1
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Barry ME, Rynkiewicz MJ, Pavadai E, Viana A, Lehman W, Moore JR. Glutamate 139 of tropomyosin is critical for cardiac thin filament blocked-state stabilization. J Mol Cell Cardiol 2024; 188:30-37. [PMID: 38266978 DOI: 10.1016/j.yjmcc.2024.01.004] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/16/2023] [Revised: 12/14/2023] [Accepted: 01/20/2024] [Indexed: 01/26/2024]
Abstract
The cardiac thin filament proteins troponin and tropomyosin control actomyosin formation and thus cardiac contractility. Calcium binding to troponin changes tropomyosin position along the thin filament, allowing myosin head binding to actin required for heart muscle contraction. The thin filament regulatory proteins are hot spots for genetic mutations causing heart muscle dysfunction. While much of the thin filament structure has been characterized, critical regions of troponin and tropomyosin involved in triggering conformational changes remain unresolved. A poorly resolved region, helix-4 (H4) of troponin I, is thought to stabilize tropomyosin in a position on actin that blocks actomyosin interactions at low calcium concentrations during muscle relaxation. We have proposed that contact between glutamate 139 on tropomyosin and positively charged residues on H4 leads to blocking-state stabilization. In this study, we attempted to disrupt these interactions by replacing E139 with lysine (E139K) to define the importance of this residue in thin filament regulation. Comparison of mutant and wild-type tropomyosin was carried out using in-vitro motility assays, actin co-sedimentation, and molecular dynamics simulations to determine perturbations in troponin-tropomyosin function caused by the tropomyosin mutation. Motility assays revealed that mutant thin filaments moved at higher velocity at low calcium with increased calcium sensitivity demonstrating that tropomyosin residue 139 is vital for proper tropomyosin-mediated inhibition during relaxation. Similarly, molecular dynamic simulations revealed a mutation-induced decrease in interaction energy between tropomyosin-E139K and troponin I (R170 and K174). These results suggest that salt-bridge stabilization of tropomyosin position by troponin IH4 is essential to prevent actomyosin interactions during cardiac muscle relaxation.
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Affiliation(s)
- Meaghan E Barry
- Department of Biological Sciences, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, United States of America
| | - Michael J Rynkiewicz
- Department of Pharmacology, Physiology & Biophysics, Boston University Chobanian and Avedisan School of Medicine, 700 Albany Street, W-408E, Boston, MA 02118, United States of America
| | - Elumalai Pavadai
- Department of Pharmacology, Physiology & Biophysics, Boston University Chobanian and Avedisan School of Medicine, 700 Albany Street, W-408E, Boston, MA 02118, United States of America
| | - Alex Viana
- Department of Biological Sciences, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, United States of America
| | - William Lehman
- Department of Pharmacology, Physiology & Biophysics, Boston University Chobanian and Avedisan School of Medicine, 700 Albany Street, W-408E, Boston, MA 02118, United States of America
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, United States of America.
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2
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Tolkatchev D, Gregorio CC, Kostyukova AS. The role of leiomodin in actin dynamics: a new road or a secret gate. FEBS J 2022; 289:6119-6131. [PMID: 34273242 PMCID: PMC8761783 DOI: 10.1111/febs.16128] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2021] [Revised: 05/10/2021] [Accepted: 07/16/2021] [Indexed: 12/29/2022]
Abstract
Leiomodin is an important emerging regulator of thin filaments. As novel molecular, cellular, animal model, and human data accumulate, the mechanisms of its action become clearer. Structural studies played a significant part in understanding the functional significance of leiomodin's interacting partners and functional domains. In this review, we present the current state of knowledge on the structural and cellular properties of leiomodin which has led to two proposed mechanisms of its function. Although it is known that leiomodin is essential for life, numerous domains within leiomodin remain unstudied and as such, we outline future directions for investigations that we predict will provide evidence that leiomodin is a multifunctional protein.
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Affiliation(s)
- Dmitri Tolkatchev
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA
| | - Carol C. Gregorio
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, The University of Arizona, Tucson, AZ 85724, USA
| | - Alla S. Kostyukova
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA
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3
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Saetang J, Tipmanee V, Benjakul S. In Silico Prediction of Cross-Reactive Epitopes of Tropomyosin from Shrimp and Other Arthropods Involved in Allergy. Molecules 2022; 27:molecules27092667. [PMID: 35566021 PMCID: PMC9104922 DOI: 10.3390/molecules27092667] [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: 03/10/2022] [Revised: 04/17/2022] [Accepted: 04/20/2022] [Indexed: 12/10/2022] Open
Abstract
Tropomyosin in shellfish is considered a major cross-reactive allergen in house dust mites and cockroaches; however, the specific epitopes have not been elucidated. Therefore, this study aimed to identify the consensus antigenic determinant among shrimp, house dust mites, and cockroaches using in silico methods. The protein sequences of tropomyosin, including Der f 10, Mac r 1, Pen a 1, Pen m 1, Per a 7, and Bla g 7, were retrieved from the UniProt database. The 3D structures were derived from the AlphaFold or modeled using the Robetta. The determination of linear epitopes was performed by AlgPRED and BepiPRED for B cell epitope, and NetMHCIIpan and NetMHCII for T cell epitope, while Ellipro was used to evaluate conformational epitopes. Fourteen peptides were discovered as the consensus linear B cell epitopes, while seventeen peptides were identified as linear T cell epitopes specific to high-frequency HLA-DR and HLA-DQ alleles. The conformational determination of B cell epitopes provided nine peptides, in which residues 209, 212, 255–256, and 258–259 were found in both linear B cell and linear T cell epitope analysis. This data could be utilized for further in vitro study and may contribute to immunotherapy for allergic diseases associated with tropomyosin.
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Affiliation(s)
- Jirakrit Saetang
- International Center of Excellence in Seafood Science and Innovation, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand;
- EZ-Mol-Design Laboratory, Faculty of Medicine, Prince of Songkla University, Songkhla 90110, Thailand;
- Correspondence: ; Tel.: +66-7428-6337
| | - Varomyalin Tipmanee
- EZ-Mol-Design Laboratory, Faculty of Medicine, Prince of Songkla University, Songkhla 90110, Thailand;
- Department of Biomedical Sciences and Biomedical Engineering, Faculty of Medicine, Prince of Songkla University, Songkhla 90110, Thailand
| | - Soottawat Benjakul
- International Center of Excellence in Seafood Science and Innovation, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand;
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4
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Tolkatchev D, Kuruba B, Smith GE, Swain KD, Smith KA, Moroz N, Williams TJ, Kostyukova AS. Structural insights into the tropomodulin assembly at the pointed ends of actin filaments. Protein Sci 2021; 30:423-437. [PMID: 33206408 PMCID: PMC7784754 DOI: 10.1002/pro.4000] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2020] [Revised: 11/09/2020] [Accepted: 11/13/2020] [Indexed: 11/11/2022]
Abstract
Tropomodulins are a family of important regulators of actin dynamics at the pointed ends of actin filaments. Four isoforms of tropomodulin, Tmod1-Tmod4, are expressed in vertebrates. Binding of tropomodulin to the pointed end is dependent on tropomyosin, an actin binding protein that itself is represented in mammals by up to 40 isoforms. The understanding of the regulatory role of the tropomodulin/tropomyosin molecular diversity has been limited due to the lack of a three-dimensional structure of the tropomodulin/tropomyosin complex. In this study, we mapped tropomyosin residues interacting with two tropomyosin-binding sites of tropomodulin and generated a three-dimensional model of the tropomodulin/tropomyosin complex for each of these sites. The models were refined by molecular dynamics simulations and validated via building a self-consistent three-dimensional model of tropomodulin assembly at the pointed end. The model of the pointed-end Tmod assembly offers new insights in how Tmod binding ensures tight control over the pointed end dynamics.
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Affiliation(s)
- Dmitri Tolkatchev
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
| | - Balaganesh Kuruba
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
| | - Garry E. Smith
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
| | - Kyle D. Swain
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
| | - Kaitlin A. Smith
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
| | - Natalia Moroz
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
- Department of Plant PathologyWashington State UniversityPullmanWashingtonUSA
| | - Trenton J. Williams
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
| | - Alla S. Kostyukova
- Voiland School of Chemical Engineering and BioengineeringWashington State UniversityPullmanWashingtonUSA
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5
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Impact of A134 and E218 Amino Acid Residues of Tropomyosin on Its Flexibility and Function. Int J Mol Sci 2020; 21:ijms21228720. [PMID: 33218166 PMCID: PMC7698929 DOI: 10.3390/ijms21228720] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2020] [Revised: 11/11/2020] [Accepted: 11/16/2020] [Indexed: 11/17/2022] Open
Abstract
Tropomyosin (Tpm) is one of the major actin-binding proteins that play a crucial role in the regulation of muscle contraction. The flexibility of the Tpm molecule is believed to be vital for its functioning, although its role and significance are under discussion. We choose two sites of the Tpm molecule that presumably have high flexibility and stabilized them with the A134L or E218L substitutions. Applying differential scanning calorimetry (DSC), molecular dynamics (MD), co-sedimentation, trypsin digestion, and in vitro motility assay, we characterized the properties of Tpm molecules with these substitutions. The A134L mutation prevented proteolysis of Tpm molecule by trypsin, and both substitutions increased the thermal stability of Tpm and its bending stiffness estimated from MD simulation. None of these mutations affected the primary binding of Tpm to F-actin; still, both of them increased the thermal stability of the actin-Tpm complex and maximal sliding velocity of regulated thin filaments in vitro at a saturating Ca2+ concentration. However, the mutations differently affected the Ca2+ sensitivity of the sliding velocity and pulling force produced by myosin heads. The data suggest that both regions of instability are essential for correct regulation and fine-tuning of Ca2+-dependent interaction of myosin heads with F-actin.
