1
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Niksirat H, Andersson L, James P, Kouba A, Kozák P. Proteomic profiling of the signal crayfish Pacifastacus leniusculus egg and spermatophore. Anim Reprod Sci 2014; 149:335-44. [DOI: 10.1016/j.anireprosci.2014.07.024] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2014] [Revised: 07/28/2014] [Accepted: 07/29/2014] [Indexed: 12/11/2022]
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2
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Van Oosterwyck H, Rodríguez JF, Doblaré M, García Aznar JM. An affine micro-sphere-based constitutive model, accounting for junctional sliding, can capture F-actin network mechanics. Comput Methods Biomech Biomed Engin 2013; 16:1002-12. [DOI: 10.1080/10255842.2011.648626] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/14/2022]
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3
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Structural dynamics of an actin spring. Biophys J 2011; 100:839-44. [PMID: 21320427 DOI: 10.1016/j.bpj.2010.12.3743] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2010] [Revised: 11/16/2010] [Accepted: 12/02/2010] [Indexed: 11/22/2022] Open
Abstract
Actin-based motility in cells is usually associated with either polymerization/depolymerization in the presence of cross-linkers or contractility in the presence of myosin motors. Here, we focus on a third distinct mechanism involving actin in motility, seen in the dynamics of an active actin spring that powers the acrosomal reaction of the horseshoe crab (Limulus polyphemus) sperm. During this process, a 60-μm bent and twisted bundle of cross-linked actin uncoils and becomes straight in a few seconds in the presence of Ca(2+). This straightening, which occurs at a constant velocity, allows the acrosome to forcefully penetrate the egg. Synthesizing ultrastructural information with the kinetics, energetics, and imaging of calcium binding allows us to construct a dynamical theory for this mechanochemical engine consistent with our experimental observations. It also illuminates the general mechanism by which energy may be stored in conformational changes and released cooperatively in ordered macromolecular assemblies.
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4
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Cowieson NP, King G, Cookson D, Ross I, Huber T, Hume DA, Kobe B, Martin JL. Cortactin adopts a globular conformation and bundles actin into sheets. J Biol Chem 2008; 283:16187-93. [PMID: 18375393 DOI: 10.1074/jbc.m708917200] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cortactin is a filamentous actin-binding protein that plays a pivotal role in translating environmental signals into coordinated rearrangement of the cytoskeleton. The dynamic reorganization of actin in the cytoskeleton drives processes including changes in cell morphology, cell migration, and phagocytosis. In general, structural proteins of the cytoskeleton bind in the N-terminal region of cortactin and regulatory proteins in the C-terminal region. Previous structural studies have reported an extended conformation for cortactin. It is therefore unclear how cortactin facilitates cross-talk between structural proteins and their regulators. In the study presented here, circular dichroism, chemical cross-linking, and small angle x-ray scattering are used to demonstrate that cortactin adopts a globular conformation, thereby bringing distant parts of the molecule into close proximity. In addition, the actin bundling activity of cortactin is characterized, showing that fully polymerized actin filaments are bundled into sheet-like structures. We present a low resolution structure that suggests how the various domains of cortactin interact to coordinate its array of binding partners at sites of actin branching.
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Affiliation(s)
- Nathan P Cowieson
- Institute for Molecular Bioscience and Australian Research Council (ARC) Special Research Centre for Functional and Applied Genomics, University of Queensland, Brisbane QLD 4072 Australia
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5
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Involvement of a gelsolin-related protein in spermatogenesis of the earthworm Lumbricus terrestris. Cell Tissue Res 2008; 332:141-50. [DOI: 10.1007/s00441-007-0561-9] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2007] [Accepted: 11/19/2007] [Indexed: 10/22/2022]
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6
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Shikinaka K, Kwon H, Kakugo A, Furukawa H, Osada Y, Gong JP, Aoyama Y, Nishioka H, Jinnai H, Okajima T. Observation of the Three-Dimensional Structure of Actin Bundles Formed with Polycations. Biomacromolecules 2007; 9:537-42. [DOI: 10.1021/bm701068n] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Kazuhiro Shikinaka
- Department of Biological Science, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Hyuckjoon Kwon
- Department of Biological Science, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Akira Kakugo
- Department of Biological Science, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Hidemitsu Furukawa
- Department of Biological Science, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Yoshihito Osada
- Department of Biological Science, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Jian Ping Gong
- Department of Biological Science, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Yoshitaka Aoyama
- JEOL Ltd., Akishima 151-0063, Japan, and Department of Polymer Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan
| | - Hideo Nishioka
- JEOL Ltd., Akishima 151-0063, Japan, and Department of Polymer Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan
| | - Hiroshi Jinnai
- JEOL Ltd., Akishima 151-0063, Japan, and Department of Polymer Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan
| | - Takaharu Okajima
- Division of Bioengineering and Bioinformatics, Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
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7
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Liu J, Koenderink GH, Kasza KE, Mackintosh FC, Weitz DA. Visualizing the strain field in semiflexible polymer networks: strain fluctuations and nonlinear rheology of F-actin gels. PHYSICAL REVIEW LETTERS 2007; 98:198304. [PMID: 17677669 DOI: 10.1103/physrevlett.98.198304] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/04/2006] [Indexed: 05/16/2023]
Abstract
We image semiflexible polymer networks under shear at the micrometer scale. By tracking embedded probe particles, we determine the local strain field, and directly measure its uniformity, or degree of affineness, on scales of 2-100 microm. The degree of nonaffine strain depends upon the polymer length and cross-link density, consistent with theoretical predictions. We also find a direct correspondence between the uniformity of the microscale strain and the nonlinear elasticity of the networks in the bulk.
