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Current Opinion in Structural Biology Apr 2010Many cellular functions depend on rapid and localized actin polymerization/depolymerization. Yet, the de novo polymerization of actin in cells is kinetically unfavorable... (Review)
Review
Many cellular functions depend on rapid and localized actin polymerization/depolymerization. Yet, the de novo polymerization of actin in cells is kinetically unfavorable because of the instability of polymerization intermediates (small actin oligomers) and the actions of actin monomer binding proteins. Cells use filament nucleation and elongation factors to initiate and sustain polymerization. Structural biology is beginning to shed light on the diverse mechanisms by which these unrelated proteins initiate polymerization, undergo regulation, and mediate the transition of monomeric actin onto actin filaments. A prominent role is played by the W domain, which in some of these proteins occurs in tandem repeats that recruit multiple actin subunits. Pro-rich regions are also abundant and mediate the binding of profilin-actin complexes, which are the main source of polymerization competent actin in cells. Filament nucleation and elongation factors frequently interact with Rho-family GTPases, which relay signals from membrane receptors to regulate actin cytoskeleton remodeling.
Topics: Actins; Animals; Humans; Models, Molecular; Protein Conformation; Protein Subunits
PubMed: 20096561
DOI: 10.1016/j.sbi.2009.12.012 -
Proceedings of the National Academy of... Jul 2010In order to understand the mechanism of muscle contraction at the atomic level, it is necessary to understand how myosin binds to actin in a reversible way. We have used...
In order to understand the mechanism of muscle contraction at the atomic level, it is necessary to understand how myosin binds to actin in a reversible way. We have used a novel molecular dynamics technique constrained by an EM map of the actin-myosin complex at 13-A resolution to obtain an atomic model of the strong-binding (rigor) actin-myosin interface. The constraining force resulting from the EM map during the molecular dynamics simulation was sufficient to convert the myosin head from the initial weak-binding state to the strong-binding (rigor) state. Our actin-myosin model suggests extensive contacts between actin and the myosin head (S1). S1 binds to two actin monomers. The contact surface between actin and S1 has increased dramatically compared with previous models. A number of loops in S1 and actin are involved in establishing the interface. Our model also suggests how the loop carrying the critical Arg 405 Glu mutation in S1 found in a familial cardiomyopathy might be functionally involved.
Topics: Actins; Molecular Dynamics Simulation; Muscle Contraction; Myosins
PubMed: 20616041
DOI: 10.1073/pnas.1003604107 -
Journal of Cell Science May 2018The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle... (Review)
Review
The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle tissues. In higher vertebrates, from birds through to mammals, actin is represented by a family of six conserved genes. Although these genes have evolved independently for more than 100 million years, they encode proteins with ≥94% sequence identity, which are differentially expressed in different tissues, and tightly regulated throughout embryogenesis and adulthood. It has been previously suggested that the existence of such similar actin genes is a fail-safe mechanism to preserve the essential function of actin through redundancy. However, knockout studies in mice and other organisms demonstrate that the different actins have distinct biological roles. The mechanisms maintaining this distinction have been debated in the literature for decades. This Review summarizes data on the functional regulation of different actin isoforms, and the mechanisms that lead to their different biological roles We focus here on recent studies demonstrating that at least some actin functions are regulated beyond the amino acid level at the level of the actin nucleotide sequence.
Topics: Actins; Amino Acid Sequence; Amino Acids; Animals; Humans; Mice; Nucleotides
PubMed: 29739859
DOI: 10.1242/jcs.215509 -
Scientific Reports Feb 2022Cortical actin plays a key role in cell movement and division, but has also been implicated in the organisation of cell surface receptors such as G protein-coupled...
Cortical actin plays a key role in cell movement and division, but has also been implicated in the organisation of cell surface receptors such as G protein-coupled receptors. The actin mesh proximal to the inner membrane forms small fenced regions, or 'corrals', in which receptors can be constrained. Quantification of the actin mesh at the nanoscale has largely been attempted in single molecule datasets and electron micrographs. This work describes the development and validation of workflows for analysis of super resolved fixed cortical actin images obtained by Super Resolved Radial Fluctuations (SRRF), Structured Illumination Microscopy (3D-SIM) and Expansion Microscopy (ExM). SRRF analysis was used to show a significant increase in corral area when treating cells with the actin disrupting agent cytochalasin D (increase of 0.31 µm ± 0.04 SEM), and ExM analysis allowed for the quantitation of actin filament densities. Thus, this work allows complex actin networks to be quantified from super-resolved images and is amenable to both fixed and live cell imaging.
