1
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Carl AG, Reynolds MJ, Gurel PS, Phua DY, Sun X, Mei L, Hamilton K, Takagi Y, Noble AJ, Sellers JR, Alushin GM. Myosin forces elicit an F-actin structural landscape that mediates mechanosensitive protein recognition. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.08.15.608188. [PMID: 39185238 PMCID: PMC11343212 DOI: 10.1101/2024.08.15.608188] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/27/2024]
Abstract
Cells mechanically interface with their surroundings through cytoskeleton-linked adhesions, allowing them to sense physical cues that instruct development and drive diseases such as cancer. Contractile forces generated by myosin motor proteins mediate these mechanical signal transduction processes through unclear protein structural mechanisms. Here, we show that myosin forces elicit structural changes in actin filaments (F-actin) that modulate binding by the mechanosensitive adhesion protein α-catenin. Using correlative cryo-fluorescence microscopy and cryo-electron tomography, we identify F-actin featuring domains of nanoscale oscillating curvature at cytoskeleton-adhesion interfaces enriched in zyxin, a marker of actin-myosin generated traction forces. We next introduce a reconstitution system for visualizing F-actin in the presence of myosin forces with cryo-electron microscopy, which reveals morphologically similar superhelical F-actin spirals. In simulations, transient forces mimicking tugging and release of filaments by motors produce spirals, supporting a mechanistic link to myosin's ATPase mechanochemical cycle. Three-dimensional reconstruction of spirals uncovers extensive asymmetric remodeling of F-actin's helical lattice. This is recognized by α-catenin, which cooperatively binds along individual strands, preferentially engaging interfaces featuring extended inter-subunit distances while simultaneously suppressing rotational deviations to regularize the lattice. Collectively, we find that myosin forces can deform F-actin, generating a conformational landscape that is detected and reciprocally modulated by a mechanosensitive protein, providing a direct structural glimpse at active force transduction through the cytoskeleton.
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Affiliation(s)
- Ayala G. Carl
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
- Tri-Institutional Program in Chemical Biology, The Rockefeller University, New York, NY, USA
| | - Matthew J. Reynolds
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
| | - Pinar S. Gurel
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
| | - Donovan Y.Z. Phua
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
| | - Xiaoyu Sun
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
| | - Lin Mei
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
- Tri-Institutional Program in Chemical Biology, The Rockefeller University, New York, NY, USA
| | - Keith Hamilton
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
| | - Yasuharu Takagi
- Laboratory of Molecular Physiology, Cell and Developmental Biology Center, National Heart, Lung, and Blood Institute, NIH, Bethesda, MD, USA
| | - Alex J. Noble
- Simons Electron Microscopy Center, New York Structural Biology Center, New York, NY, USA
| | - James R. Sellers
- Laboratory of Molecular Physiology, Cell and Developmental Biology Center, National Heart, Lung, and Blood Institute, NIH, Bethesda, MD, USA
| | - Gregory M. Alushin
- Laboratory of Structural Biophysics and Mechanobiology, The Rockefeller University, New York, NY, USA
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2
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Papasergi-Scott MM, Pérez-Hernández G, Batebi H, Gao Y, Eskici G, Seven AB, Panova O, Hilger D, Casiraghi M, He F, Maul L, Gmeiner P, Kobilka BK, Hildebrand PW, Skiniotis G. Time-resolved cryo-EM of G-protein activation by a GPCR. Nature 2024; 629:1182-1191. [PMID: 38480881 DOI: 10.1038/s41586-024-07153-1] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2023] [Accepted: 02/02/2024] [Indexed: 03/26/2024]
Abstract
G-protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by stimulating guanine nucleotide exchange in the Gα subunit1. To visualize this mechanism, we developed a time-resolved cryo-EM approach that examines the progression of ensembles of pre-steady-state intermediates of a GPCR-G-protein complex. By monitoring the transitions of the stimulatory Gs protein in complex with the β2-adrenergic receptor at short sequential time points after GTP addition, we identified the conformational trajectory underlying G-protein activation and functional dissociation from the receptor. Twenty structures generated from sequential overlapping particle subsets along this trajectory, compared to control structures, provide a high-resolution description of the order of main events driving G-protein activation in response to GTP binding. Structural changes propagate from the nucleotide-binding pocket and extend through the GTPase domain, enacting alterations to Gα switch regions and the α5 helix that weaken the G-protein-receptor interface. Molecular dynamics simulations with late structures in the cryo-EM trajectory support that enhanced ordering of GTP on closure of the α-helical domain against the nucleotide-bound Ras-homology domain correlates with α5 helix destabilization and eventual dissociation of the G protein from the GPCR. These findings also highlight the potential of time-resolved cryo-EM as a tool for mechanistic dissection of GPCR signalling events.
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MESH Headings
- Humans
- Binding Sites
- Cryoelectron Microscopy
- GTP-Binding Protein alpha Subunits, Gs/chemistry
- GTP-Binding Protein alpha Subunits, Gs/drug effects
- GTP-Binding Protein alpha Subunits, Gs/metabolism
- GTP-Binding Protein alpha Subunits, Gs/ultrastructure
- Guanosine Triphosphate/metabolism
- Guanosine Triphosphate/pharmacology
- Models, Molecular
- Molecular Dynamics Simulation
- Protein Binding
- Receptors, Adrenergic, beta-2/metabolism
- Receptors, Adrenergic, beta-2/chemistry
- Receptors, Adrenergic, beta-2/ultrastructure
- Time Factors
- Enzyme Activation/drug effects
- Protein Domains
- Protein Structure, Secondary
- Signal Transduction/drug effects
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Affiliation(s)
- Makaía M Papasergi-Scott
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Guillermo Pérez-Hernández
- Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Institute of Medical Physics and Biophysics, Berlin, Germany
| | - Hossein Batebi
- Institute of Medical Physics and Biophysics, Faculty of Medicine, Leipzig University, Leipzig, Germany
| | - Yang Gao
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Gözde Eskici
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Alpay B Seven
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Ouliana Panova
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Daniel Hilger
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
- Institute of Pharmaceutical Chemistry, Philipps-University of Marburg, Marburg, Germany
| | - Marina Casiraghi
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
- Dipartimento di Bioscienze, Università degli Studi di Milano, Milan, Italy
| | - Feng He
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Luis Maul
- Department of Chemistry and Pharmacy, Medicinal Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany
| | - Peter Gmeiner
- Department of Chemistry and Pharmacy, Medicinal Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany
| | - Brian K Kobilka
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Peter W Hildebrand
- Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Institute of Medical Physics and Biophysics, Berlin, Germany
- Institute of Medical Physics and Biophysics, Faculty of Medicine, Leipzig University, Leipzig, Germany
- Berlin Institute of Health at Charité-Universitätsmedizin Berlin, Berlin, Germany
| | - Georgios Skiniotis
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA.
- Department of Structural Biology, Stanford University School of Medicine, Stanford, CA, USA.
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3
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Montaño Romero A, Bonin C, Twomey EC. C-SPAM: an open-source time-resolved specimen vitrification device with light-activated molecules. IUCRJ 2024; 11:16-22. [PMID: 38096039 PMCID: PMC10833387 DOI: 10.1107/s2052252523010308] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/13/2023] [Accepted: 11/29/2023] [Indexed: 01/10/2024]
Abstract
Molecular structures can be determined in vitro and in situ with cryo-electron microscopy (cryo-EM). Specimen preparation is a major obstacle in cryo-EM. Typical sample preparation is orders of magnitude slower than biological processes. Time-resolved cryo-EM (TR-cryo-EM) can capture short-lived states. Here, Cryo-EM sample preparation with light-activated molecules (C-SPAM) is presented, an open-source, photochemistry-coupled device for TR-cryo-EM that enables millisecond resolution and tunable timescales across broad biological applications.
