Cytoskeleton: Structure and Functions

Cells of any eukaryotic organism are stable structures with fixed shapes that carry out different functions. The ability of the eukaryotic cells to acquire various forms and perform coordinated and directed movements depend on the cytoskeleton.

The cytoskeleton spreads across the cytoplasm and is a complex network of three protein filaments; microtubules, intermediate filaments (or actin filaments), and microfilaments or actin filaments). It is also called cytomusculature because it helps in movements like the crawling of cells on a substratum, muscle contraction, and many changes in the shapes of developing vertebrate embryos. 

The proteins present in the cytoskeleton are tubulin, actin, myosin, tropomyosin, keratins, cimentin, desmin, and lamin. 

Structure of Cytoskeleton

The cytoskeleton has three different components working together. The three components are; microtubules, microfilaments, and intermediate, formation of which occurs by filamentous proteins. The tubulin protein forms the microtubules. Actin, myosin, and tropomyosin form the microfilaments. Likewise, keratins, cimetin, desmin, and lamin form the intermediate filaments.

So, understanding the structure of the cytoskeleton requires learning about the three components. 

Microtubules

The first time observation of microtubules were in the axoplasm of the myelinated nerve fibers by Robertis and Franchi in 1953. They are also called neurotubules. The microtubules are present in almost all eukaryotic cells except human erythrocytes. These occur either freely in the cytoplasm or form a part of centrioles, cilia, and flagella. The brain of vertebrae is the most abundant source of microtubules for biochemical studies. The microtubules occur in the following parts of animal and plant cells:

1. Cilia and flagella
2. Centrioles and basal bodies
3. Nerve processes
4. Mitotic apparatus
5. Cortex of meristematic
6. Elongating cells like during the formation of the lens or spermatogenesis of some insects
7. Axostyle of parasitic flagellates, 
8. Axoneme of Exhinosphaerium,
9. Fiber system of Stentor 

Structure of Microtubules

The microtubules comprise a class of chemically and morphologically related filamentous rods common in plant and animal cells. A microtubule has long, unbranched, hollow tubules with 24-25 nm diameter, several microtubules long, and a 6 nm thick wall that has 13 subunits of protofilaments. The microtubule wall has 13 individual spiraling or liner filamentous structures of 5nm diameter, composed of tubulin.  

The protofilaments have a center-to-center spacing of 4.5 nm. The microtubules have a lumen of 14 nm width. 

Chemically, a protofilament of the microtubule consists of a protein called tubulin. It is an acidic protein with a molecular weight of 55,000 and constant sedimentation of 6S. It has two forms, α-tubulin, and β-tubulin, of about 450 amino acids. The amino acids in both forms are distinct but closely related. Tubulins are in the heterodimers that assemble to form linear protofilaments with the β-tubulin of one dimer in contact with the α-tubulin of the next. The microtubules are polar structures because all 13 protofilaments align parallelly with the same polarity. 

Microfilaments

The microfilaments are the active or motile part of the cytoskeleton made up of 5-7 nm in diameter actin proteins. They play the primary role in cyclosis and amoeboid potion. The microfilaments are sensitive cytochalasin-B that disrupts many cell activities like cell migration, cytokinesis, endocytosis, and exocytosis. 

Distribution of these are in the cortical regions of the cell beneath the plasma membrane. They are pres in the microvilli of the intestinal epithelium’s brush border and in the cells where amoeboid movement and cytoplasmic streaming occur.  

Structure of Microfilaments

Actin is the main structural component of microfilaments. Its concentration in non-muscle cells is very high (up to 10% of total cell protein). Globular actin (G-actin) and fibrillar (F-actin) are the basis for classical sol-gel transition in the cytoplasm of moving cells. There are three other actin filaments; α, β and γ. The α- actin is present in fully mature muscle tissue. Other forms are present in non-muscle cells. In non-muscle cells, the actin binds with myosin. The binding result in an arrowhead pattern of the microfilaments that point in the same direction. This suggests microfilaments possess a polarity which is a crucial property for mediating cell movements.