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6
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Tolkatchev D, Smith GE, Schultz LE, Colpan M, Helms GL, Cort JR, Gregorio CC, Kostyukova AS. Leiomodin creates a leaky cap at the pointed end of actin-thin filaments. PLoS Biol 2020; 18:e3000848. [PMID: 32898131 PMCID: PMC7500696 DOI: 10.1371/journal.pbio.3000848] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2020] [Revised: 09/18/2020] [Accepted: 08/17/2020] [Indexed: 01/26/2023] Open
Abstract
Improper lengths of actin-thin filaments are associated with altered contractile activity and lethal myopathies. Leiomodin, a member of the tropomodulin family of proteins, is critical in thin filament assembly and maintenance; however, its role is under dispute. Using nuclear magnetic resonance data and molecular dynamics simulations, we generated the first atomic structural model of the binding interface between the tropomyosin-binding site of cardiac leiomodin and the N-terminus of striated muscle tropomyosin. Our structural data indicate that the leiomodin/tropomyosin complex only forms at the pointed end of thin filaments, where the tropomyosin N-terminus is not blocked by an adjacent tropomyosin protomer. This discovery provides evidence supporting the debated mechanism where leiomodin and tropomodulin regulate thin filament lengths by competing for thin filament binding. Data from experiments performed in cardiomyocytes provide additional support for the competition model; specifically, expression of a leiomodin mutant that is unable to interact with tropomyosin fails to displace tropomodulin at thin filament pointed ends and fails to elongate thin filaments. Together with previous structural and biochemical data, we now propose a molecular mechanism of actin polymerization at the pointed end in the presence of bound leiomodin. In the proposed model, the N-terminal actin-binding site of leiomodin can act as a "swinging gate" allowing limited actin polymerization, thus making leiomodin a leaky pointed-end cap. Results presented in this work answer long-standing questions about the role of leiomodin in thin filament length regulation and maintenance.
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Affiliation(s)
- Dmitri Tolkatchev
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington, United States of America
| | - Garry E. Smith
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington, United States of America
| | - Lauren E. Schultz
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, The University of Arizona, Tucson, Arizona, United States of America
| | - Mert Colpan
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, The University of Arizona, Tucson, Arizona, United States of America
| | - Gregory L. Helms
- The Center for NMR Spectroscopy, Washington State University, Pullman, Washington, United States of America
| | - John R. Cort
- Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States of America
- Institute of Biological Chemistry, Washington State University, Pullman, Washington, United States of America
| | - Carol C. Gregorio
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, The University of Arizona, Tucson, Arizona, United States of America
| | - Alla S. Kostyukova
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington, United States of America
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7
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Yuan Y, Ayinuola YA, Singh D, Ayinuola O, Mayfield JA, Quek A, Whisstock JC, Law RHP, Lee SW, Ploplis VA, Castellino FJ. Solution structural model of the complex of the binding regions of human plasminogen with its M-protein receptor from Streptococcus pyogenes. J Struct Biol 2019; 208:18-29. [PMID: 31301349 PMCID: PMC6983471 DOI: 10.1016/j.jsb.2019.07.005] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2019] [Revised: 07/01/2019] [Accepted: 07/09/2019] [Indexed: 11/22/2022]
Abstract
VEK50 is a truncated peptide from a Streptococcal pyogenes surface human plasminogen (hPg) binding M-protein (PAM). VEK50 contains the full A-domain of PAM, which is responsible for its low nanomolar binding to hPg. The interaction of VEK50 with kringle 2, the PAM-binding domain in hPg (K2hPg), has been studied by high-resolution NMR spectroscopy. The data show that each VEK50 monomer in solution contains two tight binding sites for K2hPg, one each in the a1- (RH1; R17H18) and a2- (RH2; R30H31) repeats within the A-domain of VEK50. Two mutant forms of VEK50, viz., VEK50[RH1/AA] (VEK50ΔRH1) and VEK50[RH2/AA] (VEK50ΔRH2), were designed by replacing each RH with AA, thus eliminating one of the K2hPg binding sites within VEK50, and allowing separate study of each binding site. Using 13C- and 15N-labeled peptides, NMR-derived solution structures of VEK50 in its complex with K2hPg were solved. We conclude that the A-domain of PAM can accommodate two molecules of K2hPg docked within a short distance of each other, and the strength of the binding is slightly different for each site. The solution structure of the VEK50/K2hPg, complex, which is a reductionist model of the PAM/hPg complex, provides insights for the binding mechanism of PAM to a host protein, a process that is critical to S. pyogenes virulence.
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Affiliation(s)
- Yue Yuan
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Yetunde A Ayinuola
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Damini Singh
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Olawole Ayinuola
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Jeffrey A Mayfield
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Adam Quek
- Department of Biochemistry and Molecular Biology, Monash University, Clayton 3800 VIC, Australia
| | - James C Whisstock
- Department of Biochemistry and Molecular Biology, Monash University, Clayton 3800 VIC, Australia
| | - Ruby H P Law
- Department of Biochemistry and Molecular Biology, Monash University, Clayton 3800 VIC, Australia
| | - Shaun W Lee
- Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Victoria A Ploplis
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA; Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Francis J Castellino
- W.M. Keck Center for Transgene Research, University of Notre Dame, Notre Dame, IN 46556, USA; Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA.
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8
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Kopylova GV, Matyushenko AM, Koubassova NA, Shchepkin DV, Bershitsky SY, Levitsky DI, Tsaturyan AK. Functional outcomes of structural peculiarities of striated muscle tropomyosin. J Muscle Res Cell Motil 2019; 41:55-70. [DOI: 10.1007/s10974-019-09552-8] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2019] [Accepted: 08/17/2019] [Indexed: 12/27/2022]
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9
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Janco M, Rynkiewicz MJ, Li L, Hook J, Eiffe E, Ghosh A, Böcking T, Lehman WJ, Hardeman EC, Gunning PW. Molecular integration of the anti-tropomyosin compound ATM-3507 into the coiled coil overlap region of the cancer-associated Tpm3.1. Sci Rep 2019; 9:11262. [PMID: 31375704 PMCID: PMC6677793 DOI: 10.1038/s41598-019-47592-9] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2019] [Accepted: 07/19/2019] [Indexed: 11/30/2022] Open
Abstract
Tropomyosins (Tpm) determine the functional capacity of actin filaments in an isoform-specific manner. The primary isoform in cancer cells is Tpm3.1 and compounds that target Tpm3.1 show promising results as anti-cancer agents both in vivo and in vitro. We have determined the molecular mechanism of interaction of the lead compound ATM-3507 with Tpm3.1-containing actin filaments. When present during co-polymerization of Tpm3.1 with actin, 3H-ATM-3507 is incorporated into the filaments and saturates at approximately one molecule per Tpm3.1 dimer and with an apparent binding affinity of approximately 2 µM. In contrast, 3H-ATM-3507 is poorly incorporated into preformed Tpm3.1/actin co-polymers. CD spectroscopy and thermal melts using Tpm3.1 peptides containing the C-terminus, the N-terminus, and a combination of the two forming the overlap junction at the interface of adjacent Tpm3.1 dimers, show that ATM-3507 shifts the melting temperature of the C-terminus and the overlap junction, but not the N-terminus. Molecular dynamic simulation (MDS) analysis predicts that ATM-3507 integrates into the 4-helix coiled coil overlap junction and in doing so, likely changes the lateral movement of Tpm3.1 across the actin surface resulting in an alteration of filament interactions with actin binding proteins and myosin motors, consistent with the cellular impact of ATM-3507.
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Affiliation(s)
- Miro Janco
- School of Medical Sciences, University of New South Wales Sydney, Sydney, NSW, 2052, Australia
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA
| | - Liang Li
- School of Medical Sciences, University of New South Wales Sydney, Sydney, NSW, 2052, Australia
| | - Jeff Hook
- School of Medical Sciences, University of New South Wales Sydney, Sydney, NSW, 2052, Australia
| | - Eleanor Eiffe
- School of Medical Sciences, University of New South Wales Sydney, Sydney, NSW, 2052, Australia
| | - Anita Ghosh
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA
| | - Till Böcking
- Single Molecule Science and ARC Centre of Excellence in Advanced Molecular Imaging, University of New South Wales Sydney, Sydney, NSW, 2052, Australia
| | - William J Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA
| | - Edna C Hardeman
- School of Medical Sciences, University of New South Wales Sydney, Sydney, NSW, 2052, Australia
| | - Peter W Gunning
- School of Medical Sciences, University of New South Wales Sydney, Sydney, NSW, 2052, Australia.