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Affiliation(s)
- Alejandra Montaño Romero
- Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD USA
- The Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore MD, USA
| | - Calli Bonin
- Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD USA
- The Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore MD, USA
| | - Edward C. Twomey
- Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD USA
- The Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore MD, USA
- The Beckman Center for Cryo-EM at Johns Hopkins, Johns Hopkins University School of Medicine, Baltimore, MD USA
- Diana Helis Henry Medical Research Foundation, New Orleans, LA USA
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4
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Klebl DP, Aspinall L, Muench SP. Time resolved applications for Cryo-EM; approaches, challenges and future directions. Curr Opin Struct Biol 2023; 83:102696. [PMID: 37716094 DOI: 10.1016/j.sbi.2023.102696] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2023] [Revised: 06/14/2023] [Accepted: 08/14/2023] [Indexed: 09/18/2023]
Abstract
Developments within the cryo-EM field have allowed us to generate higher-resolution "static" structures and pull out different conformational states which exist at equilibrium within the sample. Moreover, to trap non-equilibrium states and determine conformations that are present after a defined period of time (typically in the ms time frame) new approaches have been developed for the application of time-resolved cryo-EM. Here we give an overview of these different approaches and the limitations and strengths of each whilst identifying some of the current challenges to achieve higher resolutions and trap states within faster time frames. Time-resolved applications may play an important role in the ever-expanding toolkit of cryo-EM and open up new possibilities in both single particle and tomographic studies.
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Affiliation(s)
- David P Klebl
- School of Biomedical Sciences, Astbury Centre for Structural Molecular Biology, University of Leeds, UK
| | - Louie Aspinall
- School of Molecular and Cellular Biology, University of Leeds, UK
| | - Stephen P Muench
- School of Biomedical Sciences, Astbury Centre for Structural Molecular Biology, University of Leeds, UK.
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5
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Torino S, Dhurandhar M, Stroobants A, Claessens R, Efremov RG. Time-resolved cryo-EM using a combination of droplet microfluidics with on-demand jetting. Nat Methods 2023; 20:1400-1408. [PMID: 37592181 DOI: 10.1038/s41592-023-01967-z] [Citation(s) in RCA: 16] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2022] [Accepted: 06/27/2023] [Indexed: 08/19/2023]
Abstract
Single-particle cryogenic electron microscopy (cryo-EM) allows reconstruction of high-resolution structures of proteins in different conformations. Protein function often involves transient functional conformations, which can be resolved using time-resolved cryo-EM (trEM). In trEM, reactions are arrested after a defined delay time by rapid vitrification of protein solution on the EM grid. Despite the increasing interest in trEM among the cryo-EM community, making trEM samples with a time resolution below 100 ms remains challenging. Here we report the design and the realization of a time-resolved cryo-plunger that combines a droplet-based microfluidic mixer with a laser-induced generator of microjets that allows rapid reaction initiation and plunge-freezing of cryo-EM grids. Using this approach, a time resolution of 5 ms was achieved and the protein density map was reconstructed to a resolution of 2.1 Å. trEM experiments on GroEL:GroES chaperonin complex resolved the kinetics of the complex formation and visualized putative short-lived conformations of GroEL-ATP complex.
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Affiliation(s)
- Stefania Torino
- Center for Structural Biology, Vlaams Instituut voor Biotechnologie, Brussels, Belgium
- Structural Biology Brussels, Department of Bioengineering Sciences, Vrije Universiteit Brussel, Brussels, Belgium
| | - Mugdha Dhurandhar
- Center for Structural Biology, Vlaams Instituut voor Biotechnologie, Brussels, Belgium
- Structural Biology Brussels, Department of Bioengineering Sciences, Vrije Universiteit Brussel, Brussels, Belgium
| | - Annelore Stroobants
- Center for Structural Biology, Vlaams Instituut voor Biotechnologie, Brussels, Belgium
- Structural Biology Brussels, Department of Bioengineering Sciences, Vrije Universiteit Brussel, Brussels, Belgium
| | - Raf Claessens
- Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Brussels, Belgium
| | - Rouslan G Efremov
- Center for Structural Biology, Vlaams Instituut voor Biotechnologie, Brussels, Belgium.
- Structural Biology Brussels, Department of Bioengineering Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
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6
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Papasergi-Scott MM, Pérez-Hernández G, Batebi H, Gao Y, Eskici G, Seven AB, Panova O, Hilger D, Casiraghi M, He F, Maul L, Gmeiner P, Kobilka BK, Hildebrand PW, Skiniotis G. Time-resolved cryo-EM of G protein activation by a GPCR. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.03.20.533387. [PMID: 36993214 PMCID: PMC10055275 DOI: 10.1101/2023.03.20.533387] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/31/2023]
Abstract
G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by stimulating the exchange of guanine nucleotide in the Gα subunit. To visualize this mechanism, we developed a time-resolved cryo-EM approach that examines the progression of ensembles of pre-steady-state intermediates of a GPCR-G protein complex. Using variability analysis to monitor the transitions of the stimulatory Gs protein in complex with the β 2 -adrenergic receptor (β 2 AR) at short sequential time points after GTP addition, we identified the conformational trajectory underlying G protein activation and functional dissociation from the receptor. Twenty transition structures generated from sequential overlapping particle subsets along this trajectory, compared to control structures, provide a high-resolution description of the order of events driving G protein activation upon GTP binding. Structural changes propagate from the nucleotide-binding pocket and extend through the GTPase domain, enacting alterations to Gα Switch regions and the α5 helix that weaken the G protein-receptor interface. Molecular dynamics (MD) simulations with late structures in the cryo-EM trajectory support that enhanced ordering of GTP upon closure of the alpha-helical domain (AHD) against the nucleotide-bound Ras-homology domain (RHD) correlates with irreversible α5 helix destabilization and eventual dissociation of the G protein from the GPCR. These findings also highlight the potential of time-resolved cryo-EM as a tool for mechanistic dissection of GPCR signaling events.
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7
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Amann SJ, Keihsler D, Bodrug T, Brown NG, Haselbach D. Frozen in time: analyzing molecular dynamics with time-resolved cryo-EM. Structure 2023; 31:4-19. [PMID: 36584678 PMCID: PMC9825670 DOI: 10.1016/j.str.2022.11.014] [Citation(s) in RCA: 9] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2022] [Revised: 10/10/2022] [Accepted: 11/25/2022] [Indexed: 12/30/2022]
Abstract
Molecular machines, such as polymerases, ribosomes, or proteasomes, fulfill complex tasks requiring the thermal energy of their environment. They achieve this by restricting random motion along a path of possible conformational changes. These changes are often directed through engagement with different cofactors, which can best be compared to a Brownian ratchet. Many molecular machines undergo three major steps throughout their functional cycles, including initialization, repetitive processing, and termination. Several of these major states have been elucidated by cryogenic electron microscopy (cryo-EM). However, the individual steps for these machines are unique and multistep processes themselves, and their coordination in time is still elusive. To measure these short-lived intermediate events by cryo-EM, the total reaction time needs to be shortened to enrich for the respective pre-equilibrium states. This approach is termed time-resolved cryo-EM (trEM). In this review, we sum up the methodological development of trEM and its application to a range of biological questions.
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Affiliation(s)
- Sascha Josef Amann
- IMP - Research Institute of Molecular Pathology, Campus-Vienna-Biocenter 1, 1030 Vienna, Austria; Vienna BioCenter PhD Program, a Doctoral School of the University of Vienna and Medical University of Vienna, A-1030 Vienna, Austria
| | - Demian Keihsler
- IMP - Research Institute of Molecular Pathology, Campus-Vienna-Biocenter 1, 1030 Vienna, Austria
| | - Tatyana Bodrug
- IMP - Research Institute of Molecular Pathology, Campus-Vienna-Biocenter 1, 1030 Vienna, Austria; Department of Biochemistry and Biophysics and Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
| | - Nicholas G Brown
- Department of Pharmacology and Lineberger Comprehensive Cancer Center, University of North Carolina School of Medicine, Chapel Hill, NC 27599, USA
| | - David Haselbach
- IMP - Research Institute of Molecular Pathology, Campus-Vienna-Biocenter 1, 1030 Vienna, Austria; Institute for Physical Chemistry, Albert-Ludwigs-Universität Freiburg, Albertstrasse 21, 79104 Freiburg, Germany.
