Traffic 2000 1: 19–28Munksgaard International PublishersReviewThe Polymerization MotorJulie A. TheriotDepartment of Biochemistry and Department ofMicrobiology & Immunology,Stanford University School ofMedicine,Stanford,CA 94305-5307,[email protected] and depolymerization of actin filamentsand microtubules are thought to generate force formovement in various kinds of cell motility, ranging fromlamellipodial protrusion to chromosome segregation.This article reviews the thermodynamic and physicaltheories of how a nonequilibrium polymerization reac-tion can be used to transduce chemical energy into me-chanical energy, and summarizes the evidencesuggesting that actin polymerization produces motileforce in several biological systems.Key words: Actin, Brownian ratchet, force generation,microtubule, protein polymerizationReceived and accepted for publication 1 September 1999For convenience and flexibility, cells store much of theirdisposable chemical energy in common currencies that canbe used for multiple types of cellular metabolism. Chemicalenergy can be stored in the form of nucleoside triphos-phates, transmembrane chemical and electrical gradients, ormolecules that carry high-energy reducing electrons. Biologi-cal motors, celebrated in this inaugural issue ofTraffic, areprotein machines that convert chemical energy to mechani-cal energy and force. A transmembrane proton gradient pow-ers the bacterial flagellar rotary motor, and nucleotidehydrolysis allows molecular motors such as myosin and ki-nesin to walk in a stepwise fashion along their linear cy-toskeletal tracks, as well as allowing polymerases andhelicases to run along DNA. A different type of cellular use ofchemical energy to generate mechanical force uses a spe-cialized currency rather than a general one, nonequilibriumprotein polymerization. Polymerization of small protein sub-units to form large-scale structures is an important generalfeature of biological organization in all forms of cellular life.The most striking instances of this form of organization arefound in the eukaryotic cytoskeleton, particularly in micro-tubules and actin filaments. Although the term ‘motor’ usu-ally refers to discrete protein machines like myosin, kinesin,or the flagellar rotor, protein polymerization can also convertchemical energy into mechanical force and can therefore beconsidered another type of motor.Experimental attention to the biophysical mechanism offorce generation by the protein polymerization motor haslagged far behind equivalent studies of force generation byclassical motor proteins such as myosin and kinesin. Overthe past decade, many clever techniques have been devisedto make ever more precise measurements of the amount offorce generated by individual myosin and kinesin molecules(reviewed in (1–3)). In contrast, there is only a single pub-lished report of a direct biophysical measurement of forceproduced by an individual growing microtubule (4), and noequivalent measurements for individual actin filaments.Although there is a relative paucity of quantitative experimen-tal work on force generation by protein polymerization, thereis a wealth of theoretical literature on this topic. The purposeof this review is to summarize the thermodynamic and phys-ical basis of force generation by protein polymerization,framed in a biological context. Biological evidence for forcegeneration by microtubule polymerization and depolymeriza-tion has been summarized in an excellent review (5), so hereI will focus primarily on biological examples of force genera-tion by actin polymerization. The theoretical requirementsoutlined here, however, are relevant to both microtubulesand actin.Actin’ Like a MotorActin is one of the most abundant proteins of eukaryotic cellsand has been strongly conserved throughout eukaryotic evo-lution. Actin is often associated with biological force genera-tion, either by virtue of association with a myosin (as inmuscle contraction) or due to its own polymerization motor.A few cases where actin polymerization is thought to pro-duce force are schematized in Figure 1.Cells that crawl across solid substrates must produce twotypes of force for locomotion; protrusion force to extend theleading edge of the cell margin forward, and traction force totranslocate the cell body (6,7). Both protrusion and tractionare actin-dependent. Since the best-studied form of actin-de-pendent movement is skeletal muscle contraction, wherechemomechanical energy transduction is performed bymyosin hydrolyzing ATP, the discovery of nonmuscle myosinin crawling cells engendered the proposal that force forcrawling is also produced by myosin (8). This model had tobe revised after 1987 when it was found thatDictyosteliumamoebae completely lacking myosin II heavy chain are capa-ble of crawling, with protrusion perfectly normal and tractiononly partially affected (9,10). Of course, cells contain anabundance of other myosin isoforms, including some that arelocalized to leading edge, but knockout experiments havethus far failed to provide unequivocal evidence that anymyosin isoform is required for protrusion. Drugs that prevent19Theriotactin filament elongation such as cytochalasin D, however,stop all common forms of cellular protrusion including exten-sion of pseudopodia, lamellipodia, and filopodia. It is nowwidely believed that actin polymerization rather than myosinATPase activity provides much of the force for protrusion(11,12). Quantitative measurements of cell deformability atthe leading edge are consistent with this hypothesis (13).The fact that actin polymerization alone is capable of provid-ing sufficient force to push out a lipid bilayer has beendirectly demonstrated in experiments where monomericactin is enclosed in lipid vesicles and polymerization is in-duced by increasing the salt concentration. The lipid vesiclesstart off roughly spherical, but are drastically deformed by thepolymerizing filaments, forming flattened discs with filopo-dial-like protrusions (14–16). These results lend credence tothe proposition that similar forces are at work in cellularprotrusion.Fertilization appears to be a biological function where actinpolymerization is commonly harnessed for force generation.When the sperm of the sea cucumberThyoneencountersthe egg jelly coat, an explosive actin polymerization reactionis initiated that pushes out an acrosomal process, enablingthe sperm plasma membrane to reach and fuse with theplasma
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