Steric-model for activation of muscle thin filaments

Length-dependent effects of osmotic compression on skinned rabbit psoas muscle fibers

The goal of this study was to characterize the interrelationship between sarcomere length and interfilament spacing in the control of Ca2+ sensitivity in skinned rabbit psoas muscle fibers. Measurements were made at sarcomere lengths 2.0, 2.7 and 3.4 microm. At 2.7 microm the fiber width was reduced by 17% relative to that at 2.0 microm and the pCa50 for force development was increased by approximately 0.3 pCa units. In the presence of 5% Dextran T-500 the fiber width at sarcomere length 2.0 microm was also decreased by 17% and the Ca2+ sensitivity was increased to the same value as at 2.7 microm. In contrast, at sarcomere length 2.7 microm the addition of as much as 10% Dextran T-500 had no effect on Ca2+ sensitivity. At sarcomere length 3.4 microm there was an additional 7% compression and the Ca2+ sensitivity was increased slightly (approximately 0.1 pCa units) relative to that at 2.7 microm. However at 3.4 microm the addition of 5% Dextran T-500 caused the Ca2+ sensitivity to decrease to the level seen at 2.0 microm. Given that the skinning process causes a swelling of the filament lattice it is evident that the relationship between sarcomere length and Ca2+ sensitivity observed in skinned fibers may not always be applicable to intact fibers. These data are consistent with measurements of Ca2+ in intact fibers which indicate that there might be a decline in Ca2+ sensitivity at long sarcomere lengths.

Length-dependence of cross-bridge mediated activation of the cardiac thin filament

Length-dependent Ca(2+)activation of the thin filament plays a critical role in the steep force-length relationship of cardiac muscle (Frank-Starling relation). Recent evidence indicates that the increase in myofilament Ca(2+)sensitivity and Ca(2+)-troponin C affinity that occurs with increase in sarcomere length results from a cooperative activation of the thin filament by attached cross-bridges. At short sarcomere length the Ca(2+)sensitivity is lower because the access of cross-bridges for actin is reduced. The aim of this study was to determine the length-dependence of myosin-mediated thin filament activation in skinned bovine ventricular muscle, as assayed by the generation of force with progressive reduction of MgATP concentration in the absence of Ca(2+). If the interaction between myosin and actin is weaker at short sarcomere length there should be a lower MgATP concentration needed to maintain the relaxed state. Contrary to expectation, the force-pMgATP relationship was not significantly influenced by change in sarcomere length. However this relationship became length-sensitive in the presence of phosphate analogs which stabilize weak-binding cross-bridges. We suggest that sarcomere length modulates Ca(2+)sensitivity by controlling the size of the population of thin filament regulatory units in the weakly-bound state.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Complexes of smooth muscle tropomyosin with F-actin studied by differential scanning calorimetry

Differential scanning calorimetry (DSC) and light scattering were used to analyze the interaction of duck gizzard tropomyosin (tropomyosin) with rabbit skeletal-muscle F-actin. In the absence of F-actin, tropomyosin, represented mainly by heterodimers, unfolds at 41 degrees C with a sharp thermal transition. Interaction of tropomyosin heterodimers with F-actin causes a 2-6 degrees C shift in the tropomyosin thermal transition to higher temperature, depending on the tropomyosin/actin molar ratio and protein concentration. A pronounced shift of the tropomyosin thermal transition was observed only for tropomyosin heterodimers, and not for homodimers. The most pronounced effect was observed after complete saturation of F-actin with tropomyosin molecules, at tropomyosin/actin molar ratios > 1 : 7. Under these conditions, two well-separated peaks of tropomyosin were observed on the thermogram besides the peak of F-actin, the peak characteristic of free tropomyosin heterodimer, and the peak with a maximum at 45-47 degrees C corresponding to tropomyosin bound to F-actin. By measuring the temperature-dependence of light scattering, we found that thermal unfolding of tropomyosin is accompanied by its dissociation from F-actin. Thermal unfolding of tropomyosin is almost completely reversible, whereas F-actin denatures irreversibly. The addition of tropomyosin has no effect on thermal unfolding of F-actin, which denatures with a maximum at 64 degrees C in the absence and at 78 degrees C in the presence of a twofold molar excess of phalloidin. After the F-actin-tropomyosin complex had been heated to 90 degrees C and then cooled (i.e. after complete irreversible denaturation of F-actin), only the peak characteristic of free tropomyosin was observed on the thermogram during reheating, whereas the thermal transitions of F-actin and actin-bound tropomyosin completely disappeared. Therefore, the DSC method allows changes in thermal unfolding of tropomyosin resulting from its interaction with F-actin to be probed very precisely.

Structural basis for the higher Ca(2+)-activation of the regulated actin-activated myosin ATPase observed with Dictyostelium/Tetrahymena actin chimeras

Replacement of residues 228-230 or 228-232 of subdomain 4 in Dictyostelium actin with the corresponding Tetrahymena sequence (QTA to KAY replacement: half chimera-1; QTAAS to KAYKE replacement: full chimera) leads to a higher Ca(2+)-activation of the regulated acto-myosin subfragment-1 ATPase activity. The ratio of ATPase activation in the presence of tropomyosin-troponin and Ca(2+) to that without tropomyosin-troponin becomes about four times as large as the ratio for the wild-type actin. To understand the structural basis of this higher Ca(2+)-activation, we have determined the crystal structures of the 1:1 complex of Dictyostelium mutant actins (half chimera-1 and full chimera) with gelsolin segment-1 to 2.0 A and 2.4 A resolution, respectively, together with the structure of wild-type actin as a control. Although there were local changes on the surface of the subdomain 4 and the phenolic side-chain of Tyr230 displaced the side-chain of Leu236 from a non-polar pocket to a more solvent-accessible position, the structures of the actin chimeras showed that the mutations in the 228-232 region did not introduce large changes in the overall actin structure. This suggests that residues near position 230 formed part of the tropomyosin binding site on actin in actively contracting muscle. The higher Ca(2+)-activation observed with A230Y-containing mutants can be understood in terms of a three-state model for thin filament regulation in which, in the presence of both Ca(2+) and myosin heads, the local changes of actin generated by the mutation (especially its phenolic side-chain) facilitate the transition of thin filaments from a "closed" state to an "open" state. Between 394 and 469 water molecules were identified in the different structures and it was found that actin recognizes hydrated forms of the adenine base and the Ca ion in the nucleotide binding site.

A230Y mutation of actin on subdomain 4 is sufficient for higher calcium activation of actin-activated myosin adenosinetriphosphatase in the presence of tropomyosin-troponin

To probe the mechanism by which Ca(2+) activates muscle contraction through tropomyosin and troponin, we have produced mutant actins using Dictyostelium discoideum. We focused on the sequence 228-232 (QTAAS) that is located in subdomain 4 of actin, because the chimera actin in which this sequence was replaced by KAYKE showed not only poorer tropomyosin binding but also the unexpected "higher Ca(2+) activation" [Saeki, K., et al. (1996) Biochemistry 35, 14465-14472]. We found that this higher Ca(2+) activation is solely due to the A230Y mutagenesis. Because A230Y mutant actin showed normal tropomyosin binding, the higher Ca(2+) activation is not the consequence of poorer tropomyosin binding. The significance of these results is discussed in view of a three-state model [McKillop, D. F., Geeves, M. A. (1993) Biophys. J. 65, 693-701].

