Of The Major Cytoskeletal Proteins
Eukaryotic cells have developed spatial and mechanical functions to a very high degree. Theses functions depend on a system of filaments called the cytoskeleton. The cytoskeleton pulls chromosomes apart at mitosis and then splits the dividing cell into two. It drives and guides the intracellular traffic of organelles, moving materials from one part of the cell to another. It supports the plasma membrane and provides the mechanical linkages that let the cell bear stresses and strains without being pulled apart as the environment changes. It enables cells such as sperm to swim and others, such as fibroblasts and white blood cells, to crawl across surfaces. It provides the machinery in the muscle for contraction and in the nerve cell to extend an axon and dendrites. It guides the growth of the plant cell wall and controls the amazing diversity of eukaryotic cell shapes. (1)
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A system of filaments is found in the cytoskeleton which are divided into three major groups, these are; actin filaments, which have an average diameter of 6 nanometres, microtubules which have an average diameter of 25 nanometres and finally intermediate filaments which have an average diameter of 10 nanometres. (2) Intermediate cells are made up of small elongated and fibrous subunits. Actin filaments and microtubules are made of compact and globular subunits- actin subunits for actin filaments and tubulin subunits for microtubules. All three types form helical assemblies of subunits that self-associate using a combination of end to end and side to side protein contacts. The stability and mechanical properties of each filament are determined by their structure and the strengths of the attractive forces between them. (1)
Cytoskeletal structures rarely reach from one end of the cell to the other. The cell builds up large structures by repetitive assembly of large numbers of protein subunits, which can diffuse within the cell but as they are assembled they cannot diffuse back out of the cell. This way cells can undergo structural reorganisation, by breaking apart filaments at one site and building them back up at another site. (1)
Actin is the main structural component in both muscle and non-muscle cells. Actin filaments are two parallel protofilaments that twist around each other in a right-hand helix. They appear as flexible structures but as they are cross linked and bundled together by accessory proteins the large scale actin structures are much stronger. Actin has a diameter of 5-9nm, and they are organised into a variety of linear bundles, two-dimensional networks and three-dimensional gels. Although actin filaments are dispersed throughout the cell, they are most concentrated in the cortex, just beneath the plasma membrane. (1) Actin filaments depending on the type of cell and state that it is in can assume different configurations. They extend through the cytoplasm in the form of bundles, also known as stress fibres since they determine the elongated shape of the cell and enables the cell to adhere to the substrate and the spread out on it. Actin can also exist in forms other than bundles, in round cells that do not adhere to substrate the filaments form a mesh work that is distinct from the bundles. The two states are inter convertible states of the same molecule. The bundles give the cell it's tensile strength and structural support and the mesh works gives the cell it's elastic support and force for cell locomotion.(2) Making actin filaments useful in muscle contraction.
Microtubules are hollow cylindrical structures built from 13 parallel protofilaments, each composed of alternating Î±-tubulin and Î²-tubulin molecules. When tubulin heterodimers assemble they generate two types of protein-protein contact. The first is along the longitudinal axis of the microtubule, the top of the Î²-tubulin forms an interface with the bottom of the Î±-tubulin in the adjacent heterodimer. Perpendicular to these interactions, neighbouring protofilaments form lateral contacts. The main lateral contacts are between monomers of the same type, e.g. Î± to Î± and Î² to Î². Together the longitudinal and lateral contacts are repeated in the regular helical lattice of the microtubule. Multiple contacts among subunits make microtubules stiff and difficult to bend. Therefore this helps the cell keep its shape. However the lateral bonds holding the protofilaments together are comparatively weak. For this reason microtubles break much more easily when they are bent compared to intermediate filaments.(1) They are still strong however.
