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

raldeq3ZL6THESELF-ASSEMBLYAND DYNAMICSTRUCTUREFILAMENTSOFCYTOSKELETALnu c l e a t i o n(lagphase)elongation(growth phase)steadystate( e q u i l i b r i u mp h a s e )co!elongation(growth phase)EocoEoccclfcf.ofccoot i m e a f t e r s a l ta d d i t i o n+steadystate( e q u i l i b r i u mp h a s e )c o m i n go n a n d o f fp r e f o r m e df i l a m e n ts e e d sa d d e d h e r e0(A)973(B)Nucleationls the Rate-LimitingStepin the Formationof aPolymerCytoskeletalThere is an important additional consequence of the multiple-protofilamentorganization of cltoskeletal polymers.

Short oligomers composed of a few subunits can assemble spontaneously,but they are unstable and disassemblereadily because each monomer is bonded only to a few other monomers. For a newlarge filament to form, subunits must assemble into an initial aggregate,ornucleus, that is stabilized by many subunit-subunit contacts and can then elongate rapidly by addition of more subunits. The initial process of nucleus assembly is called filament nucleation, and it can take quite a long time, depending onhow many subunits must come together to form the nucleus.The instability of smaller aggregatescreates a kinetic barrier to nucleation,which is easily observed in a solution of pure actin or tubulin-the subunits ofactin filaments and microtubules, respectively.When polymerization is initiatedin a test tube containing a solution of pure individual subunits (by raising thetemperature or raising the salt concentration), there is an initial lag phase, during which no filaments are observed.During this lag phase,however,a few of thesmall unstable aggregatessucceed in making the transition to the more stablefilament form, so that the lag phase is followed by a phase of rapid filament elongation, during which subunits add quickly onto the ends of the nucleated filaments (Figure f 6-f 0A).

Finally, the system approaches a steady state at whichthe rate of addition of new subunits to the filament ends exactly balances therate of subunit dissociation from the ends. The concentration of free subunitsleft in solution at this point is called the critical concentration, C.. As explainedin Panel f6-2 (pp. 978-979),the value ofthe critical concentration is equal to therate constant for subunit loss divided by the rate constant for subunit additionthat is, Cc = koff I kon.The lag phase in filament growth is eliminated if preexisting seeds (such asfilament fragments that have been chemically cross-linked) are added to thesolution at the beginning of the polymerizalion reaction (Figure l6-108).

Thecell takes great advantageof this nucleation requirement: it uses special proteinsto catalyze filament nucleation at specific sites, thereby determining the location at which new cytoskeletal filaments are assembled. Indeed, the regulationof filament nucleation is a primary way for cells to control their shape and theirmovement.to CreateThe Tubulinand Actin SubunitsAssembleHead-to-TailP o l a rF i l a m e n t sMicrotubulesareformedfromproteinsubunitsof tubulin.Thetubulinsubunitis itself a heterodimer formed from two closely related globular proteins calledct-tubulin and B-tubulin, tigll'tly bound together by noncovalent bonds (Figuretimeaftersaltaddition+Figure16-10 The time courseof actinpolymerizationin a test tube.(A)Polymerizationis begunby raisingthein a solutionof puresaltconcentrationisactinsubunits.(B)Polymerizationbegunin the sameway,but withpreformedfragmentsof actinfilamentspresentto act as nucleifor filamentgrowth.As indicated,the 0/ofreesubunitsreflectsthe criticalconcentration(Cc),thepoint at whichthereis no net changeinporymer.974Chapter16:TheCytoskeletonP-tubulinRf,,uotubulinheterodimer( = m i c r o t u b u l es u b u n i t )protofilamentpruseno15U nmImtnusenod-tubulinmicrotubule(A)f 6-l f ).

These two tubulin proteins are found only in this heterodimer. Each a orB monomer has a binding site for one molecule of GTP The GTP that is boundto the cr-tubulin monomer is physically trapped at the dimer interface and isnever hydrolyzed or exchanged;it can therefore be considered to be an integralpart of the tubulin heterodimer structure.

