Differential scanning calorimetry of the unfolding of myosin subfragment 1, subfragment 2, and heavy meromyosin

The thermal unfolding of rabbit skeletal heavy meromyosin (HMM), myosin subfragment 1, and subfragment 2 has been studied by differential scanning calorimetry (DSC). Two distinct endotherms are observed in the DSC scan of heavy meromyosin. The first endotherm, with a Tm of 41 degrees C at pH 7.9 in 0.1 M KCl, is assigned to the unfolding of the subfragment 2 domain of HMM based on scans of isolated subfragment 2. The unfolding of the subfragment 2 domain is reversible both in the isolated form and in HMM. The unfolding of subfragment 2 in HMM can be fit as a single two-state transition with a delta Hvh and delta Hcal of 161 kcal/mol, indicating that subfragment 2 exists as a single domain in HMM. The unfolding of subfragment 2 is characterized by an extraordinarily large delta Cp of approximately 30,000 cal/(deg.mol). In the presence of nucleotides, the high-temperature HMM endotherm with a Tm of 48 degrees C shifts to higher temperature, indicating that this peak corresponds to the unfolding of the subfragment 1 domain. This assignment has been confirmed by comparison with isolated subfragment 1. The stabilizing effect of AMPPNP was significantly greater than that of ADP. The vanadate-trapped ADP species was slightly more stable than M.AMPPNP with a Tm at 58 degrees C. The unfolding of subfragment 1, both in the isolated form and in HMM, was irreversible. Only a single endotherm was noted in the DSC scans of the subfragment 1 domain of HMM and in freshly prepared subfragment 1 complexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of the reaction of N-ethylmaleimide with myosin and heavy meromyosin subfragment 1

Myosin reacted at low ionic strength with NEM forms an actomyosin which is Ca⁺⁺ insensitive. With HMM S-1 the reaction with NEM causes a marked loss of the actin activated ATPase activity and the Ca⁺⁺ sensitivity is reduced but not eliminated. The presence of actin during the sulfhydryl reaction does not significantly alter this result. HMM S-1 prepared from myosin previously desensitized by NEM regains Ca⁺⁺ sensitivity. These results indicate that the conformations of myosin and HMM S-1 are different and could reflect a difference between insoluble (filamentous) myosin and myosin, or its fragments, in solution