Часть 2 (B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (5th edition)), страница 7

Bacterial plasmids typically encode all the gene products that are necessaryfor their owrsegregation, presumably as a strategyto ensure their faithful inheritance and propagation in their bacterial hosts. rn uiuo, parM assembles into a filamentousstructure that associatesat each end with a copy of the plasmid that encodes it,and growth of the ParM filament appears to push the replicated plasmid copiesapart, rather like a mitotic spindle operating in reverse(Figure 16-2z).AltholghParM is a structural homolog of actin, its dynamic behavior differs significantiy.ParM filaments undergo dramatic dynamic instability in uitro, more closeiyresembling microtubules than actin filaments in the way that they grow andshrink. The spindle-like structure is apparently built by the selective stabilization of spontaneously nucleated filaments that bind to specialized proteinsrecruited to the origins of replication on the plasmids.The various bacterial actin homologs share similar molecular structures buttheir amino acid sequence similarity to each other is quite low (- l0-15% identical residues).

They assemble into filaments with distinct helical packing patterns, which may also have very different dynamic behaviors. Rather than uiingthe same well-conserved actin for many different purposes, as eucaryotic cellido, bacteria have apparently opted to proliferate and specialize their actinhomologs for distinct purposes.It is now clear that the general principle of organizing cell structure by theself-associationof nucleotide-binding proteins into dynamic helical filaments isused in all cells, and that the two major families of actin and tubulin are vervmonomers(A)replicationf ilaments proteins(B)2ttmFigure 16-27 Roleof the actin homologParMin plasmidsegregation.(A)Someplasmidsbacterialdrug-resistance(yellow)encodean actin homolog,ParM,that will spontaneouslynucleateto formsmall,dynamicfilaments(green)throughoutthe bacterialcytoplasm.Asecondplasmid-encodedprotein(b/ue)bindsto specificDNAsequencesin theplasmid,and alsostabilizesthe dynamicendsof the ParMfilaments.Whentheplasmidhasduplicated,so that the ParMfilamentscan be stabilizedat both ends,the filamentsgrow and pushtheduplicatedplasmidsto oppositeendsofthe cell.(B)In thesebacterialcellsharboringa drug-resistanceplasmid,theplasmidsarelabeledin redand the ParMprotein in green.Left,a short ParMbundleconnectsthe two daughterplasmidsshortlyaftertheir duplication.Right,the fullyassembledParMfilamenthaspushedthe duplicatedplasmidstothe cellpoles.(A,adaptedfromE.C.Garner,C.S.CampbellanoR.D.Mullins,Science306:1021-1 025,2004.With permissionfrom AAAS;B,fromJ.

Moller-Jensenet al.,Mol.Cell12:1477-1 487,2003.With permissionfrom Elsevier.)FILAMENTSOFCYTOSKELETALAND DYNAMICSTRUCTURETHESELF-ASSEMBLY(A)tL__zpm(B)I2pmancient, probably predating the split between the eucaryotic and bacterial kingdoms. However, the usesto which bacteria put their cltoskeletons appear somewhat different from their eucaryotic homologs. For example, in bacteria it is thetubulin (FtsZ) that is involved in cytokinesis (the pinching apart of a dividing cellinto two daughters), while actin drives this process in eucaryotic cells. Conversely, eucaryotic microtubules are responsible for chromosome segregation,while bacterial actins (ParM and possibly MreB) help to segregatereplicatedDNA in bacteria.At least one bacterial specieswith an unusual crescent shape, Caulobactercrescentus,even appears to harbor a protein with significant structural similarity to the third major class of cytoskeletal filaments found in animal cells, theintermediate filaments.

A protein called crescentin forms a filamentous structure that seems to influence the cell shape; when the gene encoding crescentinis deleted, the Caulobacter cells grow as straight rods (Figure f 6-28)'Since we now know that bacteria do in fact have sophisticated dynamiccltoskeletons, why then do they remain so small and morphologically simple?As yet there have been no motor proleins identified that walk along the bacterialfilaments; perhaps the evolution of motor proteins was a critical step allowingmorphological elaboration in the eucaryotes.Su m m a r yThe cytoplasm of eucaryotic cells is spatielly organized by a network of protein ftla'ments known as the cytoskeleton.This network contains three principal typesof filaments:microtubules,actin filaments, and intermediatefilaments.

