Rough endoplasmic reticulum: definition, characteristics and functions

The endoplasmic reticulum is a cellular organ made up of interconnected membranes. These membranes are continuous with those of the center of the cell, the cell nucleus.

There are two types of endoplasmic reticulum: one, called rough, whose membranes form flattened cisterns and with associated ribosomes, and the other called smooth, which is organized with membranes forming tubules without ribosomes.

In this article let's talk about rough endoplasmic reticulum, what are its parts and its functions.

• Related article: "Major cell types of the human body

What is rough endoplasmic reticulum?

This organelle, in addition to rough endoplasmic reticulum, receives other names: granular endoplasmic reticulum, ergastoplasma or rough endoplasmic reticulum. This organelle can only be found in eukaryotic cells.

Structurally, it is characterized by being formed by a series of channels, flattened sacs and cisterns, which are distributed throughout the middle of the cell, the cytoplasm.

Chains made of various peptides are introduced into these flattened sacs, with which complex proteins will be formed. These same proteins travel to other parts of the cell, such as the Golgi apparatus and the smooth endoplasmic reticulum.

Around the sacs that form this organelle are numerous ribosomes associated with them. These structures are vesicles that can contain proteins and other substances. These ribosomes are what give it a rough appearance when viewed under the microscope.

This structure's main function is to synthesize proteins, which are destined for different parts of the cell to perform multiple functions, in addition to controlling their structural quality and functional.

Features

These are the main functions of the rough endoplasmic reticulum.

1. Protein synthesis

The rough endoplasmic reticulum has a function that is of vital importance for the survival of the organism: to synthesize proteins.

These proteins can perform multiple functions, whether they are structural, forming part of other organelles, acting as hormones, enzymes or transport substances. So that, the destination of these proteins can be inside the cell where they have been synthesized, forming the cell layer or going to the outside of that cell.

Most of the proteins that are part of the cell's organelles have their origin in the ribosomes of the endoplasmic reticulum. This synthesis reaches its final phase within the rough endoplasmic reticulum.

The process begins when messenger ribonucleic acid (mRNA) is attached to a small ribosomal unit and then to a large one. This is how the process called translation begins.

The first thing that is translated is the nucleotide sequence, which will synthesize a chain of about 70 amino acids. This chain is called a signal peptide. A molecule called SRP (sequence recognition particule) is responsible for recognizing this signal peptide, slowing down the translation process.

The structure formed by the two ribosomal subunits, mRNA, signal peptide, and SRP travels through the cytosol until it reaches the wall of the rough endoplasmic reticulum.

Through a special protein, called a translocator, a channel is formed in the membrane through which the peptide part of the formed structure passes. The signal peptide binds to the translocator, the rest of the peptide chain is gradually translated and introduced into the reticulum.

An enzyme, called peptidase, breaks the signal peptide from the rest of the amino acid chain, leaving this free chain inside the organelle.

Once the synthesis has been completed, the chain of amino acids acquires a three-dimensional structure, typical of a complete protein, and it folds.

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2. QA

The rough endoplasmic reticulum performs a fundamental function for good organ function. This organelle plays an important role in detecting defective proteins or that may not be useful to the body.

The process begins when a protein is detected that has been misfolded at the time of being synthesized. The enzymes in charge of this phase of the process are the group of glucosyltransferases.

Glycosyltransferase adds glucose to the defective protein, specifically in its oligosaccharide chain. The objective of this is that a chaperone, specifically calnexin, recognizes the glucose of this protein and detect it as a poorly formed protein, so it will return it to its place of origin so that it is well folded.

This process occurs multiple times. In the event that the correction is not made in this way, the next phase is passed.

The protein is directed to a part called the proteasome, where it will be degraded. In this place, multiple types of enzymes work which break down the defective protein into amino acids that can be recycled to form a new, well-folded protein.

This function of quality control and detection of what is synthesized that is not useful or that may even turn out to be toxic to the cell fulfills a very important hygienic function.

Thus, the cell can take care of ensure that well-formed proteins reach the point of maturation where they are functional, while those that are not discarded or recycled.

Ergastoplasma varieties

Depending on the cell in which it is found, this organelle has different structural characteristics, and it is also possible that it receives another name.

In secretory cells, the rough endoplasmic reticulum manifests itself in the form of numerous chains or sacks arranged in parallel and little separated from each other, enough so that the vesicles with which substances are synthesized can be formed.

In the nervous system, this organelle is called Nissl bodies, appearing in the form of widely separated cisterns with many free ribosomes in the cytosol. Some neurons, despite having this organelle, barely synthesize proteins.

Bibliographic references:

• English, A. R., Zurek, N., Voeltz, G. K. (2009). Peripheral ER structure and function. Current opinion in cell biology, 21,: 506-602.
• Daleke D. L. (2007). Phospholipid Flippases. The journal of biological chemistry. 282, 821-825.
• Nixon-Abell J, Obara, C. J., Weig V. A., Li D., Legant W. R., Xu C. S., Pasolli H. A., Harvey K., Hess H. F., Betzig E., Blackstone C., Lippincott-Schwartz3 J. (2016). Increased spatiotemporal resolution reveals highly dynamic dense tubular matrices in the peripheral ER. Science. 354, 3928-2.