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10
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Thin filament dysfunctions caused by mutations in tropomyosin Tpm3.12 and Tpm1.1. J Muscle Res Cell Motil 2019; 41:39-53. [PMID: 31270709 PMCID: PMC7109180 DOI: 10.1007/s10974-019-09532-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2019] [Accepted: 06/26/2019] [Indexed: 12/14/2022]
Abstract
Tropomyosin is the major regulator of the thin filament. In striated muscle its function is to bind troponin complex and control the access of myosin heads to actin in a Ca2+-dependent manner. It also participates in the maintenance of thin filament length by regulation of tropomodulin and leiomodin, the pointed end-binding proteins. Because the size of the overlap between actin and myosin filaments affects the number of myosin heads which interact with actin, the filament length is one of the determinants of force development. Numerous point mutations in genes encoding tropomyosin lead to single amino acid substitutions along the entire length of the coiled coil that are associated with various types of cardiomyopathy and skeletal muscle disease. Specific regions of tropomyosin interact with different binding partners; therefore, the mutations affect diverse tropomyosin functions. In this review, results of studies on mutations in the genes TPM1 and TPM3, encoding Tpm1.1 and Tpm3.12, are described. The paper is particularly focused on mutation-dependent alterations in the mechanisms of actin-myosin interactions and dynamics of the thin filament at the pointed end.
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11
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Liu GY, Mei XJ, Hu MJ, Yang Y, Liu M, Li MS, Zhang ML, Cao MJ, Liu GM. Analysis of the Allergenic Epitopes of Tropomyosin from Mud Crab Using Phage Display and Site-Directed Mutagenesis. JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY 2018; 66:9127-9137. [PMID: 30107732 DOI: 10.1021/acs.jafc.8b03466] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Mud crab ( Scylla serrata), which is widely consumed, can cause severe allergic symptoms. Eight linear epitopes and seven conformational epitopes of tropomyosin (TM) from S. serrata were identified using phage display. The conformational epitopes were formed based on the coiled-coil structure of TM. Most of the epitopes were located in the regions where primary structures were conserved among crustacean TM. Twelve synthetic peptides were designed according to the epitopes and trypsin-cutting sites of TM, among them, three synthetic peptides (including one linear epitope and two conformational epitopes) were recognized by all of the patient sera using inhibitory dot blotting. A triple-variant (R90A-E164A-Y267A) was constructed based on the critical amino acids of the TM epitope. The IgE-binding activity of the triple-variant was significantly reduced compared with that of native TM. The results of phage display and site-directed mutagenesis offered new information regarding conformational epitopes of TM.
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Affiliation(s)
- Guang-Yu Liu
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Xue-Jiao Mei
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Meng-Jun Hu
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Yang Yang
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Meng Liu
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Meng-Si Li
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Ming-Li Zhang
- Xiamen Second Hospital , Xiamen , Fujian 361021 , China
| | - Min-Jie Cao
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
| | - Guang-Ming Liu
- College of Food and Biological Engineering, Xiamen Key Laboratory of Marine Functional Food, Fujian Provincial Engineering Technology Research Center of Marine Functional Food, Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources , Jimei University , Xiamen , Fujian China
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12
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Lynn ML, Tal Grinspan L, Holeman TA, Jimenez J, Strom J, Tardiff JC. The structural basis of alpha-tropomyosin linked (Asp230Asn) familial dilated cardiomyopathy. J Mol Cell Cardiol 2017; 108:127-137. [PMID: 28600229 DOI: 10.1016/j.yjmcc.2017.06.001] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/18/2017] [Revised: 05/31/2017] [Accepted: 06/02/2017] [Indexed: 01/05/2023]
Abstract
Recently, linkage analysis of two large unrelated multigenerational families identified a novel dilated cardiomyopathy (DCM)-linked mutation in the gene coding for alpha-tropomyosin (TPM1) resulting in the substitution of an aspartic acid for an asparagine (at residue 230). To determine how a single amino acid mutation in α-tropomyosin (Tm) can lead to a highly penetrant DCM we generated a novel transgenic mouse model carrying the D230N mutation. The resultant mouse model strongly phenocopied the early onset of cardiomyopathic remodeling observed in patients as significant systolic dysfunction was observed by 2months of age. To determine the precise cellular mechanism(s) leading to the observed cardiac pathology we examined the effect of the mutation on Ca2+ handling in isolated myocytes and myofilament activation in vitro. D230N-Tm filaments exhibited a reduced Ca2+ sensitivity of sliding velocity. This decrease in sensitivity was coupled to increase in the peak amplitude of Ca2+ transients. While significant, and consistent with other DCMs, these measurements are comprised of complex inputs and did not provide sufficient experimental resolution. We then assessed the primary structural effects of D230N-Tm. Measurements of the thermal unfolding of D230N-Tm vs WT-Tm revealed an increase in stability primarily affecting the C-terminus of the Tm coiled-coil. We conclude that the D230N-Tm mutation induces a decrease in flexibility of the C-terminus via propagation through the helical structure of the protein, thus decreasing the flexibility of the Tm overlap and impairing its ability to regulate contraction. Understanding this unique structural mechanism could provide novel targets for eventual therapeutic interventions in patients with Tm-linked cardiomyopathies.
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Affiliation(s)
- M L Lynn
- Department of Physiological Sciences, University of Arizona, Tucson, AZ 85724, United States
| | - L Tal Grinspan
- Department of Medicine, Columbia University, New York, NY 10032, United States
| | - T A Holeman
- Department of Physiological Sciences, University of Arizona, Tucson, AZ 85724, United States; Department of Chemistry, University of Arizona, Tucson, AZ 85721, United States
| | - J Jimenez
- Department of Medicine, Washington University in Saint Louis, St. Louis, MO 63130, United States
| | - J Strom
- Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724, United States
| | - J C Tardiff
- Department of Physiological Sciences, University of Arizona, Tucson, AZ 85724, United States; Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724, United States; Department of Medicine, University of Arizona, Tucson, AZ 85724, United States.
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13
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Abstract
Tropomyosin is the archetypal-coiled coil, yet studies of its structure and function have proven it to be a dynamic regulator of actin filament function in muscle and non-muscle cells. Here we review aspects of its structure that deviate from canonical leucine zipper coiled coils that allow tropomyosin to bind to actin, regulate myosin, and interact directly and indirectly with actin-binding proteins. Four genes encode tropomyosins in vertebrates, with additional diversity that results from alternate promoters and alternatively spliced exons. At the same time that periodic motifs for binding actin and regulating myosin are conserved, isoform-specific domains allow for specific interaction with myosins and actin filament regulatory proteins, including troponin. Tropomyosin can be viewed as a universal regulator of the actin cytoskeleton that specifies actin filaments for cellular and intracellular functions.
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14
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Fischer S, Rynkiewicz MJ, Moore JR, Lehman W. Tropomyosin diffusion over actin subunits facilitates thin filament assembly. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2016; 3:012002. [PMID: 26798831 PMCID: PMC4714992 DOI: 10.1063/1.4940223] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/08/2015] [Accepted: 01/07/2016] [Indexed: 06/05/2023]
Abstract
Coiled-coil tropomyosin binds to consecutive actin-subunits along actin-containing thin filaments. Tropomyosin molecules then polymerize head-to-tail to form cables that wrap helically around the filaments. Little is known about the assembly process that leads to continuous, gap-free tropomyosin cable formation. We propose that tropomyosin molecules diffuse over the actin-filament surface to connect head-to-tail to partners. This possibility is likely because (1) tropomyosin hovers loosely over the actin-filament, thus binding weakly to F-actin and (2) low energy-barriers provide tropomyosin freedom for 1D axial translation on F-actin. We consider that these unique features of the actin-tropomyosin interaction are the basis of tropomyosin cable formation.
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Affiliation(s)
- Stefan Fischer
- Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg , Im Neuenheimer Feld 368, D69120 Heidelberg, Germany
| | - Michael J Rynkiewicz
- Department of Physiology and Biophysics, Boston University School of Medicine , 72 East Concord Street, Boston, Massachusetts 02118, USA
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts Lowell , One University Avenue, Lowell, Massachusetts 01854, USA
| | - William Lehman
- Department of Physiology and Biophysics, Boston University School of Medicine , 72 East Concord Street, Boston, Massachusetts 02118, USA
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15
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Rynkiewicz MJ, Schott V, Orzechowski M, Lehman W, Fischer S. Electrostatic interaction map reveals a new binding position for tropomyosin on F-actin. J Muscle Res Cell Motil 2015; 36:525-33. [PMID: 26286845 DOI: 10.1007/s10974-015-9419-z] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2015] [Accepted: 08/07/2015] [Indexed: 12/15/2022]
Abstract
Azimuthal movement of tropomyosin around the F-actin thin filament is responsible for muscle activation and relaxation. Recently a model of αα-tropomyosin, derived from molecular-mechanics and electron microscopy of different contractile states, showed that tropomyosin is rather stiff and pre-bent to present one specific face to F-actin during azimuthal transitions. However, a new model based on cryo-EM of troponin- and myosin-free filaments proposes that the interacting-face of tropomyosin can differ significantly from that in the original model. Because resolution was insufficient to assign tropomyosin side-chains, the interacting-face could not be unambiguously determined. Here, we use structural analysis and energy landscapes to further examine the proposed models. The observed bend in seven crystal structures of tropomyosin is much closer in direction and extent to the original model than to the new model. Additionally, we computed the interaction map for repositioning tropomyosin over the F-actin surface, but now extended over a much larger surface than previously (using the original interacting-face). This map shows two energy minima-one corresponding to the "blocked-state" as in the original model, and the other related by a simple 24 Å translation of tropomyosin parallel to the F-actin axis. The tropomyosin-actin complex defined by the second minimum fits perfectly into the recent cryo-EM density, without requiring any change in the interacting-face. Together, these data suggest that movement of tropomyosin between regulatory states does not require interacting-face rotation. Further, they imply that thin filament assembly may involve an interplay between initially seeded tropomyosin molecules growing from distinct binding-site regions on actin.
