Chapter 2 Actin polymerization: Regulation by divalent metal ion and nucleotide binding, ATP hydrolysis and actin binding proteins

Abstract

This chapter discusses the adenosine triphosphate (ATP) hydrolysis linked to actin polymerization, which perturbs the thermodynamics of reversible polymerization; stereochemistry of nucleotide binding to actin and tubulin—role of divalent metal ion in nucleotide binding and hydrolysis; how profilin controls monomer-polymer steady-state and promotes actin filament assembly in the presence of thymosyn β 4 ; and myosin subfragment-1 induced polymerization of G-actin. Actin filaments are major dynamic components of the cytoskeleton of eukaryotic cells. Assembly of filaments from monomeric actin occurs with expenditure of energy, the tightly bound ATP being irreversibly hydrolyzed during polymerization. This dissipation of energy perturbs the laws of reversible helical polymerization and affects the dynamics of actin filaments. Wegner showed that the free energy of ATP hydrolysis was used in actin assembly to establish an energetic difference between the two ends of the filaments. Studies have additionally shown that ATP hydrolysis destabilizes actin–actin interactions in the filament. The destabilization is linked to the liberation of P i that follows cleavage of the γ-phosphate. P i release, therefore, plays the role of a conformational switch. Because ATP hydrolysis is uncoupled from polymerization, the nucleotide content of the filaments changes during the polymerization process, and filaments grow with a stabilizing cap of terminal adenosine diphosphate (ADP)-P i subunits. The fact that the dynamic properties of F-actin are affected by ATP hydrolysis results in a nonlinear dependence of the rate of filament elongation on monomer concentration. Possible modes of regulation of filament assembly may be anticipated from the basic properties of actin.