51
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Geeves M, Griffiths H, Mijailovich S, Smith D. Cooperative [Ca²+]-dependent regulation of the rate of myosin binding to actin: solution data and the tropomyosin chain model. Biophys J 2011; 100:2679-87. [PMID: 21641313 DOI: 10.1016/j.bpj.2011.04.020] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2011] [Revised: 03/26/2011] [Accepted: 04/07/2011] [Indexed: 01/15/2023] Open
Abstract
The regulation of muscle contraction by calcium involves interactions among actin filaments, myosin-S1, tropomyosin (Tm), and troponin (Tn). We have extended our previous model in which the TmTn regulatory units are treated as a continuous flexible chain, and applied it to transient kinetic data. We have measured the time course of myosin-S1 binding to actin-Tm-Tn filaments in solution at various calcium levels with [actin]/[myosin] ratios of 10 and 0.1, which exhibit modest slowing as [Ca(2+)] is reduced and a lag phase at low calcium. These observations can be explained if myosin binds to actin in two steps, where the first step is rate-limiting and blocked by TmTnI at low calcium, and the second step is fast, reversible, and controlled by the neighboring configuration of coupled tropomyosin-troponin units. The model can describe the calcium dependence of the observed myosin binding reactions and predicts cooperative calcium binding to TnC with competition between actin and Ca-TnC for the binding of TnI. Implications for theories of thin-filament regulation in muscle are discussed.
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Affiliation(s)
- Michael Geeves
- Department of Biosciences, University of Kent, Canterbury, Kent, United Kingdom.
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52
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Bains G, Patel AB, Narayanaswami V. Pyrene: a probe to study protein conformation and conformational changes. Molecules 2011; 16:7909-35. [PMID: 22143550 PMCID: PMC6264589 DOI: 10.3390/molecules16097909] [Citation(s) in RCA: 190] [Impact Index Per Article: 14.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2011] [Revised: 09/04/2011] [Accepted: 09/06/2011] [Indexed: 11/18/2022] Open
Abstract
The review focuses on the unique spectral features of pyrene that can be utilized to investigate protein structure and conformation. Pyrene is a fluorescent probe that can be attached covalently to protein side chains, such as sulfhydryl groups. The spectral features of pyrene are exquisitely sensitive to the microenvironment of the probe: it exhibits an ensemble of monomer fluorescence emission peaks that report on the polarity of the probe microenvironment, and an additional band at longer wavelengths, the appearance of which reflects the presence of another pyrene molecule in spatial proximity (~10 Å). Its high extinction coefficient allows us to study labeled proteins in solution at physiologically relevant concentrations. The environmentally- and spatially-sensitive features of pyrene allow monitoring protein conformation, conformational changes, protein folding and unfolding, protein-protein, protein-lipid and protein-membrane interactions.
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Affiliation(s)
- Gursharan Bains
- Department of Chemistry and Biochemistry, 1250 Bellflower Boulevard, California State University Long Beach, Long Beach, CA 90840, USA
| | - Arti B. Patel
- Department of Chemistry and Biochemistry, 1250 Bellflower Boulevard, California State University Long Beach, Long Beach, CA 90840, USA
| | - Vasanthy Narayanaswami
- Department of Chemistry and Biochemistry, 1250 Bellflower Boulevard, California State University Long Beach, Long Beach, CA 90840, USA
- Children’s Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609, USA
- Author to whom correspondence should be addressed; E-Mail: ; Tel.: +1-562-985-4953; Fax: +1-562-985-8557
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53
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Ueda K, Kimura-Sakiyama C, Aihara T, Miki M, Arata T. Interaction sites of tropomyosin in muscle thin filament as identified by site-directed spin-labeling. Biophys J 2011; 100:2432-9. [PMID: 21575577 DOI: 10.1016/j.bpj.2011.03.021] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2010] [Revised: 02/23/2011] [Accepted: 03/16/2011] [Indexed: 11/30/2022] Open
Abstract
To identify interaction sites we measured the rotational motion of a spin label covalently bound to the side chain of a cysteine genetically incorporated into rabbit skeletal muscle tropomyosin (Tm) at positions 13, 36, 146, 160, 174, 190, 209, 230, 271, and 279. Upon the addition of F-actin, the mobility of all the spin labels, especially at position 13, 271, or 279, of Tm was inhibited significantly. Slow spin-label motion at the C-terminus (at the 230th and 271st residues) was observed upon addition of troponin. The binding of myosin-head S1 fragments without troponin immobilized Tm residues at 146, 160, 190, 209, 230, 271, and 279, suggesting that these residues are involved in a direct interaction between Tm and actin in its open state. As immobilization occurred at substoichiometric amounts of S1 binding to actin (a 1:7 molar ratio), the structural changes induced by S1 binding to one actin subunit must have propagated and influenced interaction sites over seven actin subunits.
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Affiliation(s)
- Keisuke Ueda
- Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan
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54
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Zhang Z, Akhter S, Mottl S, Jin JP. Calcium-regulated conformational change in the C-terminal end segment of troponin I and its binding to tropomyosin. FEBS J 2011; 278:3348-59. [PMID: 21777381 DOI: 10.1111/j.1742-4658.2011.08250.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The troponin complex plays an essential role in the thin filament regulation of striated muscle contraction. Of the three subunits of troponin, troponin I (TnI) is the actomyosin ATPase inhibitory subunit and its effect is released upon Ca(2+) binding to troponin C. The exon-8-encoded C-terminal end segment represented by the last 24 amino acids of cardiac TnI is highly conserved and is critical to the inhibitory function of troponin. Here, we investigated the function and calcium regulation of the C-terminal end segment of TnI. A TnI model molecule was labeled with Alexa Fluor 532 at a Cys engineered at the C-terminal end and used to reconstitute the tertiary troponin complex. A Ca(2+) -regulated conformational change in the C-terminus of TnI was shown by a sigmoid-shape fluorescence intensity titration curve similar to that of the CD calcium titration curve of troponin C. Such corresponding Ca(2+) responses are consistent with the function of troponin as a coordinated molecular switch. Reconstituted troponin complex containing a mini-troponin T lacking its two tropomyosin-binding sites showed a saturable binding to tropomyosin at pCa 9 but not at pCa 4. This Ca(2+) -regulated binding was diminished when the C-terminal 19 amino acids of cardiac TnI were removed. These results provided novel evidence for suggesting that the C-terminal end segment of TnI participates in the Ca(2+) regulation of muscle thin filament through interaction with tropomyosin.
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Affiliation(s)
- Zhiling Zhang
- Evanston Northwestern Healthcare and Northwestern University, Evanston, IL, USA
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55
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Borovikov YS, Avrova SV, Karpicheva OE, Robinson P, Redwood CS. The effect of the dilated cardiomyopathy-causing Glu40Lys TPM1 mutation on actin-myosin interactions during the ATPase cycle. Biochem Biophys Res Commun 2011; 411:496-500. [PMID: 21741356 DOI: 10.1016/j.bbrc.2011.06.138] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2011] [Accepted: 06/20/2011] [Indexed: 10/18/2022]
Abstract
Dilated cardiomyopathy (DCM), characterized by cardiac dilatation and contractile dysfunction, is a major cause of heart failure. DCM can result from mutations in the gene encoding cardiac α-tropomyosin (TM). In order to understand how the dilated cardiomyopathy-causing Glu40Lys mutation in TM affects actomyosin interactions, thin filaments have been reconstituted in muscle ghost fibers by incorporation of labeled Cys707 of myosin subfragment-1 and Cys374 of actin with fluorescent probe 1.5-IAEDANS and α-tropomyosin (wild-type or Glu40Lys mutant). For the first time, the effect of these α-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. The Glu40Lys mutant TM inhibited these movements at the transition from AM(∗∗)·ADP·Pi to AM state, indicating a decrease of the proportion of the strong-binding sub-states in the actomyosin population. These structural changes are likely to underlie the contractile deficit observed in human dilated cardiomyopathy.
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Affiliation(s)
- Yurii S Borovikov
- Laboratory of Mechanisms of Cell Motility, Institute of Cytology, Russian Academy of Sciences, 4 Tikhoretsky Avenue, St. Petersburg 194064, Russia.
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56
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Rao VS, Clobes AM, Guilford WH. Force spectroscopy reveals multiple "closed states" of the muscle thin filament. J Biol Chem 2011; 286:24135-41. [PMID: 21597115 DOI: 10.1074/jbc.m110.167957] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
Tropomyosin (Tm) plays a critical role in regulating the contraction of striated muscle. The three-state model of activation posits that Tm exists in three positions on the thin filament: "blocked" in the absence of calcium when myosin cannot bind, "closed" when calcium binds troponin and Tm partially covers the myosin binding site, and "open" after myosin binding forces Tm completely off neighboring sites. However, we recently showed that actin filaments decorated with phosphorylated Tm are driven by myosin with greater force than bare actin filaments. This result cannot be explained by simple steric hindrance and suggests that Tm may have additional effects on actin-myosin interactions. We therefore tested the hypothesis that Tm and its phosphorylation state affect the rate at which single actin-myosin bonds form and rupture. Using a laser trap, we measured the time necessary for the first bond to form between actin and rigor heavy meromyosin and the load-dependent durations of those bonds. Measurements were repeated in the presence of subsaturating myosin-S1 to force Tm from the closed to the open state. Maximum bond lifetimes increased in the open state, but only when Tm was phosphorylated. While the frequency with which bonds formed was extremely low in the closed state, when a bond did form it took significantly less time to do so than with bare actin. These data suggest there are at least two closed states of the thin filament, and that Tm provides additional points of contact for myosin.