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Intermediate filaments are the true cytoskeleton. Unlike the other filaments, intermediate filaments are very stable structures. They have a cytoplasmic distribution independent from the other filaments. In the cell they anchor the nucleus, positioning it within the cytoplasmic space. During mitosis they form a filamentous cage around the mitotic spindle which holds the spindle in a fixed place during chromosome movement.(2) Intermediate filaments are rope like fibres with a diameter of around 10nm; they are made of intermediate filament proteins, which constitute a large and heterogeneous family. One type of intermediate filament forms a mesh work called the nuclear lamina just beneath the inner nuclear membrane. Other types extend across the cytoplasm, giving cells mechanical strength. Intermediate filaments assemble by forming strong lateral contacts between Î±-helical coiled coils, which extend over most of the length of each elongated fibrous subunit. Because the subunits are staggered in the filament, intermediate filaments tolerate stretching and bending, forming strong rope like structures. (1)
Among the most fascinating proteins that associate with the cytoskeleton are molecular motors called motor proteins. These proteins bind to a polarized cytoskeletal filament and use the energy derived from repeated cycles of ATP hydrolysis to move it steadily along it. Many different motor proteins coexist in every eukaryotic cell. They differ in the type of filament they bind to, either actin or microtubules, the direction in which they move along the filament, and the "cargo" they carry. The cytoskeletal motor proteins associate with their filaments tracks through a "head" region, or motor domain, that binds and hydrolyse ATP. (1)
The motor proteins that move on actin filaments are members of the myosin super family. The motor proteins that move on microtubles are either members of the kinesin super family or dynein family. The myosin and kinesin super families are diverse, with about 40 genes encoding each type of protein in humans. The only structural element shared among all members of each super family is the motor "head" domain. These heads are fused to a wide variety of different "tails", which attach to different functions in the cell. These functions include the transportation and localisation of specific proteins, membrane enclosed organelles, and mRNAs. (1) There are two distinct types of specialised structures in eukaryotic cells that are formed from highly ordered arrays of motor proteins that move on stabilised filament tracks. The myosin-actin system of the sarcomere powers the contraction of various types of muscle, including skeletal, smooth and cardiac muscle. The dynein-microtubule system of the axoneme powers the beating of cilia and the undulation of flagella. (1)
Within the cell, hundreds of different cytoskeleton-associated accessory proteins regulate the spatial distribution and the dynamic behaviour of the filaments. These accessory proteins bind to the filaments or their subunits to determine the sites of assembly and disassembly, and to link filaments to one another or to other cell structures. This process brings cytoskeletal under the control of extracellular and intracellular signals. Acting together the accessory proteins enable the eukaryotic cell to maintain a highly organised but flexible internal structure and, in many cases, to move. (1)
In conclusion all of the cells movements, shaping and structuring of cells require the coordination activities of all three filament systems along with a variety of of cytoskeletal accessory proteins, including motor proteins. (1) The main components of this apparatus are microtubules which are made of stiff, hollow rods about 25 nanometres in diameter made of tubulin and these give the cell it's shape. Actin filaments which are made of thin, flexible, double-stranded helical polymers around 5 nanometres in diameter made of globular actin molecules are the main structural component in both muscle and non-muscle cells. (3) Actin filaments exist in two types; bundles and mesh works. The bundles give the cell it's tensile strength and structural support and the mesh works gives the cell it's elastic support and force for cell locomotion.(1) Intermediate filaments which are made of tough, strong filaments 10 nanometres in diameter composed of a family of insoluble proteins is the true cytoskeleton.(3) Unlike the other filaments, intermediate filaments are very stable structures and can tolerate stretching and bending, forming strong rope like structures. (1) The cytoskeleton is connected by linker proteins to both the plasma membrane enclosing the cell as well as to organelles within both the nucleus and the cytoplasm. (3) Motor proteins are the molecular motors within the cytoskeleton. These proteins bind to the cytoskeletal filament and use the energy derived from repeated cycles of ATP hydrolysis to move it steadily along it. Many different motor proteins coexist in every eukaryotic cell. Accessory proteins regulate the spatial distribution and the dynamic behaviour of the filaments and also brings the cytoskeletal under the control of extracellular and intracellular signals. (1)
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