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As we explained with our ant-trail analogy at the beginning of thechapter, the advantage to the cell seems to be the spatial and temporal flexibility that is inherent in a structural system with constant turnover. Individual subunits are small and can diffuse very rapidly; an actin or tubulin subunit can diffuse acrossthe diameter of a typical eucaryotic cell in severalseconds.As noted-Ir a p i d g r o w t h w i t h G T P - c a p p eedn d.*iCATASTROPHEaccidentallossof GTpcaoFIrapidshrinkageF|l."ouin of GTPcapREscUEtIr a p i d g r o w t h w i t h G T P - c a p p eedn drl(A) I ""GTP-tubulidnimer--;---l-c0'@soso oG T Pc a pro0,.orns t r a i g h tp r o t o f i l a m e n tII SH A N G EST ONFORMATIONG T PH Y D R O L Y SCSU B U N I CB O N DI N T H EP O L Y M E RANDWEAKENSc u r v e dp r o t o f i l a m e n tl e s ss t a b l er e g i o no fmicrotubulecontainingGDP-tubulindimersGROWINGSHRINKING(c)(B)FigureI 6-16 Dynamicinstabilitydue to the structuraldifferencesbetweena growingand a shrinkingmicrotubuleend.
(A)lf the freein solutionis betweenthe criticalvaluesindicatedin Figure16-148,a singlemicrotubuleend may undergotransitionstubulinconcentrationsubunitsat its end,forminga GTPcap'lf nucleotidegrowingand a shrinkingstate.A growingmicrotubulehasGTP-containingastatebetweenproceedsmore rapidlythan subunitaddition,ih".up is lostand the microtubulebeginsto shrink,an eventcalleda "catastrophel'hydrolysisansubunitsmay stilladd to the shrinkingend,and if enoughadd to form a new cap,then microtubulegrowthresumes,ButGTP-containingin the microtubulelattice.Theadditionof GTP-containing(B)Modelfor the structuralconsequencesof GTPhydrolysiseventcalled"rescue."wall ofthat can readilypackinto the cylindricalcausesthe end to grow in a linearconformationtubulinsubunitsto the end of a protofilamentinto a curvedof the subunitsand tendsto forcethe protofilamentchangesthe conformationHydrolysisof Gip afterassemblythe microtubule.subunitsareprotofilamentsmadefrom GDP-containingshapethat is lessableto packinto the microtubulewall.(C)In an intactmicrotubule,subunits.Lossofforcedinto a linearconformationby the manylateralbondswithinthe microtubulewall,givena stablecap of GTP-containingleadsto a progressiveprotofilamentsto relaxinto their morecurvedconformation.Thisthe GTPcap,however,allowsthe GDP-containingshowactualmicrotubuleselectronmicrographsAbovethe drawingsof a growingand a shrinkingmicrotubule,disruptionof the microtubule.GDP-containingdisintegratingparticularlythecurling,Notein vitreousice.in eachof thesetwo states,asobservedin preparations(C,courtesyof E.M.MandelkoWE.Mandelkowand R.A.Milligan,J.
CellBiol.protofilamentsat the end of the shrinkingmicrotubule.Press.)UniversityfromThe RockefellerWith permission114:977-991,1991.982Chapter16:TheCytoskeletontime 0 sec12 5 s e c3 0 7s e cpreviously, the rate-limiting step in the formation of a new filament is nucleation, so these rapidly diffusing subunits tend to assemble either on the ends ofpreexisting filaments or at particular sites where special proteins catalyze thenucleation step.The new filaments in either caseare highly dynamic, and unlessspecifically stabilized, they have only a fleeting existence.By controlling wherefilaments are nucleated and selectivelystabilized, a cell can control the locationof its filament systems,and hence its structure.
It seems that the cell is continually testing a wide variety of internal structures and only preserving those thatare useful. \Arhenexternal conditions change, or when internal signals arise (asduring the transitions in the cell cycle), the cell is poised to change its structurerapidly (seeFigures 16-2 to 16-4).Actin and tubulin have independently evolved their nucleoside triphosphate hydrolysis to enable their filaments to depolymerize readily aftei theyhave polymerized. These two proteins are completely unrelated in amino acidsequence:actin is distantly related in structure to the glycolltic enzyme hexokinase, whereas tubulin is distantly related to a large family of GTpases thatincludes the heterotrimeric G proteins and monomeric GTpases such as Rastion you are reading now is transferred into long-term memory, even a cell asstable as a neuron can grow new elongated processesto make new slmapses.Todo this, a neuron requires the dynamic, exploratory activities of its cytoikeletalfilaments.Tubulinand ActinHaveBeenHighlyconservedDuringEucaryoticEvolutiontube.
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