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Affiliation(s)
- Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA
| | - Veronika Schott
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA
- Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, 69120, Heidelberg, Germany
| | - Marek Orzechowski
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA
- Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, 69120, Heidelberg, Germany
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA, 02118, USA.
| | - Stefan Fischer
- Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, 69120, Heidelberg, Germany.
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16
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Skip residues modulate the structural properties of the myosin rod and guide thick filament assembly. Proc Natl Acad Sci U S A 2015; 112:E3806-15. [PMID: 26150528 DOI: 10.1073/pnas.1505813112] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The rod of sarcomeric myosins directs thick filament assembly and is characterized by the insertion of four skip residues that introduce discontinuities in the coiled-coil heptad repeats. We report here that the regions surrounding the first three skip residues share high structural similarity despite their low sequence homology. Near each of these skip residues, the coiled-coil transitions to a nonclose-packed structure inducing local relaxation of the superhelical pitch. Moreover, molecular dynamics suggest that these distorted regions can assume different conformationally stable states. In contrast, the last skip residue region constitutes a true molecular hinge, providing C-terminal rod flexibility. Assembly of myosin with mutated skip residues in cardiomyocytes shows that the functional importance of each skip residue is associated with rod position and reveals the unique role of the molecular hinge in promoting myosin antiparallel packing. By defining the biophysical properties of the rod, the structures and molecular dynamic calculations presented here provide insight into thick filament formation, and highlight the structural differences occurring between the coiled-coils of myosin and the stereotypical tropomyosin. In addition to extending our knowledge into the conformational and biological properties of coiled-coil discontinuities, the molecular characterization of the four myosin skip residues also provides a guide to modeling the effects of rod mutations causing cardiac and skeletal myopathies.
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17
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Fudge KR, Heeley DH. Biochemical Characterization of the Roles of Glycines 24 and 27 and Threonine 179 in Tropomyosin from the Fast Skeletal Trunk Muscle of the Atlantic Salmon. Biochemistry 2015; 54:2769-76. [DOI: 10.1021/acs.biochem.5b00156] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Affiliation(s)
- Korrina R. Fudge
- Department
of Biochemistry, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X9, Canada
| | - David H. Heeley
- Department
of Biochemistry, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X9, Canada
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18
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WAKABAYASHI T. Mechanism of the calcium-regulation of muscle contraction--in pursuit of its structural basis. PROCEEDINGS OF THE JAPAN ACADEMY. SERIES B, PHYSICAL AND BIOLOGICAL SCIENCES 2015; 91:321-50. [PMID: 26194856 PMCID: PMC4631897 DOI: 10.2183/pjab.91.321] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/16/2015] [Accepted: 06/01/2015] [Indexed: 06/10/2023]
Abstract
The author reviewed the research that led to establish the structural basis for the mechanism of the calcium-regulation of the contraction of striated muscles. The target of calcium ions is troponin on the thin filaments, of which the main component is the double-stranded helix of actin. A model of thin filament was generated by adding tropomyosin and troponin. During the process to provide the structural evidence for the model, the troponin arm was found to protrude from the calcium-depleted troponin and binds to the carboxyl-terminal region of actin. As a result, the carboxyl-terminal region of tropomyosin shifts and covers the myosin-binding sites of actin to block the binding of myosin. At higher calcium concentrations, the troponin arm changes its partner from actin to the main body of calcium-loaded troponin. Then, tropomyosin shifts back to the position near the grooves of actin double helix, and the myosin-binding sites of actin becomes available to myosin resulting in force generation through actin-myosin interactions.
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Affiliation(s)
- Takeyuki WAKABAYASHI
- Department of Physics, Graduate School of Science, the University of Tokyo, Tokyo, Japan
- Department of Biosciences, Graduate School of Science and Engineering, Teikyo University, Tochigi, Japan
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19
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Immunization with Hypoallergens of shrimp allergen tropomyosin inhibits shrimp tropomyosin specific IgE reactivity. PLoS One 2014; 9:e111649. [PMID: 25365343 PMCID: PMC4218792 DOI: 10.1371/journal.pone.0111649] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2014] [Accepted: 09/29/2014] [Indexed: 02/06/2023] Open
Abstract
Designer proteins deprived of its IgE-binding reactivity are being sought as a regimen for allergen-specific immunotherapy. Although shrimp tropomyosin (Met e 1) has long been identified as the major shellfish allergen, no immunotherapy is currently available. In this study, we aim at identifying the Met e 1 IgE epitopes for construction of hypoallergens and to determine the IgE inhibitory capacity of the hypoallergens. IgE-binding epitopes were defined by three online computational models, ELISA and dot-blot using sera from shrimp allergy patients. Based on the epitope data, two hypoallergenic derivatives were constructed by site-directed mutagenesis (MEM49) and epitope deletion (MED171). Nine regions on Met e 1 were defined as the major IgE-binding epitopes. Both hypoallergens MEM49 and MED171 showed marked reduction in their in vitro reactivity towards IgE from shrimp allergy patients and Met e 1-sensitized mice, as well as considerable decrease in induction of mast cell degranulation as demonstrated in passive cutaneous anaphylaxis assay. Both hypoallergens were able to induce Met e 1-recognizing IgG antibodies in mice, specifically IgG2a antibodies, that strongly inhibited IgE from shrimp allergy subjects and Met e 1-sensitized mice from binding to Met e 1. These results indicate that the two designer hypoallergenic molecules MEM49 and MED171 exhibit desirable preclinical characteristics, including marked reduction in IgE reactivity and allergenicity, as well as ability to induce blocking IgG antibodies. This approach therefore offers promises for development of immunotherapeutic regimen for shrimp tropomyosin allergy.
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20
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Zheng W, Barua B, Hitchcock-DeGregori SE. Probing the flexibility of tropomyosin and its binding to filamentous actin using molecular dynamics simulations. Biophys J 2014; 105:1882-92. [PMID: 24138864 DOI: 10.1016/j.bpj.2013.09.003] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2013] [Revised: 08/24/2013] [Accepted: 09/04/2013] [Indexed: 01/17/2023] Open
Abstract
Tropomyosin (Tm) is a coiled-coil protein that binds to filamentous actin (F-actin) and regulates its interactions with actin-binding proteins like myosin by moving between three positions on F-actin (the blocked, closed, and open positions). To elucidate the molecular details of Tm flexibility in relation to its binding to F-actin, we conducted extensive molecular dynamics simulations for both Tm alone and Tm-F-actin complex in the presence of explicit solvent (total simulation time >400 ns). Based on the simulations, we systematically analyzed the local flexibility of the Tm coiled coil using multiple parameters. We found a good correlation between the regions with high local flexibility and a number of destabilizing regions in Tm, including six clusters of core alanines. Despite the stabilization by F-actin binding, the distribution of local flexibility in Tm is largely unchanged in the absence and presence of F-actin. Our simulations showed variable fluctuations of individual Tm periods from the closed position toward the open position. In addition, we performed Tm-F-actin binding calculations based on the simulation trajectories, which support the importance of Tm flexibility to Tm-F-actin binding. We identified key residues of Tm involved in its dynamic interactions with F-actin, many of which have been found in recent mutational studies to be functionally important, and the rest of which will make promising targets for future mutational experiments.
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Affiliation(s)
- Wenjun Zheng
- Department of Physics, University at Buffalo, Buffalo, New York.
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21
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Barua B. Periodicities designed in the tropomyosin sequence and structure define its functions. BIOARCHITECTURE 2013; 3:51-6. [PMID: 23887197 DOI: 10.4161/bioa.25616] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Tropomyosin is an actin binding protein that regulates actin filament dynamics and its interactions with actin binding proteins such as myosin, tropomodulin, formin, Arp2/3 and ADF-cofilin in most eukaryotic cells. Tropomyosin is the prototypical two-chained, α-helical coiled coil protein that associates end-to-end and binds to both sides of the actin filament. Each tropomyosin molecule spans four to seven actin monomers in the filament, depending on the size of the tropomyosin. Tropomyosins have a periodic heptad repeat sequence that is characteristic of coiled coil proteins as well as additional periodicities required for its interaction with the actin filament, where each periodic repeat interacts with one actin molecule. This review addresses the role of periodic features of the Tm molecule in carrying out its universal functions of binding to the actin filament and its regulation and the specific features that may determine the isoform specificity of tropomyosins.
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Affiliation(s)
- Bipasha Barua
- Department of Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Piscataway, NJ, USA.
