Cytoskeleton in a cell – function, structure, and meaning

The cytoskeleton simply means the skeleton of the cytoplasm of a cell. The cytoskeleton functions to help support all these internal organs in the cell with its structure and it can also be found in all cells, though the proteins that make it up differ between organisms.

Cytoskeleton meaning

The cytoskeleton is made of a system of fibrillar structures that pervades the cytoplasm in cell biology. Therefore, it can be defined as the part of the cytoplasm that functions by providing the internal supporting framework of a cell.

The cytoskeleton is involved in various types of movements (where it anchors various cellular structures such as the flagellum) as well as the movement of cellular substances, in addition to providing structural support.

All eukaryotic cells have a cytoskeleton and they have a nucleus and organelles (Eukaryotic cells are found in plants, animals, fungi, and protists whereas prokaryotic cells are less complex, with no true nucleus or organelles other than ribosomes, and are found in bacteria and archaea, which are single-celled organisms).

Functions of the Cytoskeleton

• As previously stated, the cytoskeleton provides several purposes, first, it defines the shape of the cell and this is especially important in cells that do not have cell walls, such as animal cells, because they do not get their shape from a thick outer layer like plant cells.
• It can also cause cells to move through microfilaments and microtubules that can disassemble, reassemble, and contract allowing cells to crawl and migrate.
• Microtubules aid in the formation of structures such as cilia and flagella, which also allow for cell movement.
• The cytoskeleton not only organizes the cell and keeps its organelles in place, but it also aids in organelle movement throughout the cell. For instance, when a cell engulfs a molecule, microfilaments pull the vesicle containing the engulfed particles into the cell.
• Similarly, the cytoskeleton aids in the movement of chromosomes during cell division.

Structure of the cytoskeleton

The cytoskeleton structures aid in giving shape to the cytoplasm of the cell. Together, these structures are also known as cytoskeleton components. The description and function of each cytoskeletal component will be discussed below.

Cytoskeleton components

1. Microfilaments
2. Intermediate fibers
3. Cytoskeletal microtubules

Microfilaments

Microfilaments are the narrowest of the three types of protein fibers in the cytoskeleton. They have a diameter of about 7 nm and are composed of many linked monomers of a protein called actin that is combined in a double helix structure. Microfilaments are also known as actin filaments because they are composed of actin monomers and actin filaments are bidirectional, which means they have two structurally distinct ends.

Microfilaments are normally found at the cell periphery, where they run from the plasma membrane to the microvilli (e.g., in the pericanalicular zone, where they form the pericanalicular web/meshwork). They are found in bundles that form a three-dimensional intracellular meshwork.

Microfilament function

• Actin filaments play a variety of critical roles in the cell. For one thing, they act as tracks for the movement of myosin, a motor protein that can also form cytoskeletal filaments. Actin is involved in many cellular events that require motion due to its relationship with myosin.
• In animal cell division, for example, an actin and myosin ring pinches the cell apart to generate two new daughter cells. Actin and myosin are also abundant in muscle cells, where they form sarcomeres, which are organized structures of overlapping filaments. Your muscles contract when the actin and myosin filaments of a sarcomere slide past each other in unison.
• Actin filaments may also function as intracellular highways for the transport of cargoes such as protein-containing vesicles and even organelles. Individual myosin motors “walk” along actin filament bundles carrying these cargoes.
• Actin filaments can quickly assemble and disassemble, allowing them to play an important role in cell motility (movement), such as the crawling of a white blood cell in your immune system.
• Finally, actin filaments are important structural components of the cell as a network of actin filaments can be found in the cytoplasm at the cell’s very edge in most animal cells. The cell’s shape and structure are provided by this network, which is linked to the plasma membrane by special connector proteins.

Intermediate fibers/filaments

Intermediate filaments are cytoskeletal elements comprised of multiple strands of fibrous proteins wound together. Intermediate filaments, as the name implies, have an average diameter of 8 to 10 nm, which falls between microfilaments and microtubules.

Intermediate filaments come in a variety of forms, each of which is made up of a different type of protein. For example, keratin, a fibrous protein found in hair, nails, and skin, is one protein that forms intermediate filaments.

In contrast to actin filaments, which can grow and disassemble quickly, intermediate filaments are more permanent and play a structural role in the cell. They are designed to withstand tension, and their responsibilities include keeping the cell’s shape and anchoring the nucleus and other organelles in place. Although intermediate filament proteins do not experience dynamic instability like microtubules, they are frequently modified via phosphorylation. This is important for their assembly within the cell.

Intermediate filaments extend from the surface of the nucleus to the cell membrane in various types of cells. These filaments associate with the other components of the cytoskeleton via the elaborate network that they form in the cytoplasm, which contributes to their functions.

Functions of intermediate filaments/fibers

• Intermediate filaments, for the most part, serve to support the structural integrity of cells.
• They help provide structural support in cells (muscle and epithelial cells) that are subjected to high physical stress.
• Intermediate filaments have been shown to help support the cytoskeleton as a whole due to their more permanent stature when compared to other components of the cytoskeleton.
• They contribute to epithelial cell stretching.
• Intermediate filaments, as components of the nuclear lamina, help to strengthen the nuclear membrane and thus protect the contents of the nucleus.
• As axons grow in size, intermediate fibres provide support for them.
• They contribute to muscle contraction by forming bridges between Z discs.

Types of intermediate filaments/fibers

Groups of intermediate filaments	Structure of intermediate filaments
Type I and II	It is made up of about 15 different proteins that are found in most epithelial cells.
Type III	They can be found in smooth muscle cells, white cells, and glial cells, among other places. Vimentin and desmin are examples of proteins in this class.
Type IV	This category includes proteins found in nerve cells such as neurofilament proteins and alpha-internexin.
Type V	Lamins (a type of filaments) are an example of a protein found in this group.
Type VI	Nestin is a protein found in neurons and it is an example of intermediate fiber found in this group.

The table above shows that there are six major groups under which the different types of intermediate filaments are classified.

The cytoskeletal microtubules

Microtubules, despite their name, are the largest of the three types of cytoskeletal fibers, with a diameter of about 25 nm. A microtubule is made up of tubulin proteins arranged in a straw-like tube, and each tubulin protein is made up of two subunits, alpha-tubulin, and beta-tubulin.

Microtubules, like actin filaments, are dynamic structures that can expand and contract rapidly by adding or removing tubulin proteins. Microtubules have directionality, which means they have two ends that are structurally distinct from one another.

Microtubules function

• Microtubules play an important structural role in cells by assisting the cell in resisting compression forces.
• Microtubules play a variety of more specialized roles in a cell in the aspect of providing pathways for motor proteins known as kinesins and dyneins, which transport vesicles and other cargoes around the cell’s interior, in addition to providing structural support.
• Microtubules also function by assembling into a structure called the spindle during cell division, which pulls the chromosomes apart.

Motor Proteins

The cytoskeleton in animal cells contains a number of motor proteins and these proteins, as the name implies, actively move cytoskeleton fibers, resulting in the movement of molecules and organelles throughout the cell. Motor proteins are propelled by ATP, which is produced by cellular respiration.

Cell movement is mediated by three types of motor proteins as listed below.

1. Kinesins are proteins that move along microtubules, carrying cellular components with them. They are commonly used to attract organelles to the cell membrane.
2. Dyneins, like kinesins, are proteins that pull cellular components inward toward the nucleus. Dyneins also work to slide microtubules relative to one another, as seen in cilia and flagella movement.
3. In order to perform muscle contractions, myosins interact with actin. They also participate in cytokinesis, endocytosis (endo-cyt-osis), and exocytosis (exo-cyt-osis).

Analogy on the cytoskeleton

While thinking of these cytoskeletal structures as analogous to an animal skeleton can be helpful, perhaps a better way to remember the relative placement of the microtubules and microfilaments is when they perform their function in transporting intracellular cargo from one part of the cell to another.

By this analogy, we could think of microtubules as railroad track systems, whereas microfilaments are more like streets. Using the same analogy, we can propose that the microtubule and microfilament networks are linked at certain points so that when cargo arrives at its general destination via microtubule (rail), it can be transported to its specific address via microfilament.

We can generalize somewhat and say that kinesins drive toward the (+) end (toward the cell’s periphery) while dyneins drive toward the (-) end (toward the microtubules organizing center MTOC). The molecular motors on actin microfilaments are proteins from the myosin family. The analogies end here because the operation of these molecular motors is very different from train or truck locomotion. Finally, one could argue that there is no biological need for such a transport system.

Again, illustrating with respect to human transport, we could say that transport via simple diffusion is akin to people carrying packages around the cell at random. That is to say, the deliveries will be made eventually, but you should not rely on this method for time-sensitive materials. To keep cells (particularly larger, eukaryotic cells) alive, a directed, high-speed system is required.

Flagella, cilia, and centrosomes in cytoskeleton

Flagella are long, hair-like structures that protrude from the cell surface and are used to move an entire cell, such as sperm, and if a cell has flagella, it is usually one or a few. Motile cilia are similar to flagella, but shorter and appear in large numbers on the cell surface. When cells with motile cilia form tissues, they beat and the beating aids in the movement of materials across the tissue’s surface. A good example of this process is observed in the cilia of cells in your upper respiratory system that helps move dust and particles out towards your nostrils.

Flagella and motile cilia share a structural pattern despite their differences in length and number. Most flagella and motile cilia have 9 pairs of microtubules arranged in a circle, with an additional two microtubules in the center. This configuration is known as a 9 + 2 array.

Motor proteins called dyneins move along microtubules in flagella and motile cilia, generating a force that causes the flagellum or cilium to beat. The structural connections between the microtubule pairs, as well as the coordination of dynein movement, allow motor activity to produce a regular beating pattern.

A basal body is located at the base of the cilium or flagellum and this basal body is composed of microtubules and is essential for the assembly of the cilium or flagellum. Once assembled, the structure also controls which proteins can enter and exit the cell.

The basal body is simply a modified centriole, and a centriole is a cylinder made up of nine triplets of microtubules held together by supporting proteins. Centrioles are best known for their role in centrosomes, which are structures in animal cells that serve as microtubule-organizing centers. A centrosome is made up of two centrioles that are oriented at right angles to each other and are surrounded by a mass of “pericentriolar material” that serves as anchoring sites for microtubules.

Role of cytoskeleton in cytoplasmic streaming

The cytoskeleton in animal cells facilitates cytoplasmic streaming and this process, also known as cyclosis, involves the movement of the cytoplasm within a cell to circulate nutrients, organelles, and other substances. Endocytosis and exocytosis, or the transport of substances into and out of cells, are also aided by cyclosis.

As cytoskeletal microfilaments contract, they aid in the direction of cytoplasmic particle flow and when the microfilaments attached to organelles contract, the organelles are drawn along with them, and the cytoplasm flows in the same direction.

Prokaryotic and eukaryotic cells both have cytoplasmic streaming and in protists such as amoebae, this process results in cytoplasmic extensions known as pseudopodia. These structures are used for both food capture and locomotion.

Cytoskeleton in prokaryotes

The cytoskeleton was once thought to be unique to eukaryotic cells, but all homologs of the major proteins in the eukaryotic cytoskeleton have been discovered in prokaryotes. Although the evolutionary relationships are so distant that protein sequence analyses alone cannot reveal them, the similarity of their three-dimensional structures and functions in maintaining cell shape and polarity provides clear evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.

Structure of prokaryotic cytoskeleton

• FtsZ
• MreB and ParM
• Crescentin

The prokaryotic cytoskeleton consists of the above-listed proteins.

FtsZ

FtsZ was the first protein found in the prokaryotic cytoskeleton. In the presence of guanosine triphosphate (GTP), FtsZ forms filaments similar to tubulin, but these filaments do not group into tubules. FtsZ is the first protein to move to the division site during cell division and is required for the recruitment of other proteins that synthesize the new cell wall between the dividing cells.

MreB and ParM

MreB and other prokaryotic actin-like proteins are involved in cell shape maintenance. Actin-like proteins are encoded by genes in all non-spherical bacteria, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis.

Some plasmids encode a distinct system involving the actin-like protein ParM. ParM filaments are dynamically unstable and may partition plasmid DNA into dividing daughter cells via a mechanism similar to that used by microtubules during eukaryotic mitosis.

Crescentin

Crescententin, a third protein found in the bacterium Caulobacter crescentus, is related to intermediate filaments found in eukaryotic cells. Crescentin is also involved in the maintenance of cell shape in bacteria, such as helical and vibrioid forms, but the mechanism by which it does so is currently unknown. Curvature could also be described by the displacement of crescentic filaments after peptidoglycan synthesis is disrupted.

Cytoskeletal Dynamics

Early in animal development, there is a great deal of cellular rearrangement and migration as the blastula, a roughly spherical blob of cells begins to differentiate and form cells and tissues with specialized functions. These cells must travel from their point of origin to their final location in the fully developed animal. Some cells, such as neurons, have an additional type of cell motility in which they extend long processes (axons) from the cell body to their innervation target.

In both neurite extension and whole-cell motility, the cell must move its attachment points first, followed by the bulk of the cell. This is done gradually, and the cytoskeleton is used to speed up the process. The major components of cell motility are changing the point of forwarding adhesion, clearing internal space through myosin-powered actin microfilament rearrangement, and subsequent filling of that space with microtubules.

The membrane must be attached to the cytoskeleton in order for a force to be transmitted. In fact, signaling from membrane receptors can sometimes directly induce cytoskeleton rearrangements or movements via adapter proteins that connect actin (or other cytoskeletal elements) to transmembrane proteins like integrin receptors.

FAQ on cytoskeleton