Cytoskeleton: characteristics, structure, and functions

Characteristics and functions

The cell is equipped with a tridimensional structure, called the cytoskeleton. It has many functions such as providing support, guaranteeing mobility to the cell and the vesicles that “travel” within it, and also it is involved in the reproduction andin other basic cell functions. The architecture complexity of the cytoskeleton is based on three types of polymers, that weave together to complete different functions, and by various additional proteins. In fact, the cytoskeleton isn’t a static structure, but a very dynamic one.

Suffice it is to say that the muscle contraction is due precisely to biochemical phenomena in which the cytoskeleton is involved. Another example of the importance of the cytoskeleton is the transport of vesicles containing neurotransmitters in the cells of the nervous system. The cytoskeleton makes up also cilia and flagella, the motility parts of many bacteria, and the various railways on which the vesicles travel, bringing proteic materials maturing in the Golgi apparatus.

Structure of the cytoskeleton

The cytoskeleton is composed of different filaments of various diameters that weave together, to form tridimensional structures throughout the cell. The three main components of the cytoskeleton are microfilaments (diameter of about 6 nm), intermediate filaments (diameter of approximately 8-12nm), and microtubules (diameter of 25nm). These are polymers of various sizes characterized precisely by a protein nature.

In particular, microtubules and microfilaments represent the most dynamic part of the cytoskeleton (fast turnover structures), while the intermediate filaments are relatively stable elements.

Microfilaments

Microfilaments are the components of the cytoskeleton with smallest diameter, and have a variable length. They are polymers of G-actin, a globular protein (Fig.1). Every microfilament is constituted by two lines of globular monomers, twisted to form a double helix (this structure is called “wire of pearls”). Microfilament is a continuously renewing structure: it assembles and disintegrates assisted by protein factors, to which has been attributed a “nucleating” function. The globular actin is highly involved in the contractility of the muscle cell.

Representation of a globular actin microfilament cytoskeleton
Figure 1 – Representation of a globular actin microfilament [Bibliography: Wikimedia commons]

Intermediate filaments

The dimension of these filaments is between the microtubules and the microfilaments. In the epithelial cell, the intermediate filaments represent as much as 30% of the total proteins. They are chemically stable proteins, so the solubilization of the polypeptides that makes them up is possible only via a prolonged extraction using a solution with high denaturing power, andthey sustain and give the mechanic resistance to the cell. Strong and soft keratins, desmines, and lamins are the classes of proteins most present in intermediate filaments, based on the tissue. They have ancillary proteins, called iFAP (proteins associate with the intermediate filaments), that modify their structure when needed.

Microtubules

Those structures are empty cylindrical tubules. The wall that delimits this cavity is made up of 13 protofilaments of tubulin. These filaments are also associated with proteins that confer functional capabilities, called MAP (Microtubule Associated Proteins). In the cell, the microtubules originate from a focal point called microtubular organization center. A known example is the centrosome, a complex from which the microtubules of the aster irradiate during cell reproduction, and is a very dynamic structure. Also the flagellum, a mobility arrangement present in bacteria, flagellated protozoa and various invertebrates, is made up by microtubules, as well as vibratile cilia of the tracheal cells.

Microtubules in a cell cytoskeleton
Figure 2 – Microtubules in a cell [Bibliography: Jeffrey81, Wikimedia Commons]
Tetrahymena thermophila, ciliate organism
Figure 3 – Tetrahymena thermophila, ciliate organism [Bibliography:  Dr. Muthugapatti Kandasamy, Director of the Biomedical Microscopy Core. University of Georgia]

Original Article: “Il citoscheletro” – Aprile 10, 2021 di Silvia De Giorgi

Bibliography

  • Biologia della cellula, Colombo R., Olmo E. (link)
  • Immagine in evidenza: Glyn Nelson, via Flickr

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