The cytoskeleton.

An intricate network of cytoplasmic filamentous structures determines the maintenance of shape and the motility of animal cells. Only some fibers are permanent features, most structural elements of the versatile cytoskeleton assembling and disassembling into building blocks. Tubulin microtubules, actin microfilaments, and the varied intermediate filaments may be ranked in terms of increasing stability of the polymer. Microtubules and microfilaments are intrinsically polarized and possess directional information, that is not shared by intermediate filaments. Tubulin, actin, and the five classes of intermediate filaments each consist of highly conserved molecules composing gene families probably derived from an original gene that was duplicated and modified during evolution. The specific properties of the cytoskeletal fibers according to species or cell type mainly result from their association with binding proteins which meet different requirements, like regulation of polymerization-depolymerization rate, positioning of the cytoskeletal fiber to other homologous or heterologous fibers as well as to plasma or internal membranes, or ATP-driven motor action. In the case of intermediate filaments, the several classes of cell-specific protein subunits are expressed in a developmental and histological pattern. The regulation of cytoskeletal functioning involves ATP or GTP-binding, protein phosphorylations and local concentrations in calcium ions.