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8
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Harder OF, Voss JM, Olshin PK, Drabbels M, Lorenz UJ. Microsecond melting and revitrification of cryo samples: protein structure and beam-induced motion. Acta Crystallogr D Struct Biol 2022; 78:883-889. [PMID: 35775987 PMCID: PMC9248841 DOI: 10.1107/s205979832200554x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2022] [Accepted: 05/21/2022] [Indexed: 11/10/2022] Open
Abstract
A novel approach to time-resolved cryo-electron microscopy (cryo-EM) has recently been introduced that involves melting a cryo sample with a laser beam to allow protein dynamics to briefly occur in the liquid, before trapping the particles in their transient configurations by rapidly revitrifying the sample. With a time resolution of just a few microseconds, this approach is notably fast enough to study the domain motions that are typically associated with the activity of proteins but which have previously remained inaccessible. Here, crucial details are added to the characterization of the method. It is shown that single-particle reconstructions of apoferritin and Cowpea chlorotic mottle virus from revitrified samples are indistinguishable from those from conventional samples, demonstrating that melting and revitrification leaves the particles intact and that they do not undergo structural changes within the spatial resolution afforded by the instrument. How rapid revitrification affects the properties of the ice is also characterized, showing that revitrified samples exhibit comparable amounts of beam-induced motion. The results pave the way for microsecond time-resolved studies of the conformational dynamics of proteins and open up new avenues to study the vitrification process and to address beam-induced specimen movement.
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Affiliation(s)
- Oliver F. Harder
- Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
| | - Jonathan M. Voss
- Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
| | - Pavel K. Olshin
- Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
| | - Marcel Drabbels
- Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
| | - Ulrich J. Lorenz
- Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
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9
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Voss JM, Harder OF, Olshin PK, Drabbels M, Lorenz UJ. Rapid melting and revitrification as an approach to microsecond time-resolved cryo-electron microscopy. Chem Phys Lett 2021. [DOI: 10.1016/j.cplett.2021.138812] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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10
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Voss JM, Harder OF, Olshin PK, Drabbels M, Lorenz UJ. Microsecond melting and revitrification of cryo samples. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2021; 8:054302. [PMID: 34734102 PMCID: PMC8550802 DOI: 10.1063/4.0000129] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/17/2021] [Accepted: 10/06/2021] [Indexed: 05/29/2023]
Abstract
The dynamics of proteins that are associated with their function typically occur on the microsecond timescale, orders of magnitude faster than the time resolution of cryo-electron microscopy. We have recently introduced a novel approach to time-resolved cryo-electron microscopy that affords microsecond time resolution. It involves melting a cryo sample with a heating laser, so as to allow dynamics of the proteins to briefly occur in the liquid phase. When the laser is turned off, the sample rapidly revitrifies, trapping the particles in their transient configurations. Precise control of the temperature evolution of the sample is crucial for such an approach to succeed. Here, we provide a detailed characterization of the heat transfer occurring under laser irradiation as well as the associated phase behavior of the cryo sample. While areas close to the laser focus undergo melting and revitrification, surrounding regions crystallize. In situ observations of these phase changes therefore provide a convenient approach for assessing the temperature reached in each melting and revitrification experiment and for adjusting the heating laser power on the fly.
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Affiliation(s)
- Jonathan M. Voss
- Laboratory of Molecular Nanodynamics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
| | - Oliver F. Harder
- Laboratory of Molecular Nanodynamics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
| | - Pavel K. Olshin
- Laboratory of Molecular Nanodynamics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
| | - Marcel Drabbels
- Laboratory of Molecular Nanodynamics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
| | - Ulrich J. Lorenz
- Laboratory of Molecular Nanodynamics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
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11
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Weissenberger G, Henderikx RJM, Peters PJ. Understanding the invisible hands of sample preparation for cryo-EM. Nat Methods 2021; 18:463-471. [PMID: 33963356 DOI: 10.1038/s41592-021-01130-6] [Citation(s) in RCA: 64] [Impact Index Per Article: 21.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2020] [Accepted: 03/30/2021] [Indexed: 02/03/2023]
Abstract
Cryo-electron microscopy (cryo-EM) is rapidly becoming an attractive method in the field of structural biology. With the exploding popularity of cryo-EM, sample preparation must evolve to prevent congestion in the workflow. The dire need for improved microscopy samples has led to a diversification of methods. This Review aims to categorize and explain the principles behind various techniques in the preparation of vitrified samples for the electron microscope. Various aspects and challenges in the workflow are discussed, from sample optimization and carriers to deposition and vitrification. Reliable and versatile specimen preparation remains a challenge, and we hope to give guidelines and posit future directions for improvement.
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Affiliation(s)
- Giulia Weissenberger
- CryoSol-World, Maastricht, the Netherlands.,Maastricht Multimodal Molecular Imaging Institute (M4i), Division of Nanoscopy, Maastricht University, Maastricht, the Netherlands
| | - Rene J M Henderikx
- CryoSol-World, Maastricht, the Netherlands.,Maastricht Multimodal Molecular Imaging Institute (M4i), Division of Nanoscopy, Maastricht University, Maastricht, the Netherlands
| | - Peter J Peters
- Maastricht Multimodal Molecular Imaging Institute (M4i), Division of Nanoscopy, Maastricht University, Maastricht, the Netherlands.
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12
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Abstract
After several decades studying different acto-myosin complexes at lower and intermediate resolution - limited by the electron microscope instrumentation available then - recent advances in imaging technology have been crucial for obtaining a number of excellent high-resolution 3D reconstructions from cryo electron microscopy. The resolution level reached now is about 3-4 Å, which allows unambiguous model building of filamentous actin on its own as well as that of actin filaments decorated with strongly bound myosin variants. The interface between actin and the myosin motor domain can now be described in detail, and the function of parts of the interface (such as, e.g., the cardiomyopathy loop) can be understood in a mechanistical way. Most recently, reconstructions of actin filaments decorated with different myosins, which show a strongly bound acto-myosin complex also in the presence of the nucleotide ADP, have become available. The comparison of these structures with the nucleotide-free Rigor state provide the first mechanistic description of force sensing. An open question is still the initial interaction of the motor domain of myosin with the actin filament. Such weakly interacting states have so far not been the subject of microscopical studies, even though high-resolution structures would be needed to shed light on the initial steps of phosphate release and power stroke initiation.
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13
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Fuest M, Schaffer M, Nocera GM, Galilea-Kleinsteuber RI, Messling JE, Heymann M, Plitzko JM, Burg TP. In situ Microfluidic Cryofixation for Cryo Focused Ion Beam Milling and Cryo Electron Tomography. Sci Rep 2019; 9:19133. [PMID: 31836773 PMCID: PMC6911106 DOI: 10.1038/s41598-019-55413-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2019] [Accepted: 11/27/2019] [Indexed: 11/30/2022] Open
Abstract
We present a microfluidic platform for studying structure-function relationships at the cellular level by connecting video rate live cell imaging with in situ microfluidic cryofixation and cryo-electron tomography of near natively preserved, unstained specimens. Correlative light and electron microscopy (CLEM) has been limited by the time required to transfer live cells from the light microscope to dedicated cryofixation instruments, such as a plunge freezer or high-pressure freezer. We recently demonstrated a microfluidic based approach that enables sample cryofixation directly in the light microscope with millisecond time resolution, a speed improvement of up to three orders of magnitude. Here we show that this cryofixation method can be combined with cryo-electron tomography (cryo-ET) by using Focused Ion Beam milling at cryogenic temperatures (cryo-FIB) to prepare frozen hydrated electron transparent sections. To make cryo-FIB sectioning of rapidly frozen microfluidic channels achievable, we developed a sacrificial layer technique to fabricate microfluidic devices with a PDMS bottom wall <5 µm thick. We demonstrate the complete workflow by rapidly cryo-freezing Caenorhabditis elegans roundworms L1 larvae during live imaging in the light microscope, followed by cryo-FIB milling and lift out to produce thin, electron transparent sections for cryo-ET imaging. Cryo-ET analysis of initial results show that the structural preservation of the cryofixed C. elegans was suitable for high resolution cryo-ET work. The combination of cryofixation during live imaging enabled by microfluidic cryofixation with the molecular resolution capabilities of cryo-ET offers an exciting avenue to further advance space-time correlative light and electron microscopy (st-CLEM) for investigation of biological processes at high resolution in four dimensions.
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Affiliation(s)
- Marie Fuest
- Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077, Göttingen, Germany
| | - Miroslava Schaffer
- Max Planck Institute of Biochemistry, Am Klopferspitz 18, 82152, Martinsried, Germany
| | - Giovanni Marco Nocera
- Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077, Göttingen, Germany
| | | | - Jan-Erik Messling
- Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077, Göttingen, Germany
| | - Michael Heymann
- Max Planck Institute of Biochemistry, Am Klopferspitz 18, 82152, Martinsried, Germany.,Institute of Biomaterials and Biomolecular Systems, University of Stuttgart, Pfaffenwaldring 57, 70569, Stuttgart, Germany
| | - Jürgen M Plitzko
- Max Planck Institute of Biochemistry, Am Klopferspitz 18, 82152, Martinsried, Germany
| | - Thomas P Burg
- Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077, Göttingen, Germany. .,Technische Universität Darmstadt, Merckstrasse 25, 64283, Darmstadt, Germany.
