Microtubule

Structure of a microtubule. The ring shape depicts a microtubule in cross-section, showing the 13 protofilaments surrounding a hollow center.
Microtubules are one of the cytoskeletal filament systems in eukaryotic cells. The microtubule cytoskeleton is involved in the transport of material within cells, carried out by motor proteins that move on the surface of the microtubule.

Microtubules are tubular polymers of tubulin that form part of the cytoskeleton that provides structure and shape to the cytoplasm of eukaryotic cells and some bacteria. The tubules can grow as long as 50 micrometres and are highly dynamic. The outer diameter of a microtubule is about 24 nm while the inner diameter is about 12 nm.[1] They are formed by the polymerization of a dimer of two globular proteins, alpha and beta tubulin.[2]

Microtubules are very important in a number of cellular processes. They are involved in maintaining the structure of the cell and, together with microfilaments and intermediate filaments, they form the cytoskeleton. They also make up the internal structure of cilia and flagella. They provide platforms for intracellular transport and are involved in a variety of cellular processes, including the movement of secretory vesicles, organelles, and intracellular macromolecular assemblies (see entries for dynein and kinesin).[3] They are also involved in cell division (by mitosis and meiosis) and are the major constituents of mitotic spindles, which are used to pull eukaryotic chromosomes apart.

Microtubules are nucleated and organized by microtubule organizing centers (MTOCs), such as the centrosome found in the center of many animal cells or the basal bodies found in cilia and flagella, or the spindle pole bodies found in most fungi.

There are many proteins that bind to microtubules, including the motor proteins kinesin and dynein, severing proteins like katanin, and other proteins important for regulating microtubule dynamics.[4] Recently an actin-like protein has been found in a gram-positive bacterium Bacillus thuringiensis, which forms a microtubule-like structure and is involved in plasmid segregation.[5]

History

Tubulin and microtubule-mediated processes, like cell locomotion, were seen by early microscopists, like Leeuwenhoek (1677). However, the fibrous nature of flagella and other structures were discovered two centuries later, with improved light microscopes, and confirmed in the 20th century with the electron microscope and biochemical studies.[6]

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