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Affiliation(s)
- Vijay S Rao
- Department of Biomedical Engineering, University of Virginia, Charlottesville, Virginia 22908, USA
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57
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Nevzorov IA, Nikolaeva OP, Kainov YA, Redwood CS, Levitsky DI. Conserved noncanonical residue Gly-126 confers instability to the middle part of the tropomyosin molecule. J Biol Chem 2011; 286:15766-72. [PMID: 21454502 DOI: 10.1074/jbc.m110.209353] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Tropomyosin (Tm) is a two-stranded α-helical coiled-coil protein with a well established role in regulation of actin cytoskeleton and muscle contraction. It is believed that many Tm functions are enabled by its flexibility whose nature has not been completely understood. We hypothesized that the well conserved non-canonical residue Gly-126 causes local destabilization of Tm. To test this, we substituted Gly-126 in skeletal muscle α-Tm either with an Ala residue, which should stabilize the Tm α-helix, or with an Arg residue, which is expected to stabilize both α-helix and coiled-coil structure of Tm. We have shown that both mutations dramatically reduce the rate of Tm proteolysis by trypsin at Asp-133. Differential scanning calorimetry was used for detailed investigation of thermal unfolding of the Tm mutants, both free in solution and bound to F-actin. It was shown that a significant part of wild type Tm unfolds in a non-cooperative manner at low temperature, and both mutations confer cooperativity to this part of the Tm molecule. The size of the flexible middle part of Tm is estimated to be 60-70 amino acid residues, about a quarter of the Tm molecule. Thus, our results show that flexibility is unevenly distributed in the Tm molecule and achieves the highest extent in its middle part. We conclude that the highly conserved Gly-126, acting in concert with the previously identified non-canonical Asp-137, destabilizes the middle part of Tm, resulting in a more flexible region that is important for Tm function.
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Affiliation(s)
- Ilya A Nevzorov
- AN Bach Institute of Biochemistry, Russian Academy of Sciences, Moscow, Russia
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58
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Farman GP, Allen EJ, Schoenfelt KQ, Backx PH, de Tombe PP. The role of thin filament cooperativity in cardiac length-dependent calcium activation. Biophys J 2011; 99:2978-86. [PMID: 21044595 DOI: 10.1016/j.bpj.2010.09.003] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2010] [Revised: 08/18/2010] [Accepted: 09/01/2010] [Indexed: 11/26/2022] Open
Abstract
Length-dependent activation (LDA) is a prominent feature of cardiac muscle characterized by decreases in the Ca(2+) levels required to generate force (i.e., increases in Ca(2+) sensitivity) when muscle is stretched. Previous studies have concluded that LDA originates from the increased ability of (strong) cross-bridges to attach when muscle is lengthened, which in turn enhances Ca(2+) binding to the troponin C (TnC) subunit of the troponin complex. However, our results demonstrate that inhibition of strong cross-bridge attachment with blebbistatin had no effect on the length-dependent modulation of Ca(2+) sensitivity (i.e., EC(50)) or Ca(2+) cooperativity, suggesting that LDA originates upstream of cross-bridge attachment. To test whether LDA arises from length dependence of thin-filament activation, we replaced native cTnC with a mutant cTnC (DM-TnC) that is incapable of binding Ca(2+). Although progressive replacement of native cTnC with DM-TnC caused an expected monotonic decrease in the maximal force (F(max)), DM-TnC incorporation induced much larger increases in EC(50) and decreases in Ca(2+) cooperativity at short lengths than at long lengths. These findings support the conclusion that LDA arises primarily from the influence of length on the modulation of the Ca(2+) cooperativity arising from interaction between adjacent troponin-tropomyosin complexes on the thin filament.
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Affiliation(s)
- Gerrie P Farman
- Center for Cardiovascular Research, Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, Illinois, USA
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59
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Solaro RJ, Kobayashi T. Protein phosphorylation and signal transduction in cardiac thin filaments. J Biol Chem 2011; 286:9935-40. [PMID: 21257760 DOI: 10.1074/jbc.r110.197731] [Citation(s) in RCA: 83] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Affiliation(s)
- R John Solaro
- Department of Physiology and Biophysics and Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, Illinois 60612, USA.
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60
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Kowlessur D, Tobacman LS. Low temperature dynamic mapping reveals unexpected order and disorder in troponin. J Biol Chem 2010; 285:38978-86. [PMID: 20889975 DOI: 10.1074/jbc.m110.181305] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Troponin is a pivotal regulatory protein that binds Ca(2+) reversibly to act as the muscle contraction on-off switch. To understand troponin function, the dynamic behavior of the Ca(2+)-saturated cardiac troponin core domain was mapped in detail at 10 °C, using H/D exchange-mass spectrometry. The low temperature conditions of the present study greatly enhanced the dynamic map compared with previous work. Approximately 70% of assessable peptide bond hydrogens were protected from exchange sufficiently for dynamic measurement. This allowed the first characterization by this method of many regions of regulatory importance. Most of the TnI COOH terminus was protected from H/D exchange, implying an intrinsically folded structure. This region is critical to the troponin inhibitory function and has been implicated in thin filament activation. Other new findings include unprotected behavior, suggesting high mobility, for the residues linking the two domains of TnC, as well as for the inhibitory peptide residues preceding the TnI switch helix. These data indicate that, in solution, the regulatory subdomain of cardiac troponin is mobile relative to the remainder of troponin. Relatively dynamic properties were observed for the interacting TnI switch helix and TnC NH(2)-domain, contrasting with stable, highly protected properties for the interacting TnI helix 1 and TnC COOH-domain. Overall, exchange protection via protein folding was relatively weak or for a majority of peptide bond hydrogens. Several regions of TnT and TnI were unfolded even at low temperature, suggesting intrinsic disorder. Finally, change in temperature prominently altered local folding stability, suggesting that troponin is an unusually mobile protein under physiological conditions.
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Affiliation(s)
- Devanand Kowlessur
- Department of Medicine, University of Illinois at Chicago, Chicago, Illinois 60612, USA
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61
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Shchepkin DV, Kopylova GV, Nikitina LV, Katsnelson LB, Bershitsky SY. Effects of cardiac myosin binding protein-C on the regulation of interaction of cardiac myosin with thin filament in an in vitro motility assay. Biochem Biophys Res Commun 2010; 401:159-63. [PMID: 20849827 DOI: 10.1016/j.bbrc.2010.09.040] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2010] [Accepted: 09/09/2010] [Indexed: 11/24/2022]
Abstract
Modulatory role of whole cardiac myosin binding protein-C (сMyBP-C) in regulation of cardiac muscle contractility was studied in the in vitro motility assay with rabbit cardiac myosin as a motor protein. The effects of cMyBP-C on the interaction of cardiac myosin with regulated thin filament were tested in both in vitro motility and ATPase assays. We demonstrate that the addition of cMyBP-C increases calcium regulated Mg-ATPase activity of cardiac myosin at submaximal calcium. The Hill coefficient for 'pCa-velocity' relation in the in vitro motility assay decreased and the calcium sensitivity increased when сMyBP-C was added. Results of our experiments testifies in favor of the hypothesis that сMyBP-C slows down cross-bridge kinetics when binding to actin.
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Affiliation(s)
- D V Shchepkin
- Institute of Immunology and Physiology, Russian Academy of Sciences, Yekaterinburg 620041, Russia.
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62
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Houmeida A, Heeley DH, Belknap B, White HD. Mechanism of regulation of native cardiac muscle thin filaments by rigor cardiac myosin-S1 and calcium. J Biol Chem 2010; 285:32760-32769. [PMID: 20696756 DOI: 10.1074/jbc.m109.098228] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have studied the mechanism of activation of native cardiac thin filaments by calcium and rigor myosin. The acceleration of the rate of 2'-deoxy-3'-O-(N-methylanthraniloyl)ADP (mdADP) dissociation from cardiac myosin-S1-mdADP-P(i) and cardiac myosin-S1-mdADP by native cardiac muscle thin filaments was measured using double mixing stopped-flow fluorescence. Relative to inhibited thin filaments (no bound calcium or rigor S1), fully activated thin filaments (with both calcium and rigor-S1 bound) increase the rate of product dissociation from the physiologically important pre-power stroke myosin-mdADP-P(i) by a factor of ∼75. This can be compared with only an ∼6-fold increase in the rate of nucleotide diphosphate dissociation from nonphysiological myosin-mdADP by the fully activated thin filaments relative to the fully inhibited thin filaments. These results show that physiological levels of regulation are not only dependent on the state of the thin filament but also on the conformation of the myosin. Less than 2-fold regulation is due to a change in affinity of myosin-ADP-P(i) for thin filaments such as would be expected by a simple "steric blocking" of the myosin-binding site of the thin filament by tropomyosin. Although maximal activation requires both calcium and rigor myosin-S1 bound to the cardiac filament, association with a single ligand produces ∼70% maximal activation. This can be contrasted with skeletal thin filaments in which calcium alone only activated the rate of product dissociation ∼20% of maximum, and rigor myosin produces ∼30% maximal activation.
