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Caremani M, Reconditi M. Anisotropic Elasticity of the Myosin Motor in Muscle. Int J Mol Sci 2022; 23:ijms23052566. [PMID: 35269709 PMCID: PMC8909946 DOI: 10.3390/ijms23052566] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2021] [Revised: 02/08/2022] [Accepted: 02/23/2022] [Indexed: 11/16/2022] Open
Abstract
To define the mechanics and energetics of the myosin motor action in muscles, it is mandatory to know fundamental parameters such as the stiffness and the force of the single myosin motor, and the fraction of motors attached during contraction. These parameters can be defined in situ using sarcomere-level mechanics in single muscle fibers under the assumption that the stiffness of a myosin dimer with both motors attached (as occurs in rigor, when all motors are attached) is twice that of a single motor (as occurs in the isometric contraction). We use a mechanical/structural model to identify the constraints that underpin the stiffness of the myosin dimer with both motors attached to actin. By comparing the results of the model with the data in the literature, we conclude that the two-fold axial stiffness of the dimers with both motors attached is justified by a stiffness of the myosin motor that is anisotropic and higher along the axis of the myofilaments. A lower azimuthal stiffness of the motor plays an important role in the complex architecture of the sarcomere by allowing the motors to attach to actin filaments at different azimuthal angles relative to the thick filament.
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Affiliation(s)
- Marco Caremani
- PhysioLab, Università di Firenze, 50019 Sesto Fiorentino, Italy;
- Dipartimento di Biologia, Università di Firenze, 50019 Sesto Fiorentino, Italy
| | - Massimo Reconditi
- PhysioLab, Università di Firenze, 50019 Sesto Fiorentino, Italy;
- Dipartimento di Medicina Sperimentale e Clinica, Università di Firenze, 50134 Firenze, Italy
- Correspondence: ; Tel.: +39-055-457-4714
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TREM-1 activation is a potential key regulator in driving severe pathogenesis of enterovirus A71 infection. Sci Rep 2020; 10:3810. [PMID: 32123257 PMCID: PMC7052206 DOI: 10.1038/s41598-020-60761-5] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2019] [Accepted: 02/06/2020] [Indexed: 11/08/2022] Open
Abstract
Hand, foot and mouth disease (HFMD), caused by enterovirus A71 (EV-A71), presents mild to severe disease, and sometimes fatal neurological and respiratory manifestations. However, reasons for the severe pathogenesis remain undefined. To investigate this, infection and viral kinetics of EV-A71 isolates from clinical disease (mild, moderate and severe) from Sarawak, Malaysia, were characterised in human rhabdomyosarcoma (RD), neuroblastoma (SH-SY5Y) and peripheral blood mononuclear cells (PBMCs). High resolution transcriptomics was used to decipher EV-A71-host interactions in PBMCs. Ingenuity analyses revealed similar pathways triggered by all EV-A71 isolates, although the extent of activation varied. Importantly, several pathways were found to be specific to the severe isolate, including triggering receptor expressed on myeloid cells 1 (TREM-1) signalling. Depletion of TREM-1 in EV-A71-infected PBMCs with peptide LP17 resulted in decreased levels of pro-inflammatory genes for the moderate and severe isolates. Mechanistically, this is the first report describing the transcriptome profiles during EV-A71 infections in primary human cells, and the potential involvement of TREM-1 in the severe disease pathogenesis, thus providing new insights for future treatment targets.
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3
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Savich Y, Binder BP, Thompson AR, Thomas DD. Myosin lever arm orientation in muscle determined with high angular resolution using bifunctional spin labels. J Gen Physiol 2019; 151:1007-1016. [PMID: 31227551 PMCID: PMC6683674 DOI: 10.1085/jgp.201812210] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2018] [Revised: 02/15/2019] [Accepted: 05/29/2019] [Indexed: 02/03/2023] Open
Abstract
High-resolution structural information is invaluable for understanding muscle function. Savich et al. use bifunctional spin labeling to determine the orientation of the myosin lever arm in muscle fibers at high resolution under ambient conditions, augmenting previous insights obtained from fluorescence and EM. Despite advances in x-ray crystallography, cryo-electron microscopy (cryo-EM), and fluorescence polarization, none of these techniques provide high-resolution structural information about the myosin light chain domain (LCD; lever arm) under ambient conditions in vertebrate muscle. Here, we measure the orientation of LCD elements in demembranated muscle fibers by electron paramagnetic resonance (EPR) using a bifunctional spin label (BSL) with an angular resolution of 4°. To achieve stereoselective site-directed labeling with BSL, we engineered a pair of cysteines in the myosin regulatory light chain (RLC), either on helix E or helix B, which are roughly parallel or perpendicular to the myosin lever arm, respectively. By exchanging BSL-labeled RLC onto oriented muscle fibers, we obtain EPR spectra from which the angular distributions of BSL, and thus the lever arm, can be determined with high resolution relative to the muscle fiber axis. In the absence of ATP (rigor), each of the two labeled helices exhibits both ordered (σ ∼9–11°) and disordered (σ > 38°) populations. Using these angles to determine the orientation of the lever arm (LCD combined with converter subdomain), we observe that the oriented population corresponds to a lever arm that is perpendicular to the muscle fiber axis and that the addition of ATP in the absence of Ca2+ (inducing relaxation) shifts the orientation to a much more disordered orientational distribution. Although the detected orientation of the myosin light chain lever arm is ∼33° different than predicted from a standard “lever arm down” model based on cryo-EM of actin decorated with isolated myosin heads, it is compatible with, and thus augments and clarifies, fluorescence polarization, x-ray interference, and EM data obtained from muscle fibers. These results establish feasibility for high-resolution detection of myosin LCD rotation during muscle contraction.
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Affiliation(s)
- Yahor Savich
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN.,School of Physics and Astronomy, University of Minnesota, Minneapolis, MN
| | - Benjamin P Binder
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN.,Department of Chemistry, Augsburg University, Minneapolis, MN
| | - Andrew R Thompson
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN
| | - David D Thomas
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN
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4
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Actin-Myosin Interaction: Structure, Function and Drug Discovery. Int J Mol Sci 2018; 19:ijms19092628. [PMID: 30189615 PMCID: PMC6163256 DOI: 10.3390/ijms19092628] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2018] [Revised: 08/31/2018] [Accepted: 09/02/2018] [Indexed: 12/17/2022] Open
Abstract
Actin-myosin interactions play crucial roles in the generation of cellular force and movement. The molecular mechanism involves structural transitions at the interface between actin and myosin’s catalytic domain, and within myosin’s light chain domain, which contains binding sites for essential (ELC) and regulatory light chains (RLC). High-resolution crystal structures of isolated actin and myosin, along with cryo-electron micrographs of actin-myosin complexes, have been used to construct detailed structural models for actin-myosin interactions. However, these methods are limited by disorder, particularly within the light chain domain, and they do not capture the dynamics within this complex under physiological conditions in solution. Here we highlight the contributions of site-directed fluorescent probes and time-resolved fluorescence resonance energy transfer (TR-FRET) in understanding the structural dynamics of the actin-myosin complex in solution. A donor fluorescent probe on actin and an acceptor fluorescent probe on myosin, together with high performance TR-FRET, directly resolves structural states in the bound actin-myosin complex during its interaction with adenosine triphosphate (ATP). Results from these studies have profound implications for understanding the contractile function of actomyosin and establish the feasibility for the discovery of allosteric modulators of the actin-myosin interaction, with the ultimate goal of developing therapies for muscle disorders.
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5
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Do Actomyosin Single-Molecule Mechanics Data Predict Mechanics of Contracting Muscle? Int J Mol Sci 2018; 19:ijms19071863. [PMID: 29941816 PMCID: PMC6073448 DOI: 10.3390/ijms19071863] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2018] [Revised: 06/19/2018] [Accepted: 06/20/2018] [Indexed: 12/15/2022] Open
Abstract
In muscle, but not in single-molecule mechanics studies, actin, myosin and accessory proteins are incorporated into a highly ordered myofilament lattice. In view of this difference we compare results from single-molecule studies and muscle mechanics and analyze to what degree data from the two types of studies agree with each other. There is reasonable correspondence in estimates of the cross-bridge power-stroke distance (7–13 nm), cross-bridge stiffness (~2 pN/nm) and average isometric force per cross-bridge (6–9 pN). Furthermore, models defined on the basis of single-molecule mechanics and solution biochemistry give good fits to experimental data from muscle. This suggests that the ordered myofilament lattice, accessory proteins and emergent effects of the sarcomere organization have only minor modulatory roles. However, such factors may be of greater importance under e.g., disease conditions. We also identify areas where single-molecule and muscle data are conflicting: (1) whether force generation is an Eyring or Kramers process with just one major power-stroke or several sub-strokes; (2) whether the myofilaments and the cross-bridges have Hookean or non-linear elasticity; (3) if individual myosin heads slip between actin sites under certain conditions, e.g., in lengthening; or (4) if the two heads of myosin cooperate.
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de Vera IMS, Blackburn ME, Galiano L, Fanucci GE. Pulsed EPR distance measurements in soluble proteins by site-directed spin labeling (SDSL). ACTA ACUST UNITED AC 2013; 74:17.17.1-17.17.29. [PMID: 24510645 DOI: 10.1002/0471140864.ps1717s74] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
The resurgence of pulsed electron paramagnetic resonance (EPR) in structural biology centers on recent improvements in distance measurements using the double electron-electron resonance (DEER) technique. This unit focuses on EPR-based distance measurements by site-directed spin labeling (SDSL) of engineered cysteine residues in soluble proteins, with HIV-1 protease used as a model. To elucidate conformational changes in proteins, experimental protocols were optimized and existing data analysis programs were employed to derive distance-distribution profiles. Experimental considerations, sample preparation, and error analysis for artifact suppression are also outlined herein.
