Часть 2 (B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (5th edition)), страница 6

raideqf986FILAMENTSOFCYTOSKELETALTHESELF-ASSEMBLYAND DYNAMICSTRUCTUREthe central rod domain, demonstrating the importance of this particular part ofthe protein for correct filament assembly.A second family of intermediate filaments, called neurofilaments, is foundin 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 andNF-M proteins have lengthy C-terminal tail domains that bind to neighboringfilaments, generating aligned arrays with a uniform interfilament spacing.

During axonal growth, new neurofilament subunits are incorporated all along theaxon 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 anaxon has gro\,&'nand connected with its target cell, the diameter of the axon mayincrease as much as fivefold. The level of neurofilament gene expression seemsto directly control axonal diameter, which in turn influences how fast electricalsignals travel dor,tmthe axon.The neurodegenerative disease amyotrophic lateral sclerosis (ALS, or LouGehrig'sDisease)is associatedwith an accumulation and abnormal assembly ofneurofilaments 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-expressionof humanNF-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 smoothmuscle. Mice lacking desmin shownormal initial muscle development,but adultshave various muscle cell abnormalities, including misaligned muscle fibers.DrugsCanAlterFilamentPolymerizationBecausethe survival of eucaryotic cells depends on a balanced assembly anddisassembly of the highly conserved cytoskeletal filaments formed from actinand tubulin, the two types of filaments are frequent targets for natural toxins.These toxins are produced in self-defenseby plants, fungi, or sponges that donot wish to be eaten but cannot run away from predators, and they generallydisrupt the filament polymerization reaction.

The toxin binds tightly to eitherthe filament form or the free subunit form of a polymer, driving the assemblyreaction 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 thereby987Figure 16-22 Two types of intermediatefilaments in cellsof the nervous system.(A)Freeze-etchelectronmicroscopicin a nervecellimageof neurofilamentscross-linkingaxon,showingthe extensivethroughproteincross-bridges-anbelievedto givethis longarrangementThecellprocessgreattensilestrength.areformedby the long,cross-bridgesat the C-terminusextensionsnonhelicalproteinof the largestneurofilament(B)Freeze-etch(NF-H).imageof glialfilamentsin glialcells,showingthat thesefilamentsaresmoothandintermediate(C)Conventionalhavefew cross-bridges.electronmicrographof a crosssectionofan axonshowingthe regularside-to-sidewhichspacingof the neurofilaments,greatlyoutnumberthe microtubules.(A and B,courtesyof NobutakaHirokawa;C,courtesyof JohnHopkins.)988Chapter16:TheCytoskeletonTable16-2 DrugsThat AffectActin Filamentsand MicrotubulesPhalloidinLatrunculinbindsandstabilizesfilamentscapsfilamentplusendsseversfilamentsbindssubunitsandpreventstheirpolymerizationTaxolColchicine,colcemidVinblastine,vincristineNocodazolebindsandstabilizesmicrotubulesbindssubunitsand preventstheirpolymerizationbindssubunitsandpreventstheirpolymerizationbindssubunitsand preventstheirpolymerizationCytochalasinSwinholidestabilizes free tubulin, causing microtubule depolymerization.

In contrast,taxol, extractedfrom the bark of a rare speciesof yew tree, binds to and stabilizesmicrotubules, causing a net increasein tubulin polyrnerization. These and someother natural products that are commonly used by cell biologists to manipulatethe cyoskeleton are listed in Table 16-2.oH3C-C -OoC -NH-CH-CH-C9oHH,CoCH-Oc-oo(A)1 5p mo-c lloCH:Figure 16-23 Effectof the drug taxol onmicrotubuleorganization.(A)Molecularstructureof taxol.Recently,organicchemistshavesucceededin synthesizingthis complexmolecule,which is widelyusedfor cancertreatment.(B)|mmunofluorescencemicrographshowingthe microtubuleorganizationina liverepithelialcellbeforethe additionof taxol.(C)Microtubuleorganizationinthe sametype of cellafter taxoltreatment.Notethe thickcircumferentialbundlesof microtubulesaroundtheperipheryof the cell.(D)A Pacificyewtree,the naturalsourceof taxol.(8,C from N.A.Gloushankovaet al.,Proc.NatlAcad.Sci.U.S.A.91:8597-8601,1994.With permissionfrom NationalAcademyof Sciences;D courtesyof A.K.Mitchell2001.o HerMajestythe Queenin Rightof Canada,CanadianForestService.)989FILAMENTSOFCYTOSKELETALTHESELF-ASSEMBLYAND DYNAMICSTRUCTUREFigure16-24ThebacterialFtsZprotein,a tubulin homolog inprocaryotes.(A)A band of FtsZproteinforms a ring in a dividing bacterialcell.Thisring hasbeenlabeledby fusingthe FtsZproteinto the greenprotein(GFP),whichallowsit to be observedin livingE colifluorescentmicroscope.Iop, sideview showsthe ring asa barcellswith a fluorescencein the middleof the dividingcell.Bottom,rotatedview showingthe ringusing(B)FtsZfilamentsand rings,formedin vitro,asvisualizedstructure.Comparethis imagewith that of the microtubuleelectronmicroscopy.shownon the rightin Figure16-16C.(A,from X.