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Affiliation(s)
- Ahmed Houmeida
- From the Department of Biochemistry, University of Nouakchott, Nouakchott 5026, Mauritania
| | - David H Heeley
- Department of Biochemistry, Memorial University, St. Johns, Newfoundland A1B 3X9, Canada
| | - Betty Belknap
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, Virginia 23507
| | - Howard D White
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, Virginia 23507.
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63
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Avrova SV, Shelud'ko NS, Borovikov YS. A new property of twitchin to restrict the "rolling" of mussel tropomyosin and decrease its affinity for actin during the actomyosin ATPase cycle. Biochem Biophys Res Commun 2010; 394:126-9. [PMID: 20184863 DOI: 10.1016/j.bbrc.2010.02.128] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2010] [Accepted: 02/19/2010] [Indexed: 11/18/2022]
Abstract
A new evidence on the regulatory function of twitchin, a titin-like protein of molluscan muscles, at muscle contraction has been obtained at studying the movements of IAF-labeled mussel tropomyosin in skeletal ghost fibers during the ATP hydrolysis cycle simulated using nucleotides and non-hydrolysable ATP analogs. For the first time, myosin-induced multistep changes in mobility and in the position of mussel tropomyosin strands on the surface of the thin filament during the ATP hydrolysis cycle have been demonstrated directly. Unphosphorylated twitchin shifts the tropomyosin towards the position typical for muscle relaxation, decreases the tropomyosin affinity to actin and inhibits its movements during the ATPase cycle. Phosphorylation of twitchin by the catalytic subunit of protein kinase A reverses this effect. These data imply that twitchin is a thin filament regulator that controls actin-myosin interaction by "freezing" tropomyosin in the blocked position, resulting in the inhibition of the transformation of weak-binding states into strong-binding ones during ATPase cycle.
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Affiliation(s)
- Stanislava V Avrova
- Laboratory of Mechanisms of Cell Motility, Institute of Cytology, Russian Academy of Sciences, 4 Tikhoretsky Avenue, St Petersburg 194064, Russia
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64
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Ozawa H, Watabe S, Ochiai Y. Thermostability of striated and smooth adductor muscle tropomyosins from Yesso scallop Mizuhopecten yessoensis. J Biochem 2010; 147:823-32. [DOI: 10.1093/jb/mvq018] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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65
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Mijailovich SM, Li X, del Alamo JC, Griffiths RH, Kecman V, Geeves MA. Resolution and uniqueness of estimated parameters of a model of thin filament regulation in solution. Comput Biol Chem 2010; 34:19-33. [PMID: 20060364 PMCID: PMC6089521 DOI: 10.1016/j.compbiolchem.2009.11.002] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2009] [Revised: 11/24/2009] [Accepted: 11/24/2009] [Indexed: 11/30/2022]
Abstract
The estimation of chemical kinetic rate constants for any non-trivial model is complex due to the nonlinear effects of second order chemical reactions. We developed an algorithm to accomplish this goal based on the Damped Least Squares (DLS) inversion method and then tested the effectiveness of this method on the McKillop-Geeves (MG) model of thin filament regulation. The kinetics of MG model is defined by a set of nonlinear ordinary differential equations (ODEs) that predict the evolution of troponin-tropomyosin-actin and actin-myosin states. The values of the rate constants are estimated by integrating these ODEs numerically and fitting them to a series of stopped-flow pyrene fluorescence transients of myosin-S1 fragment binding to regulated actin in solution. The accuracy and robustness of the estimated rate constants are evaluated for DLS and two other methods, namely quasi-Newton (QN) and simulated annealing (SA). The comparison of these methods revealed that SA provides the best estimates of the model parameters because of its global optimization scheme. However it converges slowly and does quantify the uniqueness of the estimated parameters. On the other hand the QN method converges rapidly but only if the initial guess of the parameters is close to the optimum values, otherwise it diverges. Overall, the DLS method proves to be the most convenient method. It converges fast and was able to provide excellent estimates of kinetic parameters. Furthermore, DLS provides the model resolution matrix, which quantifies the interdependence of model parameters thereby evaluating the uniqueness of their estimated values. This property is essential for estimating of the dependence of the model parameters on experimental conditions (e.g. Ca(2+) concentration) when it is assessed from noisy experimental data such as pyrene fluorescence from stopped-flow transients. The advantages of the DLS method observed in this study should be further examined in other physicochemical systems to firmly establish the observed effectiveness of DSL vs. the other parameter estimation methods.
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Affiliation(s)
- Srboljub M Mijailovich
- Molecular and Integrative Physiological Sciences, Dept. of Environmental Health, Harvard School of Public Health, Boston, MA 02115, USA.
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66
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Myofilament length dependent activation. J Mol Cell Cardiol 2010; 48:851-8. [PMID: 20053351 DOI: 10.1016/j.yjmcc.2009.12.017] [Citation(s) in RCA: 214] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/18/2009] [Revised: 12/18/2009] [Accepted: 12/22/2009] [Indexed: 01/04/2023]
Abstract
The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca(2+) ions at a longer sarcomere length, commonly referred to as myofilament length-dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development.
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67
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Willott RH, Gomes AV, Chang AN, Parvatiyar MS, Pinto JR, Potter JD. Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function? J Mol Cell Cardiol 2009; 48:882-92. [PMID: 19914256 DOI: 10.1016/j.yjmcc.2009.10.031] [Citation(s) in RCA: 154] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/21/2009] [Revised: 10/19/2009] [Accepted: 10/30/2009] [Indexed: 12/25/2022]
Abstract
Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.
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Affiliation(s)
- Ruth H Willott
- Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL 33136, USA
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68
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Yang Z, Yamazaki M, Shen QW, Swartz DR. Differences between cardiac and skeletal troponin interaction with the thin filament probed by troponin exchange in skeletal myofibrils. Biophys J 2009; 97:183-94. [PMID: 19580756 DOI: 10.1016/j.bpj.2009.04.023] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2008] [Revised: 04/11/2009] [Accepted: 04/14/2009] [Indexed: 10/20/2022] Open
Abstract
Troponin (Tn) is the calcium-sensing protein of the thin filament. Although cardiac troponin (cTn) and skeletal troponin (sTn) accomplish the same function, their subunit interactions within Tn and with actin-tropomyosin are different. To further characterize these differences, myofibril ATPase activity as a function of pCa and labeled Tn exchange in rigor myofibrils was used to estimate Tn dissociation rates from the nonoverlap and overlap region as a function of pCa. Measurement of ATPase activity showed that skeletal myofibrils containing >96% cTn had a higher pCa 9 ATPase activity than, but similar pCa 4 activity to, sTn-containing myofibrils. Analysis of the pCa-ATPase activity relation showed that cTn myofibrils were more calcium sensitive but less cooperative (pCa50 = 6.14, nH = 1.46) than sTn myofibrils (pCa50= 5.90, nH = 3.36). The time course of labeled Tn exchange at pCa 9 and 4 were quite different between cTn and sTn. The apparent cTn dissociation rates were approximately 2-10-fold faster than sTn under all the conditions studied. The apparent dissociation rates for cTn were 5 x 10(-3) min(-1), 150 x 10(-3) min(-1), and 260 x 10(-3) min(-1), whereas for sTn they were 0.6 x 10(-3) min(-1), 88 x 10(-3) min(-1), and 68 x 10(-3) min(-1) for the nonoverlap region at pCa 9, nonoverlap region at pCa 4, and overlap region at pCa 4, respectively. Normalization of the apparent dissociation rates gives 1:30:50 for cTn compared with 1:150:110 for sTn (nonoverlap at pCa 9:nonoverlap at pCa 4:overlap at pCa 4) suggesting that calcium has a smaller influence, whereas strong cross-bridges have a larger influence on cTn dissociation compared with sTn. The higher cTn dissociation rate in the nonoverlap region and ATPase activity at pCa 9 suggest that it gives a less off or inactive thin filament. Analysis of the intensity ratio (after a short time of exchange) as a function of pCa showed that cTn had greater calcium sensitivity but lower cooperativity than sTn. In addition, the magnitude of the change in intensity ratio going from pCa 9 to 4 was less for cTn than sTn. These data suggest that the influence of calcium on cTn exchange is less than sTn even though calcium can activate ATPase activity to a similar extent in cTn compared with sTn myofibrils. This may be explained partially by cTn being less off or inactive at pCa 9. Modeling of the intensity profiles obtained after Tn exchange at pCa 5.8 suggest that the profiles are best explained by a model that includes a long-range cross-bridge effect that grades with distance from the rigor cross-bridge for both cTn and sTn.