Binding of troponin I and the troponin I-troponin C complex to actin-tropomyosin. Dissociation by myosin subfragment 1

Troponin I (TnI) is the component of the troponin complex, TnI, TnC, TnT, that is responsible for inhibition of actomyosin ATPase activity. Using the fluorescence of pyrene-labeled tropomyosin (Tm), we probed the interaction of TnI and TnIC with Tm on the reconstituted muscle thin filament. The results indicate that TnI and TnIC(-Ca(2+)) bind specifically and strongly to actin-Tm with a stoichiometry of 1 TnI or 1 TnIC/1 Tm/7 actin, in agreement with previous results. The binding of myosin heads (S1) to actin-Tm at low levels of saturation caused TnI and TnIC to dissociate from actin-Tm. These results are interpreted in terms of the S1-binding state allosteric-cooperative model of the actin-Tm thin filament, closed/open. Thus, TnI and TnIC(-Ca(2+)) bind to the closed state of actin-Tm and their binding is greatly weakened in the S1-induced open state, indicating that they act as allosteric inhibitors. The fluorescence change and the stoichiometry indicate that the TnI-binding site is composed of regions from both actin and Tm probably in the vicinity of Cys 190.

Actin and the smooth muscle regulatory proteins: a structural perspective

The structural details of the smooth muscle acto-myosin interaction and its functional implications have been much discussed in recent years, however other, smooth muscle specific, actin-binding proteins have received much less attention. With increasing technical advances in structural biology a great deal of structural information is now coming to light, information that can provide useful insight into the mechanism of action for many important nonmotor actin-binding proteins. The purpose of the review is to instill the current knowledge on the structure, and interaction sites on F-actin, of the major, non-motor actin-binding proteins from smooth muscle, proposed to have a role in regulation. In the light of the recent structural studies the probable roles of the various actin-binding proteins will be discussed with particular reference to structure function relationships.

Three-dimensional reconstruction of thin filaments containing mutant tropomyosin

Interactions of the components of reconstituted thin filaments were investigated using a tropomyosin internal deletion mutant, D234, in which actin-binding pseudo-repeats 2, 3, and 4 are missing. D234 retains regions of tropomyosin that bind troponin and form end-to-end tropomyosin bonds, but has a length to span only four instead of seven actin monomers. It inhibits acto-myosin subfragment 1 ATPase (acto-S-1 ATPase) and filament sliding in vitro in both the presence and absence of Ca(2+) (, J. Biol. Chem. 272:14051-14056) and lowers the affinity of S-1.ADP for actin while increasing its cooperative binding. Electron microscopy and three-dimensional reconstruction of reconstituted thin filaments containing actin, troponin, and wild-type or D234 tropomyosin were carried out to determine if Ca(2+)-induced movement of D234 occurred in the filaments. In the presence and absence of Ca(2+), the D234 position was indistinguishable from that of the wild-type tropomyosin, demonstrating that the mutation did not affect normal tropomyosin movement induced by Ca(2+) and troponin. These results suggested that, in the presence of Ca(2+) and troponin, D234 tropomyosin was trapped on filaments in the Ca(2+)-induced position and was unable to undergo a transition to a completely activated position. By adding small amounts of rigor-bonded N-ethyl-maleimide-treated S-1 to mutant thin filaments, thus mimicking the myosin-induced "open" state, inhibition could be overcome and full activation restored. This myosin requirement for full activation provides support for the existence of three functionally distinct thin filament states (off, Ca(2+)-induced, myosin-induced; cf.;, J. Mol. Biol. 266:8-14). We propose a further refinement of the three-state model in which the binding of myosin to actin causes allosteric changes in actin that promote the binding of tropomyosin in an otherwise energetically unfavorable "open" state.

The ends of tropomyosin are major determinants of actin affinity and myosin subfragment 1-induced binding to F-actin in the open state

Tropomyosin (TM) is thought to exist in equilibrium between two states on F-actin, closed and open [Geeves, M. A., and Lehrer, S. S. (1994) Biophys. J. 67, 273-282]. Myosin shifts the equilibrium to the open state in which myosin binds strongly and develops force. Tropomyosin isoforms, that primarily differ in their N- and C-terminal sequences, have different equilibria between the closed and open states. The aim of the research is to understand how the alternate ends of TM affect cooperative actin binding and the relationship between actin affinity and the cooperativity with which myosin S1 promotes binding of TM to actin in the open state. A series of rat alpha-tropomyosin variants was expressed in Escherichia coli that are identical except for the ends, which are encoded by exons 1a or 1b and exons 9a, 9c or 9d. Both the N- and C-terminal sequences, and the particular combination within a TM molecule, determine actin affinity. Compared to tropomyosins with an exon 1a-encoded N-terminus, found in long isoforms, the exon 1b-encoded sequence, expressed in 247-residue nonmuscle tropomyosins, increases actin affinity in tropomyosins expressing 9a or 9d but has little effect with 9c, a brain-specific exon. The relative actin affinities, in decreasing order, are 1b9d > 1b9a > acetylated 1a9a > 1a9d >> 1a9a > or = 1a9c congruent with 1b9c. Myosin S1 greatly increases the affinity of all tropomyosin variants for actin. In this, the actin affinity is the primary factor in the cooperativity with which myosin S1 induces TM binding to actin in the open state; generally, the higher the actin affinity, the lower the occupancy by myosin required to saturate the actin with tropomyosin: 1b9d >1a9d> 1b9a > or = acetylated 1a9a > 1a9a > 1a9c congruent with 1b9c.

Effects of tropomyosin internal deletions on thin filament function

Striated muscle tropomyosin spans seven actin monomers and contains seven quasi-repeating regions with loose sequence similarity. Each region contains a hypothesized actin binding motif. To examine the functions of these regions, full-length tropomyosin was compared with tropomyosin internal deletion mutants spanning either five or four actins. Actin-troponin-tropomyosin filaments lacking tropomyosin regions 2-3 exhibited calcium-sensitive regulation in in vitro motility and myosin S1 ATP hydrolysis experiments, similar to filaments with full-length tropomyosin. In contrast, filaments lacking tropomyosin regions 3-4 were inhibitory to these myosin functions. Deletion of regions 2-4, 3-5, or 4-6 had little effect on tropomyosin binding to actin in the presence of troponin or troponin-Ca(2+), or in the absence of troponin. However, all of these mutants inhibited myosin cycling. Deletion of the quasi-repeating regions diminished the prominent effect of myosin S1 on tropomyosin-actin binding. Interruption of this cooperative, myosin-tropomyosin interaction was least severe for the mutant lacking regions 2-3 and therefore correlated with inhibition of myosin cycling. Regions 3, 4, and 5 each contributed about 1.5 kcal/mol to this process, whereas regions 2 and 6 contributed much less. We suggest that a myosin-induced conformational change in actin facilitates the azimuthal repositioning of tropomyosin which is an essential part of regulation.