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Affiliation(s)
| | - Mandy E Blackburn
- Department of Chemistry, University of Florida, Gainesville, Florida.,Department of Biochemistry and Molecular Biology, University of Massachusetts Amherst, Amherst, Massachusetts
| | - Luis Galiano
- Department of Chemistry, University of Florida, Gainesville, Florida.,Syngenta Crop Protection, Minnetonka, Minnesota
| | - Gail E Fanucci
- Department of Chemistry, University of Florida, Gainesville, Florida
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7
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Kaya M, Higuchi H. Stiffness, working stroke, and force of single-myosin molecules in skeletal muscle: elucidation of these mechanical properties via nonlinear elasticity evaluation. Cell Mol Life Sci 2013; 70:4275-92. [PMID: 23685901 PMCID: PMC11113998 DOI: 10.1007/s00018-013-1353-x] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2012] [Revised: 02/27/2013] [Accepted: 04/25/2013] [Indexed: 10/26/2022]
Abstract
In muscles, the arrays of skeletal myosin molecules interact with actin filaments and continuously generate force at various contraction speeds. Therefore, it is crucial for myosin molecules to generate force collectively and minimize the interference between individual myosin molecules. Knowledge of the elasticity of myosin molecules is crucial for understanding the molecular mechanisms of muscle contractions because elasticity directly affects the working and drag (resistance) force generation when myosin molecules are positively or negatively strained. The working stroke distance is also an important mechanical property necessary for elucidation of the thermodynamic efficiency of muscle contractions at the molecular level. In this review, we focus on these mechanical properties obtained from single-fiber and single-molecule studies and discuss recent findings associated with these mechanical properties. We also discuss the potential molecular mechanisms associated with reduction of the drag effect caused by negatively strained myosin molecules.
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Affiliation(s)
- Motoshi Kaya
- Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan,
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8
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Three distinct actin-attached structural states of myosin in muscle fibers. Biophys J 2012; 102:1088-96. [PMID: 22404931 DOI: 10.1016/j.bpj.2011.11.4027] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2011] [Revised: 11/09/2011] [Accepted: 11/14/2011] [Indexed: 11/21/2022] Open
Abstract
We have used thiol cross-linking and electron paramagnetic resonance (EPR) to resolve structural transitions of myosin's light chain domain (LCD) and catalytic domain (CD) that are associated with force generation. Spin labels were incorporated into the LCD of muscle fibers by exchanging spin-labeled regulatory light chain for endogenous regulatory light chain, with full retention of function. To trap myosin in a structural state analogous to the elusive posthydrolysis ternary complex A.M'.D.P, we used pPDM to cross-link SH1 (Cys(707)) to SH2 (Cys(697)) on the CD. LCD orientation and dynamics were measured in three biochemical states: relaxation (A.M.T), SH1-SH2 cross-linked (A.M'.D.P analog), and rigor (A.M.D). EPR showed that the LCD of cross-linked fibers has an orientational distribution intermediate between relaxation and rigor, and saturation transfer EPR revealed slow rotational dynamics indistinguishable from that of rigor. Similar results were obtained for the CD using a bifunctional spin label to cross-link SH1-SH2, but the CD was more disordered than the LCD. We conclude that SH1-SH2 cross-linking traps a state in which both the CD and LCD are intermediate between relaxation (highly disordered and microsecond dynamics) and rigor (highly ordered and rigid), supporting the hypothesis that the cross-linked state is an A.M'D.P analog on the force generation pathway.
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9
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Xu S, Brenner B, Yu LC. State-dependent radial elasticity of attached cross-bridges in single skinned fibres of rabbit psoas muscle. J Physiol 2010; 461:283-99. [PMID: 16993186 PMCID: PMC1175258 DOI: 10.1113/jphysiol.1993.sp019514] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
1. In a single skinned fibre of rabbit psoas muscle, upon attachment of cross-bridges to actin in the presence of ADP or pyrophosphate (PP(i)), the separation between the contractile filaments, as determined by equatorial X-ray diffraction, is found to decrease, suggesting that force is generated in the radial direction.2. The single muscle fibres were subjected to compression by 0-8% of dextran T(500). The changes in lattice spacings by dextran compression were compared with changes induced by cross-bridge attachment to actin. Based on this comparison, the magnitude and the direction of the radial force generated by the attached cross-bridges were estimated. The radial cross-bridge force varied with filament separation, and the magnitude of the radial cross-bridge force reached as high as the maximal axial force produced during isometric contraction.3. One key parameter of the radial elasticity, i.e. the equilibrium spacing where the radial force is zero, was found to depend on the ligand bound to the myosin head. In the presence of ADP, the equilibrium spacing was 36 nm. In the presence of MgPP(i) the equilibrium spacing shifted to 35 nm and Ca(2+) had little effect on the equilibrium spacing.4. The equilibrium spacing was independent of the fraction of cross-bridges attached to actin. The fraction of cross-bridges attached in rigor was modulated from 100% to close to 0% by adding up to 10 mM of ATPgammaS in the rigor solution. The lattice spacing remained at 38 nm, the equilibrium spacing for nucleotide-free cross-bridges at mu = 170 mM.5. Radial force generated by cross-bridges in rigor at large lattice spacings (38 nm </= d(10) </= 46 nm) appeared to vary linearly with lattice spacing.6. The titration of ATPgammaS to fibres in rigor provided a correlation between the radial stiffness of the nucleotide-free cross-bridges and the equatorial intensities. The relation between the equatorial intensity ratio I(11)/I(10) and radial stiffness appeared to be approximately linear.7. The fibres under different conditions showed a wide range of radial stiffness, which was not proportional to the apparent axial stiffness of the fibre. If the apparent axial stiffness is a measure of the fraction of cross-bridges bound to actin, it follows that the radial elastic constant is state dependent; or vice versa.8. Differences in equilibrium lattice spacing and in radial elastic constant, most probably reflect differences in the molecular structure of the acto-myosin complex and there is more than one single conformation of the various strongly bound cross-bridge states.9. Determining equilibrium spacings of the radial elasticity appears to be an effective new approach in detecting structural differences among the attached cross-bridges, since this approach is independent of the fraction of cross-bridges attached, a factor that frequently encumbers the interpretation of structural studies of attached cross-bridge states.
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Affiliation(s)
- S Xu
- National Institutes of Health, Bethesda, MD 20892, USA
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10
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Fusi L, Reconditi M, Linari M, Brunello E, Elangovan R, Lombardi V, Piazzesi G. The mechanism of the resistance to stretch of isometrically contracting single muscle fibres. J Physiol 2009; 588:495-510. [PMID: 19948653 DOI: 10.1113/jphysiol.2009.178137] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Rapid attachment to actin of the detached motor domain of myosin dimers with one motor domain already attached has been hypothesized to explain the stretch-induced changes in X-ray interference and stiffness of active muscle. Here, using half-sarcomere mechanics in single frog muscle fibres (2.15 microm sarcomere length and 4 degrees C), we show that: (1) an increase in stiffness of the half-sarcomere under stretch is specific to isometric contraction and does not occur in rigor, indicating that the mechanism of stiffness increase is an increase in the number of attached motors; (2) 2 ms after 100 micros stretches (amplitude 2-8 nm per half-sarcomere) imposed during an isometric tetanus, the stiffness of the array of myosin motors in each half-sarcomere (e(m)) increases above the isometric value (e(m0)); (3) e(m) has a sigmoidal dependence on the distortion of the motor domains (Delta z) attached in isometric contraction, with a maximum approximately 2 e(m0) for a distortion of approximately 6 nm; e(m) is influenced by detachment of motors at z > 6 nm; (4) at the end of the 100 micros stretch the relation between e(m)/e(m0) and Delta z lies slightly but not significantly above that at 2 ms. These results support the idea that stretch-induced sliding of the actin filament distorts the actin-attached motor domain of the myosin dimers away from the centre of the sarcomere, providing the steric conditions for rapid attachment of the second motor domain. The rate of new motor attachment must be as high as 7.5 x 10(4) s(1) and explains the rapid and efficient increase of the resistance of active muscle to stretch.
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Affiliation(s)
- Luca Fusi
- Laboratory of Physiology, Department of Evolutionary Biology, University of Florence, 50019 Sesto Fiorentino, Italy
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11
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Structural dynamics of the actomyosin complex probed by a bifunctional spin label that cross-links SH1 and SH2. Biophys J 2008; 95:5238-46. [PMID: 18805936 DOI: 10.1529/biophysj.108.138982] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We have used a bifunctional spin label (BSL) to cross-link Cys(707) (SH1) and Cys(697) (SH2) in the catalytic domain of myosin subfragment 1 (S1). BSL induces the same weakened ATPase activity and actin-binding affinity that is observed when SH1 and SH2 are cross-linked with pPDM, which traps an analog of the post-hydrolysis state A.M.ADP.P. Electron paramagnetic resonance showed that BSL reports the global orientation and dynamics of S1. When bound to actin in oriented muscle fibers in the absence of ATP, BSL-S1 showed almost complete orientational disorder, as reported previously for the weakly bound A.M.ADP. In contrast, helical order is observed for the strongly bound state A.M. Saturation transfer electron paramagnetic resonance showed that the disorder of cross-linked S1 on actin is nearly static on the microsecond timescale, at least 30 times slower than that of A.M.ADP. We conclude that cross-linked S1 exhibits rotational disorder comparable to that of A.M.ADP, slow rotational mobility comparable to that of A.M, and intermediate actin affinity. These results support the hypothesis that the catalytic domain of myosin is orientationally disordered on actin in a post-hydrolysis state in the early stages of force generation.
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12
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Linari M, Caremani M, Piperio C, Brandt P, Lombardi V. Stiffness and fraction of Myosin motors responsible for active force in permeabilized muscle fibers from rabbit psoas. Biophys J 2007; 92:2476-90. [PMID: 17237201 PMCID: PMC1864836 DOI: 10.1529/biophysj.106.099549] [Citation(s) in RCA: 121] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The stiffness of the single myosin motor (epsilon) is determined in skinned fibers from rabbit psoas muscle by both mechanical and thermodynamic approaches. Changes in the elastic strain of the half-sarcomere (hs) are measured by fast mechanics both in rigor, when all myosin heads are attached, and during active contraction, with the isometric force (T0) modulated by changing either [Ca2+] or temperature. The hs compliance is 43.0+/-0.8 nm MPa-1 in isometric contraction at saturating [Ca2+], whereas in rigor it is 28.2+/-1.1 nm MPa-1. The equivalent compliance of myofilaments is 21.0+/-3.3 nm MPa-1. Accordingly, the stiffness of the ensemble of myosin heads attached in the hs is 45.5+/-1.7 kPa nm-1 in isometric contraction at saturating [Ca2+] (e0), and in rigor (er) it rises to 138.9+/-21.2 kPa nm-1. Epsilon, calculated from er and the lattice molecular dimensions, is 1.21+/-0.18 pN nm-1. epsilon estimated, using a thermodynamic approach, from the relation of T0 at saturating [Ca2+] versus the reciprocal of absolute temperature is 1.25+/-0.14 pN nm-1, similar to that estimated for fibers in rigor. Consequently, the ratio e0/er (0.33+/-0.05) can be used to estimate the fraction of attached heads during isometric contraction at saturating [Ca2+]. If the osmotic agent dextran T-500 (4 g/100 ml) is used to reduce the lateral filament spacing of the relaxed fiber to the value before skinning, both e0 and er increase by approximately 40%. Epsilon becomes approximately 1.7 pN nm-1 and the fraction and the force of myosin heads attached in the isometric contraction remain the same as before dextran application. The finding that the fraction of myosin heads attached to actin in an isometric contraction is 0.33 rules out the hypothesis of multiple mechanical cycles per ATP hydrolyzed.