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Affiliation(s)
- Zhenyun Yang
- Herman B. Wells Center for Pediatric Research, Department of Pediatrics, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA
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69
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Borovikov YS, Karpicheva OE, Avrova SV, Robinson P, Redwood CS. The effect of the dilated cardiomyopathy-causing mutation Glu54Lys of alpha-tropomyosin on actin-myosin interactions during the ATPase cycle. Arch Biochem Biophys 2009; 489:20-4. [PMID: 19646950 DOI: 10.1016/j.abb.2009.07.018] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2009] [Revised: 07/22/2009] [Accepted: 07/25/2009] [Indexed: 10/20/2022]
Abstract
In order to understand how the Glu54Lys mutation of alpha-tropomyosin affects actomyosin interactions, we labeled SH1 helix of myosin subfragment-1 (S1) and the actin subdomain-1 with fluorescent probes. These proteins were incorporated into ghost muscle fibers and their conformational states were monitored during the ATPase cycle by measuring polarized fluorescence. The addition of wild-type alpha-tropomyosin to actin filaments increases the amplitude of the SH1 helix and subdomain-1 movements during the ATPase cycle, indicating the enhancement of the efficiency of work of each cross-bridge. The Glu54Lys mutation inhibits this effect. The Glu54Lys mutation also results in the coupling of the weak-binding sub-state of S1 to the strong-binding sub-state of actin thus altering the concerted conformational changes during the ATPase cycle. We suggest that these alterations will result in reduced force production, which is likely to underlie at least in part the contractile deficit observed in human dilated cardiomyopathy.
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Affiliation(s)
- Yurii S Borovikov
- Laboratory of Mechanisms of Cell Motility, Institute of Cytology, Russian Academy of Sciences, 4 Tikhoretsky Avenue, St. Petersburg 194064, Russia.
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70
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Rao VS, Marongelli EN, Guilford WH. Phosphorylation of tropomyosin extends cooperative binding of myosin beyond a single regulatory unit. ACTA ACUST UNITED AC 2009; 66:10-23. [PMID: 18985725 DOI: 10.1002/cm.20321] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Tropomyosin (Tm) is one of the major phosphoproteins comprising the thin filament of muscle. However, the specific role of Tm phosphorylation in modulating the mechanics of actomyosin interaction has not been determined. Here we show that Tm phosphorylation is necessary for long-range cooperative activation of myosin binding. We used a novel optical trapping assay to measure the isometric stall force of an ensemble of myosin molecules moving actin filaments reconstituted with either natively phosphorylated or dephosphorylated Tm. The data show that the thin filament is cooperatively activated by myosin across regulatory units when Tm is phosphorylated. When Tm is dephosphorylated, this "long-range" cooperative activation is lost and the filament behaves identically to bare actin filaments. However, these effects are not due to dissociation of dephosphorylated Tm from the reconstituted thin filament. The data suggest that end-to-end interactions of adjacent Tm molecules are strengthened when Tm is phosphorylated, and that phosphorylation is thus essential for long range cooperative activation along the thin filament.
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Affiliation(s)
- Vijay S Rao
- Department of Biomedical Engineering, University of Virginia, Charlottesville, VA 22908, USA
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71
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Alencar AM, Butler JP, Mijailovich SM. Thermodynamic origin of cooperativity in actomyosin interactions: the coupling of short-range interactions with actin bending stiffness in an Ising-like model. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2009; 79:041906. [PMID: 19518255 DOI: 10.1103/physreve.79.041906] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/16/2008] [Revised: 02/16/2009] [Indexed: 05/27/2023]
Abstract
We present Monte Carlo simulations for a molecular motor system found in virtually all eukaryotic cells, the acto-myosin motor system, composed of a group of organic macromolecules. Cell motors were mapped to an Ising-like model, where the interaction field is transmitted through a tropomyosin polymer chain. The presence of Ca2+ induces tropomyosin to block or unblock binding sites of the myosin motor leading to its activation or deactivation. We used the Metropolis algorithm to find the transient and the equilibrium states of the acto-myosin system composed of solvent, actin, tropomyosin, troponin, Ca2+, and myosin-S1 at a given temperature, including the spatial configuration of tropomyosin on the actin filament surface. Our model describes the short- and long-range cooperativity during actin-myosin binding which emerges from the bending stiffness of the tropomyosin complex. We found all transition rates between the states only using the interaction energy of the constituents. The agreement between our model and experimental data also supports the recent theory of flexible tropomyosin.
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Affiliation(s)
- Adriano M Alencar
- Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts 02115, USA
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72
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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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73
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Patel DA, Root DD. Close proximity of myosin loop 3 to troponin determined by triangulation of resonance energy transfer distance measurements. Biochemistry 2009; 48:357-69. [PMID: 19108638 DOI: 10.1021/bi801554m] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Cooperative activation of the thin filament is known to be influenced by the tight binding of myosin to actin, but the molecular mechanism underlying this contribution of myosin is not well understood. To better understand the structural relationship of myosin with the regulatory troponin complex, resonance energy transfer measurements were used to map the location of troponin relative to a neighboring myosin bound to actin using atomic models. Using a chicken troponin T isoform that contains a single cysteine near the binding interface between troponins T, I, and C, this uniquely labeled cysteine on troponin was found to be remarkably near loop 3 of myosin. This loop has previously been localized near the actin and myosin interface by chemical cross-linking methods, but its functional contributions have not been established. The implications of this close proximity are examined by molecular modeling, which suggests that only restricted conformations of actomyosin can accommodate the presence of troponin at this location near the cross-bridge. This potential for interaction between troponin and myosin heads that bind near it along the thin filament raises the possibility of models in which direct myosin and troponin interactions may play a role in the regulatory mechanism.
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Affiliation(s)
- Dipesh A Patel
- University of North Texas, P.O. Box 305220, Denton, Texas 76203-5220, USA
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74
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Xing J, Chinnaraj M, Zhang Z, Cheung HC, Dong WJ. Structural studies of interactions between cardiac troponin I and actin in regulated thin filament using Förster resonance energy transfer. Biochemistry 2009; 47:13383-93. [PMID: 19053249 DOI: 10.1021/bi801492x] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The Ca(2+)-induced interaction between cardiac troponin I (cTnI) and actin plays a key role in the regulation of cardiac muscle contraction and relaxation. In this report we have investigated changes of this interaction in response to strong cross-bridge formation between myosin S1 and actin and PKA phosphorylation of cTnI within reconstituted thin filament. The interaction was monitored by measuring Förster resonance energy transfer (FRET) between the fluorescent donor 5-(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid (AEDANS) attached to the residues 131, 151, 160 167, 188, and 210 of cTnI and the nonfluorescent acceptor 4-(dimethylamino)phenylazophenyl-4'-maleimide (DABM) attached to cysteine 374 of actin. The FRET distance measurements showed that bound Ca(2+) induced large increases in the distances from actin to the cTnI sites, indicating a Ca(2+)-triggered separation of cTnI from actin. Strongly bound myosin S1 induced additional increases in these distances in the presence of bound Ca(2+). The two ligand-induced increases were independent of each other. These two-step changes in distances provide a direct link of structural changes at the interface between cTnI and actin to the three-state model of thin filament regulation of muscle contraction and relaxation. When cTnC was inactivated through mutations of key residues within the 12-residue Ca(2+)-binding loop, strongly bound S1 alone induced increases in the distances in spite of the fact that the filaments no longer bound regulatory Ca(2+). These results suggest bound Ca(2+) or strongly bound S1 alone can partially activate thin filament, but full activation requires both bound Ca(2+) and strongly bound S1. The distributions of the FRET distances revealed different structural dynamics associated with different regions of cTnI in different biochemical states. The second actin-binding region appears more rigid than the inhibitory/regulatory region. In the Mg(2+) state, the regulatory region appears more flexible than the inhibitory region, and in the Ca(2+) state the inhibitory region becomes more flexible. PKA phosphorylation of cTnI at Ser23 and Ser24 distance from actin to cTnI residue 131 by 2.2-5.2 A in different biochemical states and narrowed the distributions of the distances from actin to the inhibitory and regulatory regions of cTnI. The observed phosphorylation effects are likely due to an intramolecular interaction of the phosphorylated N-terminal segment and the inhibitory region of cTnI.
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Affiliation(s)
- Jun Xing
- Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, Alabama 354294, USA
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75
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Modulation of the effects of tropomyosin on actin and myosin conformational changes by troponin and Ca2+. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2008; 1794:985-94. [PMID: 19100866 DOI: 10.1016/j.bbapap.2008.11.014] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/23/2008] [Revised: 10/09/2008] [Accepted: 11/05/2008] [Indexed: 11/24/2022]
Abstract
The molecular mechanisms by which troponin (TN)-tropomyosin (TM) regulates the myosin ATPase cycle were investigated using fluorescent probes specifically bound to Cys36 of TM, Cys707 of myosin subfragment-1, and Cys374 of actin incorporated into ghost muscle fibers. Intermediate states of actomyosin were simulated by using nucleotides and non-hydrolysable ATP analogs. Multistep changes in mobility and spatial arrangement of SH1 helix of myosin motor domain and actin subdomain-1 during the ATPase cycle were observed. Each intermediate state of actomyosin induced a definite conformational state and specific position of TM strands on the surface of thin filament. TM increased the amplitude of myosin SH1 helix and actin subdomain-1 movements at transition from weak- to strong-binding states shifting to the center of thin filament at strong-binding and to the periphery of thin filament at weak-binding states. TN modulated those movements in a capital ES, Cyrillicsmall a, Cyrillic(2+)-dependent manner. At high-Ca(2+), TN enhanced the effect of TM on SH1 helix and subdomain-1 movements by transferring TM further to the center of thin filament at strong-binding states. In contrast, at low-Ca(2+), TN inhibited the effect of TM movements, "freezing" actin structure in "OFF" state and TM in the position typical for weak-binding states, resulting in disturbing the interplay of actin and myosin.