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Affiliation(s)
- J Liu
- Department of Physics & SEAS, Harvard University, Cambridge, MA 02138, USA
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8
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Chiu W, Baker ML, Jiang W, Dougherty M, Schmid MF. Electron cryomicroscopy of biological machines at subnanometer resolution. Structure 2005; 13:363-72. [PMID: 15766537 DOI: 10.1016/j.str.2004.12.016] [Citation(s) in RCA: 122] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2004] [Revised: 12/22/2004] [Accepted: 12/25/2004] [Indexed: 01/29/2023]
Abstract
Advances in electron cryomicroscopy (cryo-EM) have made possible the structural determination of large biological machines in the resolution range of 6-9 angstroms. Rice dwarf virus and the acrosomal bundle represent two distinct types of machines amenable to cryo-EM investigations at subnanometer resolutions. However, calculating the density map is only the first step, and much analysis remains to extract structural insights and the mechanism of action in these machines. This paper will review the computational and visualization methodologies necessary for analysis (structure mining) of the computed cryo-EM maps of these machines. These steps include component segmentation, averaging based on local symmetry among components, density connectivity trace, incorporation of bioinformatics analysis, and fitting of high-resolution component data, if available. The consequences of these analyses can not only identify accurately some of the secondary structure elements of the molecular components in machines but also suggest structural mechanisms related to their biological functions.
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Affiliation(s)
- Wah Chiu
- National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
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9
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Gardel ML, Shin JH, MacKintosh FC, Mahadevan L, Matsudaira PA, Weitz DA. Scaling of F-actin network rheology to probe single filament elasticity and dynamics. PHYSICAL REVIEW LETTERS 2004; 93:188102. [PMID: 15525211 DOI: 10.1103/physrevlett.93.188102] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/30/2003] [Indexed: 05/24/2023]
Abstract
The linear and nonlinear viscoelastic response of networks of cross-linked and bundled cytoskeletal filaments demonstrates remarkable scaling with both frequency and applied prestress, which helps elucidate the origins of the viscoelasticity. The frequency dependence of the shear modulus reflects the underlying single-filament relaxation dynamics for 0.1-10 rad/sec. Moreover, the nonlinear strain stiffening of such networks exhibits a universal form as a function of prestress; this is quantitatively explained by the full force-extension relation of single semiflexible filaments.
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Affiliation(s)
- M L Gardel
- Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
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10
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Schmid MF, Sherman MB, Matsudaira P, Chiu W. Structure of the acrosomal bundle. Nature 2004; 431:104-7. [PMID: 15343340 DOI: 10.1038/nature02881] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/13/2004] [Accepted: 07/20/2004] [Indexed: 11/08/2022]
Abstract
In the unactivated Limulus sperm, a 60- micro m-long bundle of actin filaments crosslinked by the protein scruin is bent and twisted into a coil around the base of the nucleus. At fertilization, the bundle uncoils and fully extends in five seconds to support a finger of membrane known as the acrosomal process. This biological spring is powered by stored elastic energy and does not require the action of motor proteins or actin polymerization. In a 9.5-A electron cryomicroscopic structure of the extended bundle, we show that twist, tilt and rotation of actin-scruin subunits deviate widely from a 'standard' F-actin filament. This variability in structural organization allows filaments to pack into a highly ordered and rigid bundle in the extended state and suggests a mechanism for storing and releasing energy between coiled and extended states without disassembly.
