The End of a Polymerizing Actin Filament Contains Numerous ATP−Subunit Segments That Are Disconnected by ADP−Subunits Resulting from ATP Hydrolysis

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Abstract
ATP hydrolysis by copolymers of ATP−actin and ADP−actin was investigated in order to analyze the effect of interfaces between ATP−subunits and ADP−subunits on hydrolysis of actin-bound ATP. Copolymers of ATP− and ADP−subunits were formed by polymerization of ATP− and ADP−actin monomers onto filaments. By changing the ratio of polymerizing ATP−actin monomers to ADP−actin monomers, the number of interfaces between ATP− and ADP−subunits and of ATP−subunits only surrounded by further ATP−subunits was varied. The rate of actin polymerization and of ATP hydrolysis was measured simultaneously on the same samples. The lag time between incorporation of actin monomers into filaments and subsequent ATP hydrolysis was found to be similar both for polymerized ATP−actin and for copolymers formed by various ratios of ATP− to ADP−actin. The experiments were performed in the presence of 1 mM MgCl2, 0.05 mM CaCl2, and 100 mM KCl or of 1 mM MgCl2 and 0.4 mM EGTA. The type of cations was found to have no major effect on the rate of ATP hydrolysis. A quantitative evaluation of the experimental data suggests that ATP at interfaces between ATP− and ADP−subunits is hydrolyzed not more than 10 times faster than ATP of subunits surrounded by further ATP−subunits. On the basis of these results, one can conclude that an actin filament onto which ATP−actin monomers polymerize contains numerous segments of ATP−subunits that are disconnected by ADP−subunits resulting from ATP hydrolysis. The average length of the numerous ATP segments of a steadily polymerizing filament is in the range of 10 ATP−subunits or below.