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European Journal of Cell Biology 2022Actin is among the most abundant proteins in eukaryotic cells and assembles into dynamic filamentous networks regulated by many actin binding proteins. The actin... (Review)
Review
Actin is among the most abundant proteins in eukaryotic cells and assembles into dynamic filamentous networks regulated by many actin binding proteins. The actin cytoskeleton must be finely tuned, both in space and time, to fulfill key cellular functions such as cell division, cell shape changes, phagocytosis and cell migration. While actin oxidation by reactive oxygen species (ROS) at non-physiological levels are known for long to impact on actin polymerization and on the cellular actin cytoskeleton, growing evidence shows that direct and reversible oxidation/reduction of specific actin amino acids plays an important and physiological role in regulating the actin cytoskeleton. In this review, we describe which actin amino acid residues can be selectively oxidized and reduced in many different ways (e.g. disulfide bond formation, glutathionylation, carbonylation, nitration, nitrosylation and other oxidations), the cellular enzymes at the origin of these post-translational modifications, and the impact of actin redox modifications both in vitro and in vivo. We show that the regulated balance of oxidation and reduction of key actin amino acid residues contributes to the control of actin filament polymerization and disassembly at the subcellular scale and highlight how improper redox modifications of actin can lead to pathological conditions.
Topics: Actin Cytoskeleton; Actins; Amino Acids; Microfilament Proteins; Oxidation-Reduction
PubMed: 35716426
DOI: 10.1016/j.ejcb.2022.151249 -
Current Opinion in Cell Biology Aug 2022Cells ingest large particles, such as bacteria, viruses, or apoptotic cells, via the process of phagocytosis, which involves formation of an actin-rich structure known... (Review)
Review
Cells ingest large particles, such as bacteria, viruses, or apoptotic cells, via the process of phagocytosis, which involves formation of an actin-rich structure known as the phagocytic cup. Phagocytic cup assembly and closure results from a concerted action of phagocytic receptors, regulators of actin polymerization, and myosin motors. Recent studies using advanced imaging approaches and biophysical techniques have revealed new information regarding phagocytic cup architecture, regulation of actin assembly, and the distribution, direction, and magnitude of the forces produced by the cytoskeletal elements that form the cup. These findings provide insights into the mechanisms leading to the assembly, expansion, and closure of phagocytic cups. The new data show that engulfment and internalization of phagocytic targets rely on several distinct yet complementary mechanisms that support the robust uptake of foreign objects and may be precisely tailored to the demands of specific phagocytic pathways.
Topics: Actins; Cell Membrane; Cytoskeleton; Phagocytes; Phagocytosis
PubMed: 35820329
DOI: 10.1016/j.ceb.2022.102112 -
Nature Communications May 2023Immune cells, such as macrophages and dendritic cells, can utilize podosomes, mechanosensitive actin-rich protrusions, to generate forces, migrate, and patrol for...
Immune cells, such as macrophages and dendritic cells, can utilize podosomes, mechanosensitive actin-rich protrusions, to generate forces, migrate, and patrol for foreign antigens. Individual podosomes probe their microenvironment through periodic protrusion and retraction cycles (height oscillations), while oscillations of multiple podosomes in a cluster are coordinated in a wave-like fashion. However, the mechanisms governing both the individual oscillations and the collective wave-like dynamics remain unclear. Here, by integrating actin polymerization, myosin contractility, actin diffusion, and mechanosensitive signaling, we develop a chemo-mechanical model for podosome dynamics in clusters. Our model reveals that podosomes show oscillatory growth when actin polymerization-driven protrusion and signaling-associated myosin contraction occur at similar rates, while the diffusion of actin monomers drives wave-like coordination of podosome oscillations. Our theoretical predictions are validated by different pharmacological treatments and the impact of microenvironment stiffness on chemo-mechanical waves. Our proposed framework can shed light on the role of podosomes in immune cell mechanosensing within the context of wound healing and cancer immunotherapy.
Topics: Podosomes; Actins; Macrophages
PubMed: 37217555
DOI: 10.1038/s41467-023-38598-z -
ELife Mar 2022Intracellular protein gradients serve a variety of functions, such as the establishment of cell polarity or to provide positional information for gene expression in...
