|CYTOSKELETON - Architectural framework of the Cell...
is a fibrillar network of microtubules, actin microfilaments, intermediate filaments, & myosin filaments which forms a dynamic, highly integrated network of structurally and biochemically distinct polymers with coordinated actions that influence cell morphology, cellular motility, intracellular transport, and many other cellular processes. The specific fucntions of the cytoskeleton iclude: establishing cell shape, providing mechanical strength, locomotion, chromosome separation in mitosis and meiosis, and intracellular transport of organelles.
Microtubules [MT]: ubiquitous polymers made of tubulin, a 120 kD protein of [Alpha] and [Beta] subunits. During MT assembly, tubulin dimers associate longitudinally to form long protofilaments, which associate to form a 13-protofilament tube 24 nm in diameter. proteins, known as microtubule-associated proteins (MAPs), form specific associations with MT's & have roles in MT assembly & cross-linking MT's to other cytoskeletal polymers. Motor MAPs, as dynein & kinesin, provide ATPase activity that move them along on Mt's. MT's arise from a microtubule organizing center (MTOC) or the Centrosome - a complex consisting of two centrioles & their associated fibrillar material.
Actin microfilaments: Globular G-actin, a 42 kD protein, is the monomer for actin polymer filamentous actin (F-actin) formation & consists of two helically organized polymer chains with a 7nm diameter. F-actin is more flexible than an MT. Actin microfilaments have a distinct polarity, possessing a plus end (+) & a minus end (-) that are defined, respectively, by dynamic states of assembly & disassembly.
Intermediate filaments: are flexible helical polymer rods that are 10 nm in diameter. There are several types of intermediate filament proteins, each of which is associated with a particular tissue or cell type. For example, several types of keratin intermediate filaments are characteristic of epithelial cells. Some of these intermediate filaments are specific for nails or hair, whereas others, known as cytokeratins, form the lining of internal cavities.
Myosin filaments: are multi-subunit enzymes that catalyze ATP-powered translocation of actin microfilaments that are widely distributed in both muscle and non-muscle cells.