Direct Visualization of Secondary Structures of F-actin by Electron Cryomicroscopy

Abstract

F-actin is a helical assembly of actin, an essential component of muscle fibers for contraction, and plays crucial roles in numerous cellular processes, such as lamellipodia and filopodia as the most abundant component and regulator of cytoskeletons by dynamic assembly and disassembly processes (from G-actin to F-actin and vice versa). While actin is a ubiquitous protein and is involved in important biological functions, the definitive high-resolution structure of F-actin remains unknown. Here we report the F-actin structure at 6.6 Å resolution, made obtainable by recent advances in electron cryomicroscopy. The density map clearly resolves all the secondary structures, such as α-helices, β-structures and loops, having made unambiguous modeling and refinement possible. Complex domain motions that open up the nucleotide binding pocket upon F-actin formation, specific D-loop and terminal conformations, and relatively tight axial but markedly loose interprotofilament interactions of hydrophilic nature revealed in the F-actin model all appear to be important for dynamic functions of actin.