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Affiliation(s)
- Marco Linari
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze, Firenze, Italy.
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13
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Naber N, Purcell TJ, Pate E, Cooke R. Dynamics of the nucleotide pocket of myosin measured by spin-labeled nucleotides. Biophys J 2006; 92:172-84. [PMID: 17028139 PMCID: PMC1697850 DOI: 10.1529/biophysj.106.090035] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We have used electron paramagnetic probes attached to the ribose of ATP (SL-ATP) to monitor conformational changes in the nucleotide pocket of myosin. Spectra for analogs bound to myosin in the absence of actin showed a high degree of immobilization, indicating a closed nucleotide pocket. In the Actin.Myosin.SL-AMPPNP, Actin.Myosin.SL-ADP.BeF(3), and Actin.Myosin.SL-ADP.AlF(4) complexes, which mimic weakly binding states near the beginning of the power stroke, the nucleotide pocket remained closed. The spectra of the strongly bound Actin.Myosin.SL-ADP complex consisted of two components, one similar to the closed pocket and one with increased probe mobility, indicating a more open pocket, The temperature dependence of the spectra showed that the two conformations of the nucleotide pocket were in equilibrium, with the open conformation more favorable at higher temperatures. These results, which show that opening of the pocket occurs only in the strongly bound states, appear reasonable, as this would tend to keep ADP bound until the end of the power stroke. This conclusion also suggests that force is initially generated by a myosin with a closed nucleotide pocket.
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Affiliation(s)
- Nariman Naber
- Department of Biochemistry and Biophysics, University of California, San Francisco, California 94158, USA.
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14
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Pronina OE, Wrzosek A, Dabrowska R, Borovikov YS. Effect of nucleotides on the orientation and mobility of myosin subfragment-1 in ghost muscle fiber. BIOCHEMISTRY (MOSCOW) 2006; 70:1140-4. [PMID: 16271031 DOI: 10.1007/s10541-005-0237-6] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Using polarization fluorimetry, the orientation and mobility of 1,5-IAEDANS specifically bound to Cys707 of myosin subfragment-1 (S1) were studied in ghost muscle tropomyosin-containing fibers in the absence and in the presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes in the structural state of the myosin head during the ATPase cycle. Maximal differences in the probe orientation by 4 degrees and its mobility by 30% were found between actomyosin states in the presence of MgADP and MgATP. It is suggested that interaction of S1 with F-actin induces nucleotide-dependent rotation of the whole motor domain of the myosin head or only the dye-binding site and also change in the head mobility.
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Affiliation(s)
- O E Pronina
- Institute of Cytology, Russian Academy of Sciences, St. Petersburg, 194064, Russia
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15
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Baumann BAJ, Liang H, Sale K, Hambly BD, Fajer PG. Myosin regulatory domain orientation in skeletal muscle fibers: application of novel electron paramagnetic resonance spectral decomposition and molecular modeling methods. Biophys J 2004; 86:3030-41. [PMID: 15111417 PMCID: PMC1304169 DOI: 10.1016/s0006-3495(04)74352-0] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2003] [Accepted: 01/20/2004] [Indexed: 10/21/2022] Open
Abstract
Reorientation of the regulatory domain of the myosin head is a feature of all current models of force generation in muscle. We have determined the orientation of the myosin regulatory light chain (RLC) using a spin-label bound rigidly and stereospecifically to the single Cys-154 of a mutant skeletal isoform. Labeled RLC was reconstituted into skeletal muscle fibers using a modified method that results in near-stoichiometric levels of RLC and fully functional muscle. Complex electron paramagnetic resonance spectra obtained in rigor necessitated the development of a novel decomposition technique. The strength of this method is that no specific model for a complex orientational distribution was presumed. The global analysis of a series of spectra, from fibers tilted with respect to the magnetic field, revealed two populations: one well-ordered (+/-15 degrees ) with the spin-label z axis parallel to actin, and a second population with a large distribution (+/-60 degrees ). A lack of order in relaxed or nonoverlap fibers demonstrated that regulatory domain ordering was defined by interaction with actin rather than the thick filament surface. No order was observed in the regulatory domain during isometric contraction, consistent with the substantial reorientation that occurs during force generation. For the first time, spin-label orientation has been interpreted in terms of the orientation of a labeled domain. A Monte Carlo conformational search technique was used to determine the orientation of the spin-label with respect to the protein. This in turn allows determination of the absolute orientation of the regulatory domain with respect to the actin axis. The comparison with the electron microscopy reconstructions verified the accuracy of the method; the electron paramagnetic resonance determined that axial orientation was within 10 degrees of the electron microscopy model.
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Affiliation(s)
- Bruce A J Baumann
- Department of Biological Sciences, Florida State University, Tallahassee, Florida, USA
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16
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Lidke DS, Thomas DD. Coordination of the two heads of myosin during muscle contraction. Proc Natl Acad Sci U S A 2002; 99:14801-6. [PMID: 12417762 PMCID: PMC137499 DOI: 10.1073/pnas.232161999] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We have used luminescence resonance energy transfer between regulatory light chains (RLC) to detect structural changes within the dimeric myosin molecule in contracting muscle fibers. Fully functional scallop muscle fibers were prepared such that each myosin molecule contained a terbium-labeled (luminescent donor) RLC on one head and a rhodamine-labeled (acceptor) RLC on the other. Time-resolved luminescence energy transfer between the two heads increased upon the transition from relaxation (ATP) to contraction (ATP plus Ca) and increased further in rigor (no ATP). Combined with experiments on mutant RLCs labeled specifically at other sites, these results support a model in which the force-generating weak-to-strong transition causes one myosin LC domain to tilt through a 30 degrees angle toward the other, thus acting as a coordinated lever arm.
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Affiliation(s)
- Diane S Lidke
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis 55455, USA
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17
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Yengo CM, De la Cruz EM, Safer D, Ostap EM, Sweeney HL. Kinetic characterization of the weak binding states of myosin V. Biochemistry 2002; 41:8508-17. [PMID: 12081502 DOI: 10.1021/bi015969u] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Myosin V is a molecular motor shown to move processively along actin filaments. We investigated the properties of the weak binding states of monomeric myosin V containing a single IQ domain (MV 1IQ) to determine if the affinities of these states are increased as compared to conventional myosin. Further, using a combination of non-hydrolyzable nucleotide analogues and mutations that block ATP hydrolysis, we sought to probe the states that are populated during ATP-induced dissociation of actomyosin. MV 1IQ binds actin with a K(d) = 4 microM in the presence of ATP gamma S at 50 mM KCl, which is 10-20-fold tighter than that of nonprocessive class II myosins. Mutations within the switch II region trapped MV 1IQ in two distinct M.ATP states with very different actin binding affinities (K(d) = 0.2 and 2 microM). Actin binding may change the conformation of the switch II region, suggesting that elements of the nucleotide binding pocket will be in a different conformation when bound to actin than is seen in any of the myosin crystal structures to date.
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Affiliation(s)
- Christopher M Yengo
- University of Pennsylvania School of Medicine, Department of Physiology and Pennsylvania Muscle Institute, Philadelphia, Pennsylvania 19104, USA
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18
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Barnett VA. Cross-bridge cooperativity during isometric contraction and unloaded shortening of skeletal muscle. J Muscle Res Cell Motil 2002; 22:415-23. [PMID: 11964067 DOI: 10.1023/a:1014572003361] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Whether the two heads of skeletal muscle myosin work independently or cooperatively remains an open question in muscle biophysics. While individual myosin heads are sufficient for ATPase activity (Reisler (1980) J Mol Biol 138: 93-107) and force production (Harada et al. (1987) Nature 326: 805-808), it has also been reported that in situ, the two heads of a myosin molecule work cooperatively (Chaen et al. (1986) J Biol Chem 261(29): 13,632-13,636). To examine the role of cross-bridge cooperativity on isometric contraction and unloaded shortening we progressively inactivated myosin cross-bridges via titration with para-phenylenedimaleimide. The resting fiber ATPase was measured to provide an estimate of the fraction of active cross-bridges remaining during the titration. Isometric force and unloaded shortening velocity decline more rapidly than the resting ATPase as the titration proceeds. This is inconsistent with models for independent force generation and suggests cooperative action of myosin cross-bridges when muscle is isometrically contracting or shortening under zero load. However the degree of cooperativity depends on the type of muscle activity. While isometric force declines in a manner consistent with pair-wise cooperative action of myosin heads, unloaded shortening velocity declines more rapidly (greater cooperativity). Therefore, myosin cross-bridges in situ may be capable of at least two types of cooperative interactions, pair-wise cooperativity (when isometric) and another form of cooperativity that is sensitive to longer range interactions transmitted from other cross-bridges in the ensemble (during unloaded shortening).
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Affiliation(s)
- V A Barnett
- Department of Physiology, University of Minnesota Medical School, Minneapolis 55455, USA.
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19
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Hopkins SC, Sabido-David C, van der Heide UA, Ferguson RE, Brandmeier BD, Dale RE, Kendrick-Jones J, Corrie JET, Trentham DR, Irving M, Goldman YE. Orientation changes of the myosin light chain domain during filament sliding in active and rigor muscle. J Mol Biol 2002; 318:1275-91. [PMID: 12083517 DOI: 10.1016/s0022-2836(02)00189-4] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Structural changes in myosin power many types of cell motility including muscle contraction. Tilting of the myosin light chain domain (LCD) seems to be the final step in transducing the energy of ATP hydrolysis, amplifying small structural changes near the ATP binding site into nanometer-scale motions of the filaments. Here we used polarized fluorescence measurements from bifunctional rhodamine probes attached at known orientations in the LCD to describe the distribution of orientations of the LCD in active contraction and rigor. We applied rapid length steps to perturb the orientations of the population of myosin heads that are attached to actin, and thereby characterized the motions of these force-bearing myosin heads. During active contraction, this population is a small fraction of the total. When the filaments slide in the shortening direction in active contraction, the long axis of LCD tilts towards its nucleotide-free orientation with no significant twisting around this axis. In contrast, filament sliding in rigor produces coordinated tilting and twisting motions.
