Proteasome

Proteasome Concept

The term Proteasome refers to a protein complex present in the cell cytosol that has proteolytic activity. It is in this complex where proteins are degraded.

Cell and Proteasome

During its life cycle, the cell needs to express innumerable proteins, since they are responsible for its most varied mechanisms and functions. Proteins are synthesized when they are needed, however, some are produced with errors in the sequence or in the three-dimensional structure and their function is not correct. In these cases, the cell has a specific mechanism to eliminate these proteins: it marks them with ubiquitin and destroys them in the proteasome.

However, it is not just the abnormal proteins that have to be destroyed from the cell. Each protein has a characteristic half-life that influences the duration of its function. Let’s look at the following example: the cell needs a particular protein to prevent passage from one phase of the cell cycle to another (in the case of poorly duplicated or replicated DNA) and, through internal signals, synthesizes that protein; when DNA is correctly duplicated or replicated, the cell may already advance in the cell cycle; therefore, the protein that prevents the advancement of the cycle has to be destroyed. This protein will have a short half-life and needs to be eliminated quickly so that the cell can divide and give rise to offspring. In these situations, the destruction of the protein is also performed with selection to the proteasome.

Proteasome Features

The proteasome is a protein complex with a tube-like morphology. It consists of two 19S subunits at the ends, and a 20S subunit located between the two subunits 19S (Figure 1). The latter function as a gate that only allows the entry of proteins labeled with ubiquitin and are composed of proteins that hydrolyze ATP, unrolling the proteins as they cross the complex. The 20S subunit is composed, among other proteins, of proteases whose active site is directed towards the interior of the tube. It is through the action of proteases that the proteins are cleaved into small peptides.

Diseases related to the accumulation of proteins with incorrect three-dimensional conformation are known. Denoting the importance of this mechanism of protein destruction, proteasomes are present in large amounts in the cell’s cytosol.

Figure 1 - Demonstrative image of the three-dimensional structure of the proteosome. It is possible to observe the 19S subunits at the extremities, with equal composition (blue, green and brown), and the 20S subunit in a central position (in red).

Figure 1 – A demonstrative image of the three-dimensional structure of the proteasome. It is possible to observe the 19S subunits at the extremities, with equal composition (blue, green and brown), and the 20S subunit in a central position (in red).

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References:

Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.

Berg J.M., Tymoczko J.L., Stryer L. (2002). Biochemistry (5th edition). W. H. Freeman, New York.

Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).

Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.

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