Essay on Cytoskeleton (For School and College Students) | Cell | Biology

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Essay # 1. Introduction to Cytoskeleton:

The existence of cytoskeleton in the structure of the protoplasm was proposed by Koltzoff in 1928. They are complex network of protein filaments extend throughout the cytoplasm and mesh of different proteins in cells. As its name implies it helps to maintain cell shape and is important in cell motility. It is a dynamic three-dimensional structure that fills the cytoplasm. This structure acts as both muscle and skeleton, for movement and stability.

The internal movement of cell organelles, locomotion and muscle fiber contraction cannot take place without the cytoskeleton. It is believed that cytoskeleton is the characteristic feature of eukaryotic cells but recent research proved that prokaryotic cells have proteins that form a cytoskeleton. In the course of the human genome project more than 800 probably cytoskele­ton related genes are found. On the basis of three types of protein filaments, cytoskeletons are of three types such as microtubules, intermediate filaments and microfilaments.

Essay # 2. Types of Cytoskeleton:

a. Microfilaments:

Microfilaments are fine, thread-like protein fibers, 5-7 nm in diameter, represent the active or motile part of the cytoskeleton (Fig. 4.62). They appear to play a major role in cyclosis and amoeboid motion. They are composed predominantly of a contractile protein called actin, which is the most abundant cellular protein. These filaments are cross-linked into networks or bundles. The semi flexible microfilaments make cells mobile, to divide in mito­sis (cytokinesis) and are responsible for muscular contraction. The flexible intermediate fila­ments strengthen the cell additionally.

In most cases a shell of microfilaments supports the plasma membrane. Microfilaments are a polymer of actin protein subunits plus attached proteins like cross-linkers. Most multi-cellular organisms have several actin iso-forms. Humans have six actin genes; four encode alpha-actin, one beta- and one gamma-actin. Alpha-actin is found in muscle cells where it plays an important role in contracting the cell, whereas beta-actin is localized in the front of moving cells and gamma-actin forms stress fibers. Actin protein as a polymer without attached proteins is called filamentous actin (F-actin), while the globular actin monomers are called G-actin.

The actin subunits are structured in two lobes with a cleft in between, where a magnesium ion (Mg+) and an ATP are located. After a G-actin is incor­porated into a filament, the ATP is hydrolysed to ADP. There are a lot of proteins that regu­late the actin assembly, filament length and stability. Microfilaments’ association with the protein myosin is responsible for muscle contraction. Microfilaments can also carry out cel­lular movements including gliding, contraction, and cytokinesis. Myosin, which consists of an ATPase active head domain and a specific tail region, can move along microfilaments. These motor proteins can transport membrane vesicles or cause contractions not only in muscle cells but also in other cells.

Actin Filaments:

Actin filaments are 8 nm in diameter and consist of two strands of the protein actin that are bound around each other. They are especially prominent in muscle cells, where they provide for the contraction of muscle tissue, monomers of the protein actin polymerize to form long, thin fibers (Fig. 4.63). Some functions of actin filaments form a band just beneath the plasma membrane that provides mechanical strength to the cell, links trans-membrane proteins (e.g., cell surface receptors) to cytoplasmic proteins, anchors the centrosomes at opposite poles of the cell during mitosis and pinches dividing animal cells during cytokinesis. It generate cyto­plasmic streaming in some cells, locomotion in cells such as white blood cells and the amoeba and interact with myosin (“thick”) filaments in skeletal muscle fibers to provide the force of muscular contraction.

Actin monomer has sub domains in which ATP binds, along with Mg⁺⁺. Actin can hydrolyze its bound ATP to ADP + P_i, releasing P_i. The actin monomer can exchange bound ADP for ATP. The conformation of actin is different, depending on whether there is ATP or ADP in the nucleotide-binding site. G-actin (globular actin) with bound ATP can polymerize, to form F-actin (filamentous actin). F-actin may hydrolyze its bound ATP to ADP + P_i, and release P_i. ADP release from the filament does not occur because the cleft opening is blocked. G-actin can release ADP and bind ATP, which is usually present in the cytosol at higher concentration than ADP.

Actin filaments have polarity. The actin monomers all orient with their cleft towards the same end of the filament (designated the minus end). Capping proteins bind at the ends of actin filaments. Different capping proteins may either stabilize an actin filament or promote disassembly. They may have a role in determining the filament length. For example- Tropomodulins cap the minus end, preventing dissociation of actin monomers.

