Structural Basis for Actin Assembly, Activation of ATP Hydrolysis, and Delayed Phosphate Release

SummaryAssembled actin filaments support cellular signaling, intracellular trafficking, and cytokinesis. ATP hydrolysis triggered by actin assembly provides the structural cues for filament turnover in vivo. Here, we present the cryo-electron microscopic (cryo-EM) structure of filamentous actin (F-actin) in the presence of phosphate, with the visualization of some α-helical backbones and large side chains. A complete atomic model based on the EM map identified intermolecular interactions mediated by bound magnesium and phosphate ions. Comparison of the F-actin model with G-actin monomer crystal structures reveals a critical role for bending of the conserved proline-rich loop in triggering phosphate release following ATP hydrolysis. Crystal structures of G-actin show that mutations in this loop trap the catalytic site in two intermediate states of the ATPase cycle. The combined structural information allows us to propose a detailed molecular mechanism for the biochemical events, including actin polymerization and ATPase activation, critical for actin filament dynamics.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (141 K)Download as PowerPoint slideHighlights► The cryo-EM structure of actin filament reveals intermolecular interactions ► Mg2+ and phosphate ions (Pi) play an important role in stabilizing the filament ► The bending of the Pro-rich loop triggers the activation of the ATPase activity ► A cavity in the catalytic site plays a role in releasing hydrolyzed γ-phosphates