| Abstract
| - The time course of actin depolymerization wasquantitatively analyzed to obtain insight intothe reactions occurring during actin disassembly. Polymeric actinwas diluted, and subsequently the timecourse of depolymerization was measured. In the presence of 0.5 mMATP, 100 mM KCl, and 1 mMMgCl2, continuous depolymerization was observed both whenthe filaments were carefully diluted andwhen the filaments were fragmented to produce short filaments. Therates of the reactions that are knownto occur during depolymerization, such as dissociation and associationof ADP− and ATP−actin moleculesand exchange of nucleotides bound to monomeric actin, were determinedby independent experiments.When the determined rate parameters were used to calculate thetime course of depolymerization,consistently in the simulations fast depolymerization of ADP−actinwas followed by slower polymerizationof ATP−actin that was formed from ADP−actin by nucleotide exchange.The lack of fast depolymerizationand subsequent slower polymerization in the experiments suggests thatour present conception about actindisassembly requires modification. Good agreement of calculatedtime courses with the experimentallydetermined continuous depolymerization was achieved if ADP bound to theterminal subunit of barbedfilament ends was assumed to be readily exchangeable for ATP. Fastnucleotide exchange at terminalsubunits may contribute to the stability of barbed filament ends and totheir role as polymerizing ends inliving cells.
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