- Research article
- Open Access
Differential scanning calorimetry study of glycerinated rabbit psoas muscle fibres in intermediate state of ATP hydrolysis
© Dergez et al; licensee BioMed Central Ltd. 2007
- Received: 03 August 2006
- Accepted: 24 June 2007
- Published: 24 June 2007
Thermal denaturation experiments were extended to study the thermal behaviour of the main motor proteins (actin and myosin) in their native environment in striated muscle fibres. The interaction of actin with myosin in the highly organized muscle structure is affected by internal forces; therefore their altered conformation and interaction may differ from those obtained in solution. The energetics of long functioning intermediate states of ATP hydrolysis cycle was studied in muscle fibres by differential scanning calorimetry (DSC).
SETARAM Micro DSC-II was used to monitor the thermal denaturation of the fibre system in rigor and in the presence of nucleotide and nucleotide analogues. The AM.ADP.Pi state of the ATP hydrolysis cycle has a very short lifetime therefore, we mimicked the different intermediate states with AMP.PNP and/or inorganic phosphate analogues Vi and AlF4 or BeFx. Studying glycerol-extracted muscle fibres from the rabbit psoas muscle by DSC, three characteristic thermal transitions were detected in rigor. The thermal transitions can be assigned to myosin heads, myosin rods and actin with transition temperatures (Tm) of 52.9 ± 0.7°C, 57.9 ± 0.7°C, 63.7 ± 1.0°C. In different intermediate states of the ATP hydrolysis mimicked by nucleotide analogues a fourth thermal transition was also detected which is very likely connected with nucleotide binding domain of myosin and/or actin filaments. This transition temperature Tm4 depended on the mimicked intermediate states, and varied in the range of 66°C – 77°C.
According to DSC measurements, strongly and weakly binding states of myosin to actin were significantly different. In the presence of ADP only a moderate change of the DSC pattern was detected in comparison with rigor, whereas in ADP.Pi state trapped by Vi, AlF4 or BeFx a remarkable stabilization was detected on the myosin head and actin filament which is reflected in a 3.0 – 10.0°C shift in Tm to higher temperature. A similar effect was observed in the case of the nonhydrolyzable AMP.PNP analogue. Differential DSC measurements suggest that stabilization actin structure in the intermediate states of ATP hydrolysis may play an additional role in actin-myosin interaction.
- Differential Scanning Calorimetry
- Fibre Bundle
- Thermal Transition
- Differential Scanning Calorimetry Measurement
- Nucleotide Binding Site
Force generation in muscle during contraction arises from direct interaction of the two main protein components of the muscle, myosin and actin. The process is driven by the energy liberated from the hydrolysis of ATP. In the presence of Ca2+, the energy released from ATP hydrolysis produces conformational changes in myosin and actin, which can be manifested as an internal motion of the myosin head while bound to actin [1–9]. The interaction results in a strain in the head portion of myosin in an ATP-dependent manner and the structural changes lead to a large rotation of the myosin neck region relieving the strain.
The powerful differential scanning calorimetry (DSC) technique allows the derivation of heat capacity of proteins as a function of temperature. From decomposition of the thermal unfolding patterns, it is possible to characterize the structural domains of macromolecules [10–12]. Using the advantage of the SETARAM microcalorimeter in this work, we tried to approach the temperature-induced unfolding processes in different intermediate states of ATP hydrolysis in striated muscle fibres. The main proteins in fibre system are subjected to stabilizing forces to maintain the ability of fibres to contract and to keep the organized structure which can modify the thermodynamic properties of the composite proteins. We studied the fibre system prepared from psoas muscle of rabbit in rigor, strongly binding and weakly binding states of myosin to actin. In the weakly binding state the inorganic phosphate (Pi) was substituted by the phosphate analogue orthovanadate, AlF4 and BeFx.
Evaluation of thermal transitions in muscle fibres
where N, D as well as F stand for the native, denatured and final state of the protein, the extent of the irreversibility is determined by the rate constant k2 of the D → F step, and its activation energy determines gives the rate of unfolding. Recently, it was shown that in some cases the thermodynamic parameters could be deduced from the standard treatment of the heat capacity curves .
DSC results on muscle fibres: transition temperatures in range of 40 – 85°C. Glycerinated muscle fibres prepared from psoas muscle of rabbit were measured in rigor, strongly and weakly binding state of myosin to actin. The transition temperatures were derived from the original DSC thermograms
Thermal transitions in rigor muscle
Earlier measurements performed on myosin showed that at least five endothermic transitions could be observed on bovine heart myosin. The peak maxima were at 17.5, 41.5, 45, 48 and 54.5°C; the estimated transition enthalpies were 150 kcal/mol, 163 kcal/mol, 277 kcal/mol, 301 kcal/mol and 747 kcal/mol [18, 19], but no assignment to the peaks was reported. Slightly different values on S1 were derived by Levitsky's group . As regards the DSC profile of the second main protein in striated muscle, the actin, it is known from literature that DSC measurements on isolated actin showed a single thermal transition. Its melting temperature varied in the range of 63°C and 70°C, depending on the form of actin and the method used in the experiments [21–23]. Structure stabilization of actin filaments in solution by phalloidin and/or jaspakinolide significantly affected the transition temperature as well . Their effects propagated cooperatively along the actin filaments . The contribution of other components of the thin filament, tropomyosin and troponin, to the total enthalpy of denaturation can be neglected in first approximation, because their estimated w% is smaller than 10 w% of the total protein content . Experiments performed on isolated tropomyosin in solution also showed a single sharp thermal transition at 41°C. Tropomyosin in complex with F-actin had no effect on the thermal unfolding of F-actin .
