Eukaryotic cytoskeletons

Microfilaments, microtubules, and intermediate filaments are the three main types of cytoskeletal filaments found in eukaryotic cells: microfilaments, microtubules, and intermediate filaments. Neurofilaments are intermediate filaments found in neurons that act as a link between two different types of proteins. In each case, the polymerization of a specific type of protein subunit results in the formation of a type with its own characteristic shape and intracellular distribution. Known as microfilaments, they are polymers made up of the protein actin that have a diameter of 7nm. Microtubules are made up of tubulin and have a diameter of 25 nanometers. Intermediate filaments are composed of a variety of proteins that vary depending on the type of cell in which they are found; they are typically 8-12 nm in diameter. Intermediate filaments are found in a variety of cell types. In addition to providing structure and shape to the cell, the cytoskeleton also contributes to the level of macromolecular crowding in this compartment by excluding macromolecules from portions of the cytosol. The cytoskeletal elements of cells have extensive and intimate interactions with their membranes.

Microfilaments

Microfilaments, also known as actin filaments, are composed of linear polymers of G-actin proteins that generate force when the growing (plus) end of the filament pushes against a barrier, such as the cell membrane. Microfilaments, also known as actin filaments, are found in the cytoplasm of all cells. They also serve as tracks for the movement of myosin molecules, which bind to the microfilament and “walk” along it when they attach to it. Actin is the primary structural component or protein of microfilaments in general. The G-actin monomer and polymer combine to form a polymer, which continues to form the microfilament as it grows in length (actin filament). Afterwards, these subunits come together to form two chains that intertwine to form what are known as F-actin chains.  In both muscle and most non-muscle cell types, contractile forces are generated by myosin motoring along F-actin filaments, which is a process known as actomyosin fibres generating contractile forces. Rho family of small GTP-binding proteins regulates actin structure, including Rho itself for contractile acto-myosin filaments (“stress fibres”), Rac for lamellipodia, and Cdc42 for filopodia. Rho family members include Rho itself, Rac, and Cdc42.

The following are some of the functions:

Muscle contraction is defined as

• Movement of the cells
• Intracellular transport/trafficking is a term used to describe the movement of materials within cells.
• The preservation of the eukaryotic cell shape
• Cytokinesis
• Cytoplasmic streaming is a term that refers to the movement of cytoplasm in the body.

Intermediate filaments 

Intermediate filaments are found in the cytoskeleton of many eukaryotic cells and are responsible for their organisation. These filaments, which have an average diameter of 10 nanometers, are more stable (strongly bound) than microfilaments, which are heterogeneous constituents of the cytoskeleton, and they are found in higher concentrations. They, like actin filaments, play a role in the maintenance of cell shape by distributing tension throughout the cell (microtubules, by contrast, resist compression but can also bear tension during mitosis and during the positioning of the centrosome). Internal tridimensional structure of the cell is organised by intermediate filaments, which act as structural components of the nuclear lamina and anchor organelles to the cell’s internal tridimensional structure. They are also involved in the formation of some cell-cell and cell-matrix junctions. Nuclear lamina can be found in all animals and all tissues, including the brain. Some animals, such as the fruit fly, do not have any intermediate filaments in their cytoplasmic vesicles. When cytoplasmic intermediate filaments are expressed in the cytoplasm of animals, these filaments are restricted to specific tissues. In epithelial cells, intermediate filaments of keratin provide protection against the various mechanical stresses that the skin may be subjected to. Organs are also protected against metabolic, oxidative, and chemical stresses as a result of their presence. The use of intermediate filaments to strengthen epithelial cells may help to prevent the onset of apoptosis, or cell death, in epithelial cells by decreasing the likelihood of stress. 

Intermediate filaments are most commonly known as the “support system” or “scaffolding” for the cell and nucleus, but they also play a role in a variety of cell functions, including the formation of blood vessels. Together with proteins and desmosomes, intermediate filaments help to form cell-cell connections and anchor cell-matrix junctions, which are essential for communication between cells as well as the performance of other vital functions within a cell. Through these connections, the cell can communicate with other cells in the desmosome, which allows the cell to adjust the structure of the tissue in response to signals from the surrounding environment. Mutations in the IF proteins have been linked to a variety of serious medical issues, including premature ageing, desmin mutations that compromise organ function, Alexander Disease, and muscular dystrophy, among others.

The following are examples of intermediate filaments:

Vimentins are the building blocks of life. In mesenchymal cells, intermediate filaments of the vimentin protein are commonly found.

Keratin is the material used to make this product. Keratin is found in abundance in epithelial cells in general.

Neurofilaments are the connective tissue of neural cells.

This laminated structure provides structural support for the nuclear envelope.

Muscle cells’ structural and mechanical support is greatly aided by desmin, which is found in abundance in them.

Microtubules

Microtubules are hollow cylinders with a diameter of approximately 23 nm (lumen diameter of approximately 15 nm), with the majority of microtubules consisting of 13 protofilaments, which are in turn polymers of alpha and beta tubulin. They have a very dynamic behaviour, bind GTP and cause polymerization to take place. The centrosome is commonly responsible for organising them.

Centrioles and flagella are formed by nine triplet sets (star-shaped), and cilia and flagella are formed by nine doublet sets (wheel-shaped) oriented around two additional microtubules (wheel-shaped). It is commonly referred to as a “9+2” arrangement because each doublet is connected to another by the protein dynein in this arrangement. In addition to being structural components of the cell, flagella and cilia are both maintained by microtubules, and therefore both can be considered components of the cytoskeleton. Cilia are two types: motile cilia and non-motile cilia. Cilia are short and more numerous than flagella, which are longer and less numerous. When compared to non-motile cilia, the motile cilia have a rhythmic waving or beating motion. The motile cilia are responsible for receiving sensory information for the cell, processing signals from other cells or the fluids surrounding it. Cilia and flagella are also controlled by microtubules, which beat (move) in response to the movement of the cilia. Aside from that, the dynein arms that are attached to the microtubules serve as molecular motors. Because microtubules slide past one another, the motion of cilia and flagella is created. This requires ATP to be done successfully. They are important in the following areas:

intracellular transport is a term used to describe the movement of materials within cells (associated with dyneins and kinesins, they transport organelles like mitochondria or vesicles).

Microtubules are arranged in the “9 + 2” configuration in this cross-sectional diagram through the cilium in the cell body.

Axoneme is a structure present in cilia and flagella.

The mitotic spindle is a structural component of the cell cycle.

In plants, the synthesis of the cell wall is called cell wall synthesis.

Furthermore, Stuart Hameroff and Roger Penrose have proposed that microtubules play a role in consciousness, in addition to the roles previously described.

Conclusion

Microfilaments, microtubules, and intermediate filaments are the three main types of cytoskeletal filaments found in eukaryotic cells: microfilaments, microtubules, and intermediate filaments. Neurofilaments are intermediate filaments found in neurons that act as a link between two different types of proteins. In each case, the polymerization of a specific type of protein subunit results in the formation of a type with its own characteristic shape and intracellular distribution. .Microtubules, microfilaments, and intermediate filaments are the structural components of the cytoskeleton of a cell. These structures give the cell its shape and assist in the organisation of the cell’s constituent parts. Aside from that, they serve as a foundation for cell movement and division.