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In thehuman body, huge numbers of cilia (10s/cmzor more) line our respiratory tract,sweeping layers of mucus, trapped particles of dust, and bacteria up to themouth where they are swallowed and ultimately eliminated. Likewise, cilia alongthe oviduct help to sweep eggs toward the uterus.The movement of a cilium or a flagellum is produced by the bending of itscore, which is called the axoneme.
The axoneme is composed of microtubulesand their associatedproteins, arranged in a distinctive and regular pattern. Ninespecial doublet microtubules (comprising one complete and one partial microtubule fused together so that they share a common tubule wall) are arranged inFigure 16-80 The contrastingmotions of flagella and cilia. (A)The wave-likemotion of the flagellumof a spermcellfrom a tunicate.The cell wasphotographedilluminationat 400 flashesper second.Notethatwith stroboscopicfrom the baseto the tip of thewavesof constantamplitudemovecontinuouslythe breaststrokeinflagellum.(B)The beatof a cilium,which resemblesswimming.A fast power stroke(redarrows),in which fluid is driven over thesurfaceof the cell,is followed by a slow recoverystroke.Eachcycletypicallyto the axisof therequires0.1-0.2 secand generatesa forceperpendicularaxoneme(the ciliarycore).(A,courtesyof C.J.Brokaw.)Figure 16-79 Effect on the heart of asubtle mutation in cardiacmyosin.left,normal heartfrom a 6-dayold mousepup.Right,heart from a pup with a pointmutation in both copiesof its cardiacmyosingene,changingArg 403to Gln.Thearrowsindicatethe atria.In the heartfrom the pup with the cardiacmyosinmutation,both atriaare greatlyenlarged(hypertrophic),and the micedie withinafew weeksof birth.
(FromD. Fatkinet al.,'103:1471999.WithJ. CIin.lnvest.,permissionfrom The RockefellerUniversityPress.)r032 Chapter16:TheCytoskeletono u t e r d y n e i na r mr a d i a ls p o k ei n n e rs h e a t hc e n t r a ls i n g l e tmicrotubulep l a s m am e m b r a n ei n n e r d y n e i na r m1 0 0n m(B)A microtubule B microtubuleo u t e r d o u b l e tm i c r o t u b u l ea ring around a pair of single microtubules (Figure r6-sr). Almost all forms ofeucaryotic flagella and cilia (from protozoans to humans) have this characteristic arrangement. The microtubules extend continuously for the length of theaxoneme, which can be f 0-200 pm.
At regular positions along the length of themicrotubules, accessoryproteins cross-link the microtubules together.Molecules of ciliary dynein form bridges between the neighboring doubletmicrotubules around the circumference of the axoneme (Figure f 6-S2). \fhenthe motor domain of this dynein is activated, the dynein molecules attached toone microtubule doublet (seeFigure 16-64) attempt to walk along the adjacentmicrotubule doublet, tending to force the adjacent doublets to slide relative toone another, much as actin thin filaments slide during muscle contraction.However, the presence of other links between the microtubule doublets prevents this sliding, and the dynein force is instead converted into a bendingmotion (Figure 16-83).The length of flagella is carefully regulared.
If one of the two flagella on achlamydomonas cell is amputated, the remaining one will transiently shrink asthe stump regrows until they reach the same length, and then the two shortenedflagella will continue to elongate until both are as long as they were on theunperturbed cell.