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14
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Fuest M, Nocera GM, Modena MM, Riedel D, Mejia YX, Burg TP. Cryofixation during live-imaging enables millisecond time-correlated light and electron microscopy. J Microsc 2018; 272:87-95. [PMID: 30088278 DOI: 10.1111/jmi.12747] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2018] [Revised: 06/29/2018] [Accepted: 07/13/2018] [Indexed: 01/13/2023]
Abstract
Correlating live-cell imaging with electron microscopy is among the most promising approaches to relate dynamic functions of cells or small organisms to their underlying ultrastructure. The time correlation between light and electron micrographs, however, is limited by the sample handling and fixation required for electron microscopy. Current cryofixation methods require a sample transfer step from the light microscope to a dedicated instrument for cryofixation. This transfer step introduces a time lapse of one second or more between live imaging and the fixed state, which is studied by electron microscopy. In this work, we cryofix Caenorhabditis elegans directly within the light microscope field of view, enabling millisecond time-correlated live imaging and electron microscopy. With our approach, the time-correlation is limited only by the sample cooling rate. C. elegans was used as a model system to establish compatibility of in situ cryofixation and subsequent transmission electron microscopy (TEM). TEM images of in situ cryofixed C. elegans show that the ultrastructure of the sample was well preserved with this method. We expect that the ability to correlate live imaging and electron microscopy at the millisecond scale will enable new paradigms to study biological processes across length scales based on real-time selection and arrest of a desired state. LAY DESCRIPTION Researchers seek to link cellular functions to their smallest structural components. Currently this requires correlation of two imaging techniques, live imaging and electron microscopy. Current correlative methods, however, have limited time resolution due to the sample preparation procedures for electron microscopy. Following live imaging, samples are transferred from the light microscope to a cryofixation, or ultra-fast freezing, instrument. The biological process progresses until the sample freezes, 1 second or more after the last live image. In this work, samples are cryofixed directly within the light microscope field of view. By eliminating the transfer step, time correlation between light and electron microscopy images of our samples is limited only by the freezing rate to the order of milliseconds rather than seconds.
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Affiliation(s)
- M Fuest
- Max Planck Institute for Biophysical Chemistry, Goettingen, Germany
| | - G M Nocera
- Max Planck Institute for Biophysical Chemistry, Goettingen, Germany
| | - M M Modena
- Max Planck Institute for Biophysical Chemistry, Goettingen, Germany
| | - D Riedel
- Max Planck Institute for Biophysical Chemistry, Goettingen, Germany
| | - Y X Mejia
- Max Planck Institute for Biophysical Chemistry, Goettingen, Germany
| | - T P Burg
- Max Planck Institute for Biophysical Chemistry, Goettingen, Germany
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15
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Chen B, Frank J. Two promising future developments of cryo-EM: capturing short-lived states and mapping a continuum of states of a macromolecule. Microscopy (Oxf) 2015; 65:69-79. [PMID: 26520784 DOI: 10.1093/jmicro/dfv344] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2015] [Accepted: 10/05/2015] [Indexed: 01/04/2023] Open
Abstract
The capabilities and application range of cryogenic electron microscopy (cryo-EM) method have expanded vastly in the last two years, thanks to the advances provided by direct detection devices and computational classification tools. We take this review as an opportunity to sketch out promising developments of cryo-EM in two important directions: (i) imaging of short-lived states (10-1000 ms) of biological molecules by using time-resolved cryo-EM, particularly the mixing-spraying method and (ii) recovering an entire continuum of coexisting states from the same sample by employing a computational technique called manifold embedding. It is tempting to think of combining these two methods, to elucidate the way the states of a molecular machine such as the ribosome branch and unfold. This idea awaits further developments of both methods, particularly by increasing the data yield of the time-resolved cryo-EM method and by developing the manifold embedding technique into a user-friendly workbench.
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Affiliation(s)
- Bo Chen
- Department of Biochemistry and Molecular Biophysics, Columbia University, 650 W. 168th Street, New York, NY 10032, USA
| | - Joachim Frank
- Department of Biochemistry and Molecular Biophysics, Columbia University, 650 W. 168th Street, New York, NY 10032, USA Department of Biological Sciences, Columbia University, New York, NY 10027, USA Howard Hughes Medical Institute, Columbia University, New York, NY 10032, USA
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16
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Lu Z, Barnard D, Shaikh TR, Meng X, Mannella CA, Yassin A, Agrawal R, Wagenknecht T, Lu TM. Gas-Assisted Annular Microsprayer for Sample Preparation for Time-Resolved Cryo-Electron Microscopy. JOURNAL OF MICROMECHANICS AND MICROENGINEERING : STRUCTURES, DEVICES, AND SYSTEMS 2014; 24:115001. [PMID: 25530679 PMCID: PMC4266110 DOI: 10.1088/0960-1317/24/11/115001] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Time-resolved cryo electron microscopy (TRCEM) has emerged as a powerful technique for transient structural characterization of isolated biomacromolecular complexes in their native state within the time scale of seconds to milliseconds. For TRCEM sample preparation, microfluidic device [9] has been demonstrated to be a promising approach to facilitate TRCEM biological sample preparation. It is capable of achieving rapidly aqueous sample mixing, controlled reaction incubation, and sample deposition on electron microscopy (EM) grids for rapid freezing. One of the critical challenges is to transfer samples to cryo-EM grids from the microfluidic device. By using microspraying method, the generated droplet size needs to be controlled to facilitate the thin ice film formation on the grid surface for efficient data collection, while not too thin to be dried out before freezing, i.e., optimized mean droplet size needs to be achieved. In this work, we developed a novel monolithic three dimensional (3D) annular gas-assisted microfluidic sprayer using 3D MEMS (MicroElectroMechanical System) fabrication techniques. The microsprayer demonstrated dense and consistent microsprays with average droplet size between 6-9 μm, which fulfilled the above droplet size requirement for TRCEM sample preparation. With droplet density of around 12-18 per grid window (window size is 58×58 μm), and the data collectible thin ice region of >50% total wetted area, we collected ~800-1000 high quality CCD micrographs in a 6-8 hour period of continuous effort. This level of output is comparable to what were routinely achieved using cryo-grids prepared by conventional blotting and manual data collection. In this case, weeks of data collection process with the previous device [9] has shortened to a day or two. And hundreds of microliter of valuable sample consumption can be reduced to only a small fraction.
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Affiliation(s)
- Zonghuan Lu
- Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180
| | - David Barnard
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
| | - Tanvir R. Shaikh
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
- CEITEC, Masaryk University, Brno, Czech Republic
| | - Xing Meng
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
| | - Carmen A. Mannella
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
| | - Aymen Yassin
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
- Department of Microbiology and Immunology, Cairo University, Cairo, Egypt
| | - Rajendra Agrawal
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
| | - Terence Wagenknecht
- Division of Translational Medicine, Wadsworth Center, New York State Department of Health, Albany, NY 12201
| | - Toh-Ming Lu
- Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180
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17
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Nicotinic acetylcholine receptor and the structural basis of neuromuscular transmission: insights from Torpedo postsynaptic membranes. Q Rev Biophys 2013; 46:283-322. [PMID: 24050525 PMCID: PMC3820380 DOI: 10.1017/s0033583513000061] [Citation(s) in RCA: 97] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
The nicotinic acetylcholine (ACh) receptor, at the neuromuscular junction, is a neurotransmitter-gated ion channel that has been fine-tuned through evolution to transduce a chemical signal into an electrical signal with maximum efficiency and speed. It is composed from three similar and two identical polypeptide chains, arranged in a ring around a narrow membrane pore. Central to the design of this assembly is a hydrophobic gate in the pore, more than 50 Å away from sites in the extracellular domain where ACh binds. Although the molecular properties of the receptor have been explored intensively over the last few decades, only recently have structures emerged revealing its complex architecture and illuminating how ACh entering the binding sites opens the distant gate. Postsynaptic membranes isolated from the (muscle-derived) electric organ of the Torpedo ray have underpinned most of the structural studies: the membranes form tubular vesicles having receptors arranged on a regular surface lattice, which can be imaged directly in frozen physiological solutions. Advances in electron crystallographic techniques have also been important, enabling analysis of the closed- and open-channel forms of the receptor in unreacted tubes or tubes reacted briefly with ACh. The structural differences between these two forms show that all five subunits participate in a concerted conformational change communicating the effect of ACh binding to the gate, but that three of them (αγ, β and δ) play a dominant role. Flexing of oppositely facing pore-lining α-helices is the principal motion determining the closed/open state of the gate. These results together with the findings of biochemical, biophysical and other structural studies allow an integrated description of the receptor and of its mode of action at the synapse.