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Affiliation(s)
- Seth C Hopkins
- Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia 19104-6083, USA
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20
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Hartvig N, Lõrinczy D, Farkas N, Belagyi J. Effect of adenosine 5'-[beta,gamma-imido]triphosphate on myosin head domain movements. EUROPEAN JOURNAL OF BIOCHEMISTRY 2002; 269:2168-77. [PMID: 11985595 DOI: 10.1046/j.1432-1033.2002.02872.x] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Conventional and saturation transfer electron paramagnetic resonance spectroscopy (EPR and ST EPR) was used to study the orientation of probe molecules in muscle fibers in different intermediate states of the ATP hydrolysis cycle. A separate procedure was used to obtain ST EPR spectra with precise phase settings even in the case of samples with low spectral intensity. Fibers prepared from rabbit psoas muscle were labeled with isothiocyanate spin labels at the reactive thiol sites of the catalytic domain of myosin. In comparison with rigor, a significant difference was detected in the orientation-dependence of spin labels in the ADP and adenosine 5'-[beta,gamma-imido]triphosphate (AdoPP[CH2]P) states, indicating changes in the internal dynamics and domain orientation of myosin. In the AdoPP[CH2]P state, approximately half of the myosin heads reflected the motional state of ADP-myosin, and the other half showed a different dynamic state with greater mobility.
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Affiliation(s)
- Nóra Hartvig
- Central Research Laboratory and Institute of Biophysics, School of Medicine, University of Pécs, Hungary
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21
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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: 23] [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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22
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Tong CW, Kolomenskii A, Lioubimov VA, Schuessler HA, Trache A, Granger HJ, Muthuchamy M. Measurements of the cross-bridge attachment/detachment process within intact sarcomeres by surface plasmon resonance. Biochemistry 2001; 40:13915-24. [PMID: 11705381 DOI: 10.1021/bi0101648] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
We have developed a surface plasmon resonance (SPR) system to monitor the cross-bridge attachment/detachment process within intact sarcomeres from mouse heart muscle. SPR occurs when laser light energy is transferred to surface plasmons that are resonantly excited in a metal (gold) film. This resonance manifests itself as a minimum in the reflection of the incident laser light and occurs at a characteristic angle. The angle of the SPR occurrence depends on the dielectric permittivity of the sample medium adjacent to the gold film. Purified sarcomeric preparations are immobilized onto the gold film in the presence of a relaxing solution. Replacement of the relaxing solution with increasing Ca(2+) concentration solution activates the cross-bridge interaction and produces an increase in the SPR angle. These results imply that the interaction of myosin heads with actin within an intact sarcomere changes the dielectric permittivity of the sarcomeric structure. In addition, we further verify that SPR measurements can detect the changes in the population of the attached cross-bridges with altered concentrations of phosphate, 2,3-butanedione monoxime, or adenosine triphosphate at a fixed calcium concentration, which have been shown to reduce the force and increase the cross-bridge population in attached state. Thus, our data provide the first evidence that the SPR technique allows the monitoring of the cross-bridge attachment/detachment process within intact sarcomeres.
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Affiliation(s)
- C W Tong
- Cardiovascular Research Institute and Department of Medical Physiology, College of Medicine, Texas A&M University System Health Science Center, 336 Reynolds Medical Building, College Station, Texas 77843-1114, USA
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23
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Chen X, Grammer J, Cooke R, Pate E, Yount RG. Synthesis and characterization of novel spin-labeled photoaffinity nonnucleoside analogues of ATP as structural and EPR probes for myosin. Bioconjug Chem 2000; 11:725-33. [PMID: 10995217 DOI: 10.1021/bc000032b] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Two new spin-labeled photoreactive nonnucleoside ATP analogues, 1-(4-azido-2-nitrophenyl)amino-3-(1-oxyl-2,2,5, 5-tetramethylpyrrolidinyl-3-carbamido)-2-propyl triphosphate (SL-NANTP) and 2-(4-azido-2-nitrophenyl)amino-2,2-(1-oxyl-2,2,6, 6-tetramethyl-4-piperidylidene)di(oxymethylene) ethyl triphosphate (SSL-NANTP), were synthesized and characterized. This study aims to develop a second generation of NANTP-based analogues containing immobile spin labels that can be used to monitor conformational changes in myosin during the contractile cycle of muscle. Previous studies have shown that both a photoaffinity nonnucleoside ATP analogue, 2-[(4-azido-2-nitrophenyl)amino] ethyl triphosphate (NANTP) [Nakamaye et al. (1985) Biochemistry 24, 5226-5235], and a photoaffinity ATP analogue, 3'(2')-O-4-[4-oxo-(4-amino-2,2,6, 6-tetramethyl-piperidino-1-oxyl)-4-benzoyl] benzoyl adenosine 5'-triphosphate (SL-Bz(2)ATP) [Wang et al. (1999) J. Muscle Res. Cell Motil. 20, 743-753], behave like ATP in their interactions with myosin. Remarkably, photolabeled myosin recovers all of its normal enzymatic properties after treatment with actin in the presence of MgATP [Luo et al. (1995) Biochemistry 34, 1978-1987]. For SL-NANTP, the spin label moiety is attached to NANTP via an aminomethyl side chain. In SSL-NANTP, attachment is via a restricted spiro ring. The two new probes interact with myosin subfragment-1 (S1) in a manner analogous to ATP, and after photoincorporation, labeled S1 recovers full activity after treatment with actin and MgATP. The electron paramagnetic resonance (EPR) spectrum resulting from S1 photolabeled with SL-NANTP shows a very high degree of probe mobility. However, the EPR spectrum of S1 photolabeled with SSL-NANTP shows that the probe is highly immobilized with respect to S1, constrained to move within a cone of angle 52 degrees (full-width, half-max). Unlike the parent, NANTP, which photolabels on the 23 kDa tryptic fragment of S1, SSL-NANTP photolabels on the 20 kDa fragment. Its highly immobile nature means that it is potentially a useful reporter group to monitor cross-bridge motion in muscle fibers.
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Affiliation(s)
- X Chen
- School of Molecular Biosciences, Department of Pure and Applied Mathematics, Department of Chemistry,Washington State University, Pullman, Washington 99164, USA
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24
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Chase PB, Chen Y, Kulin KL, Daniel TL. Viscosity and solute dependence of F-actin translocation by rabbit skeletal heavy meromyosin. Am J Physiol Cell Physiol 2000; 278:C1088-98. [PMID: 10837336 DOI: 10.1152/ajpcell.2000.278.6.c1088] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We tested the hypothesis that solvent viscosity affects translocation of rhodamine phalloidin-labeled F-actin by rabbit skeletal heavy meromyosin (HMM). When viscosity was increased using either glycerol, fructose, sucrose, or dextran (1.5, 6.0, or 15-20 kDa mol mass), there was little or no effect on the fraction of moving filaments, whereas sliding speed decreased in inverse proportion to viscosity. The results could be explained neither by an effect of osmotic pressure at high solute concentrations nor by altered solvent drag on the actin filament. Elevated viscosity inhibited HMM ATPase activity in solution, but only at much higher viscosities than were needed to reduce sliding speed. Polyethylene glycols (300, 1,000, or 3,000 mol wt) also inhibited speed via elevated viscosity but secondarily inhibited by enhancing electrostatic interactions. These results demonstrate that a diffusion-controlled process intrinsic to cross-bridge cycling can be limiting to actomyosin function.
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Affiliation(s)
- P B Chase
- Department of Radiology, University of Washington, Seattle 98195, USA.
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25
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Wang D, Luo Y, Cooke R, Grammer J, Pate E, Yount RG. Synthesis of a spin-labeled photoaffinity ATP analogue, and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers. J Muscle Res Cell Motil 1999; 20:743-53. [PMID: 10730577 DOI: 10.1023/a:1005554924153] [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: 11/12/2022]
Abstract
A spin-labeled photoaffinity ATP analogue 3'(2')-O-[4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benz oyl]benzoyl adenosine 5'-triphosphate (SL-Bz2ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3'(2')-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [alpha-32P]SL-Bz2ATP was hydrolyzed and trapped as the diphosphate analogue with Co2+ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz2ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.
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Affiliation(s)
- D Wang
- Department of Biochemistry and Biophysics, Washington State University, Pullman 99164, USA
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26
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Rome LC, Cook C, Syme DA, Connaughton MA, Ashley-Ross M, Klimov A, Tikunov B, Goldman YE. Trading force for speed: why superfast crossbridge kinetics leads to superlow forces. Proc Natl Acad Sci U S A 1999; 96:5826-31. [PMID: 10318969 PMCID: PMC21945 DOI: 10.1073/pnas.96.10.5826] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Superfast muscles power high-frequency motions such as sound production and visual tracking. As a class, these muscles also generate low forces. Using the toadfish swimbladder muscle, the fastest known vertebrate muscle, we examined the crossbridge kinetic rates responsible for high contraction rates and how these might affect force generation. Swimbladder fibers have evolved a 10-fold faster crossbridge detachment rate than fast-twitch locomotory fibers, but surprisingly the crossbridge attachment rate has remained unchanged. These kinetics result in very few crossbridges being attached during contraction of superfast fibers (only approximately 1/6 of that in locomotory fibers) and thus low force. This imbalance between attachment and detachment rates is likely to be a general mechanism that imposes a tradeoff of force for speed in all superfast fibers.
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Affiliation(s)
- L C Rome
- Biology Department, Leidy Laboratories, University of Pennsylvania, Philadelphia, PA 19104, USA.