New flagellar components including tubulin and dynein areincorporated into the growing flagella at the distal tips. Thus, even in these(A)50 nmFigure16-81The arrangementofmicrotubulesin a flagellumor cilium.(A)Electronmicrographof the flagellumof a green-algacell (Chlamydomonas)shownin crosssection,illustratingthe"9 + 2" arrangementdistinctiveof(B)Diagramof the partsofmicrotubules.a flagellumor cilium.Thevariousprojectionsfrom the microtubuleslinkthe microtubulestogetherand occuratregularintervalsalongthe lengthof theaxoneme.(A,courtesyof LewisTilney.)1 0 0n mFigure16-82 Ciliarydynein.Ciliary(axonemal)dyneinis a largeproteinassembly(nearly2 milliondaltons)composedof9-12 polypeptidechains,the largestof which is the heavychainof morethan 500,000daltons.(A)The heavychainsformthe majorportionof the globularheadand stemdomains,and manyof the smallerchainsareclusteredaroundthe baseofthe stem.Thereare two headsin the outer dynein in metazoans,but three headsin protozoa,eachformed from their ownheavychain(seeFigure16-598 for a view of an isolatedmolecule).Thetail of the moleculebindstightlyto an Amicrotubulein an ATP-|ndependentmanner,whilethe largeglobularheadshavean ATP-dependentbindingsitefor a Bmicrotubule(seeFigure16-81).Whenthe headshydrolyzetheir boundATP,they movetowardthe minusend of the Bmicrotubule,therebyproducinga slidingforcebetweenthe adjacentmicrotubuledoubletsin a ciliumor flagellum.Fordetails,seeFigure16-64.(B)Freeze-etchelectronmicrographof a cilium showingthe dynein armsprojectingat regutarintervalsfrom the doubletmicrotubules.(8,courtesyof JohnHeuser.)tuu001(8)'(IS-ZI eJnBrCees)usrueqJaru snorJels.^.ru e ^q eJnlruls-cnr1spur8rro aql o] relncrpuedrad paurrogsr ralq8nep Jellelus p qJIq ^ uI 'ssaJ-ord uoqecqdar snorrnc e ,tq asue sarpoqIESBqpup salorJtuacrvrall'sa1odalputdsaql;o ged aruof,aqpue JorralurIIaJ eql olul eloru selpoq IESeqaqt pue 'paqJosaJ('I)u!l'M'u pue Morpoo^ l'care ella8eg aq] 'aldurexa to!\auoraopfunppe31e replaclun eqt uI slso]tru; o { s a l r n o t ' V ) ' ( t € - 9 te l n 6 uaas)aLuesaql ,(;;erluessast aloulua)qcea Surrnp :alqrualuoJJalur d1puo1tf,uryaJe salorJluef,pue sepoq Ipseq 'sursrp ur salnqnlor)rlu;olueua6uellpeq1-ue8ro aruos ur 'paapul '(t8{Iarntlg; IaaqMIeJ e ur pa8uerre salnqnloJJrru'raqlabolsalnqnlor)rur1o,{erelerupur;I>pasnJJo sdnor8 auru qllm 'saurosoJluaolprurueJo Je]ual eq] lB pappequa1a1drr1eqt ploq lPq] s)iurluilo] (€ urpalpunoJ aJeleql salorJluar eql se urroJ eluEseq] e^Eq sarpoq IesBqeqJ'43€JJnsIIacur uMoqs)suralordraqlo'(selnqnlol)truaql te ela8eg pue prln crtortrecna loor l.1urg salpoq psoq pelll.l; seJntf,nJlsalalduo)ul) pup B aLlt)salnqntoj)rLuoMl ol pasnj(alnqnlor)rur]uaprJlE IeJrJolsrqaleunuoJun ueV oq])alnqnlor)rnrelaldtuotauo 6urureluotsrsnleredde Surunumrs 5o sadfi lueJaJJrp.{ran o,rnleseql elouep ol arueu arupsgaldr.r1laldu] q)ea'salnqnlorrrluaq] Jo asn aqJ '(IZ-9I arn8rg aas) IIe^i\IIaf, IerrelJpq aql ur pappequa roloruStpup aLuauoxeJOSlaSAU|UJOpaSOOUIO),fuelor prcads e dq uanrrp 'sralladord a{q a}etor ela8eg aq; 'urqa8eg ura}ordfterp>e;o uorltodramo;aql sultoJaqt Jo strunqns Surleadar ;o dn aperu 'sluauelrJ IBcIIaq p€rr '3uo1 an a77aBa7l{poq ;eseqq)el'apts aq} LuoiJpaMor^'leeqroaAeAAlou op pup urauLp Jo sepqruorJrur ureluoo louIpoq ;eseqe;o r"uel6er6Iauapaq'pealsul(g)'ueozoloid'ela8eg pallpJ seJnlJnJlsecEJJnsop eseqlSursn tur^ s osp EIJelf,EgP xauo) aql ut satpoq1nqaelql1acJoleseqq6norqtuolt)asssor)e;o qde.r6o.r>rr.uQ7 :e1deq1uorl)all (V)'salpoqlesegtrg-91arn6r1ur pessnJsrp)luaurdolanap cruo^,fuquadlrea Suunp dpoq aqtJo srxelq8u-ga1 aqrur slJaJeppup 'puelJeq puE srJqeplualJ ol IreJlpql ]ce.ufuole.rJo uoDEulr.uralep-rdsarpaz.{.predaqlurerlrco}3uu,rosuorlcaJuro1,fillqpdacsns q8rq eaqlSuny'ruradspazrreppJeqcelqorururotanp,{}mralsapru,{qsr aurorpuf.s eql'eruoJp-u,r(ss,raua8eue)fre1gcur,fuelrparaq 'suerunq uIesneJuraur{psloaJap'acuarradxa,{aq1sa8ueqc aqlo1 ,{1pcrueu,{.p pue ,r(prder puodsar o} uo}ala{so}b aq} yo ftqrqeldepe pue ,Qr-11qFaUJrsurJ]uraq] asn s11ac's.{erreJotoru-tuaruelrJalqpts pue palapro ,(UBU9NrCN3€l'rn8n10u)rnsrsnv)N E N A C: t / ! 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