Polysome characteristics, types and functions

1789
Anthony Golden

A polysome it is a group of ribosomes recruited for the translation of the same messenger RNA (mRNA). The structure is better known as the polyribosome, or the less common ergosome..

Polysomes allow the increased production of proteins from those messengers that are subjected to simultaneous translation by several ribosomes. Polysomes also participate in co-translational folding processes and in the acquisition of quaternary structures by newly synthesized proteins..

Bacterial polyribosomes. CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Polysomes, together with so-called P bodies and stress granules, control the fate and function of messengers in eukaryotic cells.. 

Polysomes have been observed in both prokaryotic and eukaryotic cells. This means that this type of macromolecular formation has a long history in the cellular world. A polysome can be made up of at least two ribosomes on the same messenger, but generally there are more than two.

In at least one mammalian cell, up to 10,000,000 ribosomes can exist. Many have been observed to be free, but a large portion is associated in the well-known polysomes.

Article index

  • 1 General characteristics
  • 2 Structure of eukaryotic polysomes
  • 3 Types of polysomes and their functions
    • 3.1 Free polysomes
    • 3.2 Polysomes associated with the endoplasmic reticulum (ER)
    • 3.3 Polysomes associated with the cytoskeleton
  • 4 Regulation of post-transcriptional gene silencing
  • 5 References

General characteristics

The ribosomes of all living things consist of two subunits: the small subunit and the large subunit. The small subunit of ribosomes is responsible for reading the messenger RNA.

The large subunit is responsible for the linear addition of amino acids to the nascent peptide. An active translational unit is one in which an mRNA has been able to recruit and allow the assembly of the ribosome. After that, the reading of triplets in the messenger and the interaction with the corresponding loaded tRNA proceeds sequentially..

Ribosomes are the building blocks of polysomes. In fact, both ways of translating a messenger can coexist in the same cell. If all the components that make up the translational machinery of the cell are purified, we would find four main fractions:

  • The first would be formed by the mRNAs associated with the proteins with which the messenger ribonucleoproteins are formed. That is, the lone messengers.
  • The second, by the ribosomal subunits, which being separated still do not translate into any messenger
  • The third would be that of monosomes. That is, the "free" ribosomes associated with some mRNA.
  • Finally, the heaviest fraction would be that of the polysomes. This is the one that actually carries out most of the translation process

Structure of eukaryotic polysomes

In eukaryotic cells, mRNAs are exported from the nucleus as messenger ribonucleoproteins. That is, the messenger is coupled with several proteins that will determine its export, mobilization and translation.. 

Among them, there are several that interact with the PABP protein attached to the polyA 3 'tail of the messenger. Others, such as the CBP20 / CBP80 complex, will bind to the 5 'hood of the mRNA.

Release of the CBP20 / CBP80 complex and recruitment of ribosomal subunits on the 5 'hood define the formation of the ribosome. 

Translation begins and new ribosomes are assembled on the 5 'hood. This happens for a limited number of times that depends on each messenger and the type of polysome in question..

Following this step, translation elongation factors associated with the cap at the 5 'end interact with the PABP protein bound to the 3' end of the mRNA. A circle is thus formed defined by the union of the non-translatable regions of the messenger. Thus, as many ribosomes are recruited as the length of the messenger, and other factors, allow.

Bound ends in the circular structure of eukaryotic polysomes. Fdardel [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Other polysomes can adopt a linear double row configuration, or a spiral configuration with four ribosomes per turn. The circular shape has been most strongly associated with free polysomes.

Types of polysomes and their functions

Polysomes are formed on active translational units (initially monosomes) with the sequential addition of other ribosomes on the same mRNA.

Depending on their subcellular location, we find three different types of polysomes, each having its own particular functions..

Free polysomes

They are found free in the cytoplasm, without apparent associations with other structures. These polysomes translate the mRNAs that code for cytosolic proteins.

Endoplasmic reticulum (ER) associated polysomes

As the nuclear envelope is an extension of the endoplasmic reticulum, this type of polysome can also be associated with the outer nuclear envelope.

In these polysomes, the mRNAs that code for two important groups of proteins are translated. Some, which are a structural part of the endoplasmic reticulum or the Golgi complex. Others, which must be post-translationally modified and / or relocated intracellularly by these organelles.

Cytoskeletal-associated polysomes

Cytoskeletal-associated polysomes translate proteins from mRNAs that are asymmetrically concentrated in certain subcellular compartments.

That is, upon leaving the nucleus, some messenger ribonucleoproteins are mobilized to the site where the product they encode is required. This mobilization is carried out by the cytoskeleton with the participation of proteins that bind to the polyA tail of the mRNA..

In other words, the cytoskeleton distributes the messengers by destination. That fate is indicated by the function of the protein and by the site where it must reside or act..

Regulation of post-transcriptional gene silencing

Even if an mRNA is transcribed, that does not necessarily mean that it must be translated. If this mRNA is specifically degraded in the cell cytoplasm, the expression of its gene is said to be post-transcriptionally regulated..

There are many ways to achieve this, and one of them is through the action of the so-called MIR genes. The end product of the transcription of a MIR gene is a microRNA (miRNA).

These are complementary or partially complementary to other messengers whose translation they regulate (post-transcriptional silencing). Silencing can also involve specific degradation of a particular messenger.

Everything related to translation, its compartmentalization, regulation, and post-transcriptional gene silencing is controlled by polysomes..

To do this, they interact with other molecular macrostructures of the cell known as P bodies and stress granules. These three bodies, mRNAs and microRNAs, thus define the proteome present in a cell at a given time..

References

  1. Afonina, Z. A., Shirokov, V. A. (2018) Three-dimensional organization of polyribosomes - A modern approach. Biochemistry (Moscow), 83: S48-S55.
  2. Akgül, B., Erdoğan, I. (2018) Intracytoplasmic re-localization of miRISC complexes. Frontiers in Genetics, doi: 10.3389 / fgene.2018.00403
  3. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., Walters, P. (2014) Molecular biology of the cell, 6th Edition. Garland Science, Taylor & Francis Group. Abingdon on Thames, United Kingdom.
  4. Chantarachot, T., Bailey-Serres, J. (2018) Polysomes, stress granules, and processing bodies: a dynamic triumvirate controlling cytoplasmic mRNA fate and function. Plant Physiology, 176: 254-269.
  5. Emmott, E., Jovanovic, M., Slavov, N. (2018) Ribosome stoichiometry: from form to function. Trends in Biochemical Sciences, doi: 10.1016 / j.tibs.2018.10.009.
  6. Wells, J. N., Bergendahl, L. T., Marsh, J. A. (2015) Co-translational assembly of protein complexes. Biohemical Society Transactions, 43: 1221-1226.

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