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( Microfilament)
Microfilaments (or actin filaments) are the thinnest filaments of the cytoskeleton found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. Microfilaments are highly versatile, functioning in (a) actoclampin-driven expansile molecular motors, where each elongating filament harnesses the hydrolysis energy of its "on-board" ATP to drive actoclampin end-tracking motors to propel cell crawling, ameboid movement, and changes in cell shape, and (b) actomyosin-driven contractile molecular motors, where the thin filaments serve as tensile platforms for myosin's ATP hydrolysis-dependent pulling action in muscle contraction and uropod advancement. Actin filaments are assembled in two general types of structures bundles and networks. Actin-binding proteins dictate the formation of either structure since they cross-link actin filaments in the double-stranded helix. In non-muscle actin bundles, the filaments are held together such that they are parallel to each other by actin-bundling and/or cationic species. Bundles play a role in many cellular processes such as cell division (cytokinesis) and cell movement. The thinnest fibers of the cytoskeleton (measuring approximately 7 nm in diameter), microfilaments are formed by the head-to-tail polymerization of actin monomers (also known as globular or G-actin). Actin subunits as part of a fiber at referred to as filamentous actin (or F-actin). Each microfilament is made up of two helical interlaced strands of subunits. Much like microtubules, actin filaments are polarized, with their fast-growing (+)-ends (also known as barbed ends, because of their appearance in electron micrographs after binding of myosin S1 sub-fragments) and a slow-growing (-)-end (or pointed end, again based on the pattern created by S1 binding).
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