All three typesof filamentsform as helical assembliesof subunits that self-associateusing a combinationof end-to-endand side-to-sideprotein contacts.Dffirences in the structureof the subunits and the manner of their self-assemblygiue the fitaments dffirent mechanicalproperties.Intermediatefilaments are rope-likeand easyto bend but hard to break.Microtubules are strong, rigid hollow tubes.Actin filaments are the thinnest of thethree and are easyto break.In liuing cells,the assemblyand disassemblyof their subunits constantly remodelsall threetypesof cytoskeletalfilaments. Microtubules and actin filaments add and losesubunitsonly at their ends,with one end (theplus end)growingfaster than the other.Tubulin and actin (the subunits of microtubulesand actin filaments, respectiuely)birul and hydrolyze nucleoside triphosphates (tubulin binds GTP and actin bindsATP).Nucleotide hydrolysisunderliesthe characteristicdynamic behauior of thesetwowherefiIaments.Actin filaments in cellsseemto predominantlyundergotreadmilling,end.theotheratdisassemblingsimultaneouslyendwhileassemblesatonea filamentMicrotubules in cellspredominantly display dynamic instability, wherea microtubuleend undergoesalternating bouts of growth and shrinkage.tnAlhereastubulin and actin haue beenstrongly conseruedin euolution, thefamilyof intermediatefilaments is uerydiuerse.Thereare many tissue-speciftcformsfound inthe cytoplasmof animal cells,including keratinfilaments in epithelialcells,neurofilamentsin neruecells,and desminfilaments in musclecells.In all thesecells,the primary job of intermediateftlaments is to prouide mechanical strength.Bacterial cellsalsocontainhomologsof tubulin, actin and intermediatefilamentsthat form dynamicfilamentous structuresinuoluedin determiningcell shapeand incell diuision.991andFigure16-28 CaulobacterbacteriumThesickle-shapedcrescentin.a protein,expressescrescenfusCaulobacterwith a seriesof coiled-coilcrescentin,todomainssimilarin sizeand organizationthe domainsof eucaryoticintermediateproteinfilaments.In cells,the crescentinformsa fiberthat runsdown the innersideof the curvingbacterialcellwall.Whenthegeneis disrupted,the bacteriaareviableform.but grow in a straightrod-shaped(FromN.Ausmees,J.R.Kuhnander,Cell115:705-713,2003'C.Jacobs-Wagnfrom Elsevier.)With oermission992Chapter16:TheCytoskeletonFILAMENTSMicrotubules, actin filaments, and intermediate filaments are much moredynamic in cells than they are in the test tube.

The cell regulatesthe length andstability of its cytoskeletal filaments, as well as their number and geometry. Itdoes so largely by regulating their attachments to one another and to other components of the cell, so that the filaments can form a wide variety of higher-orderstructures. Direct covalent modification of the filament subunits regulatessomefilament properties, but most of the regulation is performed by a large array ofaccessoryproteins that bind to either the filaments or their free subunits. Someof the most important accessory proteins associated with microtubules andactin filaments are outlined in Panel 16-3 (pp.

gg4-gg5).This section describeshow these accessoryproteins modify the dynamics and structure of cltoskeletalfilaments. we begin with a discussion of the way that microtubules and actin filaments are nucleated in cells, becausethis plays a major part in determining theoverall organization of the cell's interior.A Proteincomplexcontainingy-TubulinNucleatesMicrotubules\.{/hilecr- and B-tubulins are the regular building blocks of microtubules, anothertype of tubulin, carled y-tubulin, ]nasa more specialized role. present in muchsmaller amounts than cr- and B-tubulin, this protein is involved in the nucleation of microtubule growth in organisms ranging from yeasts to humans.Microtubules are generally nucleated from a specific intracellular locationknor,rmas a microtubule-organizing center (MToc).

Antibodies against y-tubulin stain the MToc in virtually all speciesand cell types thus far examined.MicrotubulesEmanatefrom the centrosomein AnimalcellsMost animal cells have a single, well-defined MToc called the centrosome,located near the nucleus. From this focal point, the cytoplasmic microtubulesemanate in a star-like, "astral" conformation.

Microtubules are nucleated at thecentrosome at their minus ends, so the plus ends point outward and growtoward the cell periphery. Microtubules nucleated at ihe centrosome continuously grow and shrink by dFramic instabiliry probing the entire three-dimensional volume of the cell. A centrosome is composed of a fibrous centrosomey-tubul inaccessoryproteins iny - t u b u l i nr i n g c o m p l e xFigure16-29 Polymerizationof tubulinnucleatedby ytubulin ring complexes.(A)Structureof the y-tubulinringcomplex,reconstructedfrom averagingelectronmicrographsof individualpurifiedcomplexes.(B)Modelfor thenucleationof microtubulegrowthby thelTuRC.The red outlineindicatesa pairofproteinsbound to two moleculesofy-tubulin;this groupcan be isolatedasaseparatesubcomplexof the largerring.Notethe longitudinaldiscontinuitybetweentwo protofilaments.Microtubulesgenerallyhaveone such"seam"breakingthe otherwiseuniformhelicalpackingof the protofilaments.(C)Electronmicrographof a singlemicrotubulenucleatedfrom the purifiedy-tubulinring complex.(A and C,fromM.

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