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27
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Schoenberg M. Crossbridge head detachment rate constants determined from a model that explains the behavior of both weakly- and strongly-binding crossbridges. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1999; 453:425-33; discussion 433-4. [PMID: 9889854 DOI: 10.1007/978-1-4684-6039-1_47] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/09/2023]
Abstract
Experimentally it is observed that the head regions of weakly-binding myosin crossbridges (crossbridges with ATP or ADP.Pi at the nucleotide binding site) are mobile while attached to actin, while strongly-binding crossbridge heads, such as those with PPi or AMP-PNP at the nucleotide binding site, are immobile (Pate and Cooke, Biophys. J., 1988; Fajer et al., Biophys. J., 1988). I postulate that the fundamental difference between weakly- and strongly-binding crossbridges is not their difference in affinity for actin, but the difference in mobility of the myosin heads attached to actin. Because the heads of weakly-binding crossbridges are mobile while attached to actin, the heads function independently and their behavior can be described by a simple independent-head model. With strongly-binding crossbridges, when one head detaches, it cannot re-attach in a position of lesser strain while the other is attached immobile; both heads must be detached concurrently before the crossbridge can relocate to a position of less strain and relax any tension it supports. This makes the heads appear to act cooperatively. A double-headed crossbridge model is presented which takes into account the difference between weakly- and strongly-binding crossbridges. The model is quite successful at describing the experimental data. In particular, for weakly-binding crossbridges the time constant of the response to stretch is shown to be relatively insensitive to ionic strength and for strongly-binding crossbridges, the model predicts with great accuracy the large ionic strength dependence of the rate constant for force decay. When the experimental results are interpreted according to the model, an important conclusion that emerges is that in all cases (for both weakly- and strongly-binding crossbridges) unstrained crossbridge heads in the muscle fiber detach from actin with approximately the same rate constant as myosin subfragment-1 detaches from actin in solution.
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Affiliation(s)
- M Schoenberg
- Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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28
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Frisbie SM, Xu S, Chalovich JM, Yu LC. Characterizations of cross-bridges in the presence of saturating concentrations of MgAMP-PNP in rabbit permeabilized psoas muscle. Biophys J 1998; 74:3072-82. [PMID: 9635761 PMCID: PMC1199383 DOI: 10.1016/s0006-3495(98)78014-2] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Several earlier studies have led to different conclusions about the complex of myosin with MgAMP-PNP. It has been suggested that subfragment 1 of myosin (S1)-MgAMP-PNP forms an S1-MgADP-like state, an intermediate between the myosin S1-MgATP and myosin S1-MgADP states or a mixture of cross-bridge states. We suggest that the different states observed result from the failure to saturate S1 with MgAMP-PNP. At saturating MgAMP-PNP, the interaction of myosin S1 with actin is very similar to that which occurs in the presence of MgATP. 1) At 1 degrees C and 170 mM ionic strength the equatorial x-ray diffraction intensity ratio I11/I10 decreased with an increasing MgAMP-PNP concentration and leveled off by approximately 20 mM MgAMP-PNP. The resulting ratio was the same for MgATP-relaxed fibers. 2) The two dimensional x-ray diffraction patterns from MgATP-relaxed and MgAMP-PNP-relaxed bundles are similar. 3) The affinity of S1-MgAMP-PNP for the actin-tropomyosin-troponin complex in solution in the absence of free calcium is comparable with that of S1-MgATP. 4) In the presence of calcium, I11/I10 decreased toward the relaxed value with increasing MgAMP-PNP, signifying that the affinity between cross-bridge and actin is weakened by MgAMP-PNP. 5) The degree to which the equatorial intensity ratio decreases as the ionic strength increases is similar in MgAMP-PNP and MgATP. Therefore, results from both fiber and solution studies suggest that MgAMP-PNP acts as a non hydrolyzable MgATP analogue for myosin.
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Affiliation(s)
- S M Frisbie
- National Institute of Arthritis, Musculoskeletal, and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892-7182, USA
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29
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Baker JE, Brust-Mascher I, Ramachandran S, LaConte LE, Thomas DD. A large and distinct rotation of the myosin light chain domain occurs upon muscle contraction. Proc Natl Acad Sci U S A 1998; 95:2944-9. [PMID: 9501195 PMCID: PMC19674 DOI: 10.1073/pnas.95.6.2944] [Citation(s) in RCA: 110] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/1997] [Indexed: 02/06/2023] Open
Abstract
For more than 30 years, the fundamental goal in molecular motility has been to resolve force-generating motor protein structural changes. Although low-resolution structural studies have provided evidence for force-generating myosin rotations upon muscle activation, these studies did not resolve structural states of myosin in contracting muscle. Using electron paramagnetic resonance, we observed two distinct orientations of a spin label attached specifically to a single site on the light chain domain of myosin in relaxed scallop muscle fibers. The two probe orientations, separated by a 36 degrees +/- 5 degrees axial rotation, did not change upon muscle activation, but the distribution between them changed substantially, indicating that a fraction (17% +/- 2%) of myosin heads undergoes a large (at least 30 degrees) axial rotation of the myosin light chain domain upon force generation and muscle contraction. The resulting model helps explain why this observation has remained so elusive and provides insight into the mechanisms by which motor protein structural transitions drive molecular motility.
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Affiliation(s)
- J E Baker
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis, MN 55455, USA
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30
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Gulick AM, Bauer CB, Thoden JB, Rayment I. X-ray structures of the MgADP, MgATPgammaS, and MgAMPPNP complexes of the Dictyostelium discoideum myosin motor domain. Biochemistry 1997; 36:11619-28. [PMID: 9305951 DOI: 10.1021/bi9712596] [Citation(s) in RCA: 157] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The three-dimensional structures of the truncated myosin head from Dictyostelium discoideum myosin II (S1dC) complexed with MgAMPPNP, MgATPgammaS, and MgADP are reported at 2.1, 1.9, and 2.1 A resolution, respectively. Crystals were obtained by cocrystallization and were isomorphous with respect to those of S1dC. MgADP.BeFx [Fisher, A. J., et al. (1995) Biochemistry 34, 8960-8972]. In all three structures, the electron density for the entire nucleotide was clearly discernible. The overall structures of all three complexes are very similar to that of the beryllium fluoride complex which suggests that the differences in the physiological effects of ATPgammaS and AMPPNP are due to the changes in the equilibrium between the actin-bound and actin-free states of myosin caused by the lower affinity of AMPPNP for myosin. In S1dC.MgAMPPNP, the presence of the bridging nitrogen prompts the side chain of Asn233 to rotate which disrupts the hydrogen bonding pattern in the nucleotide binding pocket and alters the water structure surrounding the ribose hydroxyl groups. It appears that this change is responsible for the reduced affinity of AMPPNP for myosin relative to ATPgammaS. In contrast to the G-proteins, there is no major change in the conformation of the ligands that coordinate the nucleotide in S1dC.MgADP. This is due to three water molecules that adopt the approximate positions of the three oxygens on the gamma-phosphate and maintain the interactions with the Mg2+ ion and protein molecule. Interestingly, the thiophosphate group is evident in S1dC.MgATPgammaS even though it is slowly hydrolyzed by myosin. This suggests that the conformation observed here and in chicken skeletal myosin subfragment-1 [Rayment, I., et al. (1993) Science 261, 50-58] is unable to hydrolyze ATP and represents the structure of the prehydrolysis weak binding state of myosin.
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Affiliation(s)
- A M Gulick
- Institute for Enzyme Research and Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53705, USA
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31
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Naber N, Cooke R, Pate E. Binding of ncd to microtubules induces a conformational change near the junction of the motor domain with the neck. Biochemistry 1997; 36:9681-9. [PMID: 9245400 DOI: 10.1021/bi9706881] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
We have covalently attached an electron paramagnetic resonance (EPR) spin probe to Cys-670 of the motor domain of ncd (nonclaret disjunctional protein) in order to investigate conformational changes associated with the chemomechanical cycle. Spin-labeling is highly specific and does not affect ncd function as monitored by either the binding affinity to microtubules or the rate of ATP hydrolysis. The EPR spectra can be deconvoluted into two components, one that is highly mobile with respect to the protein and one that is strongly immobilized. In the absence of microtubules, the relative proportions of these two components varied with temperature, showing that the transition between them involves a large change in enthalpy (DeltaH degrees = -75 kJ/mol). This result implies that the two populations represent very different protein conformations. Binding to microtubules results in virtually all probes shifting into the immobilized component, independent of the nucleotide bound. Superposition of the structures of ncd and myosin subfragment 1 reveals that the labeled cysteine is very close to the region which is homologous to the helix containing the two reactive sulfhydryls in myosin and is approximately 10 A from the junction of the motor domain with the remainder of the molecule. We conclude that the binding of ncd to microtubules results in a conformational change in this region which may be involved in the working power stroke.
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Affiliation(s)
- N Naber
- Department of Biochemistry and Biophysics, Cardiovascular Research Institute, University of California, San Francisco 94143, USA.
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32
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Roopnarine O, Thomas DD. Orientation of intermediate nucleotide states of indane dione spin-labeled myosin heads in muscle fibers. Biophys J 1996; 70:2795-806. [PMID: 8744317 PMCID: PMC1225259 DOI: 10.1016/s0006-3495(96)79849-1] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
We have used electron paramagnetic resonance to study the orientation of myosin heads in the presence of nucleotides and nucleotide analogs, to induce equilibrium states that mimic intermediates in the actomyosin ATPase cycle. We obtained electron paramagnetic resonance spectra of an indane dione spin label (InVSL) bound to Cys 707 (SH1) of the myosin head, in skinned rabbit psoas muscle fibers. This probe is rigidly immobilized on the catalytic domain of the head, and the principal axis of the probe is aligned nearly parallel to the fiber axis in rigor (no nucleotide), making it directly sensitive to axial rotation of the head. On ADP addition, all of the heads remained strongly bound to actin, but the spectral hyperfine splitting increased by 0.55 +/- 0.02 G, corresponding to a small but significant axial rotation of 7 degrees. Adenosine 5'-(adenylylim-idodiphosphate) (AMPPNP) or pyrophosphate reduced the actomyosin affinity and introduced a highly disordered population of heads similar to that observed in relaxation. For the remaining oriented population, pyrophosphate induced no significant change relative to rigor, but AMPPNP induced a slight but probably significant rotation (2.2 degrees +/- 1.6 degrees), in the direction opposite that induced by ADP. Adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) relaxed the muscle fiber, completely dissociated the heads from actin, and produced disorder similar to that in relaxation by ATP. ATP gamma S plus Ca induced a weak-binding state with most of the actin-bound heads disordered. Vanadate had negligible effect in the presence of ADP, but in isometric contraction vanadate substantially reduced both force and the fraction of oriented heads. These results are consistent with a model in which myosin heads are disordered early in the power stroke (weak-binding states) and rigidly oriented later in the power stroke (strong-binding states), whereas transitions among the strong-binding states induce only slight changes in the axial orientation of the catalytic domain.