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22
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Colpan M, Moroz NA, Kostyukova AS. Tropomodulins and tropomyosins: working as a team. J Muscle Res Cell Motil 2013; 34:247-60. [PMID: 23828180 DOI: 10.1007/s10974-013-9349-6] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2013] [Accepted: 05/24/2013] [Indexed: 11/25/2022]
Abstract
Actin filaments are major components of the cytoskeleton in eukaryotic cells and are involved in vital cellular functions such as cell motility and muscle contraction. Tmod and TM are crucial constituents of the actin filament network, making their presence indispensable in living cells. Tropomyosin (TM) is an alpha-helical, coiled coil protein that covers the grooves of actin filaments and stabilizes them. Actin filament length is optimized by tropomodulin (Tmod), which caps the slow growing (pointed end) of thin filaments to inhibit polymerization or depolymerization. Tmod consists of two structurally distinct regions: the N-terminal and the C-terminal domains. The N-terminal domain contains two TM-binding sites and one TM-dependent actin-binding site, whereas the C-terminal domain contains a TM-independent actin-binding site. Tmod binds to two TM molecules and at least one actin molecule during capping. The interaction of Tmod with TM is a key regulatory factor for actin filament organization. The binding efficacy of Tmod to TM is isoform-dependent. The affinities of Tmod/TM binding influence the proper localization and capping efficiency of Tmod at the pointed end of actin filaments in cells. Here we describe how a small difference in the sequence of the TM-binding sites of Tmod may result in dramatic change in localization of Tmod in muscle cells or morphology of non-muscle cells. We also suggest most promising directions to study and elucidate the role of Tmod-TM interaction in formation and maintenance of sarcomeric and cytoskeletal structure.
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Affiliation(s)
- Mert Colpan
- The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, 118 Dana Hall, Spokane St., Pullman, WA, 99164, USA
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23
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Yar S, Chowdhury SAK, Davis RT, Kobayashi M, Monasky MM, Rajan S, Wolska BM, Gaponenko V, Kobayashi T, Wieczorek DF, Solaro RJ. Conserved Asp-137 is important for both structure and regulatory functions of cardiac α-tropomyosin (α-TM) in a novel transgenic mouse model expressing α-TM-D137L. J Biol Chem 2013; 288:16235-16246. [PMID: 23609439 DOI: 10.1074/jbc.m113.458695] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
α-Tropomyosin (α-TM) has a conserved, charged Asp-137 residue located in the hydrophobic core of its coiled-coil structure, which is unusual in that the residue is found at a position typically occupied by a hydrophobic residue. Asp-137 is thought to destabilize the coiled-coil and so impart structural flexibility to the molecule, which is believed to be crucial for its function in the heart. A previous in vitro study indicated that the conversion of Asp-137 to a more typical canonical Leu alters flexibility of TM and affects its in vitro regulatory functions. However, the physiological importance of the residue Asp-137 and altered TM flexibility is unknown. In this study, we further analyzed structural properties of the α-TM-D137L variant and addressed the physiological importance of TM flexibility in cardiac function in studies with a novel transgenic mouse model expressing α-TM-D137L in the heart. Our NMR spectroscopy data indicated that the presence of D137L introduced long range rearrangements in TM structure. Differential scanning calorimetry measurements demonstrated that α-TM-D137L has higher thermal stability compared with α-TM, which correlated with decreased flexibility. Hearts of transgenic mice expressing α-TM-D137L showed systolic and diastolic dysfunction with decreased myofilament Ca(2+) sensitivity and cardiomyocyte contractility without changes in intracellular Ca(2+) transients or post-translational modifications of major myofilament proteins. We conclude that conversion of the highly conserved Asp-137 to Leu results in loss of flexibility of TM that is important for its regulatory functions in mouse hearts. Thus, our results provide insight into the link between flexibility of TM and its function in ejecting hearts.
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Affiliation(s)
- Sumeyye Yar
- Departments of Biochemistry and Molecular Genetics, Chicago, Illinois 60612; Physiology and Biophysics, Chicago, Illinois 60612
| | | | | | | | | | - Sudarsan Rajan
- Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267
| | - Beata M Wolska
- Physiology and Biophysics, Chicago, Illinois 60612; Medicine, Chicago, Illinois 60612; Center for Cardiovascular Research, University of Illinois, Chicago, Illinois 60612
| | - Vadim Gaponenko
- Departments of Biochemistry and Molecular Genetics, Chicago, Illinois 60612
| | - Tomoyoshi Kobayashi
- Physiology and Biophysics, Chicago, Illinois 60612; Center for Cardiovascular Research, University of Illinois, Chicago, Illinois 60612
| | - David F Wieczorek
- Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267
| | - R John Solaro
- Physiology and Biophysics, Chicago, Illinois 60612; Center for Cardiovascular Research, University of Illinois, Chicago, Illinois 60612.
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24
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Barua B, Fagnant PM, Winkelmann DA, Trybus KM, Hitchcock-DeGregori SE. A periodic pattern of evolutionarily conserved basic and acidic residues constitutes the binding interface of actin-tropomyosin. J Biol Chem 2013; 288:9602-9609. [PMID: 23420843 DOI: 10.1074/jbc.m113.451161] [Citation(s) in RCA: 46] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
Actin filament cytoskeletal and muscle functions are regulated by actin binding proteins using a variety of mechanisms. A universal actin filament regulator is the protein tropomyosin, which binds end-to-end along the length of the filament. The actin-tropomyosin filament structure is unknown, but there are atomic models in different regulatory states based on electron microscopy reconstructions, computational modeling of actin-tropomyosin, and docking of atomic resolution structures of tropomyosin to actin filament models. Here, we have tested models of the actin-tropomyosin interface in the "closed state" where tropomyosin binds to actin in the absence of myosin or troponin. Using mutagenesis coupled with functional analyses, we determined residues of actin and tropomyosin required for complex formation. The sites of mutations in tropomyosin were based on an evolutionary analysis and revealed a pattern of basic and acidic residues in the first halves of the periodic repeats (periods) in tropomyosin. In periods P1, P4, and P6, basic residues are most important for actin affinity, in contrast to periods P2, P3, P5, and P7, where both basic and acidic residues or predominantly acidic residues contribute to actin affinity. Hydrophobic interactions were found to be relatively less important for actin binding. We mutated actin residues in subdomains 1 and 3 (Asp(25)-Glu(334)-Lys(326)-Lys(328)) that are poised to make electrostatic interactions with the residues in the repeating motif on tropomyosin in the models. Tropomyosin failed to bind mutant actin filaments. Our mutagenesis studies provide the first experimental support for the atomic models of the actin-tropomyosin interface.
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Affiliation(s)
- Bipasha Barua
- Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854; Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854.
| | - Patricia M Fagnant
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont 05405
| | - Donald A Winkelmann
- Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854
| | - Kathleen M Trybus
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont 05405
| | - Sarah E Hitchcock-DeGregori
- Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854; Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854
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25
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Regulation of actin-myosin interaction by conserved periodic sites of tropomyosin. Proc Natl Acad Sci U S A 2012; 109:18425-30. [PMID: 23091026 DOI: 10.1073/pnas.1212754109] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cooperative activation of actin-myosin interaction by tropomyosin (Tm) is central to regulation of contraction in muscle cells and cellular and intracellular movements in nonmuscle cells. The steric blocking model of muscle regulation proposed 40 y ago has been substantiated at both the kinetic and structural levels. Even with atomic resolution structures of the major players, how Tm binds and is designed for regulatory function has remained a mystery. Here we show that a set of periodically distributed evolutionarily conserved surface residues of Tm is required for cooperative regulation of actomyosin. Based on our results, we propose a model of Tm on a structure of actin-Tm-myosin in the "open" (on) state showing potential electrostatic interactions of the residues with both actin and myosin. The sites alternate with a second set of conserved surface residues that are important for actin binding in the inhibitory state in the absence of myosin. The transition from the closed to open states requires the sites identified here, even when troponin + Ca(2+) is present. The evolutionarily conserved residues are important for actomyosin regulation, a universal function of Tm that has a common structural basis and mechanism.
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26
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Nevzorov IA, Levitsky DI. Tropomyosin: double helix from the protein world. BIOCHEMISTRY (MOSCOW) 2012; 76:1507-27. [PMID: 22339601 DOI: 10.1134/s0006297911130098] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
This review concerns the structure and functions of tropomyosin (TM), an actin-binding protein that plays a key role in the regulation of muscle contraction. The TM molecule is a dimer of α-helices, which form a coiled-coil. Recent views on the TM structure are analyzed, and special attention is concentrated on those structural traits of the TM molecule that distinguish it from the other coiled-coil proteins. Modern data are presented on TM functional properties, such as its interaction with actin and ability to move on the surface of actin filaments, which underlies the regulation of the actin-myosin interaction upon contraction of skeletal and cardiac muscles. Also, part of the review is devoted to analysis of the effects of mutations in TM genes associated with muscle diseases (myopathies) on the structure and functions of TM.
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Affiliation(s)
- I A Nevzorov
- Bach Institute of Biochemistry, Russian Academy of Sciences, Moscow, Russia.
