Cryo-EM of One State of F-Actin Yields a New Atomic Filament Model

Abstract

Actin is one of the most highly conserved as well as abundant eukaryotic proteins. From chickens to humans, an evolutionary separation of ∼ 350 million years, there are no amino acid changes in the skeletal muscle isoform of actin. Since the functional form of actin in most instances is a polymer (F-actin), understanding the constraints on actin sequence evolution must involve an understanding of the structure and dynamics of the actin filament. The development of direct electron detectors has allowed an unprecedented advance in the ability of cryo-EM to reach near-atomic resolution for many protein polymers and protein complexes.We have used electron cryo-microscopy and a direct electron detector and have now been able to reconstruct one state of F-actin at 4.7 A resolution. This has allowed us to build an atomic model of this state, which differs in many details from an earlier model for F-actin derived from a substantially lower resolution reconstruction. The model explains many previous observations about F-actin, such as why the “hydrophobic plug” can be structurally polymorphic. We compare this atomic model with two other distinctly different states that we have determined at ∼ 12 A resolution, and suggest that only by understanding the multiplicity of states possible for F-actin can one understand the selective pressure on many residues and why mutations of some of these residues leads to myopathies and other human disorders.