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76
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Wang H, Mao S, Chalovich JM, Marriott G. Tropomyosin dynamics in cardiac thin filaments: a multisite forster resonance energy transfer and anisotropy study. Biophys J 2008; 94:4358-69. [PMID: 18310249 PMCID: PMC2480674 DOI: 10.1529/biophysj.107.121129] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2007] [Accepted: 01/15/2008] [Indexed: 11/18/2022] Open
Abstract
Cryoelectron microscopy studies have identified distinct locations of tropomyosin (Tm) within the Ca(2+)-free, Ca(2+)-saturated, and myosin-S1-saturated states of the thin filament. On the other hand, steady-state Förster resonance energy transfer (FRET) studies using functional, reconstituted thin filaments under physiological conditions of temperature and solvent have failed to detect any movement of Tm upon Ca(2+) binding. In this investigation, an optimized system for FRET and anisotropy analyses of cardiac tropomyosin (cTm) dynamics was developed that employed a single tethered donor probe within a Tm dimer. Multisite FRET and fluorescence anisotropy analyses showed that S1 binding to Ca(2+) thin filaments triggered a uniform displacement of cTm toward F-actin but that Ca(2+) binding alone did not change FRET efficiency, most likely due to thermally driven fluctuations of cTm on the thin filament that decreased the effective separation of the donor probe between the blocked and closed states. Although Ca(2+) binding to the thin filament did not significantly change FRET efficiency, such a change was demonstrated when the thin filament was partially saturated with S1. FRET was also used to show that stoichiometric binding of S1 to Ca(2+)-activated thin filaments decreased the amplitude of Tm fluctuations and revealed a strong correlation between the cooperative binding of S1 to the closed state and the movement of cTm.
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Affiliation(s)
- Hui Wang
- Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706, USA
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77
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Reece KL, Moss RL. Intramolecular interactions in the N-domain of cardiac troponin C are important determinants of calcium sensitivity of force development. Biochemistry 2008; 47:5139-46. [PMID: 18410130 DOI: 10.1021/bi800164c] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Myocardial contraction is initiated when Ca2+ binds to site II of cardiac troponin C. This 12-residue EF-hand loop (NH2-DEDGSGTVDFDE-COOH) contains six residues (bold) that coordinate Ca2+ binding and six residues that do not appear to influence Ca2+ binding directly. We have introduced six single-cysteine substitutions (italics) within site II of cTnC to investigate whether these residues are essential for Ca2+ binding affinity in isolation and Ca2+ sensitivity of force development in single muscle fibers. Ca2+ binding properties of mutant proteins were examined in solution and after substitution into rat skinned soleus fibers. Except for the serine mutation, cysteine substitution had no effect on Ca2+ binding on cTnC in solution. However, as part of the myofilament, the threonine mutation reduced Ca2+ sensitivity while the phenylalanine mutation increased Ca2+ sensitivity. Analysis of the available crystal and NMR structures reveals specific structural mechanisms for these effects.
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Affiliation(s)
- Karen L Reece
- Department of Physiology, UniVersity of Wisconsin School of Medicine and Public Health, 123 Service Memorial Institute, 1300 University Avenue, Madison, Wisconsin 53706, USA
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78
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The unit event of sliding of the chemo-mechanical enzyme composed of myosin and actin with regulatory proteins. Biochem Biophys Res Commun 2008; 369:144-8. [DOI: 10.1016/j.bbrc.2007.11.186] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2007] [Accepted: 11/15/2007] [Indexed: 11/18/2022]
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79
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Abstract
This chapter reviews some of the many available methods for measuring the binding of myosin and other proteins to actin. Binding to actin has special considerations because actin is a long lattice and the binding site of many of its binding partners consists of multiple actin protomers. The analysis of binding to a lattice cannot be done by standard methods such as a Scatchard plot. Rational methods of analysis are described.
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80
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Ostap EM. Tropomyosins as discriminators of myosin function. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2008; 644:273-82. [PMID: 19209828 DOI: 10.1007/978-0-387-85766-4_20] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Vertebrate nonmuscle cells express multiple tropomyosin isoforms that are sorted to subcellular compartments that have distinct morphological and dynamic properties. The creation of these compartments has a role in controlling cell morphology, cell migration and polarization of cellular components. There is increasing evidence that nonmuscle myosins are regulated by tropomyosin in these compartments via the regulation of actin attachment, ATPase kinetics, or by stabilization of cytoskeletal tracks for myosin-based transport. In this chapter, I review the literature describing the regulation of various myosins by tropomyosins and consider the mechanisms for this regulation.
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Affiliation(s)
- E Michael Ostap
- Department of Physiology, University of Pennsylvania School of Medicine, B400 Richards Building, Philadelphia, PA 19104-6085, USA.
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81
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Maytum R, Hatch V, Konrad M, Lehman W, Geeves MA. Ultra Short Yeast Tropomyosins Show Novel Myosin Regulation. J Biol Chem 2008; 283:1902-10. [DOI: 10.1074/jbc.m708593200] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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82
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Skoumpla K, Coulton AT, Lehman W, Geeves MA, Mulvihill DP. Acetylation regulates tropomyosin function in the fission yeast Schizosaccharomyces pombe. J Cell Sci 2007; 120:1635-45. [PMID: 17452625 DOI: 10.1242/jcs.001115] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022] Open
Abstract
Tropomyosin is an evolutionarily conserved alpha-helical coiled-coil protein that promotes and maintains actin filaments. In yeast, Tropomyosin-stabilised filaments are used by molecular motors to transport cargoes or to generate motile forces by altering the dynamics of filament growth and shrinkage. The Schizosaccharomyces pombe tropomyosin Cdc8 localises to the cytokinetic actomyosin ring during mitosis and is absolutely required for its formation and function. We show that Cdc8 associates with actin filaments throughout the cell cycle and is subjected to post-translational modification that does not vary with cell cycle progression. At any given point in the cell cycle 80% of Cdc8 molecules are acetylated, which significantly enhances their affinity for actin. Reconstructions of electron microscopic images of actin-Cdc8 filaments establish that the majority of Cdc8 strands sit in the 'closed' position on actin filaments, suggesting a role in the regulation of myosin binding. We show that Cdc8 regulates the equilibrium binding of myosin to actin without affecting the rate of myosin binding. Unacetylated Cdc8 isoforms bind actin, but have a reduced ability to regulate myosin binding to actin. We conclude that although acetylation of Cdc8 is not essential, it provides a regulatory mechanism for modulating actin filament integrity and myosin function.
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Affiliation(s)
- Kalomoira Skoumpla
- Cell and Developmental Biology Group, Department of Biosciences, University of Kent at Canterbury, Canterbury, CT2 7NJ, UK
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83
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Boussouf SE, Geeves MA. Tropomyosin and troponin cooperativity on the thin filament. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2007; 592:99-109. [PMID: 17278359 DOI: 10.1007/978-4-431-38453-3_10] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
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84
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Mirza M, Robinson P, Kremneva E, Copeland O, Nikolaeva O, Watkins H, Levitsky D, Redwood C, El-Mezgueldi M, Marston S. The Effect of Mutations in α-Tropomyosin (E40K and E54K) That Cause Familial Dilated Cardiomyopathy on the Regulatory Mechanism of Cardiac Muscle Thin Filaments. J Biol Chem 2007; 282:13487-97. [PMID: 17360712 DOI: 10.1074/jbc.m701071200] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
E40K and E54K mutations in alpha-tropomyosin cause inherited dilated cardiomyopathy. Previously we showed, using Ala-Ser alpha-tropomyosin (AS-alpha-Tm) expressed in Escherichia coli, that both mutations decrease Ca(2+) sensitivity. E40K also reduces V(max) of actin-Tm-activated S-1 ATPase by 18%. We investigated cooperative allosteric regulation by native Tm, AS-alpha-Tm, and the two dilated cardiomyopathy-causing mutants. AS-alpha-Tm has a lower cooperative unit size (6.5) than native alpha-tropomyosin (10.0). The E40K mutation reduced the size of the cooperative unit to 3.7, whereas E54K increased it to 8.0. For the equilibrium between On and Off states, the K(T) value was the same for all actin-Tm species; however, the K(T) value of actin-Tm-troponin at pCa 5 was 50% less for AS-alpha-Tm E40K than for AS-alpha-Tm and AS-alpha-Tm E54K. K(b), the "closed" to "blocked" equilibrium constant, was the same for all tropomyosin species. The E40K mutation reduced the affinity of tropomyosin for actin by 1.74-fold, but only when in the On state (in the presence of S-1). In contrast the E54K mutation reduced affinity by 3.5-fold only in the Off state. Differential scanning calorimetry measurements of AS-alpha-Tm showed that domain 3, assigned to the N terminus of tropomyosin, was strongly destabilized by both mutations. Additionally with AS-alpha-Tm E54K, we observed a unique new domain at 55 degrees C accounting for 25% of enthalpy indicating stabilization of part of the tropomyosin. The disease-causing mechanism of the E40K mutation may be accounted for by destabilization of the On state of the thin filaments; however, the E54K mutation has a more complex effect on tropomyosin structure and function.