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18
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Lu Z, Shaikh TR, Barnard D, Meng X, Mohamed H, Yassin A, Mannella CA, Agrawal RK, Lu TM, Wagenknecht T. Monolithic microfluidic mixing-spraying devices for time-resolved cryo-electron microscopy. J Struct Biol 2009; 168:388-95. [PMID: 19683579 DOI: 10.1016/j.jsb.2009.08.004] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2009] [Revised: 07/14/2009] [Accepted: 08/10/2009] [Indexed: 11/19/2022]
Abstract
The goal of time-resolved cryo-electron microscopy is to determine structural models for transient functional states of large macromolecular complexes such as ribosomes and viruses. The challenge of time-resolved cryo-electron microscopy is to rapidly mix reactants, and then, following a defined time interval, to rapidly deposit them as a thin film and freeze the sample to the vitreous state. Here we describe a methodology in which reaction components are mixed and allowed to react, and are then sprayed onto an EM grid as it is being plunged into cryogen. All steps are accomplished by a monolithic, microfabricated silicon device that incorporates a mixer, reaction channel, and pneumatic sprayer in a single chip. We have found that microdroplets produced by air atomization spread to sufficiently thin films on a millisecond time scale provided that the carbon supporting film is made suitably hydrophilic. The device incorporates two T-mixers flowing into a single channel of four butterfly-shaped mixing elements that ensure effective mixing, followed by a microfluidic reaction channel whose length can be varied to achieve the desired reaction time. The reaction channel is flanked by two ports connected to compressed humidified nitrogen gas (at 50 psi) to generate the spray. The monolithic mixer-sprayer is incorporated into a computer-controlled plunging apparatus. To test the mixing performance and the suitability of the device for preparation of biological macromolecules for cryo-EM, ribosomes and ferritin were mixed in the device and sprayed onto grids. Three-dimensional reconstructions of the ribosomes demonstrated retention of native structure, and 30S and 50S subunits were shown to be capable of reassociation into ribosomes after passage through the device.
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Affiliation(s)
- Zonghuan Lu
- Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
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19
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Shaikh TR, Barnard D, Meng X, Wagenknecht T. Implementation of a flash-photolysis system for time-resolved cryo-electron microscopy. J Struct Biol 2008; 165:184-9. [PMID: 19114106 DOI: 10.1016/j.jsb.2008.11.007] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2008] [Revised: 11/22/2008] [Accepted: 11/24/2008] [Indexed: 10/21/2022]
Abstract
We describe here the implementation of a flash-photolysis system for time-resolved cryo-electron microscopy. A previously designed computer-controlled cryo-plunging apparatus [White, H.D., Thirumurugan, K., Walker, M.L., Trinick, J., 2003. A second generation apparatus for time-resolved electron cryo-microscopy using stepper motors and electrospray. J. Struct. Biol. 144, 246-252] was used as a hardware platform, onto which a xenon flash lamp and liquid light pipe were mounted. The irradiation initiates a reaction through cleavage of the photolabile blocking group from a biologically active compound. The timespan between flashing and freezing in cryogen is on the order of milliseconds, and defines the fastest observable reaction. Blotting of excess fluid, which takes on the order of 1s, is done before irradiation and thus does not represent a rate-limiting step. A specimen-heating problem, identified by measurements with a thermocouple, was alleviated with the use of thick, aluminum-coated grids.
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Affiliation(s)
- Tanvir R Shaikh
- Resource for the Visualization of Biological Complexity, Wadsworth Center, New York State Department of Health, P.O. Box 509, Empire State Plaza, Albany, NY 12201-0509, USA.
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20
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Jarosch R. The alpha-helix, an overlooked molecular motor. PROTOPLASMA 2005; 227:37-46. [PMID: 16389492 DOI: 10.1007/s00709-005-0136-0] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/30/2005] [Accepted: 05/31/2005] [Indexed: 05/06/2023]
Abstract
At first sight the alpha-helix appears as a rigid scaffold braced by hydrogen bonds nearly parallel to the helix axis. Looked at more closely it turned out to be highly dynamic and able to transform chemical into mechanical energy. The hydrogen bonds are fairly weak and compliant bonds. Their length, usually between 0.267 and 0.291 nm (mean value, 0.28 nm), depends on the interaction of the side chains. The most important strong interaction is the electrostatic repelling force between equally charged side chains (Glu-, Asp-, Lys+, Arg+), well known by experiments with polyamino acids. In proteins with different amino acids, repelling forces between charged side chains work in the axial direction and stretch the hydrogen bonds. Extreme shortening of the hydrogen bonds occurs when ions, e.g., Ca2+, H+, or PO3-, are added and discharge side chains. This means a cooperative pitch decrease of the alpha-helix (pitch range between 0.52 and more than 0.55 nm; mean value, 0.54 nm). This pitch change is absolutely connected by steric reasons with torque generation and torsional rotations, as demonstrated by molecular and tubular alpha-helix models. Thus, charged alpha-helices are molecular motors propelled by the electrostatic energy of added ions. The motor effect is most striking with highly charged alpha-helical coiled coils, e.g., tropomyosin, myosin, and alpha-actinin that can rotate actin filaments by winding and unwinding. For example, the shortening of muscle depends on the sliding (drilling) motion of the Ca2+-activated helical actin filaments into the cross-bridges of the A-band. Here, models are presented for the in vitro sliding of actin filaments and for cytoplasmic streaming by winding and unwinding of myosin chains, and for membrane proteins that contain nonhelical domains between membrane-penetrating alpha-helices. They may transport molecules by the described torsional rotations if they perform supercoiling. Winding and supercoiling can lead to displacement of bound ions and to a feed-back-regulated oscillation between two different coiling stages E1 and E2 that explain "eversion". The models need the torque for 1-2 rotations. They explain active and passive transports, the driving-effects of ion gradients, ATP hydrolysis by unwinding, ATP synthesis by winding up of the supercoils, etc.
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21
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Abstract
Imaging structural intermediates of biological processes is a key step in understanding biological function. Because intermediates are commonly short-lived, lasting only milliseconds, the main methods used to capture them have been conventional imaging of analog or inhibited states, having extended lifetimes, or rapid (millisecond timescale) freezing of intermediates with subsequent observation by cryo-EM. We have developed a simpler method that fixes structure on the millisecond timescale. The procedure consists of briefly (milliseconds) exposing the macromolecular structure of interest on an EM grid to conditions that initiate the structural change, then immediately fixing with uranyl acetate or tannic acid. Specimens are then observed by negative staining. The key finding that validates this approach is our demonstration that uranyl acetate, and in some cases tannic acid, fixes protein molecular structure on the millisecond timescale. This is demonstrated by our observation that exposure of actin and myosin filaments to these fixatives for as little as 10 ms is sufficient to fully preserve them against changes that normally induce rapid and major alteration in their molecular structure. Fixation appears to stabilize both ionic and hydrophobic bonds. This approach should be of general utility for studying transient molecular changes in many systems.