Cap Z capping pro­tein binds to the plus end, inhibiting polymerization. If actin monomers continue to dissoci­ate from the minus end, the actin filament will shrink. Cross-linking proteins organize actin filaments into bundles or networks. Actin-binding domains of several of the cross-linking proteins (e.g., filamin, actinin, spectrin, dystrophin and fimbrin) are homologous. Most cross -linking proteins are dimeric or have 2 actin-binding domains.

b. Intermediate Filaments:

Intermediate filaments (IFs) are tough and durable protein fibers in the cytoplasm of higher eukaryotic cells. In most animal cells, IFs form a ‘basket’ around the nucleus and extended out in gentle curving array to the cell periphery. They are constructed like woven ropes, about 10 nm in diameter (thus are “intermediate” in size between actin filaments (8 nm) and microtubules (25 nm) as well as of the thick filaments of skeletal muscle fibers) and provide tensile strength for the cell.

Intermediate filaments are found resistant to chlchicine and cytochalasin B and are sensitive to proteolysis. Some intermediate filaments (IF) are homo- polymeres of one protein, while some are hetero-polymeres of two or more proteins. All in­termediate filament proteins (67 human genes known) have a common basic structure with main differences at both ends. They form a homogenous polar fiber with a diameter of 10-12 nm.

Some IF proteins are ubiquitous (e.g. vimentin, provide mechanical strength to muscle and other cells) others are restricted, e.g., to neurons of the central nervous system (neuro­filament proteins, strengthen the long axons of neurons), muscle (desmin, syncoilin) or epithelial cells (keratin). Different kinds of epithelial cells use different keratins to build their intermediate filaments. Up to 85% of the dry weight of squamous epithelial cells can consist of keratins.

As well as the expression the organization of the intermediate filaments is cell- type-dependent. In many epithelial cells, filaments are distributed all over the cytoplasm and attach to the nucleus, while in primary fibroblasts vimentin is orientated towards the periph­ery and spans neither the whole cytoplasm nor is it connected to cell-cell-adhesion sites. The intermediate filaments network is dynamically influenced by a bunch of other proteins. The IFs provide mechanical stability as well as they take part in the assembly of the nuclear en­velope.

Intermediate filaments are not well-characterized in plants to date. In animals, these fila­ments are formed of polymers of keratin, vimentin, or lamin. In plants, filaments of these diameters (10-13 nm) are known, but the monomers that form the polymers are not well known. In one case, where a filament was identified in plants, its monomer turned out to be a glycolytic enzyme.

c. Microtubules:

Microtubules are highly dynamic protein polymers that form a crucial part of the cytoskeleton in all eukaryotic cells. Robertis and Franchi (1953) observed first time in the axoplasm of the mylineated nerve fibres, which they called them neurotubules. Microtubules were first described in detail by Ledbetter and Porter (1963). Microtubules are conveyer belts inside the cells. Microtubules are cylindrical tubes, 20-25 nm in diameter and composed of protein tubulin subunits. These subunits are termed as alpha and beta. Each micro-tubule is com­posed of eleven pairs of these tubulin subunits arranged in a ring (Table 4.9).

In animal cells, microtubules arise from a region of the cell called the microtubule organizing center (MTOC) located near the nucleus. From this center, microtubules come out across the cell, forming a network of “tracks” over which various organelles move within the cell and act as a scaffold to determine cell shape. Microtubules also form small, paired structures called centrioles within animal cells. In plant cells, microtubules are created at many sites scattered through the cell. In animal cells, the microtubules originate at the centrosome (Fig. 4.65). The relatively stiff microtubules play an important role as highway for transport of vesicles and organelles and in the separation of chromosomes during mitosis (karyo-kinesis).

Some eukaryotic cells move about by means of microtubules attached to the exterior of the plasma membrane. These microtubules are called flagella and cilia. Flagella and cilia both have the same structure: a ring of nine tubulin triplets arranged around two tubulin sub- units. The difference between flagella and cilia lies in their movement and numbers. Flagella are attached to the cell by a “crank” Mike apparatus that allows the flagella to rotate. Cilia, on the other hand, are not attached with a “crank,” and beat back and forth to provide move­ment. Ciliated cells usually have hundreds of these projections that cover their surfaces. For example, the protist Paramecium moves by means of a single flagellum, while the protist Didinium is covered with numerous cilia.

In microtubules one alpha- and one beta-tubulin form a hetero-dimer. Long chains of these hetero-dimers composed of proto-filaments, wherein always an alpha-tubulin is fol­lowed by a beta-tubulin. Every microtubule has a (-) and a (+) end. At the (+) or beta-tubulin end new heterodimers are added faster and at lower tubulin concentrations than at the (-) or alpha-tubulin end. The alpha-tubulin as well as the beta-tubulin subunit binds a small guanosine tri-phosphate (GTP).