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Affiliation(s)
- Michael F Schmid
- National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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11
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Shin JH, Gardel ML, Mahadevan L, Matsudaira P, Weitz DA. Relating microstructure to rheology of a bundled and cross-linked F-actin network in vitro. Proc Natl Acad Sci U S A 2004; 101:9636-41. [PMID: 15210969 PMCID: PMC470727 DOI: 10.1073/pnas.0308733101] [Citation(s) in RCA: 171] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2003] [Accepted: 05/12/2004] [Indexed: 11/18/2022] Open
Abstract
The organization of individual actin filaments into higher-order structures is controlled by actin-binding proteins (ABPs). Although the biological significance of the ABPs is well documented, little is known about how bundling and cross-linking quantitatively affect the microstructure and mechanical properties of actin networks. Here we quantify the effect of the ABP scruin on actin networks by using imaging techniques, cosedimentation assays, multiparticle tracking, and bulk rheology. We show how the structure of the actin network is modified as the scruin concentration is varied, and we correlate these structural changes to variations in the resultant network elasticity.
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Affiliation(s)
- J H Shin
- Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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12
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Gardel ML, Shin JH, MacKintosh FC, Mahadevan L, Matsudaira P, Weitz DA. Elastic behavior of cross-linked and bundled actin networks. Science 2004; 304:1301-5. [PMID: 15166374 DOI: 10.1126/science.1095087] [Citation(s) in RCA: 789] [Impact Index Per Article: 39.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Networks of cross-linked and bundled actin filaments are ubiquitous in the cellular cytoskeleton, but their elasticity remains poorly understood. We show that these networks exhibit exceptional elastic behavior that reflects the mechanical properties of individual filaments. There are two distinct regimes of elasticity, one reflecting bending of single filaments and a second reflecting stretching of entropic fluctuations of filament length. The mechanical stiffness can vary by several decades with small changes in cross-link concentration, and can increase markedly upon application of external stress. We parameterize the full range of behavior in a state diagram and elucidate its origin with a robust model.
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Affiliation(s)
- M L Gardel
- Department of Physics, Harvard University, Cambridge, MA 02138, USA
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13
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Shin JH, Mahadevan L, So PT, Matsudaira P. Bending Stiffness of a Crystalline Actin Bundle. J Mol Biol 2004; 337:255-61. [PMID: 15003444 DOI: 10.1016/j.jmb.2004.01.028] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2003] [Revised: 12/29/2003] [Accepted: 01/12/2004] [Indexed: 11/16/2022]
Abstract
The acrosomal process of the sperm of the horseshoe crab (Limulus polyphemus) is a unique crystalline actin bundle, consisting of multiple actin filaments cross-linked by the actin-bundling protein, scruin. For successful fertilization, the acrosomal bundle must penetrate through a 30 microm thick jelly coat surrounding the egg and thus it must be sufficiently stiff. Here, we present two measurements of the bending stiffness of a single crystalline bundle of actin. Results from these measurements indicate that the actin:scruin composite bundle has an average elastic modulus of 2 GPa, which is similar to that of a single actin filament, and a bending stiffness that is more than two orders of magnitude larger than that of a bundle of uncross-linked actin filaments due to stiffening by the scruin matrix.
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Affiliation(s)
- Jennifer H Shin
- Department of Mechanical Engineering, M.I.T., Cambridge, MA 02139, USA
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14
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Shin JH, Mahadevan L, Waller GS, Langsetmo K, Matsudaira P. Stored elastic energy powers the 60-microm extension of the Limulus polyphemus sperm actin bundle. ACTA ACUST UNITED AC 2003; 162:1183-8. [PMID: 14517201 PMCID: PMC2173959 DOI: 10.1083/jcb.200304006] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
During the 5 s of the acrosome reaction of Limulus polyphemus sperm, a 60-μm-long bundle of scruin-decorated actin filaments straightens from a coiled conformation and extends from the cell. To identify the motive force for this movement, we examined the possible sources of chemical and mechanical energy and show that the coil releases ∼10−13 J of stored mechanical strain energy, whereas chemical energy derived from calcium binding is ∼10−15 J. These measurements indicate that the coiled actin bundle extends by a spring-based mechanism, which is distinctly different from the better known polymerization or myosin-driven processes, and that calcium initiates but does not power the reaction.