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Affiliation(s)
- Fa-Qing Zhao
- Department of Cell Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, USA
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22
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Thomas DD, Prochniewicz E, Roopnarine O. Changes in actin and myosin structural dynamics due to their weak and strong interactions. Results Probl Cell Differ 2002; 36:7-19. [PMID: 11892285 PMCID: PMC10712373 DOI: 10.1007/978-3-540-46558-4_2] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Figure 3 summarizes the effects of actomyosin binding on the internal and global dynamics of either protein, as discussed in this chapter. These effects depend primarily on the strength of the interaction; which in turn depends on the state of the nucleotide at the myosin active site. When either no nucleotide or ADP is bound, the interaction is strong and the effect on each protein is maximal. When the nucleotide is ATP or ADP.Pi, or the equivalent nonhydrolyzable analogs, the interaction is weak and the effect on molecular dynamics of each protein is minimal. The weaker effects in weak-binding states are not simply the reflection of lower occupancy of binding sites--the molecular models in Fig. 3 illustrate the effects of the formation of the ternary complex, after correction for the free actin and myosin in the system. Thus EPR on myosin (Berger and Thomas 1991; Thomas et al. 1995) and pyrene fluorescence studies on actin (Geeves 1991) have shown that the formation of a ternary complex has a negligible effect on the internal dynamics of both [figure: see text] proteins (left side of Fig. 3, white arrows). As shown by both EPR (Baker et al. 1998; Roopnarine et al. 1998) and phosphorescence (Ramachandran and Thomas 1999), both domains of myosin are dynamically disordered in weak-binding states, and this is essentially unaffected by the formation of the ternary complex (left side of Fig. 3, indicated by disordered myosin domains). The only substantial effect of the formation of the weak interaction that has been reported is the EPR-detected (Ostap and Thomas 1991) restriction of the global dynamics of actin upon weak myosin binding (left column of Fig. 3, gray arrow). The effects of strong actomyosin formation are much more dramatic. While substantial rotational dynamics, both internal and global, exist in both myosin and actin in the presence of ADP or the absence of nucleotides, spin label EPR, pyrene fluorescence, and phosphorescence all show dramatic restrictions in these motions upon formation of the strong ternary complex (right column of Fig. 3). One implication of this is that the weak-to-strong transition is accompanied by a disorder-to-order transition in both actin and myosin, and this is itself an excellent candidate for the structural change that produces force (Thomas et al. 1995). Another clear implication is that the crystal structures obtained for isolated myosin and actin are not likely to be reliable representations of structures that exist in ternary complexes of these proteins (Rayment et al. 1993a and 1993b; Dominguez et al. 1998; Houdusse et al. 1999). This is clearly true of the strong-binding states, since the spectroscopic studies indicate consistently that substantial changes occur in both proteins upon strong complex formation. For the weak complexes, the problem is not that complex formation induces large structural changes, but that the structures themselves are dynamically disordered. This is probably why so many different structures have been obtained for myosin S1 with nucleotides bound--each crystal is selecting one of the many different substates represented by the dynamic ensemble. Finally, there is the problem that the structures of actomyosin complexes are probably influenced strongly by their mechanical coupling to muscle protein lattice (Baker at al. 2000). Thus, even if co-crystals of actin and myosin are obtained in the future, an accurate description of the structural changes involved in force generation will require further experiments using site-directed spectroscopic probes of both actin and myosin, in order to detect the structural dynamics of these ternary complexes under physiological conditions.
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Affiliation(s)
- David D Thomas
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
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23
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Eli-Berchoer L, Reisler E, Muhlrad A. Structural implications of the chemical modification of Cys(10) on actin. Biophys J 2000; 78:1482-9. [PMID: 10692333 PMCID: PMC1300746 DOI: 10.1016/s0006-3495(00)76701-4] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
Cys(10) is located in subdomain 1 of actin, which has an important role in the interaction of actin with myosin- and actin-binding proteins. Cys(10) was modified with fluorescence probes N-(iodoacetyl)N'-(5-sulfo-1-naphthyl)ethylene diamine (IAEDANS), 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin (CPM), or monobromo bimane (MBB) by the method of, J. Biol. Chem. 266:5508-5513). The specificity of Cys(10) modification was verified by showing that the 33-kDa subtilisin fragment of actin (residues 48-375), which contains all of the actin thiols but Cys(10), is not fluorescent. Cys(10) modification exposed a new site on actin to subtilisin cleavage. Edman degradation revealed this site to be between Ala(19) and Gly(20). The modification slightly increased the rate of epsilonATP-ATP exchange and decreased the rates of G-actin ATPase and polymerization. The activation of S1 ATPase by Cys(10)-modified F-actin showed small probe-dependent changes in the values of V(max) and K(M). The sliding speed of actin filaments in the in vitro motility assay remained unchanged upon modification of Cys(10). These results indicate that although the labeling of Cys(10) perturbs the structure of subdomain 1, the modified actin remains fully functional. The binding of S1 to actin filaments decreases the accessibility of Cys(10) probes to acrylamide and nitromethane quenchers. Because Cys(10) does not participate directly in either actin polymerization or S1 binding, our results indicate that actin-actin and actin-myosin interactions induce dynamic, allosteric changes in actin structure.
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Affiliation(s)
- L Eli-Berchoer
- Department of Oral Biology, Hadassah School of Dental Medicine, Hebrew University, Jerusalem 91120, Israel
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24
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White HD, Walker ML, Trinick J. A computer-controlled spraying-freezing apparatus for millisecond time-resolution electron cryomicroscopy. J Struct Biol 1998; 121:306-13. [PMID: 9704502 DOI: 10.1006/jsbi.1998.3968] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Apparatus is described for the kinetic investigation of biological reactions by electron cryomicroscopy with time resolution on the order of milliseconds. This involves layering a grid with one reactant and then spraying on a second reactant immediately before freezing. Two-stage mixing can be achieved by mixing two solutions, holding them in a delay line for a preset interval, and then spraying the aged solution onto a grid carrying a third reactant. The individual steps of these procedures are under software control and can be adjusted independently. Spray-freezing is widely applicable since solutions of small molecules, proteins, and protein assemblies can be delivered as aerosols. Thus the method can be used to study both the effects of small molecules on macromolecules and for monitoring protein-protein interactions. It may also be useful in other situations, for instance in light microscopy.
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Affiliation(s)
- H D White
- Department of Biochemistry, Eastern Virginia Medical School, Norfolk 23507, USA
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25
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Affiliation(s)
- G Rapp
- European Molecular Biology Laboratory, Hamburg, Germany
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26
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Woodcock CL, Horowitz RA. Electron microscopic imaging of chromatin with nucleosome resolution. Methods Cell Biol 1997; 53:167-86. [PMID: 9348509 DOI: 10.1016/s0091-679x(08)60879-1] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- C L Woodcock
- Department of Biology, University of Massachusetts, Amherst 01003, USA
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27
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Elastic and inelastic scattering cross-sections of amorphous layers of carbon and vitrified ice. Ultramicroscopy 1996. [DOI: 10.1016/0304-3991(96)00036-8] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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28
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Abstract
In this paper, the rotating cross-bridge mechanism for muscle contraction is discussed and much contradictory evidence is put forward. As an alternative, a model is given in which the motor of muscle contraction is placed in the myosin-rod hinge and/or in the actin filament. No definite choice for one of the proposed models can be made yet, although it is clear that some kind of phase transition plays an important role in the mechanism.
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Affiliation(s)
- G H Pollack
- University of Washington, Seattle 98195, USA.
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29
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Abstract
Structural intermediates in a biological reaction may be monitored by rapid spectroscopic or somewhat slower structural techniques. Intermediates may evolve in real time (no trapping), be stabilized by chemical manipulation of the reactants, the macromolecule or the solvent (chemical trapping), or be stabilized by lowering the temperature (freeze trapping). The last is beginning to be coupled with X-ray diffraction, electron cryomicroscopy and solid-state NMR approaches to characterize the trapped intermediates. Care in conducting such experiments, together with an awareness of possible artefacts, is essential if reliable structural results are to be obtained.
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Affiliation(s)
- K Moffat
- Department of Biochemistry and Molecular Biology, University of Chicago, Illinois 60637, USA
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30
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Isambert H, Venier P, Maggs AC, Fattoum A, Kassab R, Pantaloni D, Carlier MF. Flexibility of actin filaments derived from thermal fluctuations. Effect of bound nucleotide, phalloidin, and muscle regulatory proteins. J Biol Chem 1995; 270:11437-44. [PMID: 7744781 DOI: 10.1074/jbc.270.19.11437] [Citation(s) in RCA: 430] [Impact Index Per Article: 14.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Single actin filaments undergoing brownian movement in two dimensions were observed at 20 degrees C in fluorescence optical video microscopy. The persistence length (Lp) was derived from the analysis of either the cosine correlation function or the average transverse fluctuations of a series of recorded shapes of filaments assembled from rhodamine-action. Phalloidin-stabilized filaments had a persistence length of 18 +/- 1 micron, in agreement with recent observations. In the absence of phalloidin, rhodamine-labeled filaments could be observed under a variety of solution conditions once diluted in free unlabeled G-actin at the appropriate critical concentration. Such nonstabilized F-ADP-actin filaments had the same Lp of 9 +/- 0.5 microns, whether they had been assembled from ATP-G-actin or from ADP-G-actin, and independently of the tightly bound divalent metal ion. In the presence of BeF3-, which mimics the gamma-phosphate of ATP, F-ADP-BeF3-actin was appreciably more rigid, with Lp = 13.5 microns. Hence, newly formed F-ADP-Pi-actin filaments are more rigid than "old" F-ADP-actin filaments, a fact which has implications in actin-based motility processes. In the presence of skeletal tropomyosin and troponin, filaments were rigid (Lp = 20 +/- 1 micron) in the off state (-Ca2+), and flexible (Lp = 12 microns) in the on state (+Ca2+), consistent with the steric blocking model. In agreement with x-ray diffraction data, no appreciable difference was recorded between the off and on states using smooth muscle tropomyosin and caldesmon (Lp = 20 +/- 1 micron). In conclusion, this method allows accurate measurement of small (< or = 15%) changes in mechanical properties of actin filaments in correlation with their biological functions.
