Mechanism of K+-induced actin assembly.

Abstract

The assembly of highly purified actin from Dictyostelium discoideum amoebae and rabbit skeletal muscle by physiological concentrations of KCl proceeds through successive stages of rapid formation of distinct monomeric species referred to as KCl-monomer, incorporation of KCl-monomers into an ATP-containing filament, and ATP hydrolysis that occurs significantly after the incorporation event. KCl-monomer has a conformation which is distinct from that of either conventional G- or F-actin, as judged by UV spectropscopy at 210-220 nm and by changes in ATP affinity. ATP is not hydrolyzed during conversion of G-actin to KCl-monomer. KCl-monomer formation precedes filament formation and may be necessary for the assembly event. Although incorporation of KCl-monomers into filaments demonstrates lagphase kinetics by viscometry, both continuous absorbance monitoring at 232 nm and rapid sedimentation of filaments demonstrate hyperbolic assembly curves. ATP hydrolysis significantly lags the formation of actin filaments. When half of the actin has assembled, only 0.1-0.2 mol of ATP are hydrolyzed per mol of actin present as filaments.