MECHANISM OF ATP HYDROLYSIS BY POLYMERIC ACTIN

The lag between polymerization of actin and ATP hydrolysis in actin fi laments was analyzed in terms of the mechanism of the hydrolysis react ion. Under the experimental conditions (100 mM KCl and 1 mM MgCl2, or without KCl, 1 mM MgCl2 and 0.4 mM EGTA, 25 degrees C) ATP hydrolysis lagged behind polymerization by about 100 s independently of the conce ntration of polymerizing filament ends and of the actin monomer concen tration. Three models of ATP hydrolysis were compared to experimental data: (i) Random ATP hydrolysis, ATP is assumed to be hydrolyzed at a rate that is independent of the type of nucleotide bound to adjacent f ilament subunits. (ii) Cooperative hydrolysis, the rate of ATP hydroly sis is thought to depend on the type of nucleotide bound to adjacent s ubunits. (iii) Sequential hydrolysis, ATP is assumed to be hydrolyzed only at the interface between ATP-subunits and ADP-subunits. The model of sequential ATP hydrolysis could be excluded. The results were in a greement with random or cooperative ATP hydrolysis. The differences of the rates of ATP hydrolysis by a random or cooperative mechanism are so small that based on the experimental results no distinction between these two mechanisms could be made. All available evidence points tow ards a mechanism of ATP hydrolysis in which several or perhaps many in terfaces between ATP- and ADP-subunits are formed within a filament.