Tropomyosin directly modulates actomyosin mechanical performance at the level of a single actin filament

Muscle contraction is the result of myosin cross-bridges (XBs) cyclically interacting with the actin-containing thin filament. This interaction is modulated by the thin filament regulatory proteins, troponin and tropomyosin (Tm). With the use of an in vitro motility assay, the role of Tm in myosin's ability to generate force and motion was assessed. At saturating myosin surface densities, Tm had no effect on thin filament velocity. However, below 50% myosin saturation, a significant reduction in actin-Tm filament velocity was observed, with complete inhibition of movement occurring at 12. 5% of saturating surface densities. Under similar conditions, actin filaments alone demonstrated no reduction in velocity. The effect of Tm on force generation was assessed at the level of a single thin filament. In the absence of Tm, isometric force was a linear function of the density of myosin on the motility surface. At 50% myosin surface saturation, the presence of Tm resulted in a 2-fold enhancement of force relative to actin alone. However, no further potentiation of force was observed with Tm at saturating myosin surface densities. These results indicate that, in the presence of Tm, the strong binding of myosin cooperatively activates the thin filament. The inhibition of velocity at low myosin densities and the potentiation of force at higher myosin densities suggest that Tm can directly modulate the kinetics of a single myosin XB and the recruitment of a population of XBs, respectively. At saturating myosin conditions, Tm does not appear to affect the recruitment or the kinetics of myosin XBs.

Movement of smooth muscle tropomyosin by myosin heads

It has been proposed that during the activation of muscle contraction the initial binding of myosin heads to the actin thin filament contributes to switching on the thin filament and that this might involve the movement of actin-bound tropomyosin. The movement of smooth muscle tropomyosin on actin was investigated in this work by measuring the change in distance between specific residues on tropomyosin and actin by fluorescence resonance energy transfer (FRET) as a function of myosin head binding to actin. An energy transfer acceptor was attached to Cys374 of actin and a donor to the tropomyosin heterodimer at either Cys36 of the beta-chain or Cys190 of the alpha-chain. FRET changed for the donor at both positions of tropomyosin upon addition of skeletal or smooth muscle myosin heads, indicating a movement of the whole tropomyosin molecule. The changes in FRET were hyperbolic and saturated at about one head per seven actin subunits, indicating that each head cooperatively affects several tropomyosin molecules, presumably via tropomyosin's end-to-end interaction. ATP, which dissociates myosin from actin, completely reversed the changes in FRET induced by heads, whereas in the presence of ADP the effect of heads was the same as in its absence. The results indicate that myosin with and without ADP, intermediates in the myosin ATPase hydrolytic pathway, are effective regulators of tropomyosin position, which might play a role in the regulation of smooth muscle contraction.

Mutations in actin subdomain 3 that impair thin filament regulation by troponin and tropomyosin

Thin filament-mediated regulation of striated muscle contraction involves conformational switching among a few quaternary structures, with transitions induced by binding of Ca(2+) and myosin. We establish and exploit Saccharomyces cerevisiae actin as a model system to investigate this process. Ca(2+)-sensitive troponin-tropomyosin binding affinities for wild type yeast actin are seen to closely resemble those for muscle actin, and these hybrid thin filaments produce Ca(2+)-sensitive regulation of the myosin S-1 MgATPase rate. Yeast actin filament inner domain mutant K315A/E316A depresses Ca(2+) activation of the MgATPase rate, producing a 4-fold weakening of the apparent Ca(2+) affinity and a 50% decrease in the MgATPase rate at saturating Ca(2+) concentration. Observed destabilization of troponin-tropomyosin binding to actin in the presence of Ca(2+), a 1.4-fold effect, provides a partial explanation. Despite the decrease in apparent MgATPase Ca(2+) affinity, there was no detectable change in the true Ca(2+) affinity of the thin filament, measured using fluorophore-labeled troponin. Another inner domain mutant, E311A/R312A, decreased the MgATPase rate but did not change the apparent Ca(2+) affinity. These results suggest that charged residues on the surface of the actin inner domain are important in Ca(2+)- and myosin-induced thin filament activation.

Tropomyosin positions in regulated thin filaments revealed by cryoelectron microscopy

Past attempts to detect tropomyosin in electron micrograph images of frozen-hydrated troponin-regulated thin filaments under relaxing conditions have not been successful. This raised the possibility that tropomyosin may be disordered on filaments in the off-state, a possibility at odds with the steric blocking model of muscle regulation. By using cryoelectron microscopy and helical image reconstruction we have now resolved the location of tropomyosin in both relaxing and activating conditions. In the off-state, tropomyosin adopts a position on the outer domain of actin with a binding site virtually identical to that determined previously by negative staining, although at a radius of 3.8 nm, slightly higher than found in stained filaments. Molecular fitting to the atomic model of F-actin shows that tropomyosin is localized over sites on actin subdomain 1 required for myosin binding. Restricting access to these sites would inhibit the myosin-cross-bridge cycle, and hence contraction. Under high Ca(2+) activating conditions, tropomyosin moved azimuthally, away from its blocking position to the same site on the inner domain of actin previously determined by negative staining, also at 3.8 nm radius. These results provide strong support for operation of the steric mechanism of muscle regulation under near-native solution conditions and also validate the use of negative staining in investigations of muscle thin filament structure.

Role of residues 311/312 in actin-tropomyosin interaction. In vitro motility study using yeast actin mutant e311a/r312a

According to the Lorenz et al. (Lorenz, M., Poole, K. J., Popp, D., Rosenbaum, G., and Holmes, K. C. (1995) J. Mol. Biol. 246, 108-119) atomic model of the actin-tropomyosin complex, actin residue Asp-311 (Glu-311 in yeast) is predicted to have a high binding energy contribution to actin-tropomyosin binding. Using the yeast actin mutant E311A/R312A in the in vitro motility assays, we have investigated the role of these residues in such interactions. Wild type (wt) yeast actin, like skeletal alpha-actin, is fully regulated when complexed with tropomyosin (Tm) and troponin (Tn). Structure-function comparisons of the wt and E311A/R312A actins show no significant differences between them, and the unregulated F-actins slide at similar speeds in the in vitro motility assay. However, in the presence of Tm and Tn, the mutation increases both the sliding speed and the number of moving filaments at high pCa values, shifting the speed-pCa curve nearly 0.5 pCa units to the left. Tm alone (no Tn) inhibits the motilities of both actins at low heavy meromyosin densities but potentiates only the motility of the mutant actin at high heavy meromyosin densities. Actin-Tm binding measurements indicate no significant difference between wt and E311A/R312A actin in Tm binding. These results implicate allosteric effects in the regulation of actomyosin function by tropomyosin.

Isolation, electron microscopic imaging, and 3-D visualization of native cardiac thin myofilaments

An increasing number of cardiac diseases are currently pinpointed to reside at the level of the thin myofilaments (e.g., cardiomyopathies, reperfusion injury). Hence the aim of our study was to develop a new method for the isolation of mammalian thin myofilaments suitable for subsequent high-resolution electron microscopic imaging. Native cardiac thin myofilaments were extracted from glycerinated porcine myocardial tissue in the presence of protease inhibitors. Separation of thick and thin myofilaments was achieved by addition of ATP and several centrifugation steps. Negative staining and subsequent conventional and scanning transmission electron microscopy (STEM) of thin myofilaments permitted visualization of molecular details; unlike conventional preparations of thin myofilaments, our method reveals the F-actin moiety and allows direct recognition of thin myofilament-associated porcine cardiac troponin complexes. They appear as "bulges" at regular intervals of approximately 36 nm along the actin filaments. Protein analysis using SDS-polyacrylamide gel electrophoresis revealed that only approximately 20% troponin I was lost during the isolation procedure. In a further step, 3-D helical reconstructions were calculated using STEM dark-field images. These 3-D reconstructions will allow further characterization of molecular details, and they will be useful for directly visualizing molecular alterations related to diseased cardiac thin myofilaments (e.g., reperfusion injury, alterations of Ca2+-mediated tropomyosin switch).

