==== Front Biophys PhysicobiolBiophys PhysicobiolBiophysics and Physicobiology2189-4779The Biophysical Society of Japan (BSJ) 10.2142/biophysico.15.0_15115_151Review ArticleMy various thoughts on actin Oosawa Fumio 123 1 Department of Physics, Graduate School of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan 2 Institute of Molecular Biology, Graduate School of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan 3 Department of Biophysical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, JapanCorresponding author: Fumio Oosawa, 1-34-1 Kawana, Showa-ku, Nagoya, Aichi 466-0856, Japan.2018 18 7 2018 15 151 158 10 5 2018 04 6 2018 2018 © The Biophysical Society of Japan2018This article is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/4.0/.An enormous amount of research has been performed to characterize actin dynamics. Structural biology investigations have determined the localization of main chains and their changes coupled with G (Globular)-F (Filamentous) transformation of actin, whereas local thermal fluctuations that may be caused by free rotations of the tips of side chains are not yet fully investigated. This paper argues if the entropy change of actin accompanied by the G-F transformation is simply attributable to the changes in hydration. It took almost 10 years to understand that the actin filament is semi-flexible. This flexibility was visually confirmed as the development of optical microscope techniques, and the direct observation of actin severing events in the presence of actin binding proteins became possible. Finally, I expect the deep understanding of actin dynamics will lead to the elucidation of self-assembly mechanisms of the living creature. actin thermodynamicsentropy changes in G-F transformationhydration of actinactin binding proteinsflexibility of actin filament ==== Body Significance Actin research is facing a paradigm shift as a result of recent rapid advances in scientific technologies. To share the current leading actin research, the international symposium “Now in actin study” was organized in December, 2016 at Nagoya University. This paper originates from my presentation there and is dedicated to the late Profs. Michiki Kasai (1937–2015) and Sho Asakura (1927–2016). This also introduces a letter from Prof. Donald L. D. Caspar, who collaborated with leading Japanese actin scientists such as Kasai and Asakura through his sabbatical stay. He has made outstanding contributions to our understanding of the functions and mechanisms of biological macromolecular assemblies such as TMV, flagella and actin filaments. Thermodynamic Parameters of Actin Polymerization Actin is an extremely unique protein in the sense that it is possible to calculate the value of every thermodynamic quantity involved in the polymerization and depolymerization equilibrium. This calculation was performed based on the value of the critical concentration for polymerization (Cc) ([1–3] and Figs. 1 and 2), as published in the first part of Chapter V in Oosawa-Asakura’s book entitled “Thermodynamics of the Polymerization of Protein” [1]. This book chapter also describes the required experiments for obtaining thermodynamic quantities. An example of the calculations obtained by Kikumoto is shown in Table 1 (see also [4]). The estimated thermodynamic quantities were not only different between Mg2+-actin and Ca2+-actin, but also between K+ and Na+ in the polymerization buffer. Now, based on the answer to the calculation, we can obtain the value of entropy changes coupled to the monomer-polymer transition as shown in Table 1. The entropy change is quite big, and the value becomes considerably larger when actin takes a dimeric form as compared with the combined value of two independent monomers. Then the question arises: what causes such large entropy changes. Most people consider that, when actin molecules are dimerized, the amount of hydration water per molecule will become smaller than that for two separate actin monomers because the surface area of actin in contact with aqueous solution decreases, so that the entropy of the system will become larger