts to build a single polysaccharide for all the proteins which one then will transfer on the polypeptide chain in growth. This addition takes place during the synthesis of protein, inside the light of the REG.

Synthesis of polysaccharide

The dolichol is an fatty-acid synthesized in the Cytoplasme. It fits in the membrane of RE, and one finds it in the cavity.

Then one one by one adds sugars on the dolichol phosphate: 2 N-acétylglucosamines, 9 Mannose S and 3 Glucose S. All these sugars come from the cytosol by active transport and are in active form related to nucleotides.

Transfer of polysaccharide on protein in growth

Polysaccharide is always fixed on the same amino-acid of protein in growth: the Asparagine (Asn). During the synthesis, proteins hoods GRP intervene in the folding up of neosynthetized proteins. They accompany them until the Appareil by Golgi. In their absence, the proteins cannot be taken along to Golgi, they remain then in RE and are degraded.

Summary of the various types of RE

endoplasmic Réticulum rough (presence of ribosome on the surface, production of protein)
endoplasmic Réticulum smoothes (absence of ribosome on the surface)

Calciosome (strong calcium concentration contains)

Réticulum sarcoplasmic (calciosome of the muscular cells, particular localization compared to the Sarcomère)

Random links: