Components of the cytoskeleton are abundant, altogether cytoskeleton proteins account for 10-30% of all the proteins in most cells. An extremely high level of organisation of proteins is created and maintained by the cytoskeleton. The way in which these small proteins relative to the huge size of the cytoplasm, can in some cases stretch from one side of the cell to the other is by polymerisation. For each of the major cytoskeleton fibres, thousands of identical proteins assemble into linear filaments. These filaments connect protein complexes and organelles in different regions of the cell and serve as a sort of transportation system between them. They also provide mechanical support for the cell, which is especially important in animal cells since there is an absence of rigid cell wall. Each of the three major filaments that make up the cytoskeleton is a helical polymer but they have their own different arrangement within the cell and a distinct function. However, by themselves they could not provide shape or strength to the cell. The filaments are dependent upon one another and there is a type of protein known as accessory proteins, which link the filaments together and to other cell components.
Actin filaments (or microfilaments) are two stranded helical polymers of the protein actin, which is the most abundant cellular protein. They are fine, thread like protein fibres with a diameter of 5-9nm, which are organised into a variety of linear bundles, two-dimensional networks and three-dimensional gels. Actin filaments are polar structures with two different ends, a relatively inert and slow growing minus end and a faster growing plus end. It was X-ray diffraction analysis that solved the three dimensional structure of the actin molecule and this information was used to deduce the shape of actin filaments as seen above. The total length of actin filaments within the cell is 30 times grater than that of microtubules. The actin filaments are thinner, more flexible and often shorter than microtubules. They rarely occur on their own within a cell and are usually found as cross-linked bundles of fibre.
Although found throughout the cell, actin filaments are most highly concentrated at the cortex, just beneath the plasma membrane. Actin exists in two interconvertable forms globular monomers (g-actin) and double-helical filaments (f-actin). In cells around 50% of the actin may be in the g-actin form. Purified actin exists as a monomer in low ionic strength solution and assembles into actin filaments on addition of salt provided ATP is present. The polymerisation of actin is a dynamic process that is regulated by the hydrolysis of a tightly bound nucleotide i.e. ATP. This control of assembly and disassembly of actin filaments is important for movement of migratory cells and other cell processes. This balance between assembly into filaments and disassembly into monomers is mainly regulated by, actin-binding proteins. The proteins regulate the actin filaments by capping, cutting, cross-linking, bundling and stabilizing either g-actin or f-actin. Additional proteins link actin filaments to proteins in the plasma membrane.
There are many functions of the actin filaments but the key one is maintaining cells shape. For example each microvilli on an epithelial cell has a core of longitudinal actin filaments and some form a network that runs at right angles to the microvilli in the cell cortex.
The microvilli greatly increase the surface area of cells that they are present in. In the human body microvilli line the inside of the intestines so that there is a greater surface area in contact with the food to be absorbed. The actin filaments stiffen these microvilli and help them to remain extended out from the cell surface. Actin filaments play a part in cytokinesis, at the end of mitosis a ring of actin filaments forms the cleavage furrow. This ring then contracts down by the action of myosin to divide the cytoplasm of the daughter cells. They also anchor the centrosomes at opposite poles of the cell during mitosis. Actin filaments are the critical components of the submembrane cytoskeleton in red blood cells. They are also involved with the movement of cells and cellular processes. These movements occur both during development and in adults. For example they are involved in formation of the mesoderm and in the healing of wounds. These movements can also occur abnormally and are closely related to cancer. Actin filaments are also thought to play a part in the uptake and discharge of material from the cell via membrane-bound vesicles i.e. endocytosis and exocytosis. Another one of their several functions is to interact with myosin filaments (also known as thick filaments) in skeletal muscle fibres to provide the force of muscular contraction.
Intermediate filaments are rope-like fibres with a diameter of around 10nm. They are a family of related structures, all the same size, performing similar functions, and composed of related proteins, but with different forms found in different cell types. They are made of intermediate filament proteins, a group of molecularly and morphologically similar proteins with wide biochemical diversity. All intermediate filament proteins are elongated proteins that form coiled coils and assemble side-by-side.
Intermediate filaments are more stable and long-lived than actin filaments or microtubules. For example, keratin filaments (a class of intermediate filament) can remain intact even after the cell that contains them disintegrates. In most animal cells an extensive network of intermediate filaments surrounds the nucleus and extends out to the cell periphery, where they interact with the plasma membrane. Intermediate cells are found in greater numbers in the cytoplasm’s of cells that are subject to mechanical stress. For example, in epithelia where they are linked from cell to cell at specialized junctions. Also along the length of nerve cell axons and in all kinds of muscle cells.
Intermediate filament proteins fall into four major classes all highlighted in bold. Keratins (or cytokeratins) are found in epithelial cells and form keratin filaments. These are by far the most diverse class as there are over 20 distinct keratins found in human epithelia. However each kind of epithelial cell may use no more than two different keratins. It is possible that up to 85% of the dry weight of squamous epithelial cells can consist of keratins. There is even a further 8 more keratins that are specific to hair and nails.
Nerve cells contain a variety of intermediate filaments and are known as neurofilaments. These are comprised of neurofilament proteins and their main function is to provide mechanical strength to the long axons found in some neurons. Vimentin is another intermediate filament protein and is the most widely distributed of all the intermediate filament proteins. It is present in many cells of mesodermal origin, including fibroplasts, endothelial cells and white blood cells. It is also found in combination with other intermediate filament proteins in other cell types, for example, it is combined with glial fibrillary acidic protein in some types of glial cells and is combined with desmin in muscle cells. In muscle cells the function of the intermediate filaments is to anchor the thick and thin filaments, of myosin and actin respectively, in a fixed position. Nuclear lamins form a meshwork of intermediate filaments that lines the inside surface of the inner nuclear membrane of eukaryotic cells. It must be noted that it is possible to use intermediate filaments in order to determine the origin of cells in a tumour. Since different cell types/tumours often respond very differently to chemotherapy or radiation treatment, knowing the origin of the cancer can be very important in treating the patient.
Microtubules are formed from the protein tubulin. There are two types of closely related tubulin, alpha tubulin and beta tubulin, both of which are globular proteins. Although tubulin is present in almost all eukaryotic cells, the most abundant source of tubulin is in the brain. These proteins form heterodimers, which in turn self associate lengthwise to form protofilaments. All together 13 protofilaments assemble side-by-side to form a microtubule.
This microtubule is a hollow tube which is around 25nm in diameter. Microtubules are quite variable in length and can grow as much as 1000 times as long as they are thick. Microtubules are polar structures, one end (the plus end) is capable of rapid growth, while the other end (the minus end) tends to lose subunits if not stabilised. Microtubules are much more rigid than actin filaments and are typically long and straight. In most cells, the minus end of the microtubule is stabilised by being attached to a microtubule-organizing centre (MTOC) also known as a centrosome. The centrosome is located in the cytoplasm just outside the nucleus. Just before mitosis, the centrosome duplicates. The two centrosomes then 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 towards the equatorial plate forming spindle fibres.
It is understood that cancerous cells often have more than the normal number centrosomes. They are also aneuploid (have abnormal number of chromosomes). Chromosome movement in mitosis involves the polymerisation and depolymerisation of the microtubules. Taxol (a drug found in the bark of the Pacific yew) prevents depolymerisation of the microtubules of the spindle fibre. This in turn stops chromosome movement, and thus prevents the completion of mitosis. Taxol is being used with some success as an anti-cancer drug. Polymerised tubulin is in equilibrium with unpolymerised tubulin in the cell, most tubulin in cells is polymerised into microtubules but microtubules do assemble and disassemble dynamically under normal physiological conditions. The way in which microtubules assemble is by the addition of GTP containing tubulin molecules to the free end (plus end) of the microtubule. Hydrolysis of the bound GTP takes place after assembly and weakens the bonds that hold the microtubule together.
Microtubules participate in a wide variety of cell activities, and most involve motion. This motion is provided by the protein motors that use energy of ATP to move along the microtubule. There are two major groups of microtubule motors, kinesins (most of these move towards the plus end of the microtubule) and dyneins (which move to the minus end). Some examples include, the rapid transport of organelles, like vesicles and mitochondria, along the axons of neurons. This takes place along microtubules with their plus ends pointed toward the end of the axon and therefore the motors are kinesins. The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle, both kinesins and dyneins are used as motors. Several drugs used in cancer chemotherapy interfere with the polymerisation/depolymerisation of microtubules. This prevents normal separation of chromatids during mitosis and kills the cancerous cell.
Microtubules are required for the movement in cilia and flagella, which are microtubule-based organelles. Cilia and flagella are constructed from microtubules and both provide either locomotion for the cells (e.g. sperm) or move fluid past cells (e.g. ciliated epithelial cells that line the oesophagus and move a film of mucus towards the throat. Cilia and flagella both have the same basic structure. If a cell has many short ones, they are known as cilia and if the cell has few long ones they are known as flagella. Each cilium (or flagellum) is made of a cylindrical array of 9 evenly spaced microtubules, each with a partial microtubule attached. The entire assembly is sheathed by a membrane that is simply an extension of the plasma membrane. The motion of cilia and flagella is created by microtubules sliding past one another. This requires motor molecules of dynein, which link adjacent microtubules together, and the energy of ATP.
The fourth type of protein fibre that makes up the cytoskeleton, which there wasn’t a significant amount of information about is the microtrabeculae. These are tiny fibres and they have a very important function, they interconnect all of the structures within the cell. It is believed that, like intermediate fibres, the microtrabeculae help to give the cell shape.
The cytoskeleton really is a fascinating structure of which there still many things not yet understood. I predict that sometime in the near future, new discoveries concerning the cytoskeleton may play a part in finding a cure for cancer because already there are many links between the cytoskeleton and cancerous cells. I hope in this essay I have given a clear account of what the cytoskeleton is and the many components it is made up of.
