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| Cytoskeleton-the cellular scaffold |
By
Stephen Jones |
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The cytoskeleton is a network of fibers throughout the cell's cytoplasm that helps the cell maintain its shape and gives support to the cell. It is a cellular scaffolding or skeleton contained within the cytoplasm, present in both eukaryotes and prokaryotes. It is a dynamic structure that maintains cell shape, often protects the cell, enables cellular motion (using structures such as flagella, cilia and lamellipodia), and plays important roles in both intracellular transport (the movement of vesicles and organelles, for example) and cellular division.
The eukaryote cytoskeleton is composed of three different types of fibres: microtubules, microfilaments and intermediate filaments, which are differentiated by their size, with microtubules being the thickest and microfilaments being the thinnest. Microfilaments are solid rods made of globular proteins called actin and are common to all eukaryotic cells. Microfilaments range from 5 to 9 nanometers in diameter and are designed to bear large amounts of tension. In association with myosin, microfilaments help to generate the forces used in cellular contraction and basic cell movements. The filaments also enable a dividing cell to pinch off into two cells and are involved in amoeboid movements of certain types of cells. Intermediate filaments, 8 to 12 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton. They are prominent in cells that withstand mechanical stress and are the most insoluble part of the cell. Each intermediate filament monomer consists of an alpha helical rod domain which coils around another filament like a rope to form a dimer. The intermediate filaments are classified into five types: Types I-Acidic Keratin, Types II-Basic Keratin, Type III intermediate filaments are distributed in a number of cell types, including vimentin in fibroblasts, endothelial cells and leukocytes; desmin in muscle; glial fibrillary acidic factor in astrocytes and other types of glia, and peripherin in peripheral nerve fibers, Type IV-Neurofilament H (heavy), M (medium) and L (low), Type IV-internexin and Type V are the lamins which have a nuclear signal sequence so they can form a filamentous support inside the inner nuclear membrane. Microtubules, the third principal component of the cytoskeleton, are rigid hollow rods approximately 25 nm in diameter. Like actin filaments, microtubules are dynamic structures that undergo continual assembly and disassembly within the cell. Microtubules are composed of a single type of globular protein, called tubulin which is a dimer consisting of two closely related 55-kd polypeptides, alpha-tubulin and beta-tubulin. In addition, a third type of tubulin (gamma-tubulin) is specifically localized to the centrosome, where it plays a critical role in initiating microtubule assembly. Tubulin dimers polymerize to form microtubules, which generally consist of 13 linear protofilaments assembled around a hollow core. The protofilaments, which are composed of head-to-tail arrays of tubulin dimers, are arranged in parallel. Microtubules function both to determine cell shape and in a variety of cell movements, including some forms of cell locomotion, the intracellular transport of organelles, and the separation of chromosomes during mitosis.
Apart from eukaryotes, the prokaryote cytoskeleton consists of various proteins like FtsZ, MreB, ParM and crescentin which are involved in the maintenance of cell shape.
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