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urauLp 8unea1 'palenqceut sI uISauII-nurrls lBuoruroq P JeuE parelrfuoqdsoqdaruocaq suleqs 1q3u ulsauq eql uaqMtUfi OS('oureH qeal ;o {salrno)'B)'AL\6U)lla) aqrJolalua) aq] ur patebar66ero (ryal)useldov(r aqt tnoqbnorqlpasraosrpraq]rasaurosouelaurprpllr)ue)uJvs1r6urrvroqs'qsqueJo ale)se ur lla) a;6urse;o sa6eurplag-lq6;rg(8)'salnqnlor)rtu6uolern))o sauJosouelauiJo suorlnqulsrparqlog',(len$:adsar'(dwv))6yyytr;tItJelnlla)erlurur asPar)apro asear)uruPolasuodsarur sauJosouelaur;o uor1e6a.r66epue ;esradsrpaql 6ulrtoqs';1arluaubrd e'snlnurrlsleuornau,o Mar^)lleuaq)s (v)ro leuouiloqe ol asuoosalur lla) aq] u!uorle)olr;aql a6ueq>ue) saurosoueleur.ro'sa;nuerbaql' PMuq) saurosouelaura6re1ureluor'qsr;;osaoadsluar.u6rdur uorlerolo)ur>1sur sa6ueqrleranasrol a;qrsuodsa.rareLl)rqM's11artuerbasaqf 'slla) luau6rd qsg u! sluouanourourosouplaurpa1eln6egO/-9 I arn6tlc3suldslcclIv91u99V(v)dIAV)aseoJ)uldWV)aseal)epuola;a>1soy(1aql:9 L laldeLl]vzoL1025THECYTOSKELETONAND CELLBEHAVIORa c t i n - b i n d i n gs i t em y o s i nl i g h t c h a i n s@b i p o l a rf i l a m e n tof 15-20 moleculesm y o s i nt a i l r e l e a s e dI N A C T I VSET A T E :( l i g h t c h a i n sn o t p h o s p h o r y l a t e d )ACTIVESTATE:( l i g h t c h a i n sp h o s p h o r y l a t e d )(A)Figure16-72 Light-chainphosphorylationand the regulationof the assemblyof myosinll into thick filaments.(A)Theof one of the two light chains(theso-calledcontrolledphosphorylationby the enzymemyosinlight-chainkinase(MLCK)r e g u l a t o r y l i g h t c h a i n , s h o wlni gi nh t b l u e ) o n n o n m u s c l e m y o siinnal lt e s t t u b e h a s a t l e a s t t w o e f f e cittsc:a u s e s a c h a n g ethe myosintail from a "stickypatch"site,and it releasesin the conformationof the myosinhead,exposingitsactin-bindingto assembleinto short,bipolar,thickfilaments.(B)Electronon the myosinhead,therebyallowingthe myosinmoleculesmicrographof negativelystainedshortfilamentsof myosinll that havebeeninducedto assemblein a testtube byphosphorylationof their lightchains.Thesemyosinll filamentsare muchsmallerthan thosefound in skeletalmusclecells(seeFigure16-55).(8,courtesyof JohnKendrick-Jones.)of the myosin superfamily is not as well understood, but the control of thesemyosins is likewise thought to involve site-specific phosphorylations.Su m m a r yMotor proteins use the energyof ATP hydrolysis to moue along microtubules or actinfilaments.
Theymediate the sliding of ftlaments relatiue to one another and the transport of cargoalongfilament tracks.All known motor proteinsthat moueon actin filaments are members of the myosin superfamily. The motor proteins that moue onmicrotubules are either membersof the kinesin superfamily or the dynein family. Themyosin and kinesin superfamiliesare diuerse,with about 40 genesencodingeachtypeofprotein in humans. The only structural element shared among all membersof eachsuperfamily is the motor "head"domain.
Theseheadsarefused to a wide uarieryof different "tails,"which attach to different typesof cargoand enablethe uariousfamily membersto perform dffirent functions in the cell. Thesefunctions include the transportationand localization of specificproteins,membrane-enclosedorganelles,and mRNAs.Although myosin and kinesin walk along different tracks and usedifferent mech'anisms to produceforce and mouementby ATP hydrolysis,they sharea common structural core,suggestingthat they are deriuedfrom a common ancestor.Thedynein motorprotein has independentlyeuolued,and it has a distinct structureand mechanismofaction.THECYTOSKELETONANDCELLBEH IORA central challenge in all areasof cell biology is to understand how the functionsof many individual molecular components combine to produce complex cellbehaviors.
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