Intracellular protein gradients serve a variety of functions, such as the establishment of cell polarity or to provide positional information for gene expression in developing embryos. Given that cell size in a population can vary considerably, for the protein gradients to work properly they often have to be scaled to the size of the cell. Here, we examine a model of protein gradient formation within a cell that relies on cytoplasmic diffusion and cortical transport of proteins toward a cell pole. We show that the shape of the protein gradient is determined solely by the cell geometry. Furthermore, we show that the length scale over which the protein concentration in the gradient varies is determined by the linear dimensions of the cell, independent of the diffusion constant or the transport speed. This gradient provides scale-invariant positional information within a cell, which can be used for assembly of intracellular structures whose size is scaled to the linear dimensions of the cell, such as the cytokinetic ring and actin cables in budding yeast cells.
Topics: Actins; Cell Polarity; Cytoplasm; Diffusion; Saccharomycetales
PubMed: 35311649
DOI: 10.7554/eLife.71365 -
International Journal of Molecular... Oct 2022The actin cytoskeleton lies at the heart of many essential cellular processes. There are hundreds of proteins that cells use to control the size and shape of actin... (Review)
Review
The actin cytoskeleton lies at the heart of many essential cellular processes. There are hundreds of proteins that cells use to control the size and shape of actin cytoskeletal networks. As such, various pathogens utilize different strategies to hijack the infected eukaryotic host actin dynamics for their benefit. These include the control of upstream signaling pathways that lead to actin assembly, control of eukaryotic actin assembly factors, encoding toxins that distort regular actin dynamics, or by encoding effectors that directly interact with and assemble actin filaments. The latter class of effectors is unique in that, quite often, they assemble actin in a straightforward manner using novel sequences, folds, and molecular mechanisms. The study of these mechanisms promises to provide major insights into the fundamental determinants of actin assembly, as well as a deeper understanding of host-pathogen interactions in general, and contribute to therapeutic development efforts targeting their respective pathogens. This review discusses mechanisms and highlights shared and unique features of actin assembly by pathogen effectors that directly bind and assemble actin, focusing on eukaryotic actin nucleator functional mimics Sca2 (formin mimic), BimA (Ena/VASP mimic), and VopL (tandem WH2-motif mimic).
Topics: Actin Cytoskeleton; Actins; Eukaryota; Eukaryotic Cells; Formins
PubMed: 36232907
DOI: 10.3390/ijms231911606 -
Communications Biology Sep 2023Actin, an important component of eukaryotic cell cytoskeleton, regulates cell shape and transport. The morphology and biochemical properties of actin filaments are...
Actin, an important component of eukaryotic cell cytoskeleton, regulates cell shape and transport. The morphology and biochemical properties of actin filaments are determined by their structure and protein-protein contacts. Crowded environments can organize filaments into bundles, but less is known about how they affect F-actin structure. This study used 2D IR spectroscopy and spectral calculations to examine how crowding and bundling impact the secondary structure and local environments in filaments and weakly or strongly bundled networks. The results reveal that bundling induces changes in actin's secondary structure, leading to a decrease in β-sheet and an increase in loop conformations. Strongly bundled networks exhibit a decrease in backbone solvent exposure, with less perturbed α-helices and nearly "locked" β-sheets. Similarly, the loops become less hydrated but maintain a dynamic environment. These findings highlight the role of loop structure in actin network morphology and stability under morphology control by PEG.
Topics: Actins; Actin Cytoskeleton; Protein Structure, Secondary; Cytoskeleton; Cell Shape
PubMed: 37660224
DOI: 10.1038/s42003-023-05274-3 -
ELife Jul 2022Experiments using purified proteins reveal how the network of filaments that underlie cell movement becomes denser when pushing against a stronger mechanical force.
Experiments using purified proteins reveal how the network of filaments that underlie cell movement becomes denser when pushing against a stronger mechanical force.