In the case of muscle fibres, the constraint generated by filament association and protein-protein interaction increases the structural stability of the supramolecular structure, and it results in larger thermal stability of the system. The structure formation alters the dynamic and energetic properties of the contractile proteins, the consequence of that is the shift of the melting points measured in solution on intact myosin from rabbit psoas muscle to higher temperatures in rigor . This increase of transition temperatures is evidence that particular regions of myosin and actin are subjected to stabilizing forces in the filament system leading to alteration of the transition temperatures. Moreover, the generated internal forces might induce changes in the domain-domain interactions and communications as well. However, it is suggested that during domain-domain and protein-protein interaction, only the protein with a lower transition midpoint is affected by the others possessing a higher transition temperature . In the case of acto.S1 complex, S1 unfolded at much lower temperature than F-actin, but the molecules of S1 remained bound to F-actin even after their full denaturation and thereby S1 could affect the thermal stability of F-actin .
Effect of nucleotides on thermal transitions
Spectroscopic measurements based on spin labelling EPR technique and fluorescence resonance energy transfer showed little differences between the rigor and the ADP states in muscle fibres [31, 32]. Maleimide or isothiocyanate spin labels bound to Cys 707 residue of myosin head near to the ATP binding site showed almost identical rotational correlation time in millisecond time range in the saturation transfer EPR time domain in both states. However, the orientation distribution of the attached labels with respect to the longer axis of the fibres was different. This supports the view that the specific binding of ADP to myosin produces a conformational change in the environment of the nucleotide-binding site, and seemingly, this does not lead to overall dynamic change of the motor domain. In contrast, DSC measurements support the view that ADP binding induces global conformational change that leads to structure stabilizing effect in the fibre system.
EPR measurements on spin-labelled myosin in muscle fibres at room temperature showed that the ordered structure in weakly binding state of myosin to actin, which can be recorded in rigor, disappeared and the rotational correlation time of myosin heads was significantly reduced . The rotational correlation time of about 1 ms in rigor decreased with about one order of magnitude calculated from ST EPR spectra. This mobility change suggests the dissociation of myosin heads from actin or the existence of a non-specific binding of myosin heads to actin, and these actin-bound heads are mobile in the weakly binding state . The mean orientation of myosin heads to actin filaments is about 90°, and a large effect of such heads to the actin dynamics is not expected.
In order to interpret the DSC results, we might adopt the suggestion by Hozumi and Bombardier and co-workers about the existence of a second nucleotide binding site on F-actin [38, 39]. Unfortunately, little is known about the properties and the role of the second nucleotide binding site on actin. Based on former observations, it is possible to give alternative explanations. The binding of nucleotides or nucleotide analogues would affect the stability of thin filaments against heating. Carlier and co-workers reported that phosphate release in actin was reversible; Pi could rebound to actin by producing F-actin-ADP.Pi filament, which is more stable than ADP-F-actin . Orlova and Egelman [41, 42] have also shown by EM that beryllium fluoride and phosphate made the flexible filaments rigid which may support an induced communication between the nucleotide binding site and other region of actin protomers. The perturbations spread to neighbouring protomers and thereby affect the dynamic nature of F-actin . This increased stability of F-actin filaments could lead to increased heat absorption.
The second possible explanation of the DSC experiments might relay on the assumption that F-actin molecules in fibres could form two populations in the presence of nucleotides and nucleotide analogues: (i) protomers with MgADP or MgADP-Pi analogue in the nucleotide cleft, which distributed randomly on the filaments, or (ii) MgADP protomers in the nucleotide cleft and protomers with MgADP – Pi analogue bound to the second nucleotide binding sites. The binding of Vi, AlF4 or BeFx to F-actin increases the stability of filaments and leads to the shift of Tm3. The fourth heat transition Tm4 depended on the bound nucleotides as shown by Figs 1 and 5 and Table 1. The largest effect was detected in the case of BeFx, and only a small effect was obtained in the presence of Pi. DSC measurements performed on F-actin solution support the view that BeFx and a lesser extent AlF4 stabilize F-actin, and they protect it from heat denaturation [44, 45]. It cannot be excluded that there is a possibility that the increased stability of actin filaments in the presence of ATP and inorganic phosphate during contraction might have significance in the force generation . Recent DSC experiments do not support this assumption. The hydrolysis product Pi can bind to ADP-actin subunits and the newly formed ADP.Pi-actin subunits dissociates much slowly from filament ends than ADP-actin subunits . So, the change in filament flexibility is affected by the ADP.Pi-subunits at the end of filaments which might produce an additional stability in the muscle machine. However, DSC measurements showed little effect in the presence of Pi, therefore only a minor effect is expected in contrast to the increased filament stability evoked by inorganic phosphate analogues as Vi, AlF4 or BeFx.