The GTP bound to the alpha-tubulin faces the beta-tubulin subunit of the heterodimer, while the GTP of the beta-tubulin subunit directs away from the heterodimer. When a new dimer is incorporated into a microtubule the beta-tubulin bound GTP is hydrolysed to guanosine di-phosphate (GDP). If the polymerisation is faster than the hyrdolysation, a GTP-cap occupies the (+) end and causes the microtubule to de-polymerise faster than polymerise (Fig. 4.66).

In a cell the dynamics of microtubules are regulated by microtubule-associated proteins (MAPs). Some MAPs stabilize microtubules, while others destabilize microtubules. Stabi­lizing MAPs have a microtubule-binding domain and an acidic projection domain, which can bind to intermediate filaments or membranes and is suspected to determine the distance in between bundled microtubules.

An important role of microtubules is providing a pathway for intracellular movements of organelles and proteins. This is done by motor proteins (kinesins and dyneins) under consumption of ATP. Most kinesins carry their cargo along microtubules in (+) direction, while dyneins do so in (-) direction. These proteins have two head-domains, and the tail domain of the kinesin appoints the kind of cargo that can be bound and trans­ported. Dynein can bind its cargo not directly to its tail domain but needs the protein dynactin for mediation.

Cellular Motors:

Cells have protein motors that bind two molecules, and using ATP as energy, cause one molecule to shift in relationship to the other. Two types of these protein motors are myosin and actin, and dynein or kinesin and microtubules. These families of proteins all have a mo­tor end, but may have several kinds of different molecular structures on the binding end. When these proteins bind the molecules they are moved to different organelles. When linked to other microtubules, protein motors can cause motion if the ends are fixed or extend the lengths of the fiber bundles if the ends are free.

Centrosome:

The centrosome is located in the cytoplasm attached to the outside of the nucleus. It is dupli­cated during S phase of the cell cycle. Just before mitosis, the two centrosomes move apart until they are on opposite sides of the nucleus. As mitosis proceeds, microtubules grow out from each centrosome with their plus ends growing toward the metaphase plate. These clus­ters of microtubules are called spindle fibers.

Centrioles:

Each centrosome contains a pair of centrioles. Centrioles are built from a cylindrical array of 9 microtubules, each of which has attached to it 2 partial microtubules. When a cell enters the cell cycle and passes through S phase, each centriole is duplicated. A “daughter” centriole grows out of the side of each parent centriole. Centrioles appear to be needed to organize the centrosome in which they are embedded. Sperm cells contain a pair of centrioles; eggs have none. The sperm’s centrioles are absolutely essential for forming a centrosome which will form a spindle enabling the first division of the zygote to take place. Centrioles are also needed to make cilia and flagella.

Essay # 3. Cellular Movement in Cytoskeleton:

Cellular movement is accomplished by cilia and flagella. Cilia are hair-like structures that can beat in synchrony causing the movement of unicellular Paramecium. Both cilia and fla­gella are constructed from microtubules, and both provide either locomotion for the cells (e.g., sperm) or move fluid (e.g., ciliated epithelial cells that line our air passages and move a film of mucus towards the throat). Each cilium or flagellum is made of a cylindrical array of 9 evenly-spaced microtubules, each with a partial microtubule attached to it. 2 single micro­tubules run up through the centre of the bundle, completing the so-called “9+2” pattern. The entire assembly is sheathed in a membrane that is simply an extension of the plasma mem­brane. Dynein “arms” attached to the microtubules serve as the molecular motors. Defective dynein arms cause male infertility and also lead to respiratory tract and sinus problems.

The bacterial flagellum is made up of the protein flagellin. It is a 20 nm thick hollow tube. It is helical and has a sharp bend just outside the outer membrane. A shaft runs between the hook and the basal body, passing through protein rings in the cell’s membrane that act as bearings. Gram-positive organisms have 2 of these basal body rings, one in the peptidogly­can layer and other one in the plasma membrane. Gram-negative organisms have 4 such rings: the L ring associates with the lipo-polysaccharides, the P ring associates with pepti­doglycan layer, the M ring is embedded in the plasma membrane, and the S ring is directly attached to the plasma membrane. The filament ends with a capping protein (Fig. 4.67).

Each cilium or flagellum grows out from and remains attached to a basal body embed­ded in the cytoplasm. Motion of cilia and flagella is created by the microtubules sliding past one another. This requires motor molecules of dynein, which link adjacent microtubules to­gether, and the energy of ATP. At the flagellum’s anchor point on the inner cell membrane there is a rotary engine made up of protein (Mot complex), which helps the flagellum in movements.

The engine is powered by proton motive force, i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell’s metabolism. Flagella do not rotate at a constant speed but instead can increase or decrease their rotational speed in relation to the strength of the proton motive force. Flagellar rotation can move bacteria through liquid media at speeds of up to 60 cell lengths/second.