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Unformatted text preview: Chapter 19 & 20 Microtubules and Intermediate Filaments Like microfilaments, microtubules and intermediate filaments are long protein polymers. These Cytoskeletal elements fill the cytosol, spanning the distance between the nucleus and the cell membrane. They have a similar distribution pattern, but different structures. Microtubules are responsible for various cell movements including beating of cilia, flagella, and transport of membrane vesicles in cytoplasm. Movements result from the polymerization and depolymerization of microtubules and the actions of microtubule motors. In contrast to motile functions of MT, intermediate filaments is strictly structural. IF are usually attached at one end to the plasma membrane through certain cell junctions or to integral membrane proteins through IF binding proteins. IF integrate cells into tissues. 19.5 Intermediate Filaments The association of IF with nuclear and plasma membrane suggest their principal role is structural (Fig 19-31). In epithelium IFs role is to provide mechanical support for the PM in contact with ECM and other cells. In epidermial and neuronal cells IF are at least 10- fold more abundant than microtubules or other cytoskeletal components. Unlike microfilaments and microtubules, IF do not contribute to cell motility. IF differ in stability, size, and structure from other cytoskeletal fibers. IF are extremely stable even after extraction with solutions containing detergent and high salts remain intact. As opposed to MF and MT that depolymerize into subunits. They also differ in size from other two cytoskeletal fibers, they are ~ 10 nm diameter as compared to microtubules (24 nm) and MF (7 nm). IF are helical rods that assemble into ropelike filaments. In addition, IF subunits do not bind nucleotides and assembly into IF does not involve hydrolysis of ATP or GTP as does polymerization of G-actin and tubulin. IF proteins are classified according to tissue distribution. The subunits composing IF constitute a superfamily of highly helical proteins found in cytoplasm of different tissues and at nuclear membrane. Table 19-4 shows 4-groups that are based on sequence similiarity and expression pattern. Most ubiquitous group are the lamins that are found exclusively in nucleus. Whereas keratins are the most diverse classes of IF proteins. About half of this family (10) are specific for hard epithelial tissues giving rise to nails, hair, and wool. Others called cytokeratins are found in epithelia that line internal body cavities. Of the Type III Ifs the most widely distributed is vimentin which is expressed most commonly in leukocytes, blood vessel endothelium and some epithelium and mesenchymal cells including fibroblasts. Vimentin helps support cellular membranes. IT is frequently associated with MTs and keeps nucleus and other organelles in a defined place within cell. The core of neuronal axons are 1 filled with neurofilaments that are responsible for radial growth of axon and...
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This note was uploaded on 09/18/2008 for the course BIM 202 taught by Professor Simon during the Spring '06 term at UC Davis.
- Spring '06