18 Cell Organization and Movement II.pdf

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1、Chanpter18CellOrganizationandMovementII:MicrotubulesandIntermediateFilamentsIntroductionNewtlungcellinmitosisstainedforcentrosomes(magenta),microtubules(green),chromosomes(blue),keratinintermediatefilaments(red).[CourtesyofA.Khodjakor,fromNature408:423–

2、424(2000).]Threetypesoffilamentsmakeuptheanimal-cellcytoskeleton:microfilaments,microtubules,andintermediatefilaments.Whyhavethesethreedistincttypesoffilamentsevolved?Itseemslikelythattheirphysicalpropertiesaresuitedtodifferentfunctions.InChapter17wedes

3、cribedhowactinfilamentsareoftencross-linkedintonetworksofbundlestoformflexibleanddynamicstructuresandtoserveastracksforthemanydifferentclassesofmyosinmotors.Microtubulesarestifftubesthatcanexistasasinglestructureextendingupto20μmincellsorasthebundledstr

4、ucturesasseeninciliaandflagella.Aconsequenceoftheirtubulardesignistheabilityofmicrotubulestogeneratepullingandpushingforceswithoutbuckling,apropertythatallowssingletubulestoextendlargedistanceswithinacellandbundlestoslidepasteachother,asoccursinflagella

5、andinthemitoticspindle.Microtubules'abilitytoextendlongdistancesinthecell,togetherwiththeirintrinsicpolarity,isexploitedbymicrotubule-dependentmotors,whichusemicrotubulesastracksforlong-rangetransportoforganelles.Microtubulescanbehighlydynamic—beingasse

6、mbledanddisassembledfromtheirends—providingthecellwiththeflexibilitytoreorganizemicrotubuleorganizationasneeded.Incontrasttomicrofilamentsandmicrotubules,intermediatefilamentshavegreattensilestrengthandhaveevolvedtowithstandmuchlargerstressesandstrains.

7、Withpropertiesakintostrongmolecularropes,theyareideallysuitedtoendowbothcellsandtissueswithstructuralintegrityandcontributetocellularorganization.Intermediatefilamentsdonothaveanintrinsicpolaritylikemicrofilamentsandmicrotubules,soitisnotsurprisingthatt

8、herearenoknownmotorproteinsthatuseintermediatefilamentsastracks.Althoughwediscussmicrotubulesandintermediatefilamentstogetherinthischapter—andtheirlocalizationinthecytoplasmcanlooksuperficiallyquitesimilar—wewillseetheirdynamicsa