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Affiliation(s)
- Mahmooda Mirza
- National Heart and Lung Institute, Imperial College London, London SW3 6LY, United Kingdom
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85
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de Tombe PP, Belus A, Piroddi N, Scellini B, Walker JS, Martin AF, Tesi C, Poggesi C. Myofilament calcium sensitivity does not affect cross-bridge activation-relaxation kinetics. Am J Physiol Regul Integr Comp Physiol 2007; 292:R1129-36. [PMID: 17082350 DOI: 10.1152/ajpregu.00630.2006] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We employed single myofibril techniques to test whether the presence of slow skeletal troponin-I (ssTnI) is sufficient to induce increased myofilament calcium sensitivity (EC50) and whether modulation of EC50 affects the dynamics of force development. Studies were performed using rabbit psoas myofibrils activated by rapid solution switch and in which Tn was partially replaced for either recombinant cardiac Tn(cTn) or Tn composed of recombinant cTn-T (cTnT) and cTn-C (cTnC), and recombinant ssTnI (ssTnI-chimera Tn). Tn exchange was performed in rigor solution (0.5 mg/ml Tn; 20°C; 2 h) and confirmed by SDS-PAGE. cTnI exchange induced a decrease in EC50; ssTnI-chimera Tn exchange induced a further decrease in EC50 (in μM: endogenous Tn, 1.35 ± 0.08; cTnI, 1.04 ± 0.13; ssTnI-chimera Tn, 0.47 ± 0.03). EC50 was also decreased by application of 100 μM bepridil (control: 2.04 ± 0.03 μM; bepridil 1.35 ± 0.03 μM). Maximum tension was not different between any groups. Despite marked alterations in EC50, none of the dynamic activation-relaxation parameters were affected under any condition. Our results show that 1) incorporation of ssTnI into the fast skeletal sarcomere is sufficient to induce increased myofilament Ca2+ sensitivity, and 2) the dynamics of actin-myosin interaction do not correlate with EC50. This result suggests that intrinsic cross-bridge cycling rate is not altered by the dynamics of thin-filament activation.
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Affiliation(s)
- Pieter P de Tombe
- Center for Cardiovascular Research, Univ. of Illinois at Chicago, 835 S. Wolcott Avenue, MC901, Chicago IL 60612, USA.
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86
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Robinson P, Lipscomb S, Preston LC, Altin E, Watkins H, Ashley CC, Redwood CS. Mutations in fast skeletal troponin I, troponin T, and β‐tropomyosin that cause distal arthrogryposis all increase contractile function. FASEB J 2006; 21:896-905. [PMID: 17194691 DOI: 10.1096/fj.06-6899com] [Citation(s) in RCA: 81] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Distal arthrogryposes (DAs) are a group of disorders characterized by congenital contractures of distal limbs without overt neurological or muscle disease. Unexpectedly, mutations in genes encoding the fast skeletal muscle regulatory proteins troponin T (TnT), troponin I (TnI), and beta-tropomyosin (beta-TM) have been shown to cause autosomal dominant DA. We tested how these mutations affect contractile function by comparing wild-type (WT) and mutant proteins in actomyosin ATPase assays and in troponin-replaced rabbit psoas fibers. We have analyzed all four reported mutants: Arg63His TnT, Arg91Gly beta-TM, Arg174Gln TnI, and a TnI truncation mutant (Arg156ter). Thin filaments, reconstituted using actin and WT troponin and beta-TM, activated myosin subfragment-1 ATPase in a calcium-dependent, cooperative manner. Thin filaments containing either a troponin or beta-TM DA mutant produced significantly enhanced ATPase rates at all calcium concentrations without alternating calcium-sensitivity or cooperativity. In troponin-exchanged skinned fibers, each mutant caused a significant increase in Ca2+ sensitivity, and Arg156ter TnI generated significantly higher maximum force. Arg91Gly beta-TM was found to have a lower actin affinity than WT and form a less stable coiled coil. We propose the mutations cause increased contractility of developing fast-twitch skeletal muscles, thus causing muscle contractures and the development of the observed limb deformities.
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Affiliation(s)
- Paul Robinson
- Department of Cardiovascular Medicine, University of Oxford, Oxford OX3 7BN, UK
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87
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Coulton A, Lehrer SS, Geeves MA. Functional homodimers and heterodimers of recombinant smooth muscle tropomyosin. Biochemistry 2006; 45:12853-8. [PMID: 17042503 PMCID: PMC2546869 DOI: 10.1021/bi0613224] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Skeletal and smooth muscle tropomyosin (Tm) require acetylation of their N-termini to bind strongly to actin. Tm containing an N-terminal alanine-serine (AS) extension to mimic acetylation has been widely used to increase binding. The current study investigates the ability of an N-terminal AS extension to mimic native acetylation for both alpha alpha and beta beta smooth Tm homodimers. We show that (1) AS alpha-Tm binds actin 100-fold tighter than alpha-Tm and 2-fold tighter than native smooth alphabeta-Tm, (2) beta-Tm requires an AS extension to bind actin, and (3) AS beta-Tm binds actin 10-fold weaker than AS alpha-Tm. Tm is present in smooth muscle tissues as >95% heterodimer; therefore, we studied the binding of recombinant alphabeta heterodimers with different AS extensions. This study shows that recombinant Tm requires an AS extension on both alpha and beta chains to bind like native Tm and that the alpha chain contributes more to actin binding than the beta chain. Once assembled onto an actin filament, all smooth muscle Tm's regulate S1 binding to actin Tm in the same way, irrespective of the presence of an AS extension.
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Affiliation(s)
- Arthur Coulton
- Department of Biosciences, University of Kent, Canterbury, CT2 7NY, UK
| | - Sherwin S. Lehrer
- Muscle and Motility Group, Boston Biomedical Research Institute, Watertown, MA, 02472-2829 U.S.A
| | - Michael A. Geeves
- Department of Biosciences, University of Kent, Canterbury, CT2 7NY, UK
- Corresponding author: Michael A Geeves Department of Biosciences, University of Kent, Canterbury, CT2 7NY, UK E-mail: , Tel +44 1227 827597, Fax +44 1227 763912
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88
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Cantino ME, Quintanilla A. Cooperative effects of rigor and cycling cross-bridges on calcium binding to troponin C. Biophys J 2006; 92:525-34. [PMID: 17056730 PMCID: PMC1751382 DOI: 10.1529/biophysj.106.093757] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The effects of rigor and cycling cross-bridges on distributions of calcium (Ca) bound within sarcomeres of rabbit psoas muscle fibers were compared using electron probe x-ray microanalysis. Calcium in the overlap region of rigor fibers, after correction for that bound to thick filaments, was significantly higher than in the I-band at all pCa levels tested between 6.9 and 4.8, but the difference was greatest at pCa 6.9. With addition of MgATP, differences were significant at high levels of activation (pCa 5.6 and 4.9); near and below the threshold for activation, Ca was the same in I-band and overlap regions. Comparison of Ca and mass profiles at the A-I junction showed elevation of Ca extending 55-110 nm (up to three regulatory units) into the I-band. Extraction of TnC-reduced I-band and overlap Ca in rigor fibers at pCa 5.6 to the same levels found in unextracted fibers at pCa 8.9, suggesting that variations reported here reflect changes in Ca bound to troponin C (TnC). Taken together, these observations provide evidence for near-neighbor cooperative effects of both rigor and cycling cross-bridges on Ca(2+) binding to TnC.
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Affiliation(s)
- Marie E Cantino
- Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut, USA.
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89
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Alahyan M, Webb MR, Marston SB, El-Mezgueldi M. The mechanism of smooth muscle caldesmon-tropomyosin inhibition of the elementary steps of the actomyosin ATPase. J Biol Chem 2006; 281:19433-48. [PMID: 16540476 DOI: 10.1074/jbc.m507602200] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Caldesmon is a component of smooth muscle thin filaments that inhibits the actomyosin ATPase via its interaction with actin-tropomyosin. We have performed a comprehensive transient kinetic characterization of the actomyosin ATPase in the presence of smooth muscle caldesmon and tropomyosin. At physiological ratios of caldesmon to actin (1 caldesmon/7 actin monomers) actomyosin ATPase is inhibited by about 75%. Inhibitory caldesmon concentrations had little effect upon the rate of S1 binding to actin, actin-S1 dissociation by ATP, and dissociation of ADP from actin-S1 x ADP; however the rate of phosphate release from the actin-S1 x ADP x P(i) complex was decreased by more than 80%. In addition the transient of phosphate release displayed a lag of up to 200 ms. The presence of a lag phase indicates that a step on the pathway prior to phosphate release has become rate-limiting. Premixing the actin-tropomyosin filaments with myosin heads resulted in the disappearance of the lag phase. We conclude that caldesmon inhibition of the rate of phosphate release is caused by the thin filament being switched by caldesmon to an inactive state. The active and inactive states correspond to the open and closed states observed in skeletal muscle thin filaments with no evidence for the existence of a third, blocked state. Taken together these data suggest that at physiological concentrations, caldesmon controls the isomerization of the weak binding complex to the strong binding complex, and this causes the inhibition of the rate of phosphate release. This inhibition is sufficient to account for the inhibition of the steady state actomyosin ATPase by caldesmon and tropomyosin.
