Roughened endoplasmic reticulum: definition, characteristics and functions

Roughened endoplasmic reticulum: definition

The endoplasmic reticulum is a cellular organ formed by interconnected membranes. These membranes continue with those in the center of the cell, the cell nucleus.

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

In this article we will talk about the endoplasmic rugged reticulum , what are its parts and its functions.

What is the rugged endoplasmic reticulum?

This organelle, in addition to the rugged endoplasmic reticulum , is also called the granular endoplasmic reticulum, ergastoplasm or rugged endoplasmic reticulum . This organelle can only be found in eukaryotic cells.

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

Chains made of several peptides are introduced into these flattened bags, 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 observed under the microscope.

The main function of this structure is to synthesize proteins, which are destined to different parts of the cell to develop multiple functions, in addition to controlling their structural and functional quality.

Functions

These are the main functions of the rugged endoplasmic reticulum.

1. Protein synthesis

The rugged 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 structural, forming part of other organelles, acting as hormones, enzymes or transporting substances. Thus, the destination of these proteins can be the very interior of the cell where they have been synthesized , forming the cellular layer or going to the exterior of that cell.

Most of the proteins that are part of the cell’s organelles originate from the ribosomes of the endoplasmic reticulum. This synthesis reaches its final phase inside the rugged endoplasmic reticulum.

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

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

The structure formed by the two ribosomal subunits, mRNA, signal peptide and PRS, travels through the cytosol to the wall of the endoplasmic reticulum.

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

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

2. Quality control

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

The process begins when a protein is detected that has been poorly folded when it is synthesized. The enzymes in charge of this phase of the process are the group of glycosyltransferases.

Glycosyltransferase adds glucose to the defective protein , specifically in its oligosaccharide chain. The aim of this is for a chaperone, namely calnexin, to recognise the glucose of this protein and detect it as a malformed protein, thus returning it to its place of origin so that it is well folded.

This process happens several times. If the correction does not occur in this way, the next phase is carried out.

The protein is directed to a part called proteosome, where it will be degraded . This is where multiple types of enzymes work to 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 that which is synthesized that is not useful or may even prove to be toxic to the cell fulfills a very important hygienic function.

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

Ergastoplasm varieties

Depending on the cell in which it is found, this organelle has different structural characteristics, and it may also be given a different name.

In the secretory cells, the rough endoplasmic reticulum manifests itself in the form of numerous chains or sacs arranged in parallel and not very far apart from each other , sufficiently so that the vesicles with which substances are synthesized can be formed.

In the nervous system, this organelle is called the Nissl body , appearing in the form of widely separated cisterns with many free ribosomes by the cytosol. Some neurons, despite having this organelle, hardly 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.

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