The nucleotide on the p-tubulin, incontrast, may be in either the GTP or the GDP form, and it is exchangeable.Aswe shall see,the hydrolysis of GTP at this site to produce GDP has an importanteffect on microtubule dynamics.A microtubule is a hollow cylindrical structure built from l3 parallel protofilaments, each composed of alternating s-tubulin and B-tubulin molecules.\Mhen the tubulin heterodimers assemble to form the hollow cylindrical microtubule, they generate two new types of protein-protein contacts.Along the longitudinal axis of the microtubule, the "top" of one B-tubulin molecule forms aninterface with the "bottom" of the a-tubulin molecule in the adjacent heterodimer. This interface is very similar to the interface holding the a and Bmonomers together in the dimer subunit, and the binding energy is strong. perpendicular to these interactions, neighboring protofilaments form lateral contacts.

In this dimension, the main lateral contacts are between monomers of thesame type (cr-cxand 0-0). Togetnel the longitudinal and lateral contacts arerepeated in the regular helical lattice of the microtubule. Becausemultiple contacts within the lattice hold most of the subunits in a microtubule in place, theaddition and loss of subunits occurs almost exclusivelyat the microtubule ends(seeFigure 16-8). These multiple contacts among subunits make microtubulesstiff and difficult to bend. The stiffness of a filament can be characterized by itspersistencelength, a property of the filament describing how long it must bebefore random thermal fluctuations are likely to cause it to bend.

