Spontaneous fragmentation of actin filaments in physiological conditions

Abstract

Actin filaments have been shown to result from nucleation and consecutive binding of actin monomers to the ends of filaments¹. Nucleation is assumed to consist of the aggregation of a few monomers to form a small filament, but apart from the nucleation and the elongation reaction, end-to-end association of filaments has also been reported². It is not known whether in physiological conditions actin filaments can break spontaneously at subunit contacts along the filament. In view of the possible importance of spontaneous fragmentation in the determination of the length, number and turnover of thin filaments in the cell, I have now studied this reaction by measuring the polymerization kinetics of actin at low total concentrations in physiological conditions (1 mM MgCl₂, 100 mM KC1, 37 °C). Quantitative analysis of the polymerization curves suggests that actin filaments break spontaneously and consequently create more nucleation sites as the reaction proceeds. This explains the autocatalytic nature of the polymerization kinetics.