Control of Actin Dynamics in Cell MotilityIntroductionThe steady state of F-actin assembly in the presence of ATPFigure 1Control of actin sequestration/ desequestrationRegulation of actin filament turnover by capping proteins and ADF/cofilinFigure 2Figure 3Figure 4Capping proteins increase the rate of barbed end assembly at steady state: the funneled treadmilling modelActin depolymerizing factor (ADF/cofilin) enhances the rate of filament turnoverConclusions and perspectivesAcknowledgementsReferencesREVIEW ARTICLEControl of Actin Dynamics in Cell MotilityMarie-FranceCarlier*andDominiquePantaloniLaboratoire d'Enzymologie etBiochimie Structurales, CNRS91198 Gif-sur-Yvette, FranceActin polymerization plays a major role in cell movement. The controlsof actin sequestration/desequestration and of ®lament turnover are twoimportant features of cell motility. Actin binding proteins use propertiesderived from the steady-state monomer±polymer cycle of actin in thepresence of ATP, to control the F-actin/G-actin ratio and the turnoverrate of actin ®laments. Capping proteins and pro®lin regulate the size ofthe pools of F-actin and unassembled actin by affecting the steady-stateconcentration of ATP±G-actin. At steady state, the treadmilling cycle ofactin ®laments is fed by their disassembly from the pointed ends. It isregulated in two different ways by capping proteins and ADF, as fol-lows. Capping proteins, in decreasing the number of growing barbedends, increase their individual rate of growth and create a ``funneled''treadmilling process. ADF/co®lin, in increasing the rate of pointed-enddisassembly, increases the rate of ®lament turnover, hence the rate ofbarbed-end growth. In conclusion, capping proteins and ADF cooperateto increase the rate of actin assembly up to values that support the ratesof actin-based motility processes.# 1997 Academic Press LimitedKeywords: actin polymerization; treadmilling; pro®lin; ADF; cappingproteins*Corresponding authorIntroductionIt is now well accepted that cell locomotion and,more generally, changes in cell shape in responseto stimuli are powered by actin polymerization(Condeelis,1993).Physicalanalysesshowthatactin polymerization can provide a protrusiveforce suf®cient to overcome the resistance of thecellmembrane(Corteseetal.,1989;Mogilner&Oster,1996).Inrecentyears,twoaspectsofthein-volvement of actin polymerization in motility havegiven rise to intense investigations.First, a shape change of the cell in response tostimuli often necessitates a massive polymerizationof actin ®laments. The stimulation of blood plate-lets, neutrophils or chemotactic amoebae, is rapidlyfollowed by a large increase in the cellular amountof F-actin. The increase in the F-actin pool corre-lates with an identical decrease in the pool of ``se-questered actin'', i.e. G-actin in complex withproteins that prevent actin from polymerizing,such as thymosin-b4 and pro®lin. The mechanismof control of the F-actin/G-actin ratio in cells is akey issue in motility.Another fascinating feature of cell motility is theuse of rapid actin ®lament turnover to generatemovement. Continuous assembly of actin ®lamentsat the leading edge of locomoting cells builds upthe protrusive ®lopodial or lamellipodial exten-sions of the cytoplasm that determine the directionof movement. While net polymerization occurs atthe front, net depolymerization occurs at the rearof the lamella. The rate of movement is 1 to10 mm/min. For the advance of the lamellipodiumto be driven by actin polymerization, the rate of ®-lament growth at the leading edge would have tobe as fast as 10 to 100 subunits per second. A keyissue is to understand by which mechanism a cellcan ``maintain high rates of net polymerization andnet depolymerization simultaneously at differentsitesinitscytoplasm''(Fechheimer&Zigmond,1993).Insuchasteadyregimeoflocomotion,theoverall cellular F-actin content remains constant.The actin subunits coming from ®laments depoly-merizing at the rear of the lamella are recycled intonew ®laments assembled at the front in the see-mingly rapid treadmilling process observed inlocomotingkeratocytes(Wang,1985;Small,1995).Early evidence for the autonomy of the lamella asamotilemachinehasbeenprovided(Euteneuer&Schliwa,1984).Abbreviations used: Tb4, thymosin-b4; ADF, actindepolymerizing factor.J. Mol. Biol. (1997) 269, 459±4670022±2836/97/240459±9 $25.00/0/mb971062 # 1997 Academic Press LimitedBacterial pathogens such as Listeria monocyto-genesorShigella¯exneri(Higley&Way,1997,forareview) mimic the dynamic behavior of actin ®la-ments at the leading edge. They elicit their ownpropulsion in the cytoplasm by inducing actin pol-ymerization at their surface. The movement can bemonitoredinvitroinacellularextracts(Theriotetal.,1994;Marchandetal.,1995),whichprovideabasis for identifying the cellular components of themotile machinery involved in actin nucleation atthe plasma membranes and eventually reconstitut-ing actin-based motile processes in a controlledmedium.In this short review, we will survey the prin-ciples of actin polymerization that are used bydifferent actin binding proteins either to regulateactin desequestration, thus eliciting massive assem-bly of ®laments, or to control actin ®lament turn-over, thus mediating the forward movement of theleading edge.The steady state of F-actin assembly inthe presence of ATPAt the physiological ionic strength, in the pre-sence of ATP, ®laments (F-actin) coexist withmonomeric actin (G-actin) at the critical concen-tration for polymerization. Because ATP hydrolysisis associated with actin polymerization, the criticalconcentration is not, in this case, a physical mono-mer±polymer equilibrium dissociation constant. Itis the steady-state concentration at ATP±G-actin,CSS, that is maintained in the medium via mono-mer±polymer exchange reactions. In the cell med-ium, actin is assembled under these steady-stateconditions.Actin ®laments have a structural polarity, with abarbed end, at which subunits associate rapidly,and a pointed end, which has much slower dy-namics. The structural polarity is in itself suf®cientto account for the polarized dynamics of actin ®la-ments. In addition to the kinetic difference betweenthe two ends, the fact that ATP hydrolysis is as-sociated with polymerization generates a differencein critical concentration between the barbed andthe pointed ends. The critical concentration at thepointed end, CPC, can be experimentally determinedby
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