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Affiliation(s)
- Jennifer H Shin
- Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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15
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Abstract
Carbon nanotubes and biological filaments each spontaneously assemble into kinked helices, rings, and "tennis racket" shapes due to competition between elastic and interfacial effects. We show that the slender geometry is a more important determinant of the morphology than any molecular details. Our mesoscopic continuum theory is capable of quantifying observations of these structures and is suggestive of their occurrence in other filamentous assemblies as well.
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Affiliation(s)
- Adam E Cohen
- Semiconductor Physics Group, Cavendish Laboratory, Wilberforce Road, Cambridge University, Cambridge CB3 0WA, United Kingdom
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16
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Schmid MF. Cross-correlation and merging of crystallographic reflections derived from cryoelectron micrographs of 3D crystals: application to the Limulus acrosomal bundle. J Struct Biol 2003; 144:195-208. [PMID: 14643222 DOI: 10.1016/j.jsb.2003.09.024] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
A method is described for merging in reciprocal space of electron microscopic data from three-dimensional crystals of the acrosomal bundle. Permutation of indices was required to find the proper alignment of data from different bundles. The method utilizes a statistical evaluation of the significance of the cross-correlation results to indicate the proper order for merging. The three-dimensional (3D) merging is a reference-free operation that does not depend on choosing a "zero-tilt" user-defined starting point. Results from the merging are given and the merged 3D data to 9.5A resolution is evaluated for coherence. General issues such as statistical significance of cross-correlation function peaks, symmetry evaluation, and phase coherence as a function of amplitude are discussed.
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Affiliation(s)
- Michael F Schmid
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology and National Center for Macromolecular Imaging, Baylor College of Medicine, Houston, TX 77030, USA.
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17
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Lukoyanova N, VanLoock MS, Orlova A, Galkin VE, Wang K, Egelman EH. Each actin subunit has three nebulin binding sites: implications for steric blocking. Curr Biol 2002; 12:383-8. [PMID: 11882289 DOI: 10.1016/s0960-9822(02)00678-4] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Nebulin is a giant protein that spans most of the muscle thin filament. Mutations in nebulin result in myopathies and dystrophies. Nebulin contains approximately 200 copies of approximately 35 residue modules, each believed to contain an actin binding site, organized into seven-module superrepeats. The strong correlation between the number of nebulin modules and the length of skeletal muscle thin filaments in different species suggests that nebulin determines thin filament length. Little information exists about the interactions between intact nebulin and F-actin. More insight has come from working with fragments of nebulin, containing from one to hundreds of actin binding modules. However, the observed stoichiometry of binding between these fragments and actin has ranged from 0.4 to 13 modules per actin subunit. We have used electron microscopy and a novel method of helical image analysis to characterize complexes of F-actin with a nebulin fragment. The fragment binds as an extended structure spanning three actin subunits and binding to different sites on each actin. Muscle regulation involves tropomyosin movement on the surface of actin, with binding in three states. Our results suggest the intriguing possibility that intact nebulin may also be able to occupy three different sites on F-actin.
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Affiliation(s)
- Natalya Lukoyanova
- Department of Biochemistry, University of Virginia, Charlottesville 22908-0733, USA
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18
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Kelso RJ, Hudson AM, Cooley L. Drosophila Kelch regulates actin organization via Src64-dependent tyrosine phosphorylation. J Cell Biol 2002; 156:703-13. [PMID: 11854310 PMCID: PMC2174084 DOI: 10.1083/jcb.200110063] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2001] [Revised: 12/13/2001] [Accepted: 01/14/2002] [Indexed: 12/03/2022] Open
Abstract
The Drosophila kelch gene encodes a member of a protein superfamily defined by the presence of kelch repeats. In Drosophila, Kelch is required to maintain actin organization in ovarian ring canals. We set out to study the actin cross-linking activity of Kelch and how Kelch function is regulated. Biochemical studies using purified, recombinant Kelch protein showed that full-length Kelch bundles actin filaments, and kelch repeat 5 contains the actin binding site. Two-dimensional electrophoresis demonstrated that Kelch is tyrosine phosphorylated in a src64-dependent pathway. Site-directed mutagenesis determined that tyrosine residue 627 is phosphorylated. A Kelch mutant with tyrosine 627 changed to alanine (KelY627A) rescued the actin disorganization phenotype of kelch mutant ring canals, but failed to produce wild-type ring canals. Electron microscopy demonstrated that phosphorylation of Kelch is critical for the proper morphogenesis of actin during ring canal growth, and presence of the nonphosphorylatable KelY627A protein phenocopied src64 ring canals. KelY627A protein in ring canals also dramatically reduced the rate of actin monomer exchange. The phenotypes caused by src64 mutants and KelY627A expression suggest that a major function of Src64 signaling in the ring canal is the negative regulation of actin cross-linking by Kelch.
