CH16 Cytoskeleton.pdf - finedrafts

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988 Chapter 1 6: The <strong>Cytoskeleton</strong> Table 16-2 Drugs That Affect Actin Filaments and Microtubules Phalloidin Cytochalasin Swinholide Latrunculin Taxol Colchicine, colcemid Vinblastine, vincristine Nocodazole binds and stabilizes filaments caps filament plus ends severs filaments binds subunits and prevents their polymerization binds and stabilizes microtubules binds subunits and prevents their polymerization binds subunits and prevents their polymerization binds subunits and prevents their polymerization stabilizes free tubulin, causing microtubule depolymerization. In contrast, taxol, extracted from the bark of a rare species of yew tree, binds to and stabilizes microtubules, causing a net increase in tubulin polyrnerization. These and some other natural products that are commonly used by cell biologists to manipulate the cyoskeleton are listed in Table 16-2. (A) o o C -NH-CH-CH-C -O 15 pm o H3C-C -O H,C c-o o 9oH CH o-c - ll o CH: Figure 1 6-23 Effect of the drug taxol on microtubule organization. (A) Molecular structure of taxol. Recently, organic chemists have succeeded in synthesizing this complex molecule, which is widely used for cancer treatment. (B) | mmunofluorescence micrograph showing the microtubule organization in a liver epithelial cell before the addition of taxol. (C) Microtubule organization in the same type of cell after taxol treatment. Note the thick circumferential bundles of microtubules around the periphery of the cell. (D) A Pacific yew tree, the natural source of taxol. (8, C from N.A. Gloushankova et al., Proc. Natl Acad. Sci. U.S.A. 91 :8597 -8601, 1 994. With permission from National Academy of Sciences; D courtesy of A.K. Mitchell 2001. o Her Majesty the Queen in Right of Canada, Canadian Forest Service.)
THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS Figure 16-24The bacterial FtsZ protein, a tubulin homolog in procaryotes. (A) A band of FtsZ protein forms a ring in a dividing bacterial cell. This ring has been labeled by fusing the FtsZ protein to the green fluorescent protein (GFP), which allows it to be observed in living E coli cells with a fluorescence microscope. Iop, side view shows the ring as a bar in the middle of the dividing cell. Bottom, rotated view showing the ring structure. (B) FtsZ filaments and rings, formed in vitro, as visualized using electron microscopy. Compare this image with that of the microtubule shown on the right in Figure 16-16C. (A, from X. Ma, D.W. Ehrhardt and W. Margolin, Proc. Natl Acad. Sci. IJ.S.A.93:12998-13003, 1996; B, from :i,:5:Tffi'-.'#l;illir'ifl ,':?y.',i.'"::i1e-s23'lee6Arrw*h (A) Bacterial Cell Organization and Cell Division Depend on Homologs of the Eucaryotic <strong>Cytoskeleton</strong> \Mhile eucaryotic cells are typically large and morphologically complex, bacterial cells are usually only a few micrometers long and assume simple, modest shapes such as spheres or rods. Bacteria also lack the elaborate networks of intracellular membrane-enclosed organelles such as the endoplasmic reticulum and Golgi apparatus. For many years, biologists assumed that the lack of a bacterial cltoskeleton was one reason for these striking differences between cell organization in the eucaryotic and bacterial kingdoms. This assumption was challenged with the discovery in the early 1990s that nearly all bacteria and many archaea contain a homolog of tubulin, FtsZ, that can polymerize into filaments and assemble into a ring (called the Z-ring) at the site where the septum forms during cell division (Figure 16-24). The three-dimensional folded protein structure of FtsZ is remarkably simi- Iar to the structure of o or B tubulin and, like tubulin, hydrolysis of GTP is triggered by polymerization and causes a conformational change in the filament structure. Although the Z-ring itself persists for many minutes, the individual filaments within it are highly dynamic, with an average half-life of about thirty seconds. As the bacterium divides, ttre Z-ring becomes smaller until it has completely disassembled, and it is thought that the shrinkage of the Z-ring may contribute to the membrane invagination necessary for the completion of cell division. The Z-ringmay also serve as a site for localization of specialized cell wall slnthesis enzymes required for building the septum between the two daughter cells. The disassembled FtsZ subunits later reassemble at the new sites of septum formation in the daughter cells (Figure f 6-25). More recently, it has been found that many bacteria also contain homologs of actin. TWo of these, MreB and Mbl, are found primarily in rod-shaped or spiral-shaped cells, and mutations disrupting their expression cause extreme abnormalities in cell shape and defects in chromosome segregation (Figure f6-26).MreBandMblfilamentsassemble inuiuotoformlarge-scalespiralsthat (A) time 0 (min) 35 , trm Figure l6-25 Rapid rearrangements of FtsZ through the bacterial cell cycle. (A) After chromosome segregation is complete, thl ring formed by FtsZ at the middle of the cell becomes smaller as the cell pinches in two, much like the contractile ring formed by actin and myosin filaments in eucaryotic cells. The FtsZ filaments that have disassembled as the cells have separated then reassemble to form two new rings at the middle of the two daughter cells. (B) Dividing chloroplasts (red\ from a red alga also make use of a protein ring made from FtsZ (yeltow)for cleavage. (A, from Q. Sun and W. Margolin, J. Bacteriol. 1 80:2050-2056, 1 998. With peimission from American Society for Microbiology; B, from S. Miyagishima et al., Plant Cell 13:2257-2268,2001. With permission from American Society of Plant Biologists') 37 (B) (B) 1 rm 989 il;
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THE CYTOSKELETON AND CELL BEHAVIOR
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THE CYTOSKELETON AND CELL EEHAVIOR
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END.OF-CHAPTER PROBLEMS Discuss the
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