due to the contribution to the entropy of the water molecules released from actin. Is it so simple and correct? Previously, an investigation of the hydration amount and the volume change caused from G-F transformations [5] suggested that about a quarter of hydration water of an actin monomer was released during the polymerization [6,7]. The hydration state of F-actin also depends on the bound cations [8]. Can the release of hydration water molecules completely explain the entropy change caused from the polymerization equilibrium? What should we do to make this point clearer? This is our next question. On the Intramolecular Structure of Actin In the above section, I wrote that the common approach may be to consider that the entropy changes of actin molecules occur due to the increase and decrease in hydration of the whole molecule. However, what happens if intramolecular hydration exists here and there in actin? My suggestion is to examine the intramolecular structure of actin in more detail without limiting the problem so narrowly. Three-dimensional structures of many protein molecules have been revealed by conventional X-ray crystallography. In the case of actin, its structure was revealed in 1990 and has not changed much since then [9]. It is perfect. However, it is only about the conformation of main chains. All amino acids have respective side chains. So my interests are on the states and conformations of side chains. The side chains of amino acids may be fluctuating and partly rotating, so that various cases are possible because their conformations and movements may be different under different conditions. The current analysis of the molecular structure obtained, e.g., by X-ray crystallography, mainly reveals a static structure and hardly gives information of intramolecular movements. But, for example, if the tips of side chains freely rotate, the effect on the entropy changes would be large. There might be local thermal fluctuations that are explainable as local temperature. How deeply has such dynamic characteristics of the molecular structure been understood by the structural analysis so far? The papers on structural analyses of actin published since 1990 are listed in Table 2 with some comments [10–16]. Actually, in the case of actin molecule, since I imagine a local thermal ratchet in the structure, I have a deep interest in such fluctuations of internal atomic groups. In short, the dynamic analysis of the atomic structures of individual molecules is awaited in addition to the static analysis of the intramolecular structure. It also contains hydration problems. Flexibility of Actin Filaments In 1954, it was found that the thin filament aligned in parallel within the I-band of striated muscle was the actin filament (F-actin). The thin filaments were observed to “slide” against the myosin thick filaments by the interaction with myosin molecules without bending or changing the length [17,18]. Since then, researchers did not pay much attention to whether the actin filaments, i.e., the thin filaments, are flexible and pliant. However, it is natural to consider that the actin filament is not a rigid rod but is a semi-flexible polymer when the filament becomes longer and its tangent becomes measurable, because the actin filament is a helical polymer in which globular actin molecules are non-covalently linked together. The very first experiment to examine whether an actin filament is practically a string-like bendable structure in aqueous solutions was done by Fujime [19]. He applied quasi-elastic scattering of laser light to observe the thermal bending fluctuation and quantitatively determined the bending stiffness of actin filaments [19]. This method detects small Doppler broadening of frequency of laser light due to scattering from the actin filaments showing Brownian translational, rotational and bending movements. From the degree of broadening, which depended on the scattering angle, Fujime attributed a part of the broadening to the bending movement of actin filaments and estimated their flexural rigidity. The results of this experiment were reported