The DSC experiments carried out on muscle fibres are an extension of former measurements performed on isolated motor proteins (actin and myosin) and their complexes in solution. The approximation of the muscle motor system in our experiments takes into account the specific interaction between the basic motor proteins in highly organized system that allows cyclic motion in the presence of ATP.
The thermal transitions are calorimetrically irreversible in the muscle fibre system as well, therefore the DSC traces are kinetically controlled and scan-rate dependent, and the application of theory of irreversible thermal denaturation is required. The experienced scan-rate effect permitted the decomposition of the DSC traces into separate transitions and the assignment to protein subunits. In both strongly and weakly binding states of myosin to actin, nucleotides and nucleotide analogues affected specifically the thermal transitions of the myosin heads.
In order to assign the transitions to the macromolecular components, a differential method was introduced for interpretation of DSC data. The analysis showed the role of actin in this complex system containing extra ADP.Pi analogues, as ADP.Vi, ADP.AlF4 or ADP.BeFx.
ADP, ATP, 5'-adenylyl-imido-diphosphate (AMP.PNP), aluminium chloride, beryllium sulphate, P1, P5-di (adenosine-5') pentaphosphate, EGTA, glycerol, lactic dehydrogenase, 4-morpholinepropanesulfonic acid (MOPS), phosphoenol pyruvic acid, pyruvate kinase and sodium fluoride were obtained from Sigma (Germany).
Glycerol-extracted muscle fibre bundles were prepared from rabbit psoas muscle as described earlier . Strong actin binding state to myosin (AM.ADP), as well as weak actin binding transition state (AM.ADP.Pi) was monitored. In experiments involving MgADP, the activity of adenylate kinase was inhibited by the addition of 50 μM diadenosine pentaphosphate. AM.ADP.Pi state was mimicked by addition of ATP and beryllium or aluminium fluoride or orthovanadate. Beryllium fluoride and aluminium fluoride were prepared from 10 mM NaF and 3 mM AlCl3 and BeSO4 immediately before experiments. Muscle fibres were stored in solution containing 80 mM KPr, 5 mM ATP, 5 mM MgCl2, 1 mM EGTA in 20 mM MOPS pH 7.0 plus the corresponding chemicals for 15 minutes at 0°C and then a DSC measurement was taken. The fibre bundles prepared in this way were able to develop tension and they could repeatedly shorten and relax in a suitable buffer solution.
Manipulations with fibre preparations
Myosin was extracted from fibres in a solution containing 0.3 M KCl, 0.15 M K-phosphate, 2 mM EDTA and 1 mM ATP, pH 6.5 at 0°C for 15–20 min before DSC measurements.
The ATPase activity was determined using a pyruvate kinase-lactate dehydrogenase coupled optical test [48, 49] as described in an earlier paper . The decrease of absorbance at 340 nm resulted in a straight line, and from its slope the ATPase activity was estimated. The Mg2+-ATPase activity (μmmole of Pi/mg myosin.min) was 4.131 ± 0.718 μmmole of Pi/mg myosin.min (n = 4). The ATPase activity of active fibre bundles was 5.565 ± 0.816 μmmole of Pi/mg myosin.min (n = 4).
Thermal unfolding of muscle proteins in different states was monitored by a SETARAM Micro DSC-II calorimeter. Conventional Hastelloy batch vessels were used with 850 μL sample volume (muscle fibres plus buffer) in average. Typical muscle wet weights for calorimetric experiments were between 200 – 250 mg. Rigor buffer was used as a reference sample. The sample and reference vessels were equilibrated with a precision of ± 0.1 mg. The repeated scan of denatured sample was used as baseline reference, which was subtracted from the original DSC curve. The decomposition of the heat transition trace of muscle fibres was performed by the method of the 'successive annealing' suggested for the analysis of complex systems, as muscle proteins [20, 50]. A heating-cooling-heating cycle up to the subsequent heat transition maximum was repeated to derive the unfolding of the consecutive components of muscle fibres, which have different thermal stability.
Evaluation of DSC measurements
In strongly and weakly binding states of myosin to actin the thermograms could be decomposed into four separate transitions in the main transition temperature range. Deconvolution into four components was performed by using PeakFit 4.0 software from SPSS Corporation. For analysis of the single thermal transitions, Gaussian functions were assumed. The program allowed the determination of peak centre, full width at half maxima and % area of the single transitions. A square matrix presented the overlap areas between any two peaks. A laboratory developed computer program was used to subtract the single transitions. It was assumed that the different nucleotides and nucleotide analogues do not significantly affect the thermal properties of the myosin rod, and one population of actin is not perturbed by myosin in the weakly binding state of myosin to actin in the first approximation.
This work was supported in part by grants from the National Research Foundation (OTKA T 030248). The SETARAM Micro DSC-II used in the experiments were purchased with funds provided by the National Research Foundation Grant CO-272.
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