Identification of a contractile deficit in adult cardiac myocytes expressing hypertrophic cardiomyopathy-associated mutant troponin T proteins

The direct effects of expressing hypertrophic cardiomyopathy-associated (HCM-associated) mutant troponin T (TnT) proteins on the force generation of single adult cardiac myocytes have not been established. Replication-defective recombinant adenovirus vectors were generated for gene transfer of HCM-associated I79N and R92Q mutant cardiac TnT cDNAs into fully differentiated adult cardiac myocytes in primary culture. We tested the hypothesis that the mutant TnT proteins would be expressed and incorporated into the cardiac sarcomere and would behave as dominant-negative proteins to directly alter calcium-activated force generation at the level of the single cardiac myocyte. Interestingly, under identical experimental conditions, the ectopic expression of the mutant TnTs was significantly less ( approximately 8% of total) than that obtained with expression of wild-type TnT ( approximately 35%) in the myocytes. Confocal imaging of immunolabeled TnT showed a regular periodic pattern of localization of ectopic mutant TnT that was not different than that in normal controls, suggesting that mutant TnT incorporation had no deleterious effects on sarcomeric architecture. Direct measurements of isometric force production in single cardiac myocytes demonstrated marked desensitization of submaximal calcium-activated tension, with unchanged maximum tension generation in mutant TnT-expressing myocytes compared with control myocytes. Collectively, these results demonstrate an impaired expression of the mutant protein and a disabling of cardiac contraction in the submaximal range of myoplasmic calcium concentrations. Our functional results suggest that development of new pharmacological, chemical, or genetic approaches to sensitize the thin-filament regulatory protein system could ameliorate force deficits associated with expression of I79N and R92Q in adult cardiac myocytes.

Cross bridge-dependent activation of contraction in cardiac myofibrils at low pH

Striated muscle contracts in the absence of calcium at low concentrations of MgATP ([MgATP]), and this has been termed rigor activation because rigor cross bridges attach and activate adjacent actin sites. This process is well characterized in skeletal muscle but not in cardiac muscle. Rigor cross bridges are also thought to increase calcium binding to troponin C and play a synergistic role in activation. We tested the hypothesis that cross bridge-dependent activation results in an increase in contractile activity at normal and low pH values. Myofibrillar ATPase activity was measured as a function of pCa and [MgATP] at pH 7.0, and the data showed that, at pCa values of >/=5.5, there was a biphasic relationship between activity and [MgATP]. Peak activity occurred at 10-50 microM MgATP, and [MgATP] for peak activity was lower with increased pCa. The ATPase activity of rat cardiac myofibrils as a function of [MgATP] at a pCa of 9.0 was measured at several pH levels (pH 5.4-7.0). The ATPase activity as a function of [MgATP] was biphasic with a maximum at 8-10 microM MgATP. Lower pH did not result in a substantial decrease in myofibrillar ATPase activity even at pH 5.4. The extent of shortening, as measured by Z-line spacing, was greatest at 8 microM MgATP and less at both lower and higher [MgATP], and this response was observed at all pH levels. These studies suggest that the peak ATPase activity associated with low [MgATP] was coupled to sarcomere shortening. These results support the hypothesis that cross bridge-dependent activation of contraction may be responsible for contracture in the ischemic heart.

Role of Ca2+ and cross-bridges in skeletal muscle thin filament activation probed with Ca2+ sensitizers

Thin filament regulation of contraction is thought to involve the binding of two activating ligands: Ca2+ and strongly bound cross-bridges. The specific cross-bridge states required to promote thin filament activation have not been identified. This study examines the relationship between cross-bridge cycling and thin filament activation by comparing the results of kinetic experiments using the Ca2+ sensitizers caffeine and bepridil. In single skinned rat soleus fibers, 30 mM caffeine produced a leftward shift in the tension-pCa relation from 6.03 +/- 0.03 to 6.51 +/- 0.03 pCa units and lowered the maximum tension to 0.60 +/- 0.01 of the control tension. In addition, the rate of tension redevelopment (ktr) was decreased from 3.51 +/- 0.12 s-1 to 2.70 +/- 0.19 s-1, and Vmax decreased from 1.24 +/- 0.07 to 0.64 +/- 0.02 M.L./s. Bepridil produced a similar shift in the tension-pCa curves but had no effect on the kinetics. Thus bepridil increases the Ca2+ sensitivity through direct effects on TnC, whereas caffeine has significant effects on the cross-bridge interaction. Interestingly, caffeine also produced a significant increase in stiffness under relaxing conditions (pCa 9.0), indicating that caffeine induces some strongly bound cross-bridges, even in the absence of Ca2+. The results are interpreted in terms of a model integrating cross-bridge cycling with a three-state thin-filament activation model. Significantly, strongly bound, non-tension-producing cross-bridges were essential to modeling of complete activation of the thin filament.

Roles for the troponin tail domain in thin filament assembly and regulation. A deletional study of cardiac troponin T

Striated muscle contraction is regulated by Ca2+ binding to troponin, which has a globular domain and an elongated tail attributable to the NH2-terminal portion of the bovine cardiac troponin T (TnT) subunit. Truncation of the bovine cardiac troponin tail was investigated using recombinant TnT fragments and subunits TnI and TnC. Progressive truncation of the troponin tail caused progressively weaker binding of troponin-tropomyosin to actin and of troponin to actin-tropomyosin. A sharp drop-off in affinity occurred with NH2-terminal deletion of 119 rather than 94 residues. Deletion of 94 residues had no effect on Ca2+-activation of the myosin subfragment 1-thin filament MgATPase rate and did not eliminate cooperative effects of Ca2+ binding. Troponin tail peptide TnT1-153 strongly promoted tropomyosin binding to actin in the absence of TnI or TnC. The results show that the anchoring function of the troponin tail involves interactions with actin as well as with tropomyosin and has comparable importance in the presence or absence of Ca2+. Residues 95-153 are particularly important for anchoring, and residues 95-119 are crucial for function or local folding. Because striated muscle regulation involves switching among the conformational states of the thin filament, regulatory significance for the troponin tail may arise from its prominent contribution to the protein-protein interactions within these conformations.

Specific myosin heavy chain mutations suppress troponin I defects in Drosophila muscles

We show that specific mutations in the head of the thick filament molecule myosin heavy chain prevent a degenerative muscle syndrome resulting from the hdp2 mutation in the thin filament protein troponin I. One mutation deletes eight residues from the actin binding loop of myosin, while a second affects a residue at the base of this loop. Two other mutations affect amino acids near the site of nucleotide entry and exit in the motor domain. We document the degree of phenotypic rescue each suppressor permits and show that other point mutations in myosin, as well as null mutations, fail to suppress the hdp2 phenotype. We discuss mechanisms by which the hdp2 phenotypes are suppressed and conclude that the specific residues we identified in myosin are important in regulating thick and thin filament interactions. This in vivo approach to dissecting the contractile cycle defines novel molecular processes that may be difficult to uncover by biochemical and structural analysis. Our study illustrates how expression of genetic defects are dependent upon genetic background, and therefore could have implications for understanding gene interactions in human disease.