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Affiliation(s)
- Reed J Kelso
- Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06520-8005, USA
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19
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Metzler DE, Metzler CM, Sauke DJ. Growth and Development. Biochemistry 2001. [DOI: 10.1016/b978-012492543-4/50035-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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20
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Bearer EL, Prakash JM, Manchester RD, Allen PG. VASP protects actin filaments from gelsolin: an in vitro study with implications for platelet actin reorganizations. CELL MOTILITY AND THE CYTOSKELETON 2000; 47:351-64. [PMID: 11093254 PMCID: PMC3376085 DOI: 10.1002/1097-0169(200012)47:4<351::aid-cm8>3.0.co;2-8] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
An initial step in platelet shape change is disassembly of actin filaments, which are then reorganized into new actin structures, including filopodia and lamellipodia. This disassembly is thought to be mediated primarily by gelsolin, an abundant actin filament-severing protein in platelets. Shape change is inhibited by VASP, another abundant actin-binding protein. Paradoxically, in vitro VASP enhances formation of actin filaments and bundles them, activities that would be expected to increase shape change, not inhibit it. We hypothesized that VASP might inhibit shape change by stabilizing filaments and preventing their disassembly by gelsolin. Such activity would explain VASP's known physiological role. Here, we test this hypothesis in vitro using either purified recombinant or endogenous platelet VASP by fluorescence microscopy and biochemical assays. VASP inhibited gelsolin's ability to disassemble actin filaments in a dose-dependent fashion. Inhibition was detectable at the low VASP:actin ratio found inside the platelet (1:40 VASP:actin). Gelsolin bound to VASP-actin filaments at least as well as to actin alone. VASP inhibited gelsolin-induced nucleation at higher concentrations (1:5 VASP:actin ratios). VASP's affinity for actin (K(d) approximately 0.07 microM) and its ability to promote polymerization (1:20 VASP actin ratio) were greater with Ca(++)-actin than with Mg(++)-actin (K(d) approximately 1 microM and 1:1 VASP), regardless of the presence of gelsolin. By immunofluorescence, VASP and gelsolin co-localized in the filopodia and lamellipodia of platelets spreading on glass, suggesting that these in vitro interactions could take place within the cell as well. We conclude that VASP stabilizes actin filaments to the severing effects of gelsolin but does not inhibit gelsolin from binding to the filaments. These results suggest a new concept for actin dynamics inside cells: that bundling proteins protect the actin superstructure from disassembly by severing, thereby preserving the integrity of the cytoskeleton.
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Affiliation(s)
- E L Bearer
- Department of Pathology and Laboratory Medicine, Brown University, Providence, Rhode Island 02912, USA.
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21
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Abstract
Modifications can be made to F-actin that do not interfere with the binding of myosin but inhibit force generation, suggesting that actin's internal dynamics are important for muscle contraction. Observations from electron microscopy and x-ray diffraction have shown that subunits in F-actin have a relatively fixed axial rise but a variable twist. One possible explanation for this is that the actin subunits randomly exist in different discrete states of "twist, " with a significant energy barrier separating these states. This would result in very slow torsional transitions. Paracrystals impose increased order on F-actin filaments by reducing the variability in twist. By looking at filaments that have recently been dissociated from paracrystals, we find that F-actin retains a "memory" of its previous environment that persists for many seconds. This would be consistent with slow torsional transitions between discrete states of twist.
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Affiliation(s)
- A Orlova
- Department of Biochemistry and Molecular Genetics, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908-0733, USA
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22
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Abstract
Parallel actin bundles are present in a diverse array of structures, where they are critical determinants of cellular shape and physiology. In the past 18 months, new findings have solidified the concept that parallel actin bundles are assembled in cells through the sequential action of multiple actin-bundling proteins and have begun to shed light on the roles played by the individual actin-bundling proteins.
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Affiliation(s)
- J R Bartles
- Department of Cell and Molecular Biology, Ward 11-185, Northwestern University Medical School, Chicago, IL 60611, USA.
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