Topics: Actin Cytoskeleton; Actin-Related Protein 2-3 Complex; Actins; Cell Movement; Cytoskeleton
PubMed: 35894589
DOI: 10.7554/eLife.81108 -
Science Advances Apr 2020About 80% of human proteins are amino-terminally acetylated (Nt-acetylated) by one of seven Nt-acetyltransferases (NATs). Actin, the most abundant protein in the...
About 80% of human proteins are amino-terminally acetylated (Nt-acetylated) by one of seven Nt-acetyltransferases (NATs). Actin, the most abundant protein in the cytoplasm, has its own dedicated NAT, NAA80, which acts posttranslationally and affects cytoskeleton assembly and cell motility. Here, we show that NAA80 does not associate with filamentous actin in cells, and its natural substrate is the monomeric actin-profilin complex, consistent with Nt-acetylation preceding polymerization. NAA80 Nt-acetylates actin-profilin much more efficiently than actin alone, suggesting that profilin acts as a chaperone for actin Nt-acetylation. We determined crystal structures of the NAA80-actin-profilin ternary complex, representing different actin isoforms and different states of the catalytic reaction and revealing the first structure of NAT-substrate complex at atomic resolution. The structural, biochemical, and cellular analysis of mutants shows how NAA80 has evolved to specifically recognize actin among all cellular proteins while targeting all six actin isoforms, which differ the most at the amino terminus.
Topics: Acetylation; Acetyltransferases; Actins; Amino Acid Sequence; Binding Sites; Fluorescent Antibody Technique; Humans; Models, Molecular; Molecular Conformation; Profilins; Protein Binding; Protein Domains; Protein Isoforms; Protein Multimerization; Structure-Activity Relationship; Substrate Specificity
PubMed: 32284999
DOI: 10.1126/sciadv.aay8793 -
Biophysical Journal Oct 2021We used computational methods to analyze the mechanism of actin filament nucleation. We assumed a pathway where monomers form dimers, trimers, and tetramers that then...
We used computational methods to analyze the mechanism of actin filament nucleation. We assumed a pathway where monomers form dimers, trimers, and tetramers that then elongate to form filaments but also considered other pathways. We aimed to identify the rate constants for these reactions that best fit experimental measurements of polymerization time courses. The analysis showed that the formation of dimers and trimers is unfavorable because the association reactions are orders of magnitude slower than estimated in previous work rather than because of rapid dissociation of dimers and trimers. The 95% confidence intervals calculated for the four rate constants spanned no more than one order of magnitude. Slow nucleation reactions are consistent with published high-resolution structures of actin filaments and molecular dynamics simulations of filament ends. One explanation for slow dimer formation, which we support with computational analysis, is that actin monomers are in a conformational equilibrium with a dominant conformation that cannot participate in the nucleation steps.
Topics: Actin Cytoskeleton; Actins; Cytoskeleton; Kinetics; Polymerization
PubMed: 34509503
DOI: 10.1016/j.bpj.2021.09.006 -
ELife Feb 2020Transport of fluids, molecules, nutrients or nanoparticles through coral tissues are poorly documented. Here, we followed the flow of various tracers from the external...
Transport of fluids, molecules, nutrients or nanoparticles through coral tissues are poorly documented. Here, we followed the flow of various tracers from the external seawater to within the cells of all tissues in living animals. After entering the general coelenteric cavity, we show that nanoparticles disperse throughout the tissues via the paracellular pathway. Then, the ubiquitous entry gate to within the cells' cytoplasm is macropinocytosis. Most cells form large vesicles of 350-600 nm in diameter at their apical side, continuously internalizing their surrounding medium. Macropinocytosis was confirmed using specific inhibitors of PI3K and actin polymerization. Nanoparticle internalization dynamics is size dependent and differs between tissues. Furthermore, we reveal that macropinocytosis is likely a major endocytic pathway in other anthozoan species. The fact that nearly all cells of an animal are continuously soaking in the environment challenges many aspects of the classical physiology viewpoints acquired from the study of bilaterians.
Topics: Actins; Animals; Anthozoa; Cytoplasm; Dextrans; Diffusion; Models, Biological; Nanoparticles; Pinocytosis
PubMed: 32039759
DOI: 10.7554/eLife.50022