Ca2+ and cross-bridge-induced changes in troponin C in skinned skeletal muscle fibers: effects of force inhibition

Changes in skeletal troponin C (sTnC) structure during thin filament activation by Ca2+ and strongly bound cross-bridge states were monitored by measuring the linear dichroism of the 5' isomer of iodoacetamidotetramethylrhodamine (5'IATR), attached to Cys98 (sTnC-5'ATR), in sTnC-5'ATR reconstituted single skinned fibers from rabbit psoas muscle. To isolate the effects of Ca2+ and cross-bridge binding on sTnC structure, maximum Ca2+-activated force was inhibited with 0.5 mM AlF4- or with 30 mM 2,3 butanedione-monoxime (BDM) during measurements of the Ca2+ dependence of force and dichroism. Dichroism was 0.08 +/- 0.01 (+/- SEM, n = 9) in relaxing solution (pCa 9.2) and decreased to 0.004 +/- 0.002 (+/- SEM, n = 9) at pCa 4.0. Force and dichroism had similar Ca2+ sensitivities. Force inhibition with BDM caused no change in the amplitude and Ca2+ sensitivity of dichroism. Similarly, inhibition of force at pCa 4.0 with 0.5 mM AlF4- decreased force to 0.04 +/- 0.01 of maximum (+/- SEM, n = 3), and dichroism was 0.04 +/- 0.03 (+/- SEM, n = 3) of the value at pCa 9.2 and unchanged relative to the corresponding normalized value at pCa 4.0 (0.11 +/- 0.05, +/- SEM; n = 3). Inhibition of force with AlF4- also had no effect when sTnC structure was monitored by labeling with either 5-dimethylamino-1-napthalenylsulfonylaziridine (DANZ) or 4-(N-(iodoacetoxy)ethyl-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole (NBD). Increasing sarcomere length from 2.5 to 3.6 microm caused force (pCa 4.0) to decrease, but had no effect on dichroism. In contrast, rigor cross-bridge attachment caused dichroism at pCa 9.2 to decrease to 0.56 +/- 0.03 (+/- SEM, n = 5) of the value at pCa 9. 2, and force was 0.51 +/- 0.04 (+/- SEM, n = 6) of pCa 4.0 control. At pCa 4.0 in rigor, dichroism decreased further to 0.19 +/- 0.03 (+/- SEM, n = 6), slightly above the pCa 4.0 control level; force was 0.66 +/- 0.04 of pCa 4.0 control. These results indicate that cross-bridge binding in the rigor state alters sTnC structure, whereas cycling cross-bridges have little influence at either submaximum or maximum activating [Ca2+].

Defining actin filament length in striated muscle: rulers and caps or dynamic stability?

Actin filaments (thin filaments) are polymerized to strikingly uniform lengths in striated muscle sarcomeres. Yet, actin monomers can exchange dynamically into thin filaments in vivo, indicating that actin monomer association and dissociation at filament ends must be highly regulated to maintain the uniformity of filament lengths. We propose several hypothetical mechanisms that could generate uniform actin filament length distributions and discuss their application to the determination of thin filament length in vivo. At the Z line, titin may determine the minimum extent and tropomyosin the maximum extent of thin filament overlap by regulating alpha-actinin binding to actin, while a unique Z filament may bind to capZ and regulate barbed end capping. For the free portion of the thin filament, we evaluate possibilities that thin filament components (e.g. nebulin or the tropomyosin/troponin polymer) determine thin filament lengths by binding directly to tropomodulin and regulating pointed end capping, or alternatively, that myosin thick filaments, together with titin, determine filament length by indirectly regulating tropomodulin's capping activity.

Cooperativity and switching within the three-state model of muscle regulation

Thin filament regulation is mediated by the presence of tropomyosin (Tm) and troponin (Tn) on the actin filament. Binding of Tm alone induces two states, closed and open (with the equilibrium between them defined by KT), which differ in their affinity for myosin subfragment 1 (S1). Cooperative switching between the states results in characteristic sigmoidal myosin S1 binding curves. In the presence of Tn and absence of Ca2+, a third state, blocked, has previously been kinetically shown to be present, leading to the three state model of McKillop and Geeves [(1993) Biophys. J. 65, 693-701]. We have measured equilibrium binding of S1 to phalloidin-stabilized pyrene-actin filaments by monitoring the pyrene fluorescence at 50 nM, a concentration 10-fold lower than previously possible. In combination with kinetic studies, we show that the data can be fitted to a modified version of the three-state model with an additional term allowing for a varying apparent cooperative unit size (n). Our results show that the apparent cooperative unit size (n) is dependent upon both the presence of Tn and of Ca2+. Also in the absence of Ca2+, the occupancy of the blocked state (defined by KB) is accompanied by a 2-3-fold reduction in KT. These results are discussed in comparison to the Hill model [(1980) Proc. Natl. Acad. Sci. U.S.A. 77, 3186-3190] and a flexible model of thin filament regulation based upon that of Lehrer et al. [(1997) Biochemistry 36, 13449-13455].

Molecular switches in troponin

The contraction of vertebrate striated muscle contraction, and hence its work output, is controlled by Ca2+, which binds to troponin (Tn) associated with tropomyosin (TM) and actin in the thin filaments. Tn consists of three subunits: TnC, the Ca(2+)-receptor; TnI, an inhibitor of actomyosin activity; and TnT, anchoring Tn to TM. Of the four Ca(2+)-binding sites, I and II in the N-terminal domain are Ca-specific sites, while sites III and IV, the high affinity Ca-Mg sites, are in the C-domain. The former are recognized as the functionally important triggering sites. TnC, whose structure has been solved by X-ray crystallography and recently by high-resolution NMR, contains two homologous globular domains connected by an unusual single alpha-helix. The C-terminal domain exhibits an open hydrophobic area regardless of whether Ca2+ or Mg2+ is bound to sites III and IV. In contrast, the N-terminal domain is a closed structure that opens a hydrophobic patch upon Ca(2+)-binding to its two "triggering" sites producing a TnI binding area. Crosslinking and fragment binding studies indicate that, in the main, the two polypeptide chains run in opposite directions in the complex of TnC with Tn. A model of TnC-TnI interactions based on low angle X-ray and neutron scattering is discussed in light of biochemical and other physico-chemical studies. The opening of the structure in the N-terminal domain of TnC may be regarded as a molecular switch. It activates a molecular switch in TnI, reflected in the movement of portions of its C-terminal half, including Cys 133, away from actin and closer to TnC, as well as other structural changes in TnI. Finally the role of TnT in switching and transmitting the Ca(2+)-signal is discussed.

Modulating factors of calcium-free contraction at low [MgATP]: a physiological study on the steady states of skinned fibres of frog skeletal muscle

Factors that modulate Ca(2+)-free contraction at low [MgATP] were examined by analysing steady tension development in skinned fibres of frog skeletal muscle. The commonly accepted bell-shaped relationship between steady tension and log (1/[MgATP]) was found to be highly susceptible to subtle experimental conditions at the higher [MgATP] side (right limb). The limb shifted to the right with increased fibre thickness, interrupted stirring of the bathing solution, increased temperature and fibre extension, although the effects of temperature and extension were marked only in thick fibres (cross-sectional area > 6000 microns 2). The shift of the right limb was reproduced by an addition of ADP to the bathing solution. These results, together with the extreme steepness of the right limb in thick fibres, suggest that a diffusion-dependent self-regenerative activation occurs in thick fibres in which ADP accumulation and ATP depletion positively feed back through further activation of the myofibrillar ATPase. Numerical simulation supported the hypothesis of the self-regenerative activation under poor diffusion conditions, and suggested that a small rise in temperature and fibre extension can trigger the self-regenerative process at the right limb. Consequently, ADP, temperature and fibre extension are deduced to be the primary potentiators of the activation at low [MgATP]. The high efficiency of ADP in shifting the limb suggests that the activating efficiency of the MgADP-bound actomyosin complex is higher than the nucleotide-free actomyosin complex.