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Affiliation(s)
- Mustapha Alahyan
- Myocardial Systems Biology Group, National Heart and Lung Institute, Imperial College, London SW3 6LY
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90
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Stelzer JE, Larsson L, Fitzsimons DP, Moss RL. Activation dependence of stretch activation in mouse skinned myocardium: implications for ventricular function. ACTA ACUST UNITED AC 2006; 127:95-107. [PMID: 16446502 PMCID: PMC2151492 DOI: 10.1085/jgp.200509432] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Recent evidence suggests that ventricular ejection is partly powered by a delayed development of force, i.e., stretch activation, in regions of the ventricular wall due to stretch resulting from torsional twist of the ventricle around the apex-to-base axis. Given the potential importance of stretch activation in cardiac function, we characterized the stretch activation response and its Ca2+ dependence in murine skinned myocardium at 22°C in solutions of varying Ca2+ concentrations. Stretch activation was induced by suddenly imposing a stretch of 0.5–2.5% of initial length to the isometrically contracting muscle and then holding the muscle at the new length. The force response to stretch was multiphasic: force initially increased in proportion to the amount of stretch, reached a peak, and then declined to a minimum before redeveloping to a new steady level. This last phase of the response is the delayed force characteristic of myocardial stretch activation and is presumably due to increased attachment of cross-bridges as a consequence of stretch. The amplitude and rate of stretch activation varied with Ca2+ concentration and more specifically with the level of isometric force prior to the stretch. Since myocardial force is regulated both by Ca2+ binding to troponin-C and cross-bridge binding to thin filaments, we explored the role of cross-bridge binding in the stretch activation response using NEM-S1, a strong-binding, non-force–generating derivative of myosin subfragment 1. NEM-S1 treatment at submaximal Ca2+-activated isometric forces significantly accelerated the rate of the stretch activation response and reduced its amplitude. These data show that the rate and amplitude of myocardial stretch activation vary with the level of activation and that stretch activation involves cooperative binding of cross-bridges to the thin filament. Such a mechanism would contribute to increased systolic ejection in response to increased delivery of activator Ca2+ during excitation–contraction coupling.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin Medical School, Madison, WI 53706, USA
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91
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Kobayashi T, Solaro RJ. Increased Ca2+ affinity of cardiac thin filaments reconstituted with cardiomyopathy-related mutant cardiac troponin I. J Biol Chem 2006; 281:13471-13477. [PMID: 16531415 DOI: 10.1074/jbc.m509561200] [Citation(s) in RCA: 104] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To understand the molecular mechanisms whereby cardiomyopathy-related cardiac troponin I (cTnI) mutations affect myofilament activity, we have investigated the Ca2+ binding properties of various assemblies of the regulatory components that contain one of the cardiomyopahty-related mutant cTnI. Acto-S1 ATPase activities in reconstituted systems were also determined. We investigated R145G and R145W mutations from the inhibitory region and D190H and R192H mutations from the second actin-tropomyosin-binding site. Each of the four mutations sensitized the acto-S1 ATPase to Ca2+. Whereas the mutations from the inhibitory region increased the basal level of ATPase activity, those from the second actin-tropomyosin-binding site did not. The effects on the Ca2+ binding properties of the troponin ternary complex and the troponin-tropomyosin complex with one of four mutations were either desensitization or no effect compared with those with wild-type cTnI. All of the mutations, however, affected the Ca2+ sensitivities of the reconstituted thin filaments in the same direction as the acto-S1 ATPase activity. Also the thin filaments with one of the mutant cTnIs bound Ca2+ with less cooperativity compared with those with wild-type cTnI. These data indicate that the mutations found in the inhibitory region and those from the second actin-tropomyosin site shift the equilibrium of the states of the thin filaments differently. Moreover, the increased Ca2+ bound to myofilaments containing the mutant cTnIs may be an important factor in triggered arrhythmias associated with the cardiomyopathy.
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Affiliation(s)
- Tomoyoshi Kobayashi
- Department of Physiology and Biophysics and Center for Cardiovascular Research, College of Medicine, University of Illinois, Chicago, Illinois 60612.
| | - R John Solaro
- Department of Physiology and Biophysics and Center for Cardiovascular Research, College of Medicine, University of Illinois, Chicago, Illinois 60612
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92
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Kremneva E, Nikolaeva O, Maytum R, Arutyunyan AM, Kleimenov SY, Geeves MA, Levitsky DI. Thermal unfolding of smooth muscle and nonmuscle tropomyosin alpha-homodimers with alternatively spliced exons. FEBS J 2006; 273:588-600. [PMID: 16420482 DOI: 10.1111/j.1742-4658.2005.05092.x] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We used differential scanning calorimetry (DSC) and circular dichroism (CD) to investigate thermal unfolding of recombinant fibroblast isoforms of alpha-tropomyosin (Tm) in comparison with that of smooth muscle Tm. These two nonmuscle Tm isoforms 5a and 5b differ internally only by exons 6b/6a, and they both differ from smooth muscle Tm by the N-terminal exon 1b which replaces the muscle-specific exons 1a and 2a. We show that the presence of exon 1b dramatically decreases the measurable calorimetric enthalpy of the thermal unfolding of Tm observed with DSC, although it has no influence on the alpha-helix content of Tm or on the end-to-end interaction between Tm dimers. The results suggest that a significant part of the molecule of fibroblast Tm (but not smooth muscle Tm) unfolds noncooperatively, with the enthalpy no longer visible in the cooperative thermal transitions measured. On the other hand, both DSC and CD studies show that replacement of muscle exons 1a and 2a by nonmuscle exon 1b not only increases the thermal stability of the N-terminal part of Tm, but also significantly stabilizes Tm by shifting the major thermal transition of Tm to higher temperature. Replacement of exon 6b by exon 6a leads to additional increase in the alpha-Tm thermal stability. Thus, our data show for the first time a significant difference in the thermal unfolding between muscle and nonmuscle alpha-Tm isoforms, and indicate that replacement of alternatively spliced exons alters the stability of the entire Tm molecule.
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Affiliation(s)
- Elena Kremneva
- A.N. Bach Institute of Biochemistry, Russian Academy of Sciences, Moscow, Russia
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93
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Rodger A, Marrington R, Geeves MA, Hicks M, de Alwis L, Halsall DJ, Dafforn TR. Looking at long molecules in solution: what happens when they are subjected to Couette flow? Phys Chem Chem Phys 2006; 8:3161-71. [PMID: 16902709 DOI: 10.1039/b604810m] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Knowing the structure of a molecule is one of the keys to deducing its function in a biological system. However, many biomacromolecules are not amenable to structural characterisation by the powerful techniques often used namely NMR and X-ray diffraction because they are too large, or too flexible or simply refuse to crystallize. Long molecules such as DNA and fibrous proteins are two such classes of molecule. In this article the extent to which flow linear dichroism (LD) can be used to characterise the structure and function of such molecules is reviewed. Consideration is given to the issues of fluid dynamics and light scattering by such large molecules. A range of applications of LD are reviewed including (i) fibrous proteins with particular attention being given to actin; (ii) a far from comprehensive discussion of the use of LD for DNA and DNA-ligand systems; (iii) LD for the kinetics of restriction digestion of circular supercoiled DNA; and (iv) carbon nanotubes to illustrate that LD can be used on any long molecules with accessible absorption transitions.
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Affiliation(s)
- Alison Rodger
- Department of Chemistry, University of Warwick, Coventry, UKCV4 7AL
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94
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Heeley DH, Belknap B, White HD. Maximal activation of skeletal muscle thin filaments requires both rigor myosin S1 and calcium. J Biol Chem 2005; 281:668-76. [PMID: 16186114 DOI: 10.1074/jbc.m505549200] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The regulation by calcium and rigor-bound myosin-S1 of the rate of acceleration of 2'-deoxy-3'-O-(N-methylanthraniloyl)ADP (mdADP) release from myosin-mdADP-P(i) by skeletal muscle thin filaments (reconstituted from actin-tropomyosin-troponin) was measured using double mixing stopped-flow fluorescence with the nucleotide substrate 2'-deoxy-3'-O-(N-methylanthraniloyl). The predominant mechanism of regulation is the acceleration of product dissociation by a factor of approximately 200 by thin filaments in the fully activated conformation (bound calcium and rigor S1) relative to the inhibited conformation (no bound calcium or rigor S1). In contrast, only 2-3-fold regulation is due to a change in actin affinity such as would be expected by "steric blocking" of the myosin binding site of the thin filament by tropomyosin. The binding of one ligand (either calcium or rigor-S1) produces partial activation of the rate of product dissociation, but the binding of both is required to maximally accelerate product dissociation to a rate similar to that obtained with F-actin in the absence of regulatory proteins. The data support an allosteric regulation model in which the binding of either calcium or rigor S1 alone to the thin filament shifts the equilibrium in favor of the active conformation, but full activation requires binding of both ligands.