The persistence length of a microtubule is several millimeters, making microtubules thestiffest and straightest structural elements found in most animal cells.The subunits in each protofilament in a microtubule all point in the samedirection, and the protofilaments themselves are aligned in parallel (in Figure16-11, for example, the cr-tubulin is dornmand the B-tubulin up in each hetero-Figure 16-1 1 The structureof amicrotubuleand its subunit.(A)Thesubunitof eachprotofilamentis a tubulinheterodimer,formed from a very tightlylinkedpairof cr-and P-tubulinmonomers.TheGTPmoleculein the o-tubulinmonomeris so tightlyboundthat it canbe consideredan integralpart oftheprotein.TheGTPmoleculein the B-tubulinmonomer,however,is lesstightlyboundand hasan importantrolein filamentdynamics.Bothnucleotidesareshowninred.(B)One tubulinsubunit(cr-p heterodimer)and one protofilamentareshownschematically.Eachprotofilamentconsistsof manyadjacentsubunitswith the sameorientation.(C)The microtubuleis a stiffhollowtubeformedfrom 13 protofilamentsalignedin(D)A shortsegmentof aparallel.microtubuleviewedin an electron(E)Electronmicrographof amicroscope.crosssectionof a microtubuleshowingaring of 13 distinctprotofilaments.(D,courtesyof RichardWade;E,courtesyof RichardLinck.)[6rer1ra6ogurl)e paurels1o,(salrnor'))'sluaLueluIlanrle6auJo sqder6orrrr.uuort)all ())'uolleluauoeuiesaql a^eqluaLueluaq] ulqtlMslrunqnseqr llv'ruu 19,(.rana6ulleadellsrMle qtrM'xrlaqe Jo spueilslallereoo/!{lseraqlo q)ea punorepurMq)rLlM'st)eluo)lerarel,(qraqraborplaq'sluaue;ryolordorvrlJo 6urlsrsuo)seJolq6noql oq osleue) ll 'siaurouourJo xrlaqa;6urse se paqu)sapuauo srluauelr,aqt qbnoqlly'luauielue ur srououou.ljo'alnraloLuaql,o lalua)luaua6uerry(B)aql ul ual) daap e ur punoq (dcv lo dlvaplloal)nue seqraruouou]urt)eraLll!a)aqf (v)'luauieluurl)P puPJaurououru!l)e ue ,o sarnl)nJlsoql Z [-9 [ oJn6rJuru0s())'pua snullu aql pellBc sI pue Jaqlo eqlpue 'pua snld aql pelleJ sI 'lsuJ aJEa3e>1uuqspue qtMoJS qloq ereqM']uaruelJ'}selseJsezlJsru^lodep oslB pua SuIMorSe Jo spue o^N eql Jo sIIuBu,{p eJolu aqJ-lseJeql 'uollsJluaouoJ IBOIIIJJaq] Moleq sdoJp uol]eJlueJuof, llunqns eeq eqllPql os pefnlp dprder arBstuauleluJI'(tI-gI ernBIC)roqlo eql uEql ralsPJqf,nluselp8uole luelu8uJJ qcEe Jo pua euo 'slueIuBIIJ peluJoJeJdJo s}ueu8e4 pe{Ielu'.snql 'Jeqlo eqt lEoluo elquesse o] pe^ ollP sr sllunqns paulJnd Jo ssasxe ue JI'rrorypue u9l-uopelcosstpueql pue euo ]e JaleaJBqcnu uauo eJe-^lelllf,ad."tpuP uorlPrsossEJoJsluelsuos alEJlllaupl aql 'luauEIIJ JPIod[lleJn]JnJls B uI'pue qJPe lElueJeJJIprfua,raq z{erus}uels-uoc alEJ asaql Jo sanle^ alnlosqe aqr q8noql uele (spua oml aql lE lecnuapl'asPf,slq]eq ]snu gJo4luoq'slueJsuoJ eleJ pJE^DIf,EqpuB pJEl!\loJeql Jo olleJ eq]'(uopu1 'raur[1od eql Jo pua raq]Ie ]e s]runqns Jo uolllppe roJ eruBseql eq lsnur-eJluaJuoJpue 'eouaJeJJlpeqleJoJeJeqlpue)runuqiltnbaeql]uelsuoJlecrlrJc'slpnqnsd8raua eer; aq1 'srsfloJp^q dIC Jo dIVJo acuesqe eq} uII + u Jo }uau-EIrJeursllnseJslrunqns il Jo ]ueluPlg P Jo pue reqlle ol lFnqns E Jo uoplppv'saler q1,r,ror8lueuelrJ uo ]JeJJapuno;ord E eABqleql s,(E,vrul ]uaJeJJIpraurf1odqcea Jo spua oml aql seTetu uolleluelro srql Lhrrelod leJnlsnJls sapqruoJf,Iupue slueruelu ullce sanr8 slrunqns Jleql Jo uoneluelro lalered Sep8ar aqlseleuluereJJlole /v\or9leql spul l)u!ls!c o^ I e^eHsluaulelllull)v pue selnqnlol)lw'lueluellJ ullcEIenpIAIpuI uE ueql JeSuoJlsqsnlu seJnlcnJlsesaql Suqeru 'raqtaSol ueq] alpunq pue luqssorJ (lrtolaqeas)upJp elpJs-eBre1'sJe]sIuoJOIlu sue] ana;e dposuralord ,trossacce '1ac Suurg € uI ]ng3o t{}3ua1JoaouelsrsJad B qrl \ 'salnqnloJJltu pcrrpulrtc ^ olloq eql qlIA\ pareduroc luaq,i.1seapue alqFeg '(lazrqelar ere stueruelg ultov'4laq papueq-rq31J e uI reqloqcea punoJe ]suvq leql slueuellJolord lalprud orvrlJo lslsuoc ol peJeplsuooequBf, lueIuEIIJ ullJe aql firrelod leJnlcnJls ]JuIlsIp E qllm slueruelu aleraua8 olIrel-ol-peeq elquesse sllunqns uIl3E aq]'uqnqnl roJ sV'(ZI{1 arntlg) (dCC io)'epuoelcnu P JoJallsdlg uBqt JeqtBI (dCVro) dJV sI eplloelsnu aql uDce JoJ]nq(ullnqnl e{I'I 'Jarulp e ueql Jeq}EJJeluouo(uSurpurq E spq lpnqns ullJe qJeee snq] sr pue urcqc apqdaddlod re1nqo13 aputs e sI lrunqns ul]f,e aqJ'pue Jeqlo aqt pasodxa surpqnl-fl pue pue auo le pasodxa suqnqnl-nte'eJoJareq; '(rarupqlp\ ,quelod prnlcnrls ]Jullslp B seq JIeslI eFqnlorf,Ilu aql(8)(v)pua snu!urPUA SnUrUr( l u a u l e l ! 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