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Affiliation(s)
- H Isambert
- Groupe de Physicochimie Théorique, Ecole Supérieure de Physique et Chimie Industrielles, Paris, France
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Kellermayer MS, Hinds TR, Pollack GH. Persisting in vitro motility of actin filaments at nanomolar ATP concentrations after ATP pretreatment. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1995. [DOI: 10.1016/0005-2728(94)00191-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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Berriman J, Unwin N. Analysis of transient structures by cryo-microscopy combined with rapid mixing of spray droplets. Ultramicroscopy 1994; 56:241-52. [PMID: 7831735 DOI: 10.1016/0304-3991(94)90012-4] [Citation(s) in RCA: 117] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
A simple method to determine transient conformations of biological molecules is described. The two reactants (e.g. protein complex and ligand) are mixed rapidly by the coalescence of spray droplets containing one component, with a thin, grid-supported aqueous film containing the other. The transient state is then trapped by rapid freezing, and investigated later by cryo-microscopy. Images of conformations associated with reaction times of 1-100 ms can be achieved by adjusting the delay between the droplet impact and freezing. The droplets (typically 1 micron in diameter) are propelled onto the grid by an atomizer spray. It is shown that the droplets impinging on the liquid film spread rapidly over its surface under the influence of surface tension, and only weakly disturb the underlying film, partially displacing its contents away from the point of impact. Experiments with sprayed salt solutions, using vesicles derived from erythrocytes as micro-osmometers, indicate that rapid mixing occurs both through the film and laterally, by diffusion. The spraying process does not produce any detectable concentration changes due to drying in either the droplets or the film, and the method is applicable to high-resolution imaging.
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Affiliation(s)
- J Berriman
- MRC Laboratory of Molecular Biology, Cambridge, UK
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Abstract
The structures of the actin and myosin filaments of striated muscle have been studied extensively in the past by sectioning of fixed specimens. However, chemical fixation alters molecular details and prevents biochemically induced structural changes. To overcome these problems, we investigate here the potential of cryosectioning unfixed muscle. In cryosections of relaxed, unfixed specimens, individual myosin filaments displayed the characteristic helical organization of detached cross-bridges, but the filament lattice had disintegrated. To preserve both the filament lattice and the molecular structure of the filaments, we decided to section unfixed rigor muscle, stabilized by actomyosin cross-bridges. The best sections showed periodic, angled cross-bridges attached to actin and their Fourier transforms displayed layer lines similar to those in x-ray diffraction patterns of rigor muscle. To preserve relaxed filaments in their original lattice, unfixed sections of rigor muscle were picked up on a grid and relaxed before negative staining. The myosin and actin filaments showed the characteristic helical arrangements of detached cross-bridges and actin subunits, and Fourier transforms were similar to x-ray patterns of relaxed muscle. We conclude that the rigor structure of muscle and the ability of the filament lattice to undergo the rigor-relaxed transformation can be preserved in unfixed cryosections. In the future, it should be possible to carry out dynamic studies of active sacromeres by cryo-electron microscopy.
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Affiliation(s)
- J F Ménétret
- Department of Cell Biology, University of Massachusetts Medical School, Worcester 01655
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Ng CM, Ludescher RD. Microsecond rotational dynamics of F-actin in ActoS1 filaments during ATP hydrolysis. Biochemistry 1994; 33:9098-104. [PMID: 8049212 DOI: 10.1021/bi00197a011] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Rabbit skeletal muscle F-actin labeled at Cys374 with the triplet probe erythrosin-5-iodoacetamide had a steady-state phosphorescence anisotropy (rp) of 0.090 +/- 0.005 at 20 degrees C in 100 mM KCl, pH 7.0, buffer. Titration with skeletal muscle S1 fragment increased rp to 0.138 +/- 0.006 at a mole ratio of 1:1. In the presence of ATP, the anisotropy of the actoS1 complex initially decreased to 0.050 +/- 0.005, a value significantly smaller than the anisotropy of pure F-actin; rp subsequently increased to 0.126 +/- 0.002. The time course of the increase matched that expected from the measured actin-activated ATPase of S1. The plateau value at long time, 0.126, was identical to that of actoS1 in the presence of exogenous ADP or ADP plus phosphate. Characterization of the spectroscopic properties of the erythrosin probe indicated that the changes in rp were not due to changes in fast probe motions on the surface of the filament or the phosphorescence emission lifetime, or in the orientation of the probe on the surface of F-actin, suggesting that they reflect large-scale changes in the microsecond rotational dynamics of actin. ATP hydrolysis by actoS1 thus appeared to induce rotational motions of or within F-actin on the phosphorescence time scale (approximately 300 microseconds). Although the precise physical origin of the induced rotational motions is unknown, this study provides direct evidence that large-scale conformational fluctuations of the actin filament are associated with the force-generating event in actomyosin.
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Affiliation(s)
- C M Ng
- Department of Food Science, Rutgers University, New Brunswick, New Jersey
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Lheureux K, Forné T, Chaussepied P. Interaction and polymerization of the G-actin-myosin head complex: effect of DNase I. Biochemistry 1993; 32:10005-14. [PMID: 8399127 DOI: 10.1021/bi00089a016] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
The properties of polymerization and interaction of the G-actin-myosin S1 complexes (formed with either the S1(A1) or the S1(A2) isoform) have been studied by light-scattering and fluorescence measurements in the absence and in the presence of DNase I. In the absence of DNase I, the G-actin-S1(A1) and G-actin-S1(A2) complexes were found to be characterized by different limiting concentrations (l.c.), defined as the complex concentrations above which the polymerization occurs spontaneously within 20 h at 20 degrees C in a "no salt" buffer (l.c. = 0.42 and 8.8 microM for G-actin-S1(A1) and G-actin-S1(A2), respectively). The occurrence of a limiting concentration for either complex together with the kinetic properties of the polymerization led us to conclude that the G-actin-S1 polymerization occurs via a nucleation-elongation process. Fluorescence titrations and proteolysis experiments revealed that G-actin interacts with S1 with a 1:1 stoichiometry (independently of the presence of ATP) with dissociation constants, in the absence of nucleotide, of 20 and 50 nM for the G-actin-S1(A1) and G-actin-S1(A2) complexes, respectively. In the presence of at least a 1.5-fold excess of DNase I, the polymerization of the G-actin-S1 complexes was blocked even at high protein concentration or in the presence of salts. In addition, the affinity of either S1 isoform to actin was reduced 4-5-fold by DNase I, while the stoichiometry of the G-actin-S1 complexes was not changed. However, since the dissociation constants remain in the submicromolar range, we could demonstrate the existence of ternary DNase I-G-actin-S1 complexes stable under polymerizing conditions. Finally, the study of the effect of nucleotides and of various salts on the G-actin-S1 interaction further showed significant differences between the G-actin-S1 and F-actin-S1 interactions.