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27
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Oguchi Y, Ishizuka J, Hitchcock-DeGregori SE, Ishiwata S, Kawai M. The role of tropomyosin domains in cooperative activation of the actin-myosin interaction. J Mol Biol 2011; 414:667-80. [PMID: 22041451 DOI: 10.1016/j.jmb.2011.10.026] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2011] [Revised: 10/12/2011] [Accepted: 10/13/2011] [Indexed: 12/15/2022]
Abstract
To establish α-tropomyosin (Tm)'s structure-function relationships in cooperative regulation of muscle contraction, thin filaments were reconstituted with a variety of Tm mutants (Δ2Tm, Δ3Tm, Δ6Tm, P2sTm, P3sTm, P2P3sTm, P1P5Tm, and wtTm), and force and sliding velocity of the thin filament were studied using an in vitro motility assay. In the case of deletion mutants, Δ indicates which of the quasi-equivalent repeats in Tm was deleted. In the case of period (P) mutants, an Ala cluster was introduced into the indicated period to strengthen the Tm-actin interaction. In P1P5Tm, the N-terminal half of period 5 was substituted with that of period 1 to test the quasi-equivalence of these two Tm periods. The reconstitution included bovine cardiac troponin. Deletion studies revealed that period 3 is important for the positive cooperative effect of Tm on actin filament regulation and that period 2 also contributes to this effect at low ionic strength, but to a lesser degree. Furthermore, Tm with one extra Ala cluster at period 2 (P2s) or period 3 (P3s) did not increase force or velocity, whereas Tm with two extra Ala clusters (P2P3s) increased both force and velocity, demonstrating interaction between these periods. Most mutants did not move in the absence of Ca(2+). Notable exceptions were Δ6Tm and P1P5Tm, which moved near at the full velocity, but with reduced force, which indicate impaired relaxation. These results are consistent with the mechanism that the Tm-actin interaction cooperatively affects actin to result in generation of greater force and velocity.
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Affiliation(s)
- Yusuke Oguchi
- Department of Physics, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
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Evolutionarily conserved surface residues constitute actin binding sites of tropomyosin. Proc Natl Acad Sci U S A 2011; 108:10150-5. [PMID: 21642532 DOI: 10.1073/pnas.1101221108] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Tropomyosin (Tm) is a two-chained, α-helical coiled-coil protein that associates end-to-end to form a continuous strand along actin filaments and regulates the functions and stability of actin in eukaryotic muscle and nonmuscle cells. Mutations in Tm cause skeletal and cardiac myopathies. We applied a neoteric molecular evolution approach to gain insight into the fundamental unresolved question of what makes the Tm coiled coil an actin binding protein. We carried out a phylogenetic analysis of 70 coding sequences of Tm genes from 26 animal species, from cnidarians to chordates, and evaluated the substitution rates (ω) at individual codons to identify conserved sites. The most conserved residues at surface b, c, f heptad repeat positions were mutated in rat striated muscle αTm and expressed in Escherichia coli. Each mutant had 3-4 sites mutated to Ala within the first half or the second half of periods 2-6. Actin affinity and thermodynamic stability were determined in vitro. Mutations in the first half of periods 2, 4, and 5 resulted in the largest reduction in actin affinity (> 4-fold), indicating these mutations include residues in actin-binding sites. Mutations in the second half of the periods had a ≤ 2-fold effect on affinity indicating these residues may be involved in other conserved regulatory functions. The structural relevance of these results was assessed by constructing molecular models for the actin-Tm filament. Molecular evolution analysis is a general approach that may be used to identify potential binding sites of a protein for a conserved protein.
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Klenchin VA, Frye JJ, Jones MH, Winey M, Rayment I. Structure-function analysis of the C-terminal domain of CNM67, a core component of the Saccharomyces cerevisiae spindle pole body. J Biol Chem 2011; 286:18240-50. [PMID: 21454609 PMCID: PMC3093896 DOI: 10.1074/jbc.m111.227371] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2011] [Revised: 03/09/2011] [Indexed: 11/06/2022] Open
Abstract
The spindle pole body of the budding yeast Saccharomyces cerevisiae has served as a model system for understanding microtubule organizing centers, yet very little is known about the molecular structure of its components. We report here the structure of the C-terminal domain of the core component Cnm67 at 2.3 Å resolution. The structure determination was aided by a novel approach to crystallization of proteins containing coiled-coils that utilizes globular domains to stabilize the coiled-coils. This enhances their solubility in Escherichia coli and improves their crystallization. The Cnm67 C-terminal domain (residues Asn-429-Lys-581) exhibits a previously unseen dimeric, interdigitated, all α-helical fold. In vivo studies demonstrate that this domain alone is able to localize to the spindle pole body. In addition, the structure reveals a large functionally indispensable positively charged surface patch that is implicated in spindle pole body localization. Finally, the C-terminal eight residues are disordered but are critical for protein folding and structural stability.
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Affiliation(s)
- Vadim A. Klenchin
- From the Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706 and
| | - Jeremiah J. Frye
- From the Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706 and
| | - Michele H. Jones
- the Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, Colorado 80309
| | - Mark Winey
- the Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, Colorado 80309
| | - Ivan Rayment
- From the Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706 and
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Śliwińska M, Żukowska M, Borys D, Moraczewska J. Different positions of tropomyosin isoforms on actin filament are determined by specific sequences of end-to-end overlaps. Cytoskeleton (Hoboken) 2011; 68:300-12. [DOI: 10.1002/cm.20513] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
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Tu D, Li Y, Song HK, Toms AV, Gould CJ, Ficarro SB, Marto JA, Goode BL, Eck MJ. Crystal structure of a coiled-coil domain from human ROCK I. PLoS One 2011; 6:e18080. [PMID: 21445309 PMCID: PMC3061879 DOI: 10.1371/journal.pone.0018080] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2010] [Accepted: 02/21/2011] [Indexed: 11/19/2022] Open
Abstract
The small GTPase Rho and one of its targets, Rho-associated kinase (ROCK), participate in a variety of actin-based cellular processes including smooth muscle contraction, cell migration, and stress fiber formation. The ROCK protein consists of an N-terminal kinase domain, a central coiled-coil domain containing a Rho binding site, and a C-terminal pleckstrin homology domain. Here we present the crystal structure of a large section of the central coiled-coil domain of human ROCK I (amino acids 535-700). The structure forms a parallel α-helical coiled-coil dimer that is structurally similar to tropomyosin, an actin filament binding protein. There is an unusual discontinuity in the coiled-coil; three charged residues (E613, R617 and D620) are positioned at what is normally the hydrophobic core of coiled-coil packing. We speculate that this conserved irregularity could function as a hinge that allows ROCK to adopt its autoinhibited conformation.
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Affiliation(s)
- Daqi Tu
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, United States of America
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts, United States of America
| | - Yiqun Li
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts, United States of America
| | - Hyun Kyu Song
- School of Life Sciences and Biotechnology, Korea University, Seoul, Korea
| | - Angela V. Toms
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, United States of America
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts, United States of America
| | - Christopher J. Gould
- Department of Biology and Rosenstiel Basic Medical Science Research Center, Brandeis University, Waltham, Massachusetts, United States of America
| | - Scott B. Ficarro
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, United States of America
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts, United States of America
| | - Jarrod A. Marto
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, United States of America
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts, United States of America
| | - Bruce L. Goode
- Department of Biology and Rosenstiel Basic Medical Science Research Center, Brandeis University, Waltham, Massachusetts, United States of America
| | - Michael J. Eck
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts, United States of America
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, Massachusetts, United States of America
- * E-mail:
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Ali LF, Cohen JM, Tobacman LS. Push and pull of tropomyosin's opposite effects on myosin attachment to actin. A chimeric tropomyosin host-guest study. Biochemistry 2010; 49:10873-80. [PMID: 21114337 DOI: 10.1021/bi101632f] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Tropomyosin is a ubiquitous actin-binding protein with an extended coiled-coil structure. Tropomyosin-actin interactions are weak and loosely specific, but they potently influence myosin. One such influence is inhibitory and is due to tropomyosin's statistically preferred positions on actin that sterically interfere with actin's strong attachment site for myosin. Contrastingly, tropomyosin's other influence is activating. It increases myosin's overall actin affinity ∼4-fold. Stoichiometric considerations cause this activating effect to equate to an ∼4(7)-fold effect of myosin on the actin affinity of tropomyosin. These positive, mutual, myosin-tropomyosin effects are absent if Saccharomyces cerevisiae tropomyosin replaces mammalian tropomyosin. To investigate these phenomena, chimeric tropomyosins were generated in which 38-residue muscle tropomyosin segments replaced a natural duplication within S. cerevisiae tropomyosin TPM1. Two such chimeric tropomyosins were sufficiently folded coiled coils to allow functional study. The two chimeras differed from TPM1 but in opposite ways. Consistent with steric interference, myosin greatly decreased the actin affinity of chimera 7, which contained muscle tropomyosin residues 228-265. On the other hand, myosin S1 increased by an order of magnitude the actin affinity of chimera 3, which contained muscle tropomyosin residues 74-111. Similarly, myosin S1-ADP binding to actin was strengthened 2-fold by substitution of chimera 3 tropomyosin for wild-type TPM1. Thus, a yeast tropomyosin was induced to mimic the activating behavior of mammalian tropomyosin by inserting a mammalian tropomyosin sequence. The data were not consistent with direct tropomyosin-myosin binding. Rather, they suggest an allosteric mechanism, in which myosin and tropomyosin share an effect on the actin filament.