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Affiliation(s)
- O Roopnarine
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455, USA
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33
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Adhikari BB, Fajer PG. Myosin head orientation and mobility during isometric contraction: effects of osmotic compression. Biophys J 1996; 70:1872-80. [PMID: 8785347 PMCID: PMC1225157 DOI: 10.1016/s0006-3495(96)79752-7] [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: 02/02/2023] Open
Abstract
We have correlated the mobility and the generation of force of myosin heads by applying radial compression to isometrically contracting muscle fibers. Osmotic pressure was produced by dextran T-500, and its effect on the orientation and mobility of myosin heads labeled with N-(1-oxy-2,2,5,5-tetramethyl-4-pyperidinyl)maleimide was observed by conventional and saturation-transfer electron paramagnetic resonance methods. A biphasic behavior is spectral changes coinciding with the tension dependence was observed as the fibers were compressed. At diameters above the equilibrium spacing, the large myosin head disorder characteristic during contraction in the absence of compression was largely maintained, whereas the mobility decreased threefold, from tauR approximately 25 microseconds to approximately 80-90 microseconds. The inhibition of fast microsecond motions was not accompanied by tension loss, implying that these motions are not necessary for force generation. At diameters below the equilibrium spacing, both the disorder and the mobility decreased dramatically in parallel with the tension inhibition, suggesting that slower microsecond motions and the disorder of the myosin head are necessary for muscle function.
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Affiliation(s)
- B B Adhikari
- Institute of Molecular Biophysics, Florida State University, Tallahassee 32306, USA
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34
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Ostap EM, Barnett VA, Thomas DD. Resolution of three structural states of spin-labeled myosin in contracting muscle. Biophys J 1995; 69:177-88. [PMID: 7669895 PMCID: PMC1236236 DOI: 10.1016/s0006-3495(95)79888-5] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
We have used electron paramagnetic resonance (EPR) spectroscopy to detect ATP- and calcium-induced changes in the structure of spin-labeled myosin heads in glycerinated rabbit psoas muscle fibers in key physiological states. The probe was a nitroxide iodoacetamide derivative attached selectively to myosin SH1 (Cys 707), the conventional EPR spectra of which have been shown to resolve several conformational states of the myosin ATPase cycle, on the basis of nanosecond rotational motion within the protein. Spectra were acquired in rigor and during the steady-state phases of relaxation and isometric contraction. Spectral components corresponding to specific conformational states and biochemical intermediates were detected and assigned by reference to EPR spectra of trapped kinetic intermediates. In the absence of ATP, all of the myosin heads were rigidly attached to the thin filament, and only a single conformation was detected, in which there was no sub-microsecond probe motion. In relaxation, the EPR spectrum resolved two conformations of the myosin head that are distinct from rigor. These structural states were virtually identical to those observed previously for isolated myosin and were assigned to the populations of the M*.ATP and M**.ADP.Pi states. During isometric contraction, the EPR spectrum resolves the same two conformations observed in relaxation, plus a small fraction (20-30%) of heads in the oriented actin-bound conformation that is observed in rigor. This rigor-like component is a calcium-dependent, actin-bound state that may represent force-generating cross-bridges. As the spin label is located near the nucleotide-binding pocket in a region proposed to be pivotal for large-scale force-generating structural changes in myosin, we propose that the observed spectroscopic changes indicate directly the key steps in energy transduction in the molecular motor of contracting muscle.
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Affiliation(s)
- E M Ostap
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455, USA
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35
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Zhao L, Naber N, Cooke R. Muscle cross-bridges bound to actin are disordered in the presence of 2,3-butanedione monoxime. Biophys J 1995; 68:1980-90. [PMID: 7612840 PMCID: PMC1282101 DOI: 10.1016/s0006-3495(95)80375-9] [Citation(s) in RCA: 47] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Electron paramagnetic resonance spectroscopy was used to monitor the orientation of muscle cross-bridges attached to actin in a low force and high stiffness state that may occur before force generation in the actomyosin cycle of interactions. 2,3-butanedione monoxime (BDM) has been shown to act as an uncompetitive inhibitor of the myosin ATPase that stabilizes a myosin.ADP.P(i) complex. Such a complex is thought to attach to actin at the beginning of the powerstroke. Addition of 25 mM BDM decreases tension by 90%, although stiffness remains high, 40-50% of control, showing that cross-bridges are attached to actin but generate little or no force. Active cross-bridge orientation was monitored via electron paramagnetic resonance spectroscopy of a maleimide spin probe rigidly attached to cys-707 (SH-1) on the myosin head. A new labeling procedure was used that showed improved specificity of labeling. In 25 mM BDM, the probes have an almost isotropic angular distribution, indicating that cross-bridges are highly disordered. We conclude that in the pre-powerstroke state stabilized by BDM, cross-bridges are attached to actin, generating little force, with a large portion of the catalytic domain of the myosin heads disordered.
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Affiliation(s)
- L Zhao
- Department of Biochemistry and Biophysics, University of California, San Francisco 94143, USA
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36
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Thomas DD, Ramachandran S, Roopnarine O, Hayden DW, Ostap EM. The mechanism of force generation in myosin: a disorder-to-order transition, coupled to internal structural changes. Biophys J 1995; 68:135S-141S. [PMID: 7787056 PMCID: PMC1281895] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We propose a molecular mechanism of force generation in muscle, based primarily on site-specific spectroscopic probe studies of myosin heads in contracting muscle fibers and myofibrils. Electron paramagnetic resonance (EPR) and time-resolved phosphorescence anisotropy (TPA) of probes attached to SH1 (Cys 707, in the catalytic domain of the head) have consistently shown that most myosin heads in contracting muscle are dynamically disordered, undergoing large-amplitude rotations in the microsecond time range. Some of these disordered heads are bound to actin, especially in the early (weak-binding, preforce) phase of the ATPase cycle. The small ordered population (10-20%) is rigidly oriented precisely as in rigor, with no other distinct angle observed in contraction or in the presence of intermediate states trapped by nucleotide analogs. These results are not consistent with the classical model in which the entire head undergoes a 45 degree transition between two distinct orientations. Therefore, it has been proposed that the catalytic domain of the myosin head has only one stereospecific (rigor-like) actin-binding angle, and that the head's internal structure changes during force generation, causing the distal light-chain-binding domain to rotate. To test this model, we have performed EPR and TPA studies of probes attached to regulatory light chains (RLCs) in rabbit and scallop myofibrils and fibers. The RLC results confirm the predominance of dynamic (microsecond) rotational disorder in both relaxation and contraction, and show that the different mechanisms of calcium regulation in the two muscles produce different rotational dynamics. In rabbit myofibrils, RLC probes are more dynamically disordered than SH1 probes, especially in rigor and contraction,indicating that the light-chain-binding domain undergoes rotational motions relative to the catalytic domain when myosin heads interact with actin. An SH1-bound spin label, which is sensitive to myosin's internal dynamics, resolves three distinct conformations during contraction, and time-resolved EPR shows that these transitions are coupled to specific steps in the ATPase cycle. We propose that force is generated during contraction by a disorder-to-order transition, in which myosin heads first attach weakly to actin in a nonstereospecific mode characterized by large-scale dynamic disorder, then undergo at least two conformational transitions involving large-scale structural (rotational) changes within the head, culminating in a highly ordered strong-binding state that bears force.
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Affiliation(s)
- D D Thomas
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455, USA
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37
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Raucher D, Fajer EA, Sár C, Hideg K, Zhao Y, Kawai M, Fajer PG. A novel electron paramagnetic resonance spin label and its application to study the cross-bridge cycle. Biophys J 1995; 68:128S-133S; discussion 134S. [PMID: 7787055 PMCID: PMC1281893] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We have used a novel alpha-iodoketone spin-label (IKSL) to study myosin head orientation and cross-bridge dynamics in the putative pre-powerstroke state. Possible perturbation of the cross-bridge cycle by the label was assayed by the sinusoidal analysis method (Kawai and Brandt, 1980; Kawai and Zhao, 1993), which determines the rate constants of the elementary steps in the cycle. A comparison of the rates obtained from unlabeled and IKSL fibers revealed small (10-20%) changes in the ATP hydrolysis rate and in the rate constants of the elementary steps. The labeling induced small changes (< 13%) in the distribution of the cross-bridges among the various intermediate states. Pre-powerstroke cross-bridges were induced by aluminum fluoride in the presence of Ca2+ and ATP. In this state, force development is inhibited, but a large proportion (40%) of the cross-bridges are still attached to the thin filament. We have used conventional electron paramagnetic resonance to measure orientation, and have found that the pre-powerstroke heads are as disordered as in relaxation. Their mobility, as determined by saturation transfer electron paramagnetic resonance, was significantly restricted. Assuming that stiffness is proportional to the fraction of strongly attached heads, the rotational correlation time of the bound heads is estimated to be tau r = approximately 150-400 microseconds.
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Affiliation(s)
- D Raucher
- Institute of Molecular Biophysics, Florida State University, Tallahassee 32306, USA
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38
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Roopnarine O, Thomas DD. Orientational dynamics of indane dione spin-labeled myosin heads in relaxed and contracting skeletal muscle fibers. Biophys J 1995; 68:1461-71. [PMID: 7787032 PMCID: PMC1282041 DOI: 10.1016/s0006-3495(95)80319-x] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We have used electron paramagnetic resonance (EPR) spectroscopy to study the orientation and rotational motions of spin-labeled myosin heads during steady-state relaxation and contraction of skinned rabbit psoas muscle fibers. Using an indane-dione spin label, we obtained EPR spectra corresponding specifically to probes attached to Cys 707 (SH1) on the catalytic domain of myosin heads. The probe is rigidly immobilized, so that it reports the global rotation of the myosin head, and the probe's principal axis is aligned almost parallel with the fiber axis in rigor, making it directly sensitive to axial rotation of the head. Numerical simulations of EPR spectra showed that the labeled heads are highly oriented in rigor, but in relaxation they have at least 90 degrees (Gaussian full width) of axial disorder, centered at an angle approximately equal to that in rigor. Spectra obtained in isometric contraction are fit quite well by assuming that 79 +/- 2% of the myosin heads are disordered as in relaxation, whereas the remaining 21 +/- 2% have the same orientation as in rigor. Computer-simulated spectra confirm that there is no significant population (> 5%) of heads having a distinct orientation substantially different (> 10 degrees) from that in rigor, and even the large disordered population of heads has a mean orientation that is similar to that in rigor. Because this spin label reports axial head rotations directly, these results suggest strongly that the catalytic domain of myosin does not undergo a transition between two distinct axial orientations during force generation. Saturation transfer EPR shows that the rotational disorder is dynamic on the microsecond time scale in both relaxation and contraction. These results are consistent with models of contraction involving 1) a transition from a dynamically disordered preforce state to an ordered (rigorlike) force-generating state and/or 2) domain movements within the myosin head that do not change the axial orientation of the SH1-containing catalytic domain relative to actin.
