Rough Endoplasmic Reticulum (RER): Structure, Functions

The endoplasmic reticulum (ER) which consists of ribosomes on its surface is known as the rough endoplasmic reticulum. So it is also called the granular endoplasmic reticulum.

Translocon is the binding site of the ribosome on the rough endoplasmic reticulum.
These ribosomes look like studs and they can distinguish the organelle from the smooth sections of the ER.
Proteins are synthesized from amino acids. It is with the aid ribosome.
Ribosomes consist of four RNA molecules.
The attachment of ribosomes over the surface of RER through two types of glycoproteins. They are:
Riboprotein I (6500 daltons) and Riboprotein II (6400 daltons).
RER consists of more cisternae and fewer tubules and vesicles.
Near the nucleus, it is more abundant where it is connected with its outer membrane.
RER is basophilic.
So, it is also called ergastoplasm by Garnier, 1897.
RER is present in the cells which are involved in the active transport of protein as well as in the synthesis of enzymes. It includes:
- Acinar cells of the pancreas
- Plasma cells
- Goblet cells
- Cells of endocrine glands
In conjunction with the Golgi complex, RER helps in the formation of primary lysosomes.
It involves the synthesis, folding, and modification of proteins.
It includes those cells which need to be transported to different cellular organelles in the cell.
It is involved in the response of the cell to unfolded protein.
It plays a role in the induction of apoptosis.
It is due to the close interaction with mitochondria.
Polysomes are strings of ribosomes that synthesize the proteins.
Morphology also aids in the identification. Its structure is often convoluted and flattened which seems like the sac.
It originates in proximity to the nucleus.
RER has the membrane in association with the outer nuclear membrane.
It forms large membrane sheets which are double.
It is best studied within the secretory cells specialized in these functions.

Rough Endoplasmic Reticulum (RER). Created with BioRender.com

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Structure of Rough Endoplasmic Reticulum (RER)

It consists of three structures. They are:

Cisternae

Its diameter is 40 to 50 µm.
They are long, flattened, sac-like, and unbranched tubules.
In the bundles, they are aligned parallel.
RER usually is present as cisternae, They are found in cells like the pancreas, brain, and notochord where the synthesis usually takes place.

Vesicles

The diameter of the vesicle is 25 to 500 µm.
They are oval-shaped.
They seem like the vacuole which is bound by the membrane.
They are usually found in the cytoplasm in isolated form.
It is found in most cells. As compared to SER they are less abundant.

Tubules

They are branched structures.
It forms the reticular system.
They are usually present in all cells.
Its diameter is 50 to 190 µm.
These structures are usually found in SER.

Functions of Rough Endoplasmic Reticulum (RER)

It is the site for the synthesis of protein.
Polypeptides are synthesized.
Ribosomes synthesize proteins and enzymes.
It then enters through the RER’s channel which is used both for intracellular use and extracellular transport.
Some proteins are to be secreted from the cell or they need to be exported.
Some proteins are essential in the synthesis of cellular membranes.
Such proteins are assumed to be synthesized by the RER.
In the nascent protein at the side of COOH, about 40 amino acids are present. They are protected inside the tunnel of free or bound ribosomes.
Similarly, it is found that the lumen of the RER is involved in the protection of the rest of the chain at the NH₂ end.
During the translation, nascent polypeptide chai pass to the cisternae of ER.
When the growing polypeptide chain, reaches the cisternae, it gets trapped in it by folding it into the secondary and tertiary structure.
It provides the surface area for the association of many things.
It includes metabolically active enzymes, amino acids, and ribosomes.
It protects the secretory proteins from the protease enzymes which are present in the cytoplasm.
So they pass in the cisternae of RER instead of passing into the cytoplasm
By losing the ribosomes, it forms the smooth endoplasmic reticulum.
For holding the ribosomes, it consists of ribophorins.
Rough endoplasmic reticulum synthesizes the zymogens of lysosome enzymes.
The RER provides enzyme precursors for the formation of lysosomes with the help of the Golgi complex.

Protein glycosylation

It is the process of the addition of sugar in secretory proteins.
During this process, oligosaccharides get transported in the proteins.
This oligosaccharide is always transferred to the NH2 group on the side chain of an asparagines residue of the protein.
So, this oligosaccharide is said as asparagines-linked or the N-linked.
This transfer is aided by the enzyme glycosyltransferase. It is a membrane-bound enzyme. On the luminal surface of the ER membrane, its active site is exposed.
In the Endoplasmic membrane, the precursor oligosaccharide is held by dolicol.
Dolicol is the carrier which is a special lipid molecule.

Comparison of Rough Endoplasmic Reticulum (RER) with Smooth Endoplasmic Reticulum (SER)

Ribosomes: Present in rough ER and absent in smooth ER.
Composition: Rough ER is made up of more cisternae and few tubules. Smooth ER is made up of more tubules and vesicles.
Presence: Rough ER is present in protein and enzyme-forming cells like Pancreatic cells, new cells, etc. Smooth ER is found mainly in lipid forming cells: adipocytes, interstitial cells, adrenal cortical cells, etc.
Ribophorins: Present in rough ER and absent in smooth ER.
Formation: Rough ER is believed to be formed from the outer nuclear membrane. Smooth ER is developed from rough ER by loss of ribosomes.
Function: Rough ER helps information and transportation of proteins and enzymes. Smooth ER helps in the formation of lipid, glycogen, and steroids.

References

Shakya, M., Mehata, K.R., Gautam, M.K., Pokhrel, K.R., and Khanal, K. (2020 ) “ Principles of Biology”, Asmita Books Publisher and Distributors Ltd, Bhotahity, Nepal
Verma, P. S., and Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S.Chand and Company Ltd.
https://biologydictionary.net/rough-endoplasmic-reticulum/
https://www.britannica.com/science/cell-biology/The-endoplasmic-reticulum#ref313734
https://en.wikipedia.org/wiki/Endoplasmic_reticulum