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Affiliation(s)
- Laith F Ali
- Department of Medicine, University of Illinois at Chicago,Chicago, Illinois 60612, United States
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Brown JH. How sequence directs bending in tropomyosin and other two-stranded alpha-helical coiled coils. Protein Sci 2010; 19:1366-75. [PMID: 20506487 PMCID: PMC2974828 DOI: 10.1002/pro.415] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2009] [Revised: 04/15/2010] [Accepted: 05/02/2010] [Indexed: 12/31/2022]
Abstract
A quantitative analysis of the direction of bending of two-stranded alpha-helical coiled coils in crystal structures has been carried out to help determine how the amino acid sequence of the coiled coil influences its shape and function. Change in the axial staggering of the coiled coil, occurring at the boundaries of either clusters of core alanines in tropomyosin or of clusters of core bulky residues in the myosin rod, causes bending within the plane of the local dimer. The results also reveal that large gaps in the core of the coiled coil, which are seen for small core residues near large core residues or for unbranched core residues near canonical branched core residues, are correlated with bending out of the local dimeric plane. Comparison of tropomyosin structures determined in independent crystal environments provides further evidence for the concept that sequence directs the bending of the coiled coil, but that crystal environment is at least as important as sequence for determining the magnitude of bending. Tropomyosin thus appears to consist of more directionally restrained hinge-like joints rather than directionally variable universal joints, which helps account for and predicts the geometric and dynamic nature of its binding to F-actin.
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Affiliation(s)
- Jerry H Brown
- Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02454-9110, USA.
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The relationship between curvature, flexibility and persistence length in the tropomyosin coiled-coil. J Struct Biol 2010; 170:313-8. [PMID: 20117217 DOI: 10.1016/j.jsb.2010.01.016] [Citation(s) in RCA: 67] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2009] [Revised: 01/25/2010] [Accepted: 01/25/2010] [Indexed: 01/29/2023]
Abstract
The inherent flexibility of rod-like tropomyosin coiled-coils is a significant factor that constrains tropomyosin's complex positional dynamics on actin filaments. Flexibility of elongated straight molecules typically is assessed by persistence length, a measure of lengthwise thermal bending fluctuations. However, if a molecule's equilibrium conformation is curved, this formulation yields an "apparent" persistence length ( approximately 100nm for tropomyosin), measuring deviations from idealized straight conformations which then overestimate actual dynamic flexibility. To obtain the "dynamic" persistence length, a true measurement of flexural stiffness, the average curvature of the molecule must be taken into account. Different methods used in our studies for measuring the dynamic persistence length directly from Molecular Dynamics (MD) simulations of tropomyosin are described here in detail. The dynamic persistence length found, 460+/-40nm, is approximately 12-times longer than tropomyosin and 5-times the apparent persistence length, showing that tropomyosin is considerably stiffer than previously thought. The longitudinal twisting behavior of tropomyosin during MD shows that the amplitude of end-to-end twisting fluctuation is approximately 30 degrees when tropomyosin adopts its near-average conformation. The measured bending and twisting flexibilities are used to evaluate different models of tropomyosin motion on F-actin.
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Wang CLA, Coluccio LM. New insights into the regulation of the actin cytoskeleton by tropomyosin. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2010; 281:91-128. [PMID: 20460184 DOI: 10.1016/s1937-6448(10)81003-2] [Citation(s) in RCA: 70] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
The actin cytoskeleton is regulated by a variety of actin-binding proteins including those constituting the tropomyosin family. Tropomyosins are coiled-coil dimers that bind along the length of actin filaments. In muscles, tropomyosin regulates the interaction of actin-containing thin filaments with myosin-containing thick filaments to allow contraction. In nonmuscle cells where multiple tropomyosin isoforms are expressed, tropomyosins participate in a number of cellular events involving the cytoskeleton. This chapter reviews the current state of the literature regarding tropomyosin structure and function and discusses the evidence that tropomyosins play a role in regulating actin assembly.
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36
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The Shape and Flexibility of Tropomyosin Coiled Coils: Implications for Actin Filament Assembly and Regulation. J Mol Biol 2010; 395:327-39. [DOI: 10.1016/j.jmb.2009.10.060] [Citation(s) in RCA: 100] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2009] [Revised: 10/19/2009] [Accepted: 10/27/2009] [Indexed: 12/18/2022]
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Hitchcock-DeGregori SE, Singh A. What makes tropomyosin an actin binding protein? A perspective. J Struct Biol 2009; 170:319-24. [PMID: 20036744 DOI: 10.1016/j.jsb.2009.12.013] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2009] [Accepted: 12/16/2009] [Indexed: 01/08/2023]
Abstract
Tropomyosin is a two-chained alpha-helical coiled coil that binds along the length of the actin filament and regulates its function. The paper addresses the question of how a "simple" coiled-coil sequence encodes the information for binding and regulating the actin filament, its universal target. Determination of the tropomyosin sequence confirmed Crick's predicted heptapeptide repeat of hydrophobic interface residues and revealed additional features that have been shown to be important for its function: a 7-fold periodicity predicted to correspond to actin binding sites and interruptions of the canonical interface with destabilizing residues, such as Ala. Evidence from published work is summarized, leading to the proposal of a paradigm that binding of tropomyosin to the actin filament requires local instability as well as regions of flexibility. The flexibility derives from bends and local unfolding at regions with a destabilized coiled-coil interface, as well as from the dynamic end-to-end complex. The features are required for tropomyosin to assume the form of the helical actin filament, and to bind to actin monomers along its length. The requirement of instability/flexibility for binding may be generalized to the binding of other coiled coils to their targets.
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Affiliation(s)
- Sarah E Hitchcock-DeGregori
- Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, 675 Hoes Lane, Piscataway, NJ 08854, USA.
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Li XE, Lehman W, Fischer S, Holmes KC. Curvature variation along the tropomyosin molecule. J Struct Biol 2009; 170:307-12. [PMID: 20026408 DOI: 10.1016/j.jsb.2009.12.017] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2009] [Revised: 12/07/2009] [Accepted: 12/16/2009] [Indexed: 01/03/2023]
Abstract
Complementarity between the tropomyosin supercoil and the helical contour of actin-filaments is required for the binding interaction of actin and tropomyosin (Li et al., 2010). Clusters of small alanine residues in place of canonical leucines along coiled-coil tropomyosin may be responsible for pre-shaping tropomyosin and promoting conformational complementarity to F-actin. A longitudinal displacement between the two chains of the tropomyosin coiled-coil induced by the alanine clusters could produce localized bending or limited flexibility along tropomyosin needed to shape tropomyosin (Brown and Cohen, 2005). To evaluate the influence of alanine clusters on tropomyosin curvature, we calculated the longitudinal displacement between amino acid residues on adjacent chains of the tropomyosin coiled-coil and related this "Z-displacement" to the position of the alanine clusters. Measurements were made on high-resolution crystal structures of tropomyosin fragments and on trajectories from molecular dynamics simulations of full-length alphaalpha-tropomyosin. We found no strict one-for-one spatial correlation between alanine cluster position and the Z-displacement. Neither did we find any direct correspondence between the clusters and the local curvature of tropomyosin. Rather than just causing specific local structural effects, the overall influence of alanine clusters is complex and delocalized, leading to a gradually changing bending pattern along the length of tropomyosin.
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Affiliation(s)
- Xiaochuan Edward Li
- Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA; Computational Biochemistry Group, University of Heidelberg, Im Neuenheimer Feld 368, Heidelberg D-69120, Germany
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Bulheller BM, Rodger A, Hicks MR, Dafforn TR, Serpell LC, Marshall KE, Bromley EHC, King PJS, Channon KJ, Woolfson DN, Hirst JD. Flow Linear Dichroism of Some Prototypical Proteins. J Am Chem Soc 2009; 131:13305-14. [DOI: 10.1021/ja902662e] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Benjamin M. Bulheller
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Alison Rodger
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Matthew R. Hicks
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Timothy R. Dafforn
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Louise C. Serpell
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Karen E. Marshall
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Elizabeth H. C. Bromley
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Patrick J. S. King
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Kevin J. Channon
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Derek N. Woolfson
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
| | - Jonathan D. Hirst
- School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K., School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Biochemistry, School of Life Sciences, University of Sussex, Falmer BN1 9QG, U.K., School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and Department of Biochemistry, University of Bristol, Bristol BS8 1TD, U.K
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Singh A, Hitchcock-Degregori SE. A peek into tropomyosin binding and unfolding on the actin filament. PLoS One 2009; 4:e6336. [PMID: 19629180 PMCID: PMC2710508 DOI: 10.1371/journal.pone.0006336] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2009] [Accepted: 06/21/2009] [Indexed: 11/19/2022] Open
Abstract
Background Tropomyosin is a prototypical coiled coil along its length with subtle variations in structure that allow interactions with actin and other proteins. Actin binding globally stabilizes tropomyosin. Tropomyosin-actin interaction occurs periodically along the length of tropomyosin. However, it is not well understood how tropomyosin binds actin. Principal Findings Tropomyosin's periodic binding sites make differential contributions to two components of actin binding, cooperativity and affinity, and can be classified as primary or secondary sites. We show through mutagenesis and analysis of recombinant striated muscle α-tropomyosins that primary actin binding sites have a destabilizing coiled-coil interface, typically alanine-rich, embedded within a non-interface recognition sequence. Introduction of an Ala cluster in place of the native, more stable interface in period 2 and/or period 3 sites (of seven) increased the affinity or cooperativity of actin binding, analysed by cosedimentation and differential scanning calorimetry. Replacement of period 3 with period 5 sequence, an unstable region of known importance for cooperative actin binding, increased the cooperativity of binding. Introduction of the fluorescent probe, pyrene, near the mutation sites in periods 2 and 3 reported local instability, stabilization by actin binding, and local unfolding before or coincident with dissociation from actin (measured using light scattering), and chain dissociation (analyzed using circular dichroism). Conclusions This, and previous work, suggests that regions of tropomyosin involved in binding actin have non-interface residues specific for interaction with actin and an unstable interface that is locally stabilized upon binding. The destabilized interface allows residues on the coiled-coil surface to obtain an optimal conformation for interaction with actin by increasing the number of local substates that the side chains can sample. We suggest that local disorder is a property typical of coiled coil binding sites and proteins that have multiple binding partners, of which tropomyosin is one type.