Intermediate filaments

Intermediate filaments (IFs) are tough protein fibers in the cytoplasm of higher eukaryotic cells. These are woven ropes 8-10 nm in diameter, an intermediate between the thin and thick filaments in muscle cells. IFs are resistant to colchicine and cytochalasin B but sensitive to proteolysis. In animal cells, IFs form a basket around the nucleus and extend to the cell periphery in gentle curving arrays. These are present in epithelia linking one cell to another at desmosomal junctions, along axons, and cytoplasm of smooth muscle cells. Other names given to IFs are tonofilaments (in epidermal), glial filaments (in neuroglial cells), and neurofilaments (in nerve cells). IFs have a tubular appearance; each tubule comprises 4 or 5 protofilaments arranged in parallel. 

Structure of Intermediate Filaments

IFs are made up of polypeptides of a wide range of sizes (40,000 to 130,000 daltons). Although they have an extensive size range, they mainly consists of amino acids encoded by the same multigene family. They have a central region of 310 amino acid residue, forming an extended α -helix with three α -helical interruptions.

Types of Intermediate Filaments 

The types of intermediate filaments have mainly four kinds of proteins based on morphology and localization: type I IF proteins, type II IF proteins, type III IF proteins, and type IV proteins.

1. Type I IF proteins: They are present in epithelial cells and have two subfamilies of keratin; acidic keratin and neutral or basic keratin. The keratin filaments are heteropolymers formed by an equal number of subunits from each of the two keratin subfamilies. The keratins are groups of a complex class of IF proteins, with 19 distinct forms in human epithelial and eight more in hair and nails. Mammalian cytokeratin is an α-fibrous protein synthesized in living cells of epidermis and dead stratum corneum layers.  
2. Type II IF proteins: These include four types of proteins; Vimentin, desmin, synemin, and glial fibrillary acidic protein (or glial filaments). The cells of mesenchymal origin have Vimentin, like fibroblasts, blood vessels, endothelial cells, and white blood cells. Desmin is present in both striated and smooth muscle cells. Glial filaments occur in some glial cells, like astrocytes and Schwann cells. Synemin protein is a protein of 230,000 daltons in the intermediate filaments of muscle along with Vimentin and desmin. 
3. Type III IF proteins: These IF proteins assemble into neurofilaments so termed as neurofilament proteins. 
4. Type IV IF proteins: They are the nuclear lamins that form two-dimensional, highly organized sheets of filaments. 

Functions of Cytoskeleton

The main function of the cytoskeleton is maintaining the shape of the cells. The functions of the cytoskeleton are differentiated based on its components.  

Functions of microtubules 

Microtubules are the most functionally active components of the cytoskeleton of eukaryotic cells. The functions are as follows:

• The shape of the cell and some cell processes like axons, dendrites, microvilli, etc., have microtubules in a proper orientation.
• During cell differentiation, the microtubules help to determine the shape of developing cells. For example, the production of an array of microtubules wrapped around the nucleus in a double spiral arrangement helps elongate the spermatid’s nucleus during spermiogenesis.
• Microtubules help in determining the polarity of some cells.
• The movement of cells in the right direction also depends on the microtubules.
• The microtubules play a crucial role in the contraction of the spindle fibers and the movement of chromosomes and centrioles in ciliary and flagellar motion.
• Microtubules help transport macromolecules, granules, and vesicles within the cell.

Function of intermediate filaments

The intermediate filaments provide mechanical support to the cell and its nucleus. IFs in epithelia form a cellular network to resist external forces. For example, the neurofilaments in the axons resist stress caused by the animal’s motion, which in other cases, can destroy the cytoplasm.

Desmin filaments support the sarcomeres in muscle cells mechanically, and vimentin filaments surround and support the fat droplets of the fat cells.   

Functions of microfilaments

Microfilaments are involved in movement associated with furrow formation in cell division, cytoplasmic streaming in plants, and cell migration during embryonic development.

References

• Microtubules, Filaments | Learn Science at Scitable. Nature.com. Retrieved 26 August 2022, from https://www.nature.com/scitable/topicpage/microtubules-and-filaments-14052932/.
• Iwasa, J., & Marshall, W. (2018). Karp’s cell biology (8th ed.). Wiley.
• Verma, P., & Agarwal, V. (2004). Cell biology, genetics, molecular biology, evolution and ecology (24th ed., pp. 293-303). S. CHAND & COMPANY LTD.