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Affiliation(s)
- O Roopnarine
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455, USA
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39
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Abstract
Conventional EPR studies of muscle fibers labeled with a novel alpha-iodoketo spin label at Cys-707 of the myosin head revealed substantial internal domain reorganization on the addition of ADP to rigor fibers. The spin probes that are well-ordered in the rigor state become disordered and form two distinct populations. These orientational changes do not correspond to rotation of the myosin catalytic domain as a whole because other probes (maleimide and iodoacetamide nitroxides attached to the same Cys-707 of myosin head) report only a small (5-10 degrees) torsional rotation and little or no change in the tilt angle [Ajtai et al. (1992) Biochemistry 31, 207-17; Fajer (1994) Biophys. J. 66, 2039-50]. In the presence of ADP, the labeled domain becomes more flexible and executes large-amplitude microsecond motions, as measured by saturation-transfer EPR with rates (tau r = 150 microseconds) intermediate between the rotations of detached (tau r = 7 microseconds) and rigor heads (tau r = 2500 microseconds). This finding contrasts with an absence of global motion of the myosin head in ADP (tau r = 2200 microseconds) as reported by the maleimide spin label. Our results imply that the myosin head in a single chemical state (AM.ADP) is capable of attaining many internal configurations, some of which are dynamic. The presence of these slow structural fluctuations might be related to the slow release of the hydrolysis products of actomyosin ATPase.
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Affiliation(s)
- D Raucher
- Institute of Molecular Biophysics, Florida State University, Tallahassee 32306-3015
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40
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Hirose K, Franzini-Armstrong C, Goldman YE, Murray JM. Structural changes in muscle crossbridges accompanying force generation. J Biophys Biochem Cytol 1994; 127:763-78. [PMID: 7962058 PMCID: PMC2120236 DOI: 10.1083/jcb.127.3.763] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
We have investigated the structure of the crossbridges in muscles rapidly frozen while relaxed, in rigor, and at various times after activation from rigor by flash photolysis of caged ATP. We used Fourier analysis of images of cross sections to obtain an average view of the muscle structure, and correspondence analysis to extract information about individual crossbridge shapes. The crossbridge structure changes dramatically between relaxed, rigor, and with time after ATP release. In relaxed muscle, most crossbridges are detached. In rigor, all are attached and have a characteristic asymmetric shape that shows strong left-handed curvature when viewed from the M-line towards the Z-line. Immediately after ATP release, before significant force has developed (20 ms) the homogeneous rigor population is replaced by a much more diverse collection of crossbridge shapes. Over the next few hundred milliseconds, the proportion of attached crossbridges changes little, but the distribution of the crossbridges among different structural classes continues to evolve. Some forms of attached crossbridge (presumably weakly attached) increase at early times when tension is low. The proportion of several other attached non-rigor crossbridge shapes increases in parallel with the development of active tension. The results lend strong support to models of muscle contraction that have attributed force generation to structural changes in attached crossbridges.
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Affiliation(s)
- K Hirose
- Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia 19104-6058
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41
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Raucher D, Fajer PG. Orientation and dynamics of myosin heads in aluminum fluoride induced pre-power stroke states: an EPR study. Biochemistry 1994; 33:11993-9. [PMID: 7918418 DOI: 10.1021/bi00205a039] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
We have determined the orientation and dynamics of the putative pre-power stroke crossbridges in skinned muscle fibers labeled with maleimide spin-label at Cys-707 of myosin. Orientation was measured using electron paramagnetic resonance (EPR) and mobility by saturation transfer EPR. The crossbridges are trapped in the pre-power stroke conformation in the presence of aluminum fluoride, Ca, and ATP. In agreement with data published for unlabeled fibers (Chase et al., 1994), spin-labeled muscle fibers display 42.5% of rigor stiffness, without the generation of force. The trapped crossbridges are as disordered as the relaxed heads, but their microsecond dynamics are significantly restricted. Modeling of the immobile fraction (35%), in terms of attached heads as estimated from stiffness, suggests that the bound heads rotate with a correlation time tau r = 150-400 microseconds, as compared to tau r = 3 microseconds for the heads in relaxed fibers. These "strongly" attached myosin heads, at orientations other than in rigor, are a candidate for the state from which head rotation generates force, as postulated by H. E. Huxley (1969). Ordering of the heads may well be the structural event driving the generation of force.
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Affiliation(s)
- D Raucher
- Institute of Molecular Biophysics, Florida State University, Tallahassee 32306
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42
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Roopnarine O, Thomas DD. A spin label that binds to myosin heads in muscle fibers with its principal axis parallel to the fiber axis. Biophys J 1994; 67:1634-45. [PMID: 7819495 PMCID: PMC1225525 DOI: 10.1016/s0006-3495(94)80636-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We have used an indane-dione spin label (2-[-oxyl-2,2,5,5-tetramethyl-3-pyrrolin-3-yl)methenyl]in dane-1,3-dione), designated InVSL, to study the orientation of myosin heads in bundles of chemically skinned rabbit psoas muscle fibers, with electron paramagnetic resonance (EPR) spectroscopy. After reversible preblocking with 5,5'-dithiobis(2-nitro-benzoic acid) (DTNB), we were able to attach most of the spin label covalently and rigidly to either Cys 707 (SH1) or Cys 697 (SH2) on myosin heads. EPR spectra of labeled fibers contained substantial contributions from both oriented and disordered populations of spin labels. Similar spectra were obtained from fibers decorated with InVSL-labeled myosin heads (subfragment 1), indicating that virtually all the spin labels in labeled fibers are on the myosin head. We specifically labeled SH2 with InVSL after reversible preblocking of the SH1 sites with 1-fluoro-2,4-dinitrobenzene (FDNB), resulting in a spectrum that indicated only disordered spin labels. Therefore, the oriented and disordered populations correspond to labels on SH1 and SH2, respectively. The spectrum of SH2-bound labels was subtracted to produce a spectrum corresponding to SH1-bound labels, which was used for further analysis. For this corrected spectrum, the angle between the fiber axis and the principal axis of the spin label was fitted well by a Gaussian distribution centered at theta o = 11 +/- 1 degree, with a full width at half-maximum of delta theta = 15 +/- 2 degrees. The unique orientation of InVSL, with its principal axis almost parallel to the fiber axis, makes it complementary to spin labels previously studied in this system. This label can provide unambiguous information about axial rotations of myosin heads, since any axial rotation of the head must be reflected in the same axial rotation of the principal axis of the probe, thus changing the hyperfine splitting. Therefore, InVSL-labeled fibers have ideal properties needed for further exploration myosin head orientation and rotational motion in muscle.
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Affiliation(s)
- O Roopnarine
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455
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43
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Belágyi J, Frey I, Pótó L. ADP-induced changes in ordering of spin-labelled myosin heads in muscle fibres. EUROPEAN JOURNAL OF BIOCHEMISTRY 1994; 224:215-22. [PMID: 8076642 DOI: 10.1111/j.1432-1033.1994.tb20014.x] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Rotational dynamics and ordering of myosin heads in glycerinated skeletal muscle fibres were studied using an isothiocyanate-based spin label attached to the fast-reacting thiol sites of myosin and were compared with data obtained for maleimide and iodoacetamide spin labels attached to the same sites. The ordering of probe molecules on the millisecond time scale in the rigor state, at sarcomere length 2.2-2.3 +/- 0.1 microns, was static. Isothiocyanate probe molecules showed greater mobility; the segment holding the label rotated in the microsecond time range. In the saturation transfer EPR time domain, MgADP did not produce a significant change in the mobility of spin labels. The spectra of isothiocyanate spin-labelled fibres were analyzed in terms of two narrow distributions with mean angles of 75 degrees and 56 degrees. In the rigor state, the fractions represented approximately 76% and 24% of the total EPR absorbance. In the presence of MgADP, the conventional EPR spectra showed large changes in the ordering of isothiocyanate probe molecules towards a new distribution, the population with a theta value of 56% increased from 24% to 71% at the expense of the 75% population with no change in the mean angles of the distributions. In the case of maleimide and iodoacetamide spin-labelled fibres, however, the effect of MgADP on the probe angular distribution was small.
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Affiliation(s)
- J Belágyi
- Central Research Laboratory, University Medical School, Pécs, Hungary
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44
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Berger CL, Thomas DD. Rotational dynamics of actin-bound intermediates of the myosin adenosine triphosphatase cycle in myofibrils. Biophys J 1994; 67:250-61. [PMID: 7918993 PMCID: PMC1225355 DOI: 10.1016/s0006-3495(94)80476-x] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We have used saturation transfer electron paramagnetic resonance (ST-EPR) to measure the microsecond rotational motion of actin-bound myosin heads in spin-labeled myofibrils in the presence of the ATP analogs AMPPNP (5'-adenylylimido-diphosphate) and ATP gamma S (adenosine-5'-O-(3-thiotriphosphate)). AMPPNP and ATP gamma S are believed to trap myosin in two major conformational intermediates of the actomyosin ATPase cycle, respectively known as the weakly bound and strongly bound states. Previous ST-EPR experiments with solutions of acto-S1 have demonstrated that actin-bound myosin heads are rotationally mobile on the microsecond time scale in the presence of ATP gamma S, but not in the presence of AMPPNP. However, it is not clear that results obtained with acto-S1 in solution can be extended to actomyosin constrained within the myofibrillar lattice. Therefore, ST-EPR spectra of spin-labeled myofibrils were analyzed explicitly in terms of the actin-bound component of myosin heads in the presence of AMPPNP and ATP gamma S. The fraction of actin-attached myosin heads was determined biochemically in the spin-labeled myofibrils, using the proteolytic rates actomyosin binding assay. At physiological ionic strength (mu = 165 mM), actin-bound myosin heads were found to be rotationally mobile on the microsecond time scale (tau r = 24 +/- 8 microseconds) in the presence of ATP gamma S, but not AMPPNP. Similar results were obtained at low ionic strength, confirming the acto-S1 solution studies. The microsecond rotational motions of actin-attached myosin heads in the presence of ATP gamma S are similar to those observed for spin-labeled myosin heads during the steady-state cycling of the actomyosin ATPase, both in solution and in an active isometric muscle fiber. These results indicate that weakly bound myosin heads, in the pre-force phase of the ATPase cycle, are rotationally mobile, while strongly bound heads, in the force-generating phase, are rotationally immobile. We propose that force generation involves a transition from a dynamically disordered crossbridge to a rigid and stereospecific one.