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Affiliation(s)
- Abhishek Singh
- Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, Piscataway, New Jersey, United States of America.
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41
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Greenfield NJ, Kotlyanskaya L, Hitchcock-DeGregori SE. Structure of the N terminus of a nonmuscle alpha-tropomyosin in complex with the C terminus: implications for actin binding. Biochemistry 2009; 48:1272-83. [PMID: 19170537 PMCID: PMC4410877 DOI: 10.1021/bi801861k] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Tropomyosin is a coiled-coil actin binding protein that stabilizes the filament, protects it from severing, and cooperatively regulates actin's interaction with myosin. Depending on the first coding exon, tropomyosins are low molecular weight (LMW), found in the cytoskeleton and predominant in transformed cells, or high molecular weight (HMW), found in muscle and nonmuscle cells. The N- and C-terminal ends form a complex that allows tropomyosin to associate N terminus-to-C terminus along the actin filament. We determined the structure of a LMW tropomyosin N-terminal model peptide complexed with a smooth/nonmuscle tropomyosin C-terminal peptide. Using NMR and circular dichroism we showed that both ends become more helical upon complex formation but that the C-terminal peptide is partially unfolded at 20 degrees C. The first five residues of the N terminus that are disordered in the free peptide are more helical and are part of the overlap complex. NMR data indicate residues 2-17 bind to the C terminus in the complex. The data support a model for the LMW overlap complex that is homologous to the striated muscle tropomyosin complex in which the ends are oriented in parallel N terminus-to-C terminus with the plane of the N-terminal coiled coil perpendicular to the plane of the C terminus. The main difference is that the overlap spans 16 residues in the LMW tropomyosin complex compared to 11 residues in the HMW striated muscle overlap complex. We discuss the relevance of a stable but dynamic intermolecular junction for high-affinity binding to actin.
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Affiliation(s)
- Norma J. Greenfield
- Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, Piscataway, NJ 08854
| | - Lucy Kotlyanskaya
- Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, Piscataway, NJ 08854
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Gestalt-binding of tropomyosin to actin filaments. J Muscle Res Cell Motil 2008; 29:213-9. [PMID: 19116763 DOI: 10.1007/s10974-008-9157-6] [Citation(s) in RCA: 104] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2008] [Accepted: 12/01/2008] [Indexed: 01/26/2023]
Abstract
We argue that the overall behavior of tropomyosin on F-actin cannot be easily discerned by examining thin filaments reduced to their smallest interacting units. In isolation, the individual interactions of actin and tropomyosin, by themselves, are too weak to account for the specificity of the system. Instead the association of tropomyosin on actin can only be fully explained after considering the concerted action of the entire acto-tropomyosin system. We propose that the low K ( a ) describing tropomyosin:actin interaction, when taken together with the form-fitting complementarity of tropomyosin strands on F-actin and the tendency for tropomyosin to polymerize end-to-end, make possible unique thin filament functions both locally and at higher levels of filament organization.
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Structural basis for tropomyosin overlap in thin (actin) filaments and the generation of a molecular swivel by troponin-T. Proc Natl Acad Sci U S A 2008; 105:7200-5. [PMID: 18483193 DOI: 10.1073/pnas.0801950105] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Head-to-tail polymerization of tropomyosin is crucial for its actin binding, function in actin filament assembly, and the regulation of actin-myosin contraction. Here, we describe the 2.1 A resolution structure of crystals containing overlapping tropomyosin N and C termini (TM-N and TM-C) and the 2.9 A resolution structure of crystals containing TM-N and TM-C together with a fragment of troponin-T (TnT). At each junction, the N-terminal helices of TM-N were splayed, with only one of them packing against TM-C. In the C-terminal region of TM-C, a crucial water in the coiled-coil core broke the local 2-fold symmetry and helps generate a kink on one helix. In the presence of a TnT fragment, the asymmetry in TM-C facilitates formation of a 4-helix bundle containing two TM-C chains and one chain each of TM-N and TnT. Mutating the residues that generate the asymmetry in TM-C caused a marked decrease in the affinity of troponin for actin-tropomyosin filaments. The highly conserved region of TnT, in which most cardiomyopathy mutations reside, is crucial for interacting with tropomyosin. The structure of the ternary complex also explains why the skeletal- and cardiac-muscle specific C-terminal region is required to bind TnT and why tropomyosin homodimers bind only a single TnT. On actin filaments, the head-to-tail junction can function as a molecular swivel to accommodate irregularities in the coiled-coil path between successive tropomyosins enabling each to interact equivalently with the actin helix.
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Two-crystal structures of tropomyosin C-terminal fragment 176-273: exposure of the hydrophobic core to the solvent destabilizes the tropomyosin molecule. Biophys J 2008; 95:710-9. [PMID: 18339732 DOI: 10.1529/biophysj.107.126144] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Tropomyosin (Tm) is a two-stranded alpha-helical coiled-coil protein, and when associated with troponin, it is responsible for the actin filament-based regulation of muscle contraction in vertebrate skeletal and cardiac muscles. It is widely believed that Tm adopts a flexible rod-like structure in which the flexibility must play a crucial role in its functions. To obtain more information about the flexibility of Tm, we solved and compared two crystal structures of the identical C-terminal segments, spanning approximately 40% of the entire length. We also compared these structures with our previously reported crystal structure of an almost identical Tm segment in a distinct crystal form. The parameters specifying the local coiled-coil geometry, such as the separation between two helices and the local helical pitch, undulate along the length of Tm in the same way as among the three crystal structures, indicating that these parameters are defined by the amino acid sequence. In the region of increased separation, around Glu-218 and Gln-263, the hydrophobic core is disrupted by three holes. Moreover, for the first time to our knowledge, for Tm, water molecules have been identified in these holes. In some structures, the B-factors are higher around the holes than in the rest of the molecule. The Tm coiled-coil must be destabilized and therefore may be flexible, not only in the alanine clusters but also in the regions of the broken core. A closer look at the local staggering between the two chains and the local bending revealed that the strain accumulates at the alanine cluster and may be relaxed in the broken core region. Moreover, the strain is distributed over a long range, even when a deformation like bending may occur at a limited number of spots. Thus, Tm should not be regarded as a train of short rigid rods connected by flexible linkers, but rather as a seamless rubber rod patched with relatively more flexible regions.
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Hitchcock-DeGregori SE. Tropomyosin: Function Follows Structure. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2008; 644:60-72. [DOI: 10.1007/978-0-387-85766-4_5] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
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Tropomyosin and the steric mechanism of muscle regulation. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2008; 644:95-109. [PMID: 19209816 DOI: 10.1007/978-0-387-85766-4_8] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Contraction in all muscles must be precisely regulated and requisite control systems must be able to adjust to changes in physiological and myopathic stimuli. In this chapter, we outline the structural evidence for a steric mechanism that governs muscle activity. The mechanism involves calcium and myosin induced changes in the position of tropomyosin along actin-based thin filaments. This process either blocks or uncovers myosin crossbridge binding sites on actin and consequently regulates crossbridge cycling on thin filaments, the sliding of thin and thick filaments and muscle shortening and force production.
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Sumida JP, Wu E, Lehrer SS. Conserved Asp-137 imparts flexibility to tropomyosin and affects function. J Biol Chem 2007; 283:6728-34. [PMID: 18165684 DOI: 10.1074/jbc.m707485200] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Tropomyosin (Tm) is an alpha-helical coiled-coil that controls muscle contraction by sterically regulating the myosin-actin interaction. Tm moves between three states on F-actin as either a uniform or a non-uniform semi-flexible rod. Tm is stabilized by hydrophobic residues in the "a" and "d" positions of the heptad repeat. The highly conserved Asp-137 is unusual in that it introduces a negative charge on each chain in a position typically occupied by hydrophobic residues. The occurrence of two charged residues in the hydrophobic region is expected to destabilize the region and impart flexibility. To determine whether this region is unstable, we have substituted hydrophobic Leu for Asp-137 and studied changes in Tm susceptibility to limited proteolysis by trypsin and changes in regulation. We found that native and Tm controls that contain Asp-137 were readily cleaved at Arg-133 with t 1/2 of 5 min. In contrast, the Leu-137 mutant was not cleaved under the same conditions. Actin stabilized Tm, causing a 10-fold reduction in the rate of cleavage at Arg-133. The actin-myosin subfragment S1 ATPase activity was greater for the Leu mutant compared with controls in the absence of troponin and in the presence of troponin and Ca2+. We conclude that the highly conserved Asp-137 destabilizes the middle of Tm, resulting in a more flexible region that is important for the cooperative activation of the thin filament by myosin. We thus have shown a link between the dynamic properties of Tm and its function.
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Affiliation(s)
- John P Sumida
- Cardiovascular Program, Boston Biomedical Research Institute, Watertown, Massachusetts 02472, USA
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