CH16 Cytoskeleton.pdf - finedrafts

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THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS the central rod domain, demonstrating the importance of this particular part of the protein for correct filament assembly. A second family of intermediate filaments, called neurofilaments, is found in high concentrations along the axons of vertebrate neurons (Figure 16-22). Three types of neurofilament proteins (NF-L, NF-M, NF-H) coassemble in uiuo, forming heteropolymers that contain NF-L plus one of the others. The NF-H and NF-M proteins have lengthy C-terminal tail domains that bind to neighboring filaments, generating aligned arrays with a uniform interfilament spacing. During axonal growth, new neurofilament subunits are incorporated all along the axon in a dgramic process that involves the addition of subunits along the filament length, as well as the addition of subunits at the filament ends. After an axon has gro\,&'n and connected with its target cell, the diameter of the axon may increase as much as fivefold. The level of neurofilament gene expression seems to directly control axonal diameter, which in turn influences how fast electrical signals travel dor,tm the axon. The neurodegenerative disease amyotrophic lateral sclerosis (ALS, or Lou Gehrig's Disease) is associated with an accumulation and abnormal assembly of neurofilaments in motor neuron cell bodies and in the axon, which may interfere with normal axonal transport. The degeneration of the axons leads to muscle weakness and atrophy, which is usually fatal. The over-expression of human NF-L or NF-H in mice results in mice that have an ALS-like disease. The vimentin-like filaments are a third family of intermediate filaments. Desmin, a member of this family, is expressed in skeletal, cardiac, and smooth muscle. Mice lacking desmin shownormal initial muscle development, but adults have various muscle cell abnormalities, including misaligned muscle fibers. Drugs Can Alter Filament Polymerization Because the survival of eucaryotic cells depends on a balanced assembly and disassembly of the highly conserved cytoskeletal filaments formed from actin and tubulin, the two types of filaments are frequent targets for natural toxins. These toxins are produced in self-defense by plants, fungi, or sponges that do not wish to be eaten but cannot run away from predators, and they generally disrupt the filament polymerization reaction. The toxin binds tightly to either the filament form or the free subunit form of a polymer, driving the assembly reaction in the direction that favors the form to which the toxin binds. For example, the drug latrunculin, extracled from the sea sponge Latrunculia magnifica, binds to actin monomers and prevents their assembly into filaments; it thereby 987 Figure 16-22 Two types of intermediate filaments in cells of the nervous system. (A) Freeze-etch electron microscopic image of neurofilaments in a nerve cell axon, showing the extensive cross-linking through protein cross-bridges-an arrangement believed to give this long cell process great tensile strength. The cross-bridges are formed by the long, nonhelical extensions at the C-terminus of the largest neurofilament protein (NF-H). (B) Freeze-etch image of glial filaments in glial cells, showing that these intermediate filaments are smooth and have few cross-bridges. (C) Conventional electron micrograph of a cross section of an axon showing the regular side-to-side spacing of the neurofilaments, which greatly outnumber the microtubules. (A and B, courtesy of Nobutaka Hirokawa; C, courtesy of John Hopkins.)
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THE CYTOSKELETON AND CELL BEHAVIOR
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END.OF-CHAPTER PROBLEMS Discuss the
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