Ca2+ regulates the kinetics of tension development in intact cardiac muscle

The goal of this study was to determine whether Ca2+ plays a role in regulating tension development kinetics in intact cardiac muscle. In cardiac muscle, this fundamental issue of Ca2+ regulation has been controversial. The approach was to induce steady-state tetanic contractions of intact right ventricular trabeculae from rat hearts at varying external Ca2+ concentrations ([Ca2+]) at 22 degreesC. During tetani, cross bridges were mechanically disrupted and the kinetics of tension redevelopment were assessed from the rate constant of exponential tension redevelopment (ktr). There was a relationship between ktr and external [Ca2+] that was similar in form to the relationship between tension and [Ca2+]. Thus a close relationship also existed between ktr and tension (r = 0.88; P < 0. 001); whereas at maximal tetanic tension (saturating cytosolic [Ca2+]), ktr was 16.4 +/- 2.2 s-1 (mean +/- SE, n = 7), at zero tension (low cytosolic [Ca2+]), ktr extrapolated to 20% of maximum (3.3 +/- 0.7 s-1). Qualitatively similar results were obtained using different mechanical protocols to disrupt cross bridges. These data demonstrate that tension redevelopment kinetics in intact cardiac muscle are influenced by the level of Ca2+ activation. These findings contrast with the findings of one previous study of intact cardiac muscle. Activation dependence of tension development kinetics may play an important role in determining the rate and extent of myocardial tension rise during the cardiac cycle in vivo.

A new look at thin filament regulation in vertebrate skeletal muscle

It is 30 years since Ebashi and colleagues showed that Ca2+ ions directly affect regulation of the myosin-actin interaction in muscle through the action of tropomyosin and troponin on muscle thin filaments. It is more than 20 years since the idea was put forward that tropomyosin might act, at least in part, by changing its position on actin, thus uncovering or modifying the myosin binding site on actin when troponin molecules take up Ca2+. Since that time, a great deal of evidence for and against this steric blocking mechanism has been published: a structure for actin filaments at close to atomic resolution has been proposed, and the whole regulation story has become both more complicated and more subtle. Here we review structural and biochemical aspects of regulation in vertebrate skeletal muscle. We show that some basic ideas of the steric blocking mechanism remain valid. We also show that additional factors, such as troponin movements and structural changes within the actin monomers themselves, may be crucial. A number of the resulting regulation scenarios need to be distinguished.

Tropomyosin and troponin regulation of wild type and E93K mutant actin filaments from Drosophila flight muscle. Charge reversal on actin changes actin-tropomyosin from on to off state

In the Drosophila flight muscle actin mutant E93K there is a charge reversal on the surface of actin close to the proposed position of tropomyosin when it is in the off state. Using a quantitative in vitro motility assay we have found that the wild type Drosophila ACT88F actin behaved like rabbit skeletal muscle actin when tropomyosin and troponin were added at pCa5 and pCa9. In contrast the effect of tropomyosin upon the E93K mutant actin filament movement was completely different from wild type and resembled the response of wild type with tropomyosin+troponin at pCa9 (i.e. the filaments were switched off). Velocity of E93K actin did not increase, and the fraction of filaments motile was reduced to less than 15% by adding up to 30 nM tropomyosin. When myosin subfragment-1 modified by N-ethylmaleimide was mixed with mutant E93K actin-tropomyosin filaments we observed that it restored motility of the filaments to the level observed with E93K actin alone. We conclude that electrostatic charge on the surface of domain 2 of actin plays a critical role in determining the state of actin-tropomyosin that is a central feature of the steric blocking mechanism of actin filament regulation.

Structural coupling of troponin C and actomyosin in muscle fibers

EPR of spin labeled TnC at Cys98 was used to explore the possible structural coupling between TnC in the thin filament and myosin trapped in the intermediate states of ATPase cycle. Weakly attached myosin heads (trapped by low ionic strength, low temperature and ATP) did not induce structural changes in TnC as compared to relaxed muscle, as spin labeled TnC displayed the same narrow orientational distribution [Li, H.-C., and Fajer, P. G. (1994) Biochemistry 33, 14324]. Ca2+-binding alone resulted in disordering of the labeled domain of TnC. Additional conformational changes of TnC occurred upon the attachment of strongly bound, prepower stroke myosin heads (trapped by AlF4-). These changes were not present in ghost fibers which myosin had been removed, excluding direct effects of AlF4- on the orientation of TnC in muscle fibers. The postpower stroke heads (rigor.ADP/Ca2+ and rigor/Ca2+) induced further changes in the orientational distribution of labeled domain of TnC irrespective of the degree of cooperativity in thin filaments. We thus conclude that troponin C in thin filaments detects structural changes in myosin during force generation, implying that there is a structural coupling between actomyosin and TnC.

Probing the coupling of Ca2+ and rigor activation of rabbit psoas myofibrillar ATPase with ethylene glycol

We have exploited solvent perturbation to probe the coupling of Ca2+ and rigor activation of the ATPase of myofibrils from rabbit psoas. Three techniques were used: overall myofibrillar ATPases by the rapid-flow quench method; kinetics of the interaction of ATP with myofibrils by fluorescence stopped-flow; and myofibrillar shortening by optical microscopy. Because of its extensive use with muscle systems, ranging from myosin subfragment-1 to muscle fibres, we chose 40% ethylene glycol as the relaxing agent. At 4 degrees C, the glycol had little effect on the myofibrillar ATPase at low [Ca2+], but at high [Ca2+] the activity was reduced 50-fold, close to the level found under relaxing conditions, and there was no shortening. However, the ATPase of chemically cross-linked myofibrils (permanently activated even without Ca2+) was reduced only 3-4-fold. The lesser reduction of the ATPase of permanently activated myofibrils was also observed in single turnover experiments in which activation occurs by a few heads in the rigor state activating the remaining heads. The addition of ADP, which also promotes strong head-thin filament interactions, also activated the ATPase but only in the presence of Ca2+. Further experiments revealed that in 40% ethylene glycol, Ca2+ does initiate shortening but only with the aid of strong interactions and at temperatures above 15 degrees C. This confirms that in the organized and intact myofibril, Ca2+ and rigor activation are coupled, as proposed previously for regulated actomyosin subfragment-1.