Ma,D.W.Ehrhardtand1996;B,fromW. Margolin,Proc.NatlAcad.Sci.IJ.S.A.93:12998-13003,(A):i,:5:Tffi'-.'#l;illir'ifl,':?y.',i.'"::i1e-s23'lee6Arrw*h1 rmand CellDivisionDependonCellOrganizationBacterialCytoskeletonHomologsof the Eucaryotic\Mhile eucaryotic cells are typically large and morphologically complex, bacterialcells are usually only a few micrometers long and assume simple, modest shapessuch as spheres or rods. Bacteria also lack the elaborate networks of intracellular membrane-enclosed organelles such as the endoplasmic reticulum andGolgi apparatus.

For many years, biologists assumed that the lack of a bacterialcltoskeleton 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 manyarchaea contain a homolog of tubulin, FtsZ, that can polymerize into filamentsand assemble into a ring (called the Z-ring) at the site where the septum formsduring cell division (Figure 16-24).The three-dimensional folded protein structure of FtsZ is remarkably simiIar 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 filamentstructure.

Although the Z-ring itself persistsfor 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 necessaryfor the completion of cell division. The Z-ringmay also serve as a site for localization of specialized cell wallslnthesis enzymes required for building the septum between the two daughtercells. 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 homologsof actin. TWo of these, MreB and Mbl, are found primarily in rod-shaped or spiral-shaped cells, and mutations disrupting their expression cause extremeabnormalities in cell shape and defects in chromosome segregation (Figuref6-26).MreBandMblfilamentsassemble inuiuotoformlarge-scalespiralsthat(B)(B)(A)il;time 0(min)3537, trmFigure l6-25 Rapidrearrangementsof FtsZthrough the bacterialcell cycle.(A)After chromosomesegregationis complete,thl ringformedby FtsZat the middleof the cellbecomessmallerasthe cellpinchesin two, much likethe contractileringasthe cellshaveseparatedcells.The FtsZfilamentsthat havedisassembledformedby actinand myosinfilamentsin eucaryotic(red\from a redalga(B)chloroplastsDividingcells.daughtertwoofthemiddleringsattheto form two newthen reassemblealsomake useof a protein ring madefrom FtsZ(yeltow)forcleavage.(A,from Q.

Sunand W. Margolin,J. Bacteriol.180:2050-2056,1998.With peimissionfrom AmericanSocietyfor Microbiology;B,from S.Miyagishimaet al.,PlantCellWith permissionfrom AmericanSocietyof PlantBiologists')13:2257-2268,2001.990Chapter16:TheCytoskeletonr(c)L-----.1 (B)5umFigure16-26 Actin homologsinbacteriadetermine cell shape.(A)Thecommon soil bacteriumBacillussubtilisnormallyformscellswith a regularrodlikeshape.(B)B.subtiliscellslackingtheactinhomologMbl grow into irregulartwistedtubesand eventuallydie.(C)TheMbl proteinformslong helicesmadeofup manyshortfilamentsthat run thelengthof the bacterialcelland helptodirectthe sitesof cellwall synthesis.(FromL.J.Jones,R.Carbadillo-LopezandJ.

Errington,Cell104:913-922,2001.Withpermissionfrom Elsevier.)5[mspan the length of the cell and apparently contribute to cell shape determination by serving as a scaffold to direct the synthesisof the peptidoglycan cell wall,in much the same way that microtubules help organize the synthesis of the cellulose cell wall in higher plant cells (see Figure 19-82). As with Ftsz, the filaments within the MreB and Mbl spirals are highly dynamic, with half-lives of afew minutes; as for actin, ATP hydrolysis accompanies the polymerization proCCSS.Diverse relatives of MreB and Mbl have more specializedroles.A particularlyintriguing bacterial actin homolog is parM, which is encoded on certain bacterial plasmids that also carry genes responsible for antibiotic resistanceand frequently cause the spread of multi-drug resistancein epidemics.