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Affiliation(s)
- David H Heeley
- Department of Biochemistry, Memorial University, St. John's, Newfoundland A1B 3X9 Canada
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95
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Galkin VE, Esashi F, Yu X, Yang S, West SC, Egelman EH. BRCA2 BRC motifs bind RAD51-DNA filaments. Proc Natl Acad Sci U S A 2005; 102:8537-42. [PMID: 15937124 PMCID: PMC1150802 DOI: 10.1073/pnas.0407266102] [Citation(s) in RCA: 108] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Germ-line mutations in BRCA2 account for approximately half the cases of autosomal dominant familial breast cancers. BRCA2 has been shown to interact directly with RAD51, an essential component of the cellular machinery for homologous recombination and the maintenance of genome stability. Interactions between BRCA2 and RAD51 take place by means of the conserved BRC repeat regions of BRCA2. Previously, it was shown that peptides corresponding to BRC3 or BRC4 bind RAD51 monomers and block RAD51-DNA filament formation. In this work, we further analyze these interactions and find that at lower molar ratios BRC3 or BRC4 actually bind and form stable complexes with RAD51-DNA nucleoprotein filaments. Only at high concentrations of the BRC repeats are filaments disrupted. The specific protein-protein contacts occur in the RAD51 filament by means of the N-terminal domain of RAD51 for BRC3 and the nucleotide-binding core of RAD51 for BRC4. These observations show that the BRC repeats bind distinct regions of RAD51 and are nonequivalent in their mode of interaction. The results provide insight into why mutation in just one of the eight BRC repeats would affect the way that BRCA2 protein interacts with the RAD51 filament. Disruption of a single RAD51 interaction site, one of several simultaneous interactions occurring throughout the BRC repeat-containing exon 11 of BRCA2, might modulate the ability of RAD51 to promote recombinational repair and lead to an increased risk of breast cancer.
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Affiliation(s)
- Vitold E Galkin
- Department of Biochemistry and Molecular Genetics, University of Virginia Health Sciences Center, Charlottesville, VA 22908-0733, USA
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96
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Abstract
Although well known as the location of the mechanism by which the cardiac sarcomere is activated by Ca2+ to generate force and shortening, the thin filament is now also recognized as a vital component determining the dynamics of contraction and relaxation. Molecular signaling in the thin filament involves steric, allosteric, and cooperative mechanisms that are modified by protein phosphorylation, sarcomere length and load, the chemical environment, and isoform composition. Approaches employing transgenesis and mutagenesis now permit investigation of these processes at the level of the systems biology of the heart. These studies reveal that the thin filaments are not merely slaves to the levels of Ca2+ determined by membrane channels, transporters and exchangers, but are actively involved in beat to beat control of cardiac function by neural and hormonal factors and by the Frank-Starling mechanism.
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Affiliation(s)
- Tomoyoshi Kobayashi
- Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, Illinois 60612, USA.
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97
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Graceffa P, Mazurkie A. Effect of Caldesmon on the Position and Myosin-induced Movement of Smooth Muscle Tropomyosin Bound to Actin. J Biol Chem 2005; 280:4135-43. [PMID: 15504719 DOI: 10.1074/jbc.m410375200] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
It is known that the actin-binding protein caldesmon inhibits actomyosin ATPase activity and might in this way take part in the thin filament regulation of smooth muscle contraction. Although the molecular mechanism of this inhibition is unknown, it is clear that the presence of actin-bound tropomyosin is necessary for full inhibition. Recent evidence also suggests that the myosin-induced movement of tropomyosin plays a key role in regulation. In this work, fluorescence studies provide evidence to show that caldesmon interacts with and alters the position of tropomyosin in a reconstituted actin thin filament and thereby limits the ability of myosin heads to move tropomyosin. Caldesmon interacts with the Cys-190 region in the COOH-terminal half of tropomyosin, resulting in the movement of this part of tropomyosin to a new position on actin. Additionally, this constrains the myosin-induced movement of this region of tropomyosin. On the other hand, caldesmon does not appear to interact with the Cys-36 region in the NH2-terminal half of tropomyosin and neither alters the position of nor significantly constrains the myosin-induced movement of this part of tropomyosin. The ability of caldesmon to limit the myosin-induced movement of tropomyosin provides a possible molecular basis for the inhibitory function of caldesmon. The different movements of the two halves of tropomyosin indicate that actin-bound tropomyosin moves as a flexible molecule and not as a rigid rod. Interestingly, caldesmon, which inhibits tropomyosin's potentiation of actomyosin ATPase activity, moves tropomyosin in one direction, whereas myosin heads, which enhance potentiation, move tropomyosin in the opposite direction.
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Affiliation(s)
- Philip Graceffa
- Boston Biomedical Research Institute, Watertown, Massachusetts 02472, USA.
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98
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Gafurov B, Fredricksen S, Cai A, Brenner B, Chase PB, Chalovich JM. The Delta 14 mutation of human cardiac troponin T enhances ATPase activity and alters the cooperative binding of S1-ADP to regulated actin. Biochemistry 2004; 43:15276-85. [PMID: 15568820 PMCID: PMC1351011 DOI: 10.1021/bi048646h] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The complex of tropomyosin and troponin binds to actin and inhibits activation of myosin ATPase activity and force production of striated muscles at low free Ca(2+) concentrations. Ca(2+) stimulates ATP activity, and at subsaturating actin concentrations, the binding of NEM-modified S1 to actin-tropomyosin-troponin increases the rate of ATP hydrolysis even further. We show here that the Delta14 mutation of troponin T, associated with familial hypertrophic cardiomyopathy, results in an increase in ATPase rate like that seen with wild-type troponin in the presence of NEM-S1. The enhanced ATPase activity was not due to a decreased incorporation of mutant troponin T with troponin I and troponin C to form an active troponin complex. The activating effect was more prominent with a hybrid troponin (skeletal TnI, TnC, and cardiac TnT) than with all cardiac troponin. Thus it appears that changes in the troponin-troponin contacts that result from mutations or from forming hybrids stabilize a more active state of regulated actin. An analysis of the effect of the Delta14 mutation on the equilibrium binding of S1-ADP to actin was consistent with stabilization of an active state of actin. This change in activation may be important in the development of cardiac disease.
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99
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Shitaka Y, Kimura C, Miki M. The rates of switching movement of troponin T between three states of skeletal muscle thin filaments determined by fluorescence resonance energy transfer. J Biol Chem 2004; 280:2613-9. [PMID: 15548522 DOI: 10.1074/jbc.m408553200] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Troponin (Tn) plays the key roles in the regulation of striated muscle contraction. Tn consists of three subunits (TnT, TnC, and TnI). In combination with the stopped-flow method, fluorescence resonance energy transfer between probes attached to Cys-60 or Cys-250 of TnT and Cys-374 of actin was measured to determine the rates of switching movement of the troponin tail domain (Cys-60) and of the TnT-TnI coiled-coil C terminus (Cys-250) between three states (relaxed, closed, and open) of the thin filament. When the free Ca(2+) concentration was rapidly changed, these domains moved with rates of approximately 450 and approximately 85 s(-1) at pH 7.0 on Ca(2+) up and down, respectively. When myosin subfragment 1 (S1) was dissociated from thin filaments by rapid mixing with ATP, these domains moved with a single rate constant of approximately 400 s(-1) in the presence and absence of Ca(2+). The light scattering measurements showed that ATP-induced S1 dissociation occurred with a rate constant >800 s(-1). When S1 was rapidly mixed with the thin filament, these domains moved with almost the same or slightly faster rates than those of S1 binding measured by light scattering. In most but not all aspects, the rates of movement of the troponin tail domain and of the TnT-TnI coiled-coil C terminus were very similar to those of certain TnI sites (N terminus, Cys-133, and C terminus) previously characterized (Shitaka, Y., Kimura, C., Iio, T., and Miki, M. (2004) Biochemistry 43, 10739-10747), suggesting that a series of conformational changes in the Tn complex during switching on or off process occurs synchronously.
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Affiliation(s)
- Yuji Shitaka
- Department of Applied Chemistry and Biotechnology, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan
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100
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Kremneva E, Boussouf S, Nikolaeva O, Maytum R, Geeves MA, Levitsky DI. Effects of two familial hypertrophic cardiomyopathy mutations in alpha-tropomyosin, Asp175Asn and Glu180Gly, on the thermal unfolding of actin-bound tropomyosin. Biophys J 2004; 87:3922-33. [PMID: 15454401 PMCID: PMC1304903 DOI: 10.1529/biophysj.104.048793] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Differential scanning calorimetry was used to investigate the thermal unfolding of native alpha-tropomyosin (Tm), wild-type alpha-Tm expressed in Escherichia coli and the wild-type alpha-Tm carrying either of two missense mutations associated with familial hypertrophic cardiomyopathy, D175N or E180G. Recombinant alpha-Tm was expressed with an N-terminal Ala-Ser extension to substitute for the essential N-terminal acetylation of the native Tm. Native and Ala-Ser-Tm were indistinguishable in our assays. In the absence of F-actin, the thermal unfolding of Tm was reversible and the heat sorption curve of Tm with Cys-190 reduced was decomposed into two separate calorimetric domains with maxima at approximately 42 and 51 degrees C. In the presence of phalloidin-stabilized F-actin, a new cooperative transition appears at 46-47 degrees C and completely disappears after the irreversible denaturation of F-actin. A good correlation was found to exist between the maximum of this peak and the temperature of half-maximal dissociation of the F-actin/Tm complex as determined by light scattering experiments. We conclude that Tm thermal denaturation only occurs upon its dissociation from F-actin. In the presence of F-actin, D175N alpha-Tm shows a melting profile and temperature dependence of dissociation from F-actin similar to those for wild-type alpha-Tm. The actin-induced stabilization of E180G alpha-Tm is significantly less than for wild-type alpha-Tm and D175N alpha-Tm, and this property could contribute to the more severe myopathy phenotype reported for this mutation.
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Affiliation(s)
- Elena Kremneva
- A. N. Bach Institute of Biochemistry, Russian Academy of Sciences, Moscow 119071, Russia
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