The muscle thin filament as a classical cooperative/allosteric regulatory system

It is generally accepted that the regulation of muscle contraction involves cooperative and allosteric interactions among the protein components, actin, myosin, tropomyosin and troponin. But, as yet, the individual role of each component has not been clearly identified. Here we compare the properties of the components of the muscle regulatory system with the corresponding components of two systems, hemoglobin and aspartate transcarbamylase, that are well described by the classical Monod, Wyman and Changeux (MWC) model. The analogy indicates that actin is the catalytic subunit, tropomyosin is the regulatory subunit and troponin in the absence and presence of Ca2+ is the allosteric inhibitor and activator, respectively. The analogy additionally indicates that the substrate is myosin-ATP (or myosin-ADP-Pi) rather than ATP. Also, in contrast to other MWC systems, the activating ligand for actin-tropomyosin is a myosin-nucleotide intermediate or product that binds tightly to actin, rather than the substrate which binds weakly. This tightly bound intermediate switches the system from the off-state to the on-state (T to R-state in MWC nomenclature) in a concerted transition, affecting n actin subunits, allowing force to be developed.

F-actin-binding proteins

The study of proteins that bind filamentous actin (F-actin) is entering an exciting stage as more and more structures are determined. After more than 50 years in which the focus was on muscle proteins, emphasis has recently shifted towards understanding the complex interplay among actin-binding molecules in non-muscle cells. To date, the binding sites for eight classes of filament-binding molecules have been determined by combining low- to intermediate-resolution maps obtained by electron microscopy with atomic structures determined by X-ray crystallography and NMR. Recent results have dramatically accentuated the importance of filament geometry and actin conformation in defining these interactions.

Towards an atomic model of the thick filaments of muscle

The thick filaments of muscle and non-muscle cells are polymers of myosin molecules whose energy-transducing heads lie on the filament surface, where they interact with actin to generate force. A key structural question is how the myosin heads are arranged in the relaxed state, and how this arrangement changes on activation of contraction. We have fitted the atomic structure of the myosin head to the three-dimensional structure of myosin filaments of tarantula muscle determined by electron microscopy to produce a near-atomic model of the head arrangement. A good fit is obtained only when the two heads from a myosin molecule run along the helical tracks antiparallel to each other. Oppositely oriented heads from axially adjacent molecules in a helix interact with each other, with their nucleotide-binding pockets opposed. This arrangement, supported also by crosslinking evidence, suggests a simple mechanism for the stabilization of myosin head helices in relaxed muscle via the formation of intermolecular "dimers" of heads from axially adjacent myosin molecules.

Visualization of caldesmon on smooth muscle thin filaments

Caldesmon, a narrow, elongated actin-binding protein, is found in both nonmuscle and smooth muscle cells. It inhibits actomyosin ATPase and filament severing in vitro, and is thus a putative regulatory protein. To elucidate its function, we have used electron microscopy and three-dimensional image reconstruction to reveal the location of caldesmon on isolated smooth muscle thin filaments. Caldesmon density was clearly delineated in reconstructions and found to occur peripherally, on the extreme outer edge of actin subdomains-1 and 2, without making obvious contacts with tropomyosin strands on the inner domains of actin. When the reconstructions were fitted to the atomic model of F-actin, caldesmon appeared to cover potentially weak sites of myosin interaction with actin, while, together with tropomyosin, it flanked strong sites of myosin interaction, without covering them. These interactions are unlike those of troponin-tropomyosin and therefore inhibition of actomyosin ATPase by caldesmon-tropomyosin and by troponin-tropomyosin cannot occur in the same way. The location of caldesmon would allow it to compete with a number of cellular actin-binding proteins, including those known to sever or sequester actin.

Interaction of F-actin with synthetic peptides spanning the loop region of human cardiac beta-myosin heavy chain containing Arg403

The atomic model of the F-actin-myosin subfragment 1 complex (acto-S-1) from skeletal muscle suggests that the transition of the complex from a weakly to a strongly binding state, generating mechanical force during the contractile cycle, may involve the attachment of the upper 50-kDa subdomain of myosin subfragment 1 (S-1) to the interface between subdomains 1 and 3 of actin. For the human cardiac myosin, this putative interaction would take place at the ordered loop including Arg403 of the beta-heavy chain sequence, a residue whose mutation into Gln is known to elicit a severe hypertrophic cardiomyopathy caused by a decrease of the rate of the actomyosin ATPase activity. Moreover, in several nonmuscle myosins the replacement of a Glu residue within the homolog loop by Ser or Thr also results in the reduction of the actomyosin ATPase rate that is alleviated by phosphorylation. As an approach to the characterization of the unknown interaction properties of F-actin with this particular S-1 loop region, we have synthesized four 17-residue peptides corresponding to the sequence Gly398-Gly414 of the human beta-cardiac myosin. Three peptides included Arg403 (GG17) or Gln403 (GG17Q) or Ser409 (GG17S) and the fourth peptide (GG17sc) was a scrambled version of the normal GG17 sequence. Using fluorescence polarization, cosedimentation analyses and photocross-linking, we show that the three former peptides, but not the scrambled sequence, directly associate in solution to F-actin, at a nearly physiological ionic strength, with almost identical affinities (Kd approximately 40 microM). The binding strength of the F-actin-GG17 peptide complex was increased fivefold (Kd = 8 microM) in the presence of subsaturating concentrations of added skeletal S-1 relative to actin, without apparent competition between the peptide and S-1. Each of the three actin-binding peptides inhibited the steady-state actin-activated MgATPase of skeletal S-1 by specifically decreasing about twofold the Vmax of the reaction without changing the actin affinity for the S-1-ATP intermediate. Cosedimentation assays indicated the binding of about 0.65 mol peptide/mol actin under conditions inducing 70% inhibition. Collectively, the data point to a specific and stoichiometric interaction of the peptides with F-actin that uncouples its binding to S-1 from ATP hydrolysis, probably by interfering with the proper attachment of the S-1 loop segment to the interdomain connection of actin.

Actin-tropomyosin activation of myosin subfragment 1 ATPase and thin filament cooperativity. The role of tropomyosin flexibility and end-to-end interactions

Tropomyosin (Tm) bound to actin induces cooperative activation of actomyosin subfragment 1 (actin-S1) ATPase, observed as a sigmoid ATPase vs [S1] dependence. The activation is much steeper for gizzard muscle Tm (GTm) than for rabbit skeletal Tm (RSTm). To investigate if this greater cooperativity is due to increased communication between GTms along the thin filament, we studied effects of S1 binding on the state of actin-Tm using the fluorescence of pyrene-labeled Tm. Kinetic and equilibrium studies provided values for n, the apparent cooperative unit size [Geeves, M. A., and Lehrer, S. S. (1994) Biophys. J. 67, 273]. We report comparative studies of Tm-actin-S1 ATPase with values of n using GTm, RSTm, and 5aTm, a 1/7 shorter nonmuscle Tm from rat fibroblast cells [Pittenger, M. F., et al. (1994) Curr. Opin. Cell Biol., 6, 96]. 5aTm and GTm produce similar cooperative activation of actin-S1 ATPase and have similar n values that are 2-fold greater than RSTm, indicating a correlation between ATPase activation and n value. This appears to be due to the similarity of the C-terminal amino acid sequences of 5a and GTm which produce strong end-to-end interactions. The results are discussed in terms of a continuous flexible Tm strand on the actin filament.

