The cytoskeleton is not just a static structural entity but a participant in cellular signaling pathways. Microfilaments, intermediate filaments and microtubules comprise the cytoskeleton. Together, these filaments regulate transmembrane ion fluxes, the organization of the cytoplasm, the shape of the cell, and cell motility and secretion. Proteins associated with cytoskeletal filaments regulate the organization of the cytoskeleton. Modification of cytoskeletal and cytoskeleton associated proteins by intracellular second messengers modulates their function and help to explain the morphological changes that accompany certain extracellular signals. For example, phosphorylation of component proteins alters the structure and function of focal adhesions. While cytoskeletal motor proteins derive energy from nucleotide hydrolysis to move along actin or tubulin filaments, no evidence of intermediate filament motors has been found. Anchoring proteins connect the cytoskeleton to cell membranes to maintain the organization of the cytoplasm, but their specific functions vary widely depending on their location and the type of cell.
Microfilaments, comprised of actin, are responsible for cell motility, alterations in cell morphology during cytokinesis and stimulus response, and the contractile force generated by cells. The actin monomer (globular or G-actin) is 385 residues long, with four domains that surround a deep cleft in the protein where a divalent cation and ATP or ADP are bound. Although actin polymerizes into filaments (F-actin) after hydrolysis of the bound ATP, polymerization does not require this reaction. Nucleation or the presence of a certain number of monomers already assembled into a filament, initiates polymerization, which is mediated by actin-binding proteins (ABPs). There are 48 classes of ABPs. Monomer binding proteins regulate actin polymerization by sequestering G-actin and thereby regulating monomer availability. Small severing proteins like gelsolin bind to the filaments, cleave them and cap the free ends to prevent further polymerization. Barbed-end capping proteins prevent polymerization by capping the ends of polymers and preventing further monomer addition. Other ABPs include crosslinking proteins, membrane-associated proteins and the motor proteins known as myosins. Because of the variety of functions of the actin cytoskeleton, and the required precision of the timing, these molecules are important targets of intracellular signaling pathways.
Tubulin monomers, approximately 50kDa, are expressed as α- and β-isoforms, both of which have a bound guanosine triphosphate (GTP) that is only hydrolyzable on the β subunit. The heterodimer of these two isoforms is the subunit of the microtubule. The amount of monomer present in the cell regulates intracellular levels of tubulin protein. In vitro, microtubules spontaneously assemble from tubulin into cylindrical polymers in the presence of GTP. Microtubule assembly in cells is promoted by MAPs, GTP, Mg2+, elevated temperatures, and the drug taxol. Disassembly is induced by Ca2+, guanosine diphosphate (GDP), low temperatures, phosphorylation of MAPs, and drugs like colchicine, vinblastine, griseofulvin and nocodazole. Located to one side of the nucleus, the microtubule organizing center (MTOC), the site of microtubule nucleation, also orients the microtubules so that the fast growing end points to the periphery of the cell. Microtubules assemble and disassemble constantly, a behavior known as dynamic instability, in which polymerization of some filaments compensates for depolymerization of other filaments.
Two criteria define microtubule-associated proteins (MAPs): (1) subcellular fractionation with microtubules and (2) the ability to stimulate polymer nucleation and elongation in vitro. MAP2 and tau were first described as microtubule binding proteins, but further study has revealed their association with all three types of cytoskeletal filaments. Since the function of tau has already been discussed at length, this section will focus on microtubule motors. Microtubules and their associated motor proteins (of the kinesin and dynein families) mediate most of the trafficking of intracellular vesicles and organelles in eukaryotic cells. Kinesin motors transport towards the plusend of the microtubules at the periphery of the cell, and dynein motors transport towards the minus-end at the MTOC. Motor proteins also regulate microtubule polymerization. In some systems, localization of organelles depends on the actin cytoskeleton as well, indicating that it may be the interaction between the two systems that specifies the location of the organelle in the cell.
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