The structure of the F-actin filament and the actin molecule

doi:10.1016/0955-0674(92)90054-G

Current Opinion in Cell Biology

Volume 4, Issue 1, February 1992, Pages 20-26

https://doi.org/10.1016/0955-0674(92)90054-G Get rights and content

Abstract

A consensus view on the three-dimensional structure of the F-actin filament and the relative strength of the intersubunit contacts in the filament has been established from an atomic filament model and recent three-dimensional reconstructions from electron micrographs of F-actin filaments. Functional implications of recent structural and biochemical data indicating a rather dynamic filament structure are discussed.

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This suggests that processed beta actin is otherwise functional and can incorporate into filaments and undergo disassembly regardless of its N-terminus. An attractive possibility is that processed actin subunits within filaments and/or the monomer pool can affect interactions with actin-binding proteins that are attracted or repelled by the negatively charged actin N-terminal tails exposed on the actin subunit surface (Holmes et al., 1982; Kabsch et al., 1985; Bremer and Aebi, 1992; Chik et al., 1996; Rebowski et al., 2020). Locally applied, such attraction or repulsion can potentially have a profound effect on actin function.
Cytoplasmic beta- and gamma-actin are ubiquitously expressed in every eukaryotic cell. They are encoded by different genes, but their amino acid sequences differ only by four conservative substitutions at the N-termini, making it difficult to dissect their individual regulation. Here, we analyzed actin from cultured cells and tissues by mass spectrometry and found that beta, unlike gamma actin, undergoes sequential removal of N-terminal Asp residues, leading to truncated actin species found in both F- and G-actin preparations. This processing affects up to ∼3% of beta actin in different cell types. We used CRISPR/Cas-9 in cultured cells to delete two candidate enzymes capable of mediating this type of processing. This deletion abolishes most of the beta actin N-terminal processing and results in changes in F-actin levels, cell spreading, filopodia formation, and cell migration. Our results demonstrate previously unknown isoform-specific actin regulation that can potentially affect actin functions in cells.
Dynamical mechanism of stepping of the molecular motor myosin V along actin filament and simulation in an actual system
2019, Physica A: Statistical Mechanics and its Applications
Citation Excerpt :
Also, I verify the applicability of the dynamical mechanism with a simulation experiment preserving most of the known characteristics of myosin–actin complex [2]. The structure of the actin filament comprises two twisted strands of the actin monomers [27–29]. The actin monomer can be considered as a brick with a square face that is 5.5 nm in width and 3.5 nm in height [24].
On the base of the random motion of myosin, the structural asymmetry of the actin filament, and the interaction between the motor and the track, the present study proposes a physical mechanism for the processive stepping of the two-headed molecular motor myosin V along the actin filament. The detachment from actin filament and forward swing of the trailing head, which are caused by increased entropy and the release of the bending energy in the neck region, are the initial steps of the stepping process. The detached head then rotates and randomly moves around the still attached head. A step is completed after the detached head detects the next binding site. In the mechanism, the direction of myosin V movement was determined on the basis of the configuration of the actomyosin system. The role of ATP is the energy provider for the local random motion. The applicability of the dynamic mechanism was evaluated on a simulation system that preserved most of the characteristics of the actomyosin system.
Unconventional actin configurations step into the limelight
2013, Advances in Protein Chemistry and Structural Biology
Citation Excerpt :
These interactions, together with its inherent plasticity, render actin one of the most versatile proteins in the eukaryotic cell, both with respect to structure and function. Due to its propensity to form noncrystalline filamentous aggregates or paracrystalline arrays under the conditions normally used for crystallization (Bremer & Aebi, 1992), monomeric actin has for long resisted crystallization attempts. Different strategies employed to prevent actin from polymerizing, including point mutations, covalent modification, or association with other proteins or ligands, finally allowed its structure to be determined at atomic resolution (Fig. 5.1; Kabsch et al., 1990).
The existence of a cellular machinery that is based on the reversible polymerization of globular nucleotide-bound protomers into polar microfilaments is a persistent feature from prokaryotes to higher vertebrates. However, while in bacteria, actin-like proteins with such properties have evolved into a large family with divergent sequences and polymeric structures, eukaryotes express only a small number of highly conserved actins. Indeed, the sequence of actin is one of the best conserved among eukaryotes and yet actin carries out many different functions at distinct cellular sites. Because of the notorious conservation and lack of suitable tools to examine structural plasticity, the vast majority of studies on cellular actin functions consider mainly two structural states, G-actin and F-actin. However, there is more to the structural plasticity of actin than first meets the eye. On one hand, more than 200 actin-binding proteins shape the conformation of actin and thereby regulate functional diversity. On the other hand, unconventional actin conformations that differ from monomeric G-actin are stepping into the limelight. In addition, supramolecular actin structures that extend beyond classical F-actin are emerging. Herein, we recapitulate the current knowledge on the structure and conformations of monomeric actin and its polymerization into higher order structures, paying special attention to less known forms and their involvement in actin function.
Two deafness-causing (DFNA20/26) actin mutations affect Arp2/3-dependent actin regulation
2012, Journal of Biological Chemistry
Hearing requires proper function of the auditory hair cell, which is critically dependent upon its actin-based cytoskeletal structure. Currently, ten point mutations in nonmuscle γ-actin have been identified as causing progressive autosomal dominant nonsyndromic hearing loss (DFNA20/26), highlighting these ten residues as functionally important to actin structure and/or regulation. Two of the mutations, K118M and K118N, are located near the putative binding site for the ubiquitously expressed Arp2/3 complex. We therefore hypothesized that these mutations may affect Arp2/3-dependent regulation of the actin cytoskeleton. Using in vitro bulk polymerization assays, we show that the Lys-118 mutations notably reduce actin + Arp2/3 polymerization rates compared with WT. Further in vitro analysis of the K118M mutant using TIRF microscopy indicates the actual number of branches formed per filament is reduced compared with WT and, surprisingly, branch location is altered such that the majority of K118M branches form near the pointed end of the filament. These results highlight a previously unknown role for the Lys-118 residue in the actin-Arp2/3 interaction and also further suggest that Lys-118 may play a more significant role in intra- and intermonomer interactions than was initially hypothesized.
Three-dimensional cryo-electron microscopy on intermediate filaments
2010, Methods in Cell Biology
Together with microtubules and actin filaments (F-actin), intermediate filaments (IFs) form the cytoskeleton of metazoan cells. However, unlike the other two entities that are extremely conserved, IFs are much more diverse and are grouped into five different families. In contrast to microtubules and F-actin, IFs do not exhibit a polarity, which may be the reason that no molecular motors travel along them. The molecular structure of IFs is less well resolved than that of the other cytoskeletal systems. This is partially due to their functional variability, tissue-specific expression, and their intrinsic structural properties. IFs are composed mostly of relatively smooth protofibrils formed by antiparallel arranged α-helical coiled–coil bundles flanked by small globular domains at either end. These features make them difficult to study by various electron microscopy methods or atomic force microscopy (AFM). Furthermore, the elongated shape of monomeric or dimeric IF units interferes with the formation of highly ordered three-dimensional (3-D) crystals suitable for atomic resolution crystallography. So far, most of the data we currently have on IF macromolecular structures come from electron microscopy of negatively stained samples, and fragmented α-helical coiled–coil units solved by X-ray diffraction. In addition, AFM allows the observation of the dynamic states of IFs in solution and delivers a new view into the assembly properties of IFs. Here, we discuss the applicability of cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) for the field. Both methods are strongly related and have only recently been applied to IFs. However, cryo-EM revealed distinct new features within IFs that have not been seen before, and cryo-ET adds a 3-D view of IFs revealing the path and number of protofilaments within the various IF assemblies.
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Intermediate filaments are a large and structurally diverse group of cellular filaments that are classified into five different groups. They are referred to as intermediate filaments (IFs) because they are intermediate in diameter between the two other cytoskeletal filament systems that is filamentous actin and microtubules. The basic building block of IFs is a predominantly α-helical rod with variable length globular N- and C-terminal domains. On the ultra-structural level there are two major differences between IFs and microtubules or actin filaments: IFs are non-polar, and they do not exhibit large globular domains. IF molecules associate via a coiled-coil interaction into dimers and higher oligomers. Structural investigations into the molecular building plan of IFs have been performed with a variety of biophysical and imaging methods such as negative staining and metal-shadowing electron microscopy (EM), mass determination by scanning transmission EM, X-ray crystallography on fragments of the IF stalk and low-angle X-ray scattering. The actual packing of IF dimers into a long filament varies between the different families. Typically the dimers form so called protofibrils that further assemble into a filament. Here we introduce new cryo-imaging methods for structural investigations of IFs in vitro and in vivo, i.e., cryo-electron microscopy and cryo-electron tomography, as well as associated techniques such as the preparation and handling of vitrified sections of cellular specimens.

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The structure of the F-actin filament and the actin molecule

Abstract

Curr Opin Cell Biol

J Mol Biol

J Mol Biol

J Muscle Res Cell Motil

Annu Rev Biochem

Science

Nature

J Microsc

Nature

Nature

Ultramicroscopy

Plants Contain Highly Divergent Actin Isovariants

Cell Motil Cytoskeleton

The Molecular Evolution of Actin

Genetics

Atomic Structure of the Actin: DNase I Complex

Nature

Muscle Proteins: Actin

Curr Opin Struct Biol

Atomic Model of the Actin Filament

Nature

Molecular Structure of F-Actin and Location of Surface Binding Sites

Nature

The Structural Basis for the Intrinsic Disorder of the F-actin Filament: the Lateral Slipping Model

J Cell Biol

Actin

Curr Opin Cell Biol

Molecular Aspects of Microfilament Structure and Assembly

Curr Opin Struct Biol