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Affiliation(s)
- C L Berger
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455
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Abstract
The determination of the iodoacetamide spin label orientation in myosin heads (Fajer, 1994) allows us for the first time to determine directly protein orientation from EPR spectra. Computational simulations have been used to determine the sensitivity of EPR to both torsional and tilting motions of myosin heads. For rigor heads (no nucleotide), we can detect 0.2 degree changes in the tilt angle and 4 degrees in the torsion of the head. Sensitivity decreases with increasing head disorder, but even in the presence of +/- 30 degrees disorder as expected for detached heads, 10 degree changes in the center of the orientational distribution can be detected. We have combined these numerical simulations with a Simplex optimization to compare the orientation of intrinsic heads, with the orientation of labeled extrinsic heads that have been infused into unlabeled muscle fibers. The near identity (within 2 degrees) of the orientational distribution in the two instances can be attributed to myosin elasticity taking up the mechanical strain induced by the mismatch of myosin and actin filament periodicity. A similar analysis of the spectra of fibers with ADP bound to myosin revealed a small (approximately 5 degrees-10 degrees) torsional reorientation, without a substantial change of the tilt angle (< 2 degrees).
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Affiliation(s)
- P G Fajer
- Institute of Molecular Biophysics, Florida State University, Tallahassee 32300
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Abstract
Current methods of analyzing EPR spectra of spin-labeled muscle fibers allow the determination of spin-label orientation within the fiber, rather than the orientation of the myosin head itself. In order to describe the orientational distribution of spin labeled myosin heads within the muscle fibers, the orientation of the spin label within the myosin head must be known. The iodoacetamide label orientation in the myosin head was determined to be (16.8 degrees, 28.3 degrees, 4.2 degrees) or (16.6 degrees, 72.0 degrees, 4.3 degrees). These Eulerian angles were obtained from the analysis of EPR spectra of fibers decorated with labeled myosin heads in the absence of ATP, with the assumption that the head's tilt angle is 40 degrees, as observed in a recent EM study [Pollard, T., Bhandari, D., Maupin, P., Wachsstock, D., Weeds, A. & Zot, H. (1993) Biophys. J. 64, 454-471]. Knowledge of spin-label orientation will allow for quantitative determination of myosin head orientation in the various states of the contractile cycle.
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Affiliation(s)
- P G Fajer
- Institute of Molecular Biophysics, Florida State University, Tallahassee 32306
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Hirose K, Lenart TD, Murray JM, Franzini-Armstrong C, Goldman YE. Flash and smash: rapid freezing of muscle fibers activated by photolysis of caged ATP. Biophys J 1993; 65:397-408. [PMID: 8369445 PMCID: PMC1225734 DOI: 10.1016/s0006-3495(93)81061-0] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
A new approach was used to study transient structural states of cross-bridges during activation of muscle fibers. Rabbit skinned muscle fibers were rapidly and synchronously activated from the rigor state by photolysis of caged ATP in the presence of Ca2+. At several different times during the switch from rigor to fully active tension development, the fibers were rapidly frozen on a liquid helium-cooled metal block, freeze-substituted, and examined in an electron microscope. The limits of structural preservation and resolution with this technique were analyzed. We demonstrate that the resolution of our images is sufficient to draw the following conclusions about cross-bridge structure. Rigor cross-bridges point away from the Z-line and most of them are wider near the thin filaments than near the backbone of the thick filaments. In contrast, cross-bridges in actively contracting fibers stretch between the thick and thin filaments at a variable angle, and are uniformly thin. Diffraction patterns computed from contracting muscle show layer lines both at 38 and 43 nm indicating that active cross-bridges contribute mass to both the actin- and myosin-based helical periodicities. The images obtained from fibers frozen 20 ms after release of ATP show a mixture of rigor and active type cross-bridge configurations. There is little evidence of cross-bridges with the rigor shape by 50 ms, and the difference in configurations between 50 and 300 ms after photolysis is surprisingly subtle.
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Affiliation(s)
- K Hirose
- Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia 19104
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Xu S, Brenner B, Yu LC. State-dependent radial elasticity of attached cross-bridges in single skinned fibres of rabbit psoas muscle. J Physiol 1993; 465:749-65. [PMID: 7693922 PMCID: PMC1175457 DOI: 10.1113/jphysiol.1993.sp019704] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
1. In a single skinned fibre of rabbit psoas muscle, upon attachment of cross bridges to actin in the presence of ADP or pyrophosphate (PPi), the separation between the contractile filaments, as determined by equatorial X-ray diffraction, is found to decrease, suggesting that force is generated in the radial direction. 2. The single muscle fibres were subjected to compression by 0-8% of dextran T500. The changes in lattice spacings by dextran compression were compared with changes induced by cross-bridge attachment to actin. Based on this comparison, the magnitude and the direction of the radial force generated by the attached cross-bridges were estimated. The radial cross-bridge force varied with filament separation, and the magnitude of the radial cross-bridge force reached as high as the maximal axial force produced during isometric contraction. 3. One key parameter of the radial elasticity, i.e. the equilibrium spacing where the radial force is zero, was found to depend on the ligand bound to the myosin head. In the presence of ADP, the equilibrium spacing was 36 nm. In the presence of MgPPi the equilibrium spacing shifted to 35 nm and Ca2+ had little effect on the equilibrium spacing. 4. The equilibrium spacing was independent of the fraction of cross-bridges attached to actin. The fraction of cross-bridges attached in rigor was modulated from 100% to close to 0% by adding up to 10 mM of ATP gamma S in the rigor solution. The lattice spacing remained at 38 nm, the equilibrium spacing for nucleotide-free cross-bridges at mu = 170 mM. 5. Radial force generated by cross-bridges in rigor at large lattice spacings (38 nm < or = d10 < or = 46 nm) appeared to vary linearly with lattice spacing. 6. The titration of ATP gamma S to fibres in rigor provided a correlation between the radial stiffness of the nucleotide-free cross-bridges and the equatorial intensities. The relation between the equatorial intensity ratio I11/I10 and radial stiffness appeared to be approximately linear. 7. The fibres under different conditions showed a wide range of radial stiffness, which was not proportional to the apparent axial stiffness of the fibre. If the apparent axial stiffness is a measure of the fraction of cross-bridges bound to actin, it follows that the radial elastic constant is state dependent; or vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)
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Affiliation(s)
- S Xu
- National Institutes of Health, Bethesda, MD 20892
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Howard EC, Lindahl KM, Polnaszek CF, Thomas DD. Simulation of saturation transfer electron paramagnetic resonance spectra for rotational motion with restricted angular amplitude. Biophys J 1993; 64:581-93. [PMID: 8386008 PMCID: PMC1262370 DOI: 10.1016/s0006-3495(93)81417-6] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
We have simulated both conventional (V1) and saturation transfer (V'2) electron paramagnetic resonance spectra for the case of Brownian rotational diffusion restricted in angular amplitude. Numerical solutions of the diffusion-coupled Bloch equations were obtained for an axially symmetric 14N nitroxide spin label with its principal axis rotating within a Gaussian angular distribution of full width delta theta at half maximum. Spectra were first calculated for a macroscopically oriented system with cylindrical symmetry (e.g., a bundle of muscle fibers or a stack of membrane bilayers), with the Gaussian angular distribution centered at theta 0 with respect to the magnetic field. These spectra were then summed over theta 0 to obtain the spectrum of a randomly oriented sample (e.g., a dispersion of myofibrils or membrane vesicles). The angular amplitude delta theta was varied from 0 degrees, corresponding to isotropic motion (order parameter = 0). For each value of delta theta, the rotational correlation time, tau r, was varied from 10(-7) to 10(-2) s, spanning the range from maximal to minimal saturation transfer. We provide plots that illustrate the dependence of spectral parameters on delta theta and tau r. For an oriented system, the effects of changing delta theta and tau r are easily distinguishable, and both parameters can be determined unambiguously by comparing simulated and experimental spectra. For a macroscopically disordered system, the simulated spectra are still quite sensitive to delta theta, but a decrease in tau r produces changes similar to those from an increase in delta theta. If delta theta can be determined independently, then the results of the present study can be used to determine tau r from experimental spectra. Similarly, if tau r is known, then delta theta can be determined.
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Affiliation(s)
- E C Howard
- Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455
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Burghardt TP, Ajtai K. Luminescent/paramagnetic probes of order and orientation in biological assemblies: the transformation of luminescent probes into pi-radicals by photochemical reduction within the contractile apparatus. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1993; 332:225-33. [PMID: 7509109 DOI: 10.1007/978-1-4615-2872-2_21] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Several unsubstituted xanthene dyes (eosin, erythrosin, and fluorescein) were irradiated by laser light at their absorption maximum in the presence of different reducing agents. Due to photochemical reduction the quinoidal structure of the xanthene ring is transformed into a semiquinone and a pi-radical is formed having a characteristic electron paramagnetic resonance (EPR) signal of an unpaired electron spin with proton hyperfine interactions. A strong EPR signal is observed from the dye in solution or when specifically attached to myosin following irradiation in the presence of dithiothreitol or cysteine. The spectroscopic methods of fluorescence polarization and EPR are useful in the study of ordered biological assemblies. These methods generate complementary information about the order of the system but a consistent quantitative interpretation of the related data is complicated because the signals arise from different donors. Our method allows us to detect both signals from the same donor. We applied our new technique to the study of skeletal muscle fibers. The fluorescent dye iodoacetamidofluorescein was covalently attached to the reactive thiol of the myosin molecule in muscle fibers. Fluorescence polarization and EPR spectroscopy were performed on the labeled fibers in rigor. Both signals indicate a highly ordered system characteristic of cross-bridges bound to actin.
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Affiliation(s)
- T P Burghardt
- Department of Biochemistry and Molecular Biology, Mayo Foundation, Rochester, MN 55905
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