in 1970 [19]. Since then, it took nearly 10 years for the notion “the actin filament is semi-flexible” to be recognized. It was finally confirmed by direct visualization of the Brownian motions of individual actin filaments in solution by technical advances in dark-field optical microscopy by Nagashima and Asakura [20]. This technique allowed us to perform quantitative measurements of the average amplitude and temporal frequency of thermal bending motions of actin filaments, and the results were consistent with those obtained by quasi-elastic scattering of laser light [20]. The direct observation and analysis that demonstrated the semi-flexible structure of actin filament were reported in 1978 [20]. Although it may sound easy, it actually took 10 years to convince researchers about the flexibility of the filaments. Nowadays, no one sticks to the idea that actin filaments are “rigid rods”. Then, the recent advances in electron cryo-microscopy techniques have made it possible to examine the detailed intramolecular structure of actin filament. Each filament looks like a straight rod in electron cryo-micrographs of ice-embedded frozen-hydrated specimens. Not only the intramolecular structures but also intermolecular binding interactions between actin subunits within the filament have been visualized in detail [11–15]. To apply this technique, the solution of actin filaments has to be quickly frozen by plunging the electron microscopy grid into liquid ethane to avoid crystallization of ice. It is considered that the reason why each filament looks straight instead of curved seems to be attributable to this method of quick freezing without chemical fixation including staining. By recent technical advances, it has become possible to custom-design the structure of actin filaments. For example, a circular bundle of actin filaments similar to the contractile ring, which appears on the occasion of cell division, has been reconstituted within a water-in-oil droplet covered with a mono-layered phospholipid membrane by Miyazaki et al. [21]. The rotational movement of actin rings was accidentally found by Higashi-Fujime when she squeezed out the cytoplasm of Nitella cells and observed the dynamics of filamentous structures under an optical microscope [22]. This ring showed a rapid rotational movement when ATP was present. I still do not know how such an ultra-high speed rotation can be achieved by myosin motors of Nitella. Formation and Severing of Actin Filaments Actin filaments take various conformations from moment to moment, especially beneath the plasma membrane in living cells. Through elongation, branching, shortening by polymerization, severing, depolymerization and so on, actin filaments form a network structure over a wide area in a cell. This actin-based cell motility is orchestrated in cooperation with various regulatory proteins and other kinds of filaments formed by various cytoskeletal proteins. More than several tens of proteins are involved in the formation of such network structures. Here I will describe proteins having a severing effect on actin filaments, on which research has recently been advanced. This is the case of actin extracted from Physarum polycephalum by Hatano et al. [23]. In particular, in the presence of Ca2+, this actin filament becomes short and flexible. Later, a novel protein that severs actin filaments was extracted from P. polycephalum and named fragmin, because Physarum actin filaments become more flexible and are easily severed when fragmin is added [24]. Upon adding fragmin and ATP to the solution of Physarum actin filaments in the presence of Ca2+, the filaments are immediately broken to small fragments and scattered around. When I saw this phenomenon, I remembered the experiment done by Asakura a long time ago as described below. To visualize individual actin filaments under a dark-field microscope, he decorated actin filaments with heavy meromyosin (HMM: a proteolytic fragment of myosin molecule). This increased the light scattering intensity and enabled him to observe Brownian movements of actin filaments. It was not possible