Perturbations of functional interactions with myosin induce long-range allosteric and cooperative structural changes in actin

The role of the rotational dynamics of actin filaments in their interaction with myosin was studied by comparing the effect of myosin subfragment 1 (S1) with two other structural perturbations, which have substantial inhibitory effects on activation of myosin ATPase and in vitro motility of F-actin: (1) binding of the antibody fragment Fab(1-7) against the first seven N-terminal residues and (2) copolymerization with monomers treated with the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC), referred to as EDC-actin. The rotational motion of actin was measured by time-resolved phosphorescence anisotropy (TPA) of erythrosin iodoacetamide (ErIA) attached to Cys 374 on actin. The binding of S1 in a rigor complex (no nucleotide) induced intramonomer (allosteric) and intermonomer (cooperative) structural changes that increased the residual anisotropy of labeled F-actin, indicating a conformational change in the region of the C terminus. Similar allosteric and cooperative changes were induced by binding of Fab(1-7) and by copolymerization of the ErIA-labeled actin monomers with EDC-actin. This suggests that the functional perturbations transform actin to a form resembling the rigor actomyosin complex. The correlation of the perturbation-induced changes in TPA of actin with the functional effects suggests that the actomyosin interaction can be inhibited by stabilization of actin in one of its structural intermediates.

3-D image reconstruction of reconstituted smooth muscle thin filaments containing calponin: visualization of interactions between F-actin and calponin

Calponin is a putative thin filament regulatory protein of smooth muscle that inhibits actomyosin ATPase in vitro. We have used electron microscopy and three-dimensional reconstruction to elucidate the structural organization of calponin on actin and actin-tropomyosin filaments. Calponin density was clearly delineated in the reconstructions and found to occur peripherally along the long-pitch actin-helix. The main calponin mass was located over sub-domain 2 of actin, and connected axially adjacent actin monomers by binding to the "upper" and "lower" edges of sub-domains 1 of each actin. When the reconstructions were fitted to the atomic model of F-actin, calponin appeared to contact actin near the N terminus and at residues 349 to 352 close to the C terminus of sub-domain 1 on one monomer. It also touched residues 92 to 95 of sub-domain 1 on the axially neighboring actin and continued up the side of this monomer as far as residues 43 to 48 of sub-domain 2. These positions are consensus binding sites for a number of actin-associated proteins and are also near to sites of weak myosin interaction. Calponin did not appear to block strong myosin binding sites on actin. In contrast to the calponin mass which appeared monomeric in reconstructions, tropomyosin formed a continuous strand of added density along F-actin. When added to tropomyosin-containing filaments, calponin caused a shift of tropomyosin away from sub-domain 1 towards sub-domain 3 of actin, exposing strong myosin-binding sites that were previously covered by tropomyosin. This structural effect is unlike that of troponin and therefore inhibition of actomyosin ATPase by calponin and troponin cannot be strictly analogous. The location of calponin would allow it to directly compete or interact with a number of actin-binding proteins.

Restoration of defective mechanochemical properties of cleaved actins by native tropomyosin: involvement of the 40-50 loop in subdomain 2 of actin in interaction with myosin and tropomyosin

Native tropomyosin activated sliding movement in vitro of F-actin with ATP by 30%. Actin cleaved at the 40-50 loop by subtilisin or proteinase K slid on HMM much slower than intact actin, but native tropomyosin strikingly recovered this defective motility of cleaved actin by 2 to 3 times. On the other hand, with ATP analogues of CTP and ITP, sliding movements of cleaved actin and particularly intact actin were inhibited by native tropomyosin, indicating that native tropomyosin augmented specificity of the myosin substrate of NTP. These results suggested that the 40-50 loop in the small domain 2 of actin interacted directly or indirectly with tropomyosin and play a significant role in cross talk between myosin and native tropomyosin.

Three-dimensional image reconstruction of reconstituted smooth muscle thin filaments: effects of caldesmon

Caldesmon inhibits actomyosin ATPase and filament sliding in vitro, and therefore may play a role in modulating smooth and non-muscle motile activities. A bacterially expressed caldesmon fragment, 606C, which consists of the C-terminal 150 amino acids of the intact molecule, possesses the same inhibitory properties as full-length caldesmon and was used in our structural studies to examine caldesmon function. Three-dimensional image reconstruction was carried out from electron micrographs of negatively stained, reconstituted thin filaments consisting of actin and smooth muscle tropomyosin both with and without added 606C. Helically arranged actin monomers and tropomyosin strands were observed in both cases. In the absence of 606C, tropomyosin adopted a position on the inner edge of the outer domain of actin monomers, with an apparent connection to sub-domain 1 of actin. In 606C-containing filaments that inhibited acto-HMM ATPase activity, tropomyosin was found in a different position, in association with the inner domain of actin, away from the majority of strong myosin binding sites. The effect of caldesmon on tropomyosin position therefore differs from that of troponin on skeletal muscle filaments, implying that caldesmon and troponin act by different structural mechanisms.

The active state of the thin filament is destabilized by an internal deletion in tropomyosin

The function of three of tropomyosin's sequential quasiequivalent regions was studied by deletion from skeletal muscle alpha-tropomyosin of internal residues 49-167. This deletion mutant tropomyosin spans four instead of the normal seven actins, and most of the tropomyosin region believed to interact with troponin is retained and uninterrupted in the mutant. The mutant tropomyosin was compared with a full-length control molecule that was modified to functionally resemble muscle tropomyosin (Monteiro, P. B., Lataro, R. C., Ferro, J. A., and Reinach, F. C. (1994) J. Biol. Chem. 269, 10461-10466). The tropomyosin deletion suppressed the actin-myosin subfragment 1 MgATPase rate and the in vitro sliding of thin filaments over a heavy meromyosin-coated surface. This inhibition was not reversed by troponin plus Ca2+. Comparable tropomyosin affinities for actin, regardless of the deletion, suggest that the deleted region has little interaction with actin in the absence of other proteins. Similarly, the deletion did not weaken binding of the troponin-tropomyosin complex to actin. Furthermore, Ca2+ had a 2-fold effect on troponin-tropomyosin's affinity for actin, regardless of the deletion. Notably, the deletion greatly weakened tropomyosin binding to myosin subfragment 1-decorated actin, with the full-length tropomyosin having a 100-fold greater affinity. The inhibitory properties resulting from the deletion are attributed to defective stabilization of the myosin-induced active state of the thin filament.

Cooperative effect of calcium binding to adjacent troponin molecules on the thin filament-myosin subfragment 1 MgATPase rate

The myosin subfragment 1 (S1) MgATPase rate was measured using thin filaments with known extents of Ca2+ binding controlled by varying the ratio of native cardiac troponin versus an inhibitory troponin with a mutation in the sole regulatory Ca2+ binding site of troponin C. Fractional MgATPase activation was less than the fraction of troponins that bound Ca2+, implying a cooperative effect of bound Ca2+ on cross-bridge cycling. Addition of phalloidin did not alter cooperative effects between bound Ca2+ molecules in the presence or absence of myosin S1. When the myosin S1 concentration was raised sufficiently to introduce cooperative myosin-myosin effects, lower Ca2+ concentrations were needed to activate the MgATPase rate. MgATPase activation remained less than Ca2+ binding, implying a true, not just an apparent, increase in Ca2+ affinity. MgATPase activation by Ca2+ was more cooperative than could be explained by cooperativeness of overall Ca2+ binding, the discrepancy between Ca2+ binding and MgATPase activation, or interactions between myosins. The results suggest the thin filament-myosin S1 MgATPase cycle requires calcium binding to adjacent troponin molecules and that this binding is cooperatively promoted by a single cycling cross-bridge. This mechanism is a potential explanation for Ca2+-mediated regulation of cross-bridge kinetics in muscle fibers.