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Affiliation(s)
- K Lheureux
- Centre de Recherches de Biochimie Macromoléculaire, CNRS-UPR9008, INSERM-U249, Montpellier, France
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Funatsu T, Kono E, Tsukita S. Time-resolved electron microscopic analysis of the behavior of myosin heads on actin filaments after photolysis of caged ATP. J Biophys Biochem Cytol 1993; 121:1053-64. [PMID: 8501115 PMCID: PMC2119686 DOI: 10.1083/jcb.121.5.1053] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
The interaction between myosin subfragment 1 (S1) and actin filaments after the photolysis of P3-1-(2-nitrophenyl)ethyl ester of ATP (caged ATP) was analyzed with a newly developed freezing system using liquid helium. Actin and S1 (100 microM each) formed a ropelike double-helix characteristic of rigor in the presence of 5 mM caged ATP at room temperature. At 15 ms after photolysis, the ropelike double helix was partially disintegrated. The number of S1 attached to actin filaments gradually decreased up to 35 ms after photolysis, and no more changes were detected from 35 to 200 ms. After depletion of ATP, the ropelike double helix was reformed. Taking recent analyses of actomyosin kinetics into consideration, we concluded that most S1 observed on actin filaments at 35-200 ms are so called "weakly bound S1" (S1.ATP or S1.ADP.Pi) and that the weakly bound S1 under a rapid association-dissociation equilibrium with actin filaments can be captured by electron microscopy by means of our newly developed freezing system. This enabled us to directly compare the conformation of weakly and strongly bound S1. Within the resolution of deep-etch replica technique, there were no significant conformational differences between weakly and strongly bound S1, and neither types of S1 showed any positive cooperativity in their binding to actin filaments. Close comparison revealed that the weakly and strongly bound S1 have different angles of attachment to actin filaments. As compared to strongly bound S1, weakly bound S1 showed a significantly broader distribution of attachment angles. These results are discussed with special reference to the molecular mechanism of acto-myosin interaction in the presence of ATP.
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Affiliation(s)
- T Funatsu
- Department of Information Physiology, National Institute for Physiological Sciences, Aichi, Japan
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Pollard TD, Bhandari D, Maupin P, Wachsstock D, Weeds AG, Zot HG. Direct visualization by electron microscopy of the weakly bound intermediates in the actomyosin adenosine triphosphatase cycle. Biophys J 1993; 64:454-71. [PMID: 8457671 PMCID: PMC1262348 DOI: 10.1016/s0006-3495(93)81387-0] [Citation(s) in RCA: 58] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
We used a novel stopped-flow/rapid-freezing machine to prepare the transient intermediates in the actin-myosin adenosine triphosphatase (ATPase) cycle for direct observation by electron microscopy. We focused on the low affinity complexes of myosin-adenosine triphosphate (ATP) and myosin-adenosine diphosphate (ADP)-Pi with actin filaments since the transition from these states to the high affinity actin-myosin-ADP and actin-myosin states is postulated to generate the molecular motion that drives muscle contraction and other types of cellular movements. After rapid freezing and metal replication of mixtures of myosin subfragment-1, actin filaments, and ATP, the structure of the weakly bound intermediates is indistinguishable from nucleotide-free rigor complexes. In particular, the average angle of attachment of the myosin head to the actin filament is approximately 40 degrees in both cases. At all stages in the ATPase cycle, the configuration of most of the myosin heads bound to actin filaments is similar, and the part of the myosin head preserved in freeze-fracture replicas does not tilt by more than a few degrees during the transition from the low affinity to high affinity states. In contrast, myosin heads chemically cross-linked to actin filaments differ in their attachment angles from ordered at 40 degrees without ATP to nearly random in the presence of ATP when viewed by negative staining (Craig, R., L.E. Greene, and E. Eisenberg. 1985. Proc. Natl. Acad. Sci. USA. 82:3247-3251, and confirmed here), freezing in vitreous ice (Applegate, D., and P. Flicker. 1987. J. Biol. Chem. 262:6856-6863), and in replicas of rapidly frozen samples. This suggests that many of the cross-linked heads in these preparations are dissociated from but tethered to the actin filaments in the presence of ATP. These observations suggest that the molecular motion produced by myosin and actin takes place with the myosin head at a point some distance from the actin binding site or does not involve a large change in the shape of the myosin head.
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Affiliation(s)
- T D Pollard
- Department of Cell Biology and Anatomy, Johns Hopkins Medical School, Baltimore, Maryland 21205
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40
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Abstract
Native myosin filaments from scallop striated muscle that have been rapidly frozen in relaxing solutions appear to be well preserved in vitreous ice. Electron micrographs of samples at -177 degrees C were recorded with an electron dose of 10 e/A2 at 1.5 microns defocus. After filament images were straightened by spline-fitting, several transforms showed well-defined layer-lines arising from the helical structure of the filament. A set of 17 near-meridional layer-lines has been collected and corrected for background and for phase and amplitude contrast functions. Preliminary helical reconstructions from this still incomplete data set reveal aspects of structure that were not apparent from earlier analysis of negatively stained filaments from scallop muscle. Individual pear-shaped myosin heads now appear to be well resolved from each other and from the filament backbone. The two heads of each myosin molecule appear to be splayed apart axially. The reconstructions also reveal that the filament backbone has a polygonal shape in cross-section, and that it appears to contain seven peripherally located subfilaments.
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Affiliation(s)
- P Vibert
- Rosenstiel Basic Medical Sciences Research Center, Brandies University, Waltham, MA 02254-9110
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Cano ML, Cassimeris L, Joyce M, Zigmond SH. Characterization of tetramethylrhodaminyl-phalloidin binding to cellular F-actin. CELL MOTILITY AND THE CYTOSKELETON 1992; 21:147-58. [PMID: 1559266 DOI: 10.1002/cm.970210208] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Fluorescent derivatives of phalloidin are widely used to measure filamentous actin (F-actin) levels and to stabilize F-actin. We have characterized the kinetics and affinity of binding of tetramethylrhodaminyl (TRITC)-phalloidin to rabbit skeletal muscle F-actin and to F-actin in lysates of rabbit polymorphonuclear leukocytes (PMNs). We have defined conditions where TRITC-phalloidin can be used to inhibit F-actin depolymerization and to quantify F-actin without prior fixation. By equilibrium measurements, the affinity of TRITC-phalloidin binding to rabbit skeletal muscle F-actin (pyrene labeled) or to PMN lysate F-actin was 1-4 x 10(-7) M. In both cases, the stoichiometry of binding was approximately 1:1. Kinetic measurements of TRITC-phalloidin binding to PMN lysate F-actin resulted in an association rate constant of 420 +/- 120 M-1 sec-1 and a dissociation rate constant of 8.3 +/- 0.9 x 10(-5) sec-1. The affinity calculated from the kinetic measurements (2 +/- 1 x 10(-7) M) agreed well with that obtained by equilibrium measurements. The rate with which 0.6 microM TRITC-phalloidin inhibited 0.1 microM pyrenyl F-actin depolymerization (90% inhibition in 10 sec) was much faster than the rate of binding to pyrenyl F-actin (less than 1% bound in 10 sec), suggesting that phalloidin binds to filament ends more rapidly than to the rest of the filament. We show that TRITC-phalloidin can be used to measure F-actin levels in cell lysates when G-actin is also present (i.e., in cell lysates at high concentrations) if DNase I is included to prevent phalloidin-induced polymerization.
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Affiliation(s)
- M L Cano
- Department of Chemical Engineering, University of Pennsylvania, Philadelphia 19104
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Schröder RR, Hofmann W, Menetret JF, Holmes KC, Goody RS. Cryo-electron microscopy of vitrified muscle samples. ELECTRON MICROSCOPY REVIEWS 1992; 5:171-92. [PMID: 1730075 DOI: 10.1016/0892-0354(92)90009-f] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
A great deal of information on the 3-dimensional structure of the protein assemblies involved in muscle contraction has been obtained using conventional transmission electron microscopy. In recent years, developments in cryo-electron microscopy have facilitated work with fully hydrated, non-chemically fixed specimens. It is shown how this technique can be used to visualize muscle sarcomere filaments in quasi-native conditions, to access hitherto inaccessible states of the crossbridge cycle, and to obtain new high resolution structural information on their 3-dimensional protein structure. A short introduction to the crossbridge cycle and its biochemically accessible states illustrates the problems amenable to studies using the electron microscope, as well as the possibilities offered by cryo-microscopy on vitrified samples. Work on vitrified cryo-sections and myosin filament suspensions demonstrates the accessibility of crossbridge states and gives implications on the gross structural features of myosin filaments. Recent studies on actin filaments and myosin (S1) decorated actin filaments provide the first high resolution data on vitrified samples. The use of photolabile nucleotide precursors allows the trapping of short lived states in the millisecond time range, thereby visualizing intermediate states of the crossbridge cycle.
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Affiliation(s)
- R R Schröder
- Max-Planck-Institute for Medical Research, Department of Biophysics, Heidelberg F.R.G
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