to visualize individual actin filaments in solution without HMM decoration when this experiment was done in 1978 [20]. This was an epoch-making experiment that made it possible to directly visualize individual actin filaments even though the filaments were thickened by decoration with HMM. This experiment proved that actin filaments are semi-flexible and show bending Brownian movements in solution. During a series of these experiments, Asakura succeeded in visualizing individual actin filaments under a fluorescence microscope by labeling them with a fluorescent dye, i.e., fluorescein (FITC) [25]. Asakura investigated the effects of myosin subfragment-1 (S1: a proteolytic fragment of myosin molecule or a myosin motor domain, as a part of HMM having the ability to interact with actin and ATPase) on fluorescent actin filaments in the presence of ATP. Actin filaments were severed rapidly into small fragments. I vividly remember that Asakura showed me this phenomenon several times. There was no need to use actin purified from Physarum or use fragmin. S1 (with ATP) possesses an intrinsic ability to sever actin filaments. The reason why I focused on such properties would be because I had been carefully observing the “super precipitation phenomenon” that occurs in the mixed solution of actin filaments and myosin molecules with ATP. In this assay, we often observed the severing and fragmentation of actin filaments by using electron microscopy. Note that, in 1969, Tawada and myself published a paper showing that the elongation of actin filaments stops when S1 and ATP are added at the same time [26] (This was the last paper that described my own experiments). Many interesting experiments remain to be done to carefully investigate behaviors of actin filament and myosin in the presence of ATP at various levels of combinations and conditions. I wonder if there is a point overlapping between the properties summarized in the Proceedings of Ciba Foundation Symposium 50 years ago (Table 3, [27]) and those presented by Uyeda and his colleagues in the Actin Symposium held in 2016 in Nagoya [28,29]. It was a great pleasure to see many presentations on various severing proteins of actin filaments, such as fragmin [24], gelsolin (e.g., [30]), villin (e.g., [31]), ADF/cofilin (e.g., [32]) and Pf ADF1 (e.g., [33]), and electron microscopic observations on actin polymers at this Actin Symposium (see [4,29,34–40]). At the end, it is my pleasure to share a letter from Prof. Donald L. D. Caspar at Florida State University, USA. He collaborated with Kasai and Asakura through his stay in Japan and has made outstanding contributions to our understanding of the functions and mechanisms of biological macromolecular assemblies such as TMV, flagella and actin filaments. Acknowledgment I would like to thank Ikuko Fujiwara and Timothy F. Day for their valuable contributions on completing the manuscript. I also thank Taro Uyeda for providing lists of actin binding proteins with structural information, and Hajime Honda and Kingo Takiguchi for helping to confirm previous experimental setups. I am grateful to Mahito Kikumoto for preparing the modified version of the plots in Figures 1 and 2. I am also grateful to Keiichi Namba, Katsuzo Wakabayashi and Shin’ichi Ishiwata for valuable contributions on the manuscript and discussions. Conflicts of Interest The author declares that he has no conflict of interest. Author Contribution F. O. gave various thoughts on actin and wrote the manuscript. Appendix Figure 1 The specific features of the polymerization of actin as a condensation phenomenon. (a) The experimental relation between the degree of flow birefringence (Δn) (dependent on the concentration of F-actin) and the total actin concentration (C0) at various temperatures, i.e., 20°C (1), 6°C (2) and 0°C (3). Actin concentration, 3.4 mg mL−1; 0.4 mM MgCl2; 7.5 mM veronal-HCl buffer of pH 8.3; 0.75 mM ATP. (b) The theoretical relation between the concentrations of G- (Ci) or F-actin (Ch) and the total actin concentration (C0). The critical concentration of actin polymerization (Cc) is inversely