Topics: A549 Cells; Actin Cytoskeleton; Actins; Cell Membrane; Cytochalasin D; Humans; Image Processing, Computer-Assisted; Microscopy, Fluorescence
PubMed: 35177729
DOI: 10.1038/s41598-022-06702-w -
Bioarchitecture 2013Tropomyosin is an actin binding protein that regulates actin filament dynamics and its interactions with actin binding proteins such as myosin, tropomodulin, formin,... (Review)
Review
Tropomyosin is an actin binding protein that regulates actin filament dynamics and its interactions with actin binding proteins such as myosin, tropomodulin, formin, Arp2/3 and ADF-cofilin in most eukaryotic cells. Tropomyosin is the prototypical two-chained, α-helical coiled coil protein that associates end-to-end and binds to both sides of the actin filament. Each tropomyosin molecule spans four to seven actin monomers in the filament, depending on the size of the tropomyosin. Tropomyosins have a periodic heptad repeat sequence that is characteristic of coiled coil proteins as well as additional periodicities required for its interaction with the actin filament, where each periodic repeat interacts with one actin molecule. This review addresses the role of periodic features of the Tm molecule in carrying out its universal functions of binding to the actin filament and its regulation and the specific features that may determine the isoform specificity of tropomyosins.
Topics: Actins; Amino Acid Sequence; Animals; Humans; Protein Binding; Protein Structure, Secondary; Tropomyosin
PubMed: 23887197
DOI: 10.4161/bioa.25616 -
Scientific Reports Nov 2019Actin is a ubiquitous, essential, and highly abundant protein in all eukaryotic cells that performs key roles in contractility, adhesion, migration, and leading edge...
Actin is a ubiquitous, essential, and highly abundant protein in all eukaryotic cells that performs key roles in contractility, adhesion, migration, and leading edge dynamics. The two non-muscle actins, β- and γ-, are ubiquitously present in every cell type and are nearly identical to each other at the amino acid level, but play distinct intracellular roles. The mechanisms regulating this distinction have been the focus of recent interest in the field. Work from our lab has previously shown that β-, but not γ-, actin undergoes N-terminal arginylation on Asp3. While functional evidence suggest that this arginylation may be important to actin's function, progress in these studies so far has been hindered by difficulties in arginylated actin detection, precluding estimations of the abundance of arginylated actin in cells, and its occurrence in different tissues and cell types. The present study represents the first antibody-based quantification of the percentage of arginylated actin in migratory non-muscle cells under different physiological conditions, as well as in different cells and tissues. We find that while the steady-state level of arginylated actin is relatively low, it is consistently present in vivo, and is somewhat more prominent in migratory cells. Inhibition of N-terminal actin acetylation dramatically increases the intracellular actin arginylation level, suggesting that these two modifications may directly compete in vivo. These findings constitute an essential step in our understanding of actin regulation by arginylation, and in uncovering the dynamic interplay of actin's N-terminal modifications in vivo.
Topics: Acetylation; Actins; Animals; Arginine; Cells, Cultured; Cytoplasm; Embryo, Mammalian; Fibroblasts; Mice; Protein Domains; Protein Processing, Post-Translational
PubMed: 31723207
DOI: 10.1038/s41598-019-52848-5 -
Canadian Journal of Physiology and... Feb 2015Recent studies have demonstrated a novel molecular mechanism for the regulation of airway smooth muscle (ASM) contraction by RhoA GTPase. In ASM tissues, both myosin... (Review)
Review
Recent studies have demonstrated a novel molecular mechanism for the regulation of airway smooth muscle (ASM) contraction by RhoA GTPase. In ASM tissues, both myosin light chain (MLC) phosphorylation and actin polymerization are required for active tension generation. RhoA inactivation dramatically suppresses agonist-induced tension development and completely inhibits agonist-induced actin polymerization, but only slightly reduces MLC phosphorylation. The inhibition of MLC phosphatase does not reverse the effects of RhoA inactivation on contraction or actin polymerization. Thus, RhoA regulates ASM contraction through its effects on actin polymerization rather than MLC phosphorylation. Contractile stimulation of ASM induces the recruitment and assembly of paxillin, vinculin, and focal adhesion kinase (FAK) into membrane adhesion complexes (adhesomes) that regulate actin polymerization by catalyzing the activation of cdc42 GTPase by the G-protein-coupled receptor kinase-interacting target (GIT) - p21-activated kinase (PAK) - PAK-interacting exchange factor (PIX) complex. Cdc42 is a necessary and specific activator of the actin filament nucleation activator, N-WASp. The recruitment and activation of paxillin, vinculin, and FAK is prevented by RhoA inactivation, thus preventing cdc42 and N-WASp activation. We conclude that RhoA regulates ASM contraction by catalyzing the assembly and activation of membrane adhesome signaling modules that regulate actin polymerization, and that the RhoA-mediated assembly of adhesome complexes is a fundamental step in the signal transduction process in response to a contractile agonist.
Topics: Actins; GTP Phosphohydrolases; Humans; Lung; Muscle Contraction; Muscle, Smooth; Signal Transduction; rhoA GTP-Binding Protein
PubMed: 25531582
DOI: 10.1139/cjpp-2014-0388 -
Journal of Computational Neuroscience Feb 2022In this article, we elucidate the roles of divalent ion condensation and highly polarized immobile water molecules on the propagation of ionic calcium waves along actin...
In this article, we elucidate the roles of divalent ion condensation and highly polarized immobile water molecules on the propagation of ionic calcium waves along actin filaments. We introduced a novel electrical triple layer model and used a non-linear Debye-Huckel theory with a non-linear, dissipative, electrical transmission line model to characterize the physicochemical properties of each monomer in the filament. This characterization is carried out in terms of an electric circuit model containing monomeric flow resistances and ionic capacitances in both the condensed and diffuse layers. We considered resting and excited states of a neuron using representative mono and divalent electrolyte mixtures. Additionally, we used 0.05V and 0.15V voltage inputs to study ionic waves along actin filaments in voltage clamp experiments. Our results reveal that the physicochemical properties characterizing the condensed and diffuse layers lead to different electrical conductive mediums depending on the ionic species and the neuron state. This region specific propagation mechanism provides a more realistic avenue of delivery by way of cytoskeleton filaments for larger charged cationic species. A new direct path for transporting divalent ions might be crucial for many electrical processes found in localized neuron elements such as at mitochondria and dendritic spines.
Topics: Actin Cytoskeleton; Actins; Cations; Cytoskeleton; Models, Neurological
PubMed: 34392446
DOI: 10.1007/s10827-021-00795-4 -
International Journal of Molecular... May 2024Actin filaments, as key components of the cytoskeleton, have aroused great interest due to their numerous functional roles in eukaryotic cells, including intracellular...
Actin filaments, as key components of the cytoskeleton, have aroused great interest due to their numerous functional roles in eukaryotic cells, including intracellular electrical signaling. The aim of this research is to characterize the alternating current (AC) conduction characteristics of both globular and polymerized actin and quantitatively compare their values to those theoretically predicted earlier. Actin filaments have been demonstrated to act as conducting bionanowires, forming a signaling network capable of transmitting ionic waves in cells. We performed conductivity measurements for different concentrations of actin, considering both unpolymerized and polymerized actin to identify potential differences in their electrical properties. These measurements revealed two relevant characteristics: first, the polymerized actin, arranged in filaments, has a lower impedance than its globular counterpart; second, an increase in the actin concentration leads to higher conductivities. Furthermore, from the data collected, we developed a quantitative model to represent the electrical properties of actin in a buffer solution. We hypothesize that actin filaments can be modeled as electrical resistor-inductor-capacitor (RLC) circuits, where the resistive contribution is due to the viscous ion flows along the filaments; the inductive contribution is due to the solenoidal flows along and around the helix-shaped filament and the capacitive contribution is due to the counterion layer formed around each negatively charged filament.
Topics: Actin Cytoskeleton; Actins; Electric Conductivity; Animals; Polymerization
PubMed: 38791524
DOI: 10.3390/ijms25105485 -
Proceedings of the Japan Academy.... 2010The actin cytoskeleton drives cell locomotion and tissue remodeling. The invention of live-cell fluorescence single-molecule imaging opened a window for direct viewing... (Review)
Review
The actin cytoskeleton drives cell locomotion and tissue remodeling. The invention of live-cell fluorescence single-molecule imaging opened a window for direct viewing of the actin remodeling processes in the cell. Since then, a number of unanticipated molecular functions have been revealed. One is the mechanism of F-actin network breakdown. In lamellipodia, one third of newly polymerized F-actin disassembles within 10 seconds. This fast F-actin turnover is facilitated by the filament severing/disrupting activity involving cofilin and AIP1. Astoundingly fast dissociation kinetics of the barbed end interactors including capping protein suggests that F-actin turnover might proceed through repetitive disruption/reassembly of the filament near the barbed end. The picture of actin polymerization is also being revealed. At the leading edge of the cell, Arp2/3 complex is highly activated in a narrow edge region. In contrast, mDia1 and its related Formin homology proteins display a long-distance directional molecular movement using their processive actin capping ability. Recently, these two independently-developed projects converged into a discovery of the spatiotemporal coupling between mDia1-mediated filament nucleation and actin disassembly. Presumably, the local concentration fluctuation of G-actin regulates the actin nucleation efficiency of specific actin nucleators including mDia1. Pharmacological perturbation and quantitative molecular behavior analysis synergize to reveal hidden molecular linkages in the actin turnover cycle and cell signaling.
Topics: Actins; Animals; Cell Movement; Humans; Kinetics; Protein Multimerization; Protein Structure, Quaternary; Pseudopodia
PubMed: 20075609
DOI: 10.2183/pjab.86.62