proportional to the absolute temperature. The figure is reproduced from previously published figure [3]. Figure 2 van’t Hoff plot for the critical concentration (Cc) of actin polymerization originally obtained by Michiki Kasai. The figure is reproduced from previously published figure [1]. Table 1 Experimentally estimated thermodynamic quantities of actin polymerization equilibrium #Conditions temp. (°C) T (K) Cc (μM) ΔG$ (cal/mol) ΔH$ (cal/mol) ΔS$ (cal/molK) K 30 mM Mg-actin 4 277.15 1.44 −7399 5334 45.9 〃 18 291.15 0.7 −8192 〃 46.5 〃 23.5 296.65 0.84 −8240 〃 45.8 Na 30 mM Mg-actin 4 277.15 0.43 −8069 2434 38.5 〃 18 291.15 0.26 −8752 〃 38.4 〃 23.5 296.65 0.35 −8751 〃 37.7 K 30 mM Ca-actin 4 277.15 3.01 −6993 3906 39.3 〃 18 291.15 2.41 −7475 〃 39.1 〃 23.5 296.65 1.81 −7786 〃 39.4 Na 30 mM Ca-actin 4 277.15 2.06 −7203 2652 35.6 〃 18 291.15 1.85 −7628 〃 35.3 〃 23.5 296.65 1.43 −7923 〃 35.6 *Low ionic condition (Kasai’s [1,2]) temp. (°C) T (K) Cc* (μM) ΔG* (cal/mol) ΔH* (cal/mol) ΔS* (cal/molK) 0.4 mM MgCl2 0 273.15 61.9 −5254 14407 72 〃 6 279.15 40.8 −5600 〃 71.7 〃 20 293.15 10.4 −6674 〃 71.9 T; absolute temperature (K), Cc; critical concentration for polymerization, ΔG; Gibbs’ free energy change of polymerization equilibrium, ΔH; enthalpy change of polymerization equilibrium, ΔS; entropy change of polymerization equilibrium. The 5% pyrene-labeled actin that bound Ca2+ or Mg2+ was polymerized in buffers including potassium or sodium ions overnight at each indicated temperature to measure Cc by monitoring pyrene fluorescence with excitation wavelength of 366 nm and emission wavelength of 407 nm [4]. Because the actin polymerization satisfies a two-phase thermodynamic equilibrium known as the Clausius-Clapeyron relation [1, 2], the thermodynamic quantities were determined from the slope and the 1/T axis-intercept of the linear regression line of the van’t Hoff plot (ln Cc=ΔG/RT=ΔH/RT−ΔS/R, where R is the gas constant [4]) (see Fig. 2). $ Other thermodynamic parameters calculated from Cc and T; ΔG=RT lnCc, ΔH=R·(slope of lnCc-1/T plot), ΔS=(ΔG−ΔH)/T * Calculated and read data from the figures in refs [1] and [2]. # Conditions: K 30 mM Mg-actin and Na 30 mM Mg-actin indicate that Mg2+-bound actin molecules were polymerized in 5 mM Tris-HCl, pH 8.0 and 30 mM KCl or NaCl, respectively. K 30 mM Ca-actin and Na 30 mM Ca-actin indicate that Ca2+-bound actin molecules were polymerized in 5 mM Tris-HCl, pH 8.0 with 30 mM KCl or NaCl, respectively. Table 2 List of F-actin structures at near-atomic levels, associating with dynamics of actin polymerization and interactions with other actin binding proteins 1) In 1990, Kabsch et al. and Holmes et al. showed that structures of G-actin [9] and F-actin [10] are not so extremely different, except the hydrophobic plug. This plug region was thought to stabilize the two strands of filament structure, but it was proved later by the structural analysis of F-actin by electron cryo-microscopy that the plug is actually a hydrophilic one by von der Ecken et al. [16]. 2) In 2009, Oda et al. showed the nature of G- to F-actin conformational transition by using X-ray fiber diffraction analysis together with cryo-electron microscope’s data by Narita [11]. The transition is caused by a rotation of two major domains of actin, and actin molecules become flat by this rotation upon formation of the filament. 3) In 2010, Fujii et al. used a novel electron cryomicroscopy technique and reported a three-dimensional density map of actin filaments [12]. The map showed all secondary structures of actin molecule in the filament structure. This map confirmed that actin is flat in the filament and that it forms one conformation. The map also visualized four residues at the N-terminus, which play important roles in the binding of other proteins such as myosin. 4) In 2010, Murakami et al. showed the electron cryomicroscopic structure of actin filaments with intermolecular interactions mediated by Mg2+ or inorganic phosphate (Pi) [13]. They showed conformational changes orchestrated with actin polymerization and suggested four steps in actin ATPase cycle. 5) In 2010, Galkin et al. found six different modes in the skeletal muscle F-actin structure [14]. 6) In 2015, Galkin et al. improved the resolution of electron cryomicrographs and reconstituted two classes of F-actin structure [15]. Both F-actin maps possess a significant opening of the ATP-binding cleft. These two actin structures are different from each other regarding the movement of subdomain 4 (SD4) toward SD2. One of these actin structures takes a form to easily release Pi after ATP hydrolysis. 7) In 2015, von der Ecken et al. found that the binding of G-actin to the barbed-end of F-actin is initiated by the intra-strand binding of SD4 of G-actin to SD3 of actin molecule in F-actin [16]. Then the D-loop of G-actin is trapped in the hydrophobic cleft of SD3 of actin molecule in F-actin and pulls on SD2. However, the binding of G-actin at the pointed-end is more complicated. Including the processes described above, in total seven processes are required for G-actin to bind to the pointed-end of F-actin. After 2010, the resolution of F-actin structural analyses is significantly improved. This allows us to discuss the dynamics of F-actin structure at a near-atomic level. Table 3 Rate regulation of polymerization of actin* Polymerization process (Bond formation) Depolymerization process (Bond breaking) Nucleotides Mechanical agitation Divalent cations Thermal agitation Myosin Myosin+ATP Polymer nuclei α-actinin, one of the actin accessory proteins, that was found more than 50 years ago [41, 42] has a regulatory function for polymerization of actin. * Adopted from Ciba Foundation Symposium at 1966 [27]. ==== Refs References 1 Oosawa F Asakura S Thermodynamics of the Polymerization of Proteins Academic Press New York 1975 2 Asakura S Kasai M Oosawa F The effect of temperature on the equilibrium state of actin solutions J Polymer Sci Part A: General Papers 44 35 49 1960 3 Oosawa F Macromolecular assembly of actin Muscle and nonmuscle motility Stracher A 1 151 216 Academic Press New York 1983 4 Kikumoto M Oosawa F Thermodynamic measurements of actin polymerization with various cation species Cytoskeleton (Hoboken) 74 465 471 2017 29080281 5 Ikkai T Ooi T Noguchi H Actin: volume change on transformation of G-form to F-form Science 152 1756 1757 1966 5938415 6 Gekko K Hasegawa Y Compressibility-structure relationship of globular proteins Biochemistry 25 6563 6571 1986 3790543 7 Kabir SR Yokoyama K Mihashi K Kodama T Suzuki M Hyper-mobile water is induced around actin filaments Biophys J 85 3154 3161 2003 14581215 8 Suzuki M Imao A Mogami G Chishima R Watanabe T Yamaguchi T Strong dependence of hydration state of F-actin on the bound Mg(2+)/Ca(2+) ions J Phys Chem B 120 6917 6928 2016 27332748 9 Kabsch W Mannherz HG Suck D Pai EF Holmes KC Atomic structure of the actin: DNase I complex Nature 347 37 44 1990 2395459 10 Holmes KC Popp D Gebhard W Kabsch W Atomic model of the actin filament Nature 347 44 49 1990 2395461 11 Oda T Iwasa M Aihara T Maéda Y Narita A The nature of the globular- to fibrous-actin transition Nature 457 441 445 2009 19158791 12 Fujii T Iwane AH Yanagida T Namba K Direct visualization of secondary structures of F-actin by electron cryomicroscopy Nature 467 724 728 2010 20844487 13 Murakami K Yasunaga T Noguchi TQ Gomibuchi Y Ngo KX Uyeda TQ Structural basis for actin assembly, activation of ATP hydrolysis, and delayed phosphate release Cell 143 275 287 2010 20946985 14 Galkin VE Orlova A Schröder GF Egelman EH Structural polymorphism in F-actin Nat Struct Mol Biol 17 1318 1323 2010 20935633 15 Galkin VE Orlova A Vos MR Schröder GF Egelman EH Near-atomic resolution for one state of F-actin Structure 23 173 182 2015 25533486 16 von der Ecken J Müller M Lehman W Manstein DJ Penczek PA Raunser S Structure of the F-actin-tropomyosin complex Nature 519 114 117 2015 25470062 17 Huxley AF Niedergerke R Structural changes in muscle during contraction; interference microscopy of living muscle fibres Nature 173 971 973 1954 13165697 18 Huxley H Hanson J Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation Nature 173 973 976 1954 13165698 19 Fujime S Quasi-elastic light scattering from solutions of macromolecules II. Doppler broadening of light scattered from solutions of semi-flexible polymers, F-actin J Phys Soc Japan 29 751 759 1970 20 Nagashima H Asakura S Dark-field light microscopic study of the flexibility of F-actin complexes J Mol Biol 136 169 182 1980 6990003 21 Miyazaki M Chiba M Eguchi H Ohki T Ishiwata S Cell-sized spherical confinement induces the spontaneous formation of contractile actomyosin rings in vitro Nat Cell Biol 17 480 489 2015 25799060 22 Higashi-Fujime S Active movement in vitro of bundle of microfilaments isolated from Nitella cell J Cell Biol 87 569 578 1980 7193210 23 Hatano S Oosawa F Isolation and characterization of plasmodium actin Biochim Biophys Acta 127 488 498 1966 6007360 24 Hasegawa T Takahashi S Hayashi H Hatano S Fragmin: a calcium ion sensitive regulatory factor on the formation of actin filaments Biochemistry 19 2677 2683 1980 6893158 25 Oosawa F Physicochemical properties of single filaments of F-actin Actin, structure and function in muscle and nonmuscle cells Dos Remedious C Barden J 69 80 Academic Press Sydney 1983 26 Tawada K Oosawa F Effect of the H-meromyosin plus ATP system on F-actin Biochim Biophys Acta 180 199 201 1969 5787267 27 Oosawa F Kasai M Hatano S Asakura S Polymerization of actin and flagellin Ciba Foundation Symposium Wolstenholme GEW O’Connor M 273 307 1966 28 Ngo KX Umeki N Kijima ST Kodera N Ueno H Furutani-Umezu N Allosteric regulation by cooperative conformational changes of actin filaments drives mutually exclusive binding with cofilin and myosin Sci Rep 6 35449 2016 27762277 29 Fujiwara I Narita A Keeping the focus on biophysics and actin filaments in Nagoya: A report of the 2016 “now in actin” symposium Cytoskeleton (Hoboken) 74 450 464 2017 28681410 30 McGough A Chiu W Way M Determination of the gelsolin binding site on F-actin: implications for severing and capping Biophys J 74 764 772 1998 9533689 31 Hampton CM Liu J Taylor DW DeRosier DJ Taylor KA The 3D structure of villin as an unusual F-actin crosslinker Structure 16 1882 1891 2008 19081064 32 Galkin VE Orlova A Kudryashov DS Solodukhin A Reisler E Schröder GF Remodeling of actin filaments by ADF/cofilin proteins Proc Natl Acad Sci USA 108 20568 20572 2011 22158895 33 Wong W Webb AI Olshina MA Infusini G Tan YH Hanssen E A mechanism for actin filament severing by malaria parasite actin depolymerizing factor 1 via a low affinity binding interface J Biol Chem 289 4043 4054 2014 24371134 34 Vavylonis D Actin biophysics in the tradition of Fumio Oosawa: A special issue with contributions from participants at the 2016 “Now in Actin” meeting in Nagoya Cytoskeleton (Hoboken) 74 445 2017 29171931 35 Pollard TD Tribute to Fumio Oosawa the pioneer in actin biophysics Cytoskeleton (Hoboken) 74 446 449 2017 28618211 36 Popp D Loh ND Zorgati H Ghoshdastider U Liow LT Ivanova MI Flow-aligned, single-shot fiber diffraction using a femtosecond X-ray free-electron laser Cytoskeleton (Hoboken) 74 472 481 2017 28574190 37 Hirakawa R Nishikawa Y Uyeda TQP Tokuraku K Unidirectional growth of heavy meromyosin clusters along actin filaments revealed by real-time fluorescence microscopy Cytoskeleton (Hoboken) 74 482 489 2017 28888060 38 Ryan GL Holz D Yamashiro S Taniguchi D Watanabe N Vavylonis D Cell protrusion and retraction driven by fluctuations in actin polymerization: A two-dimensional model Cytoskeleton (Hoboken) 74 490 503 2017 28752950 39 Ohnuki J Yodogawa A Takano M Electrostatic balance between global repulsion and local attraction in reentrant polymerization of actin Cytoskeleton (Hoboken) 74 504 511 2017 28768064 40 Suzuki M Mogami G Ohsugi H Watanabe T Matubayasi N Physical driving force of actomyosin motility based on the hydration effect Cytoskeleton (Hoboken) 74 512 527 2017 29087038 41 Ebashi S Ebashi F Alpha-actinin, a new structural protein from striated muscle. I. Preparation and action on actomyosin-ATP interaction J Biochem 58 7 12 1965 5857104 42 Maruyama K Ebashi S Alpha-actinin, a new structural protein from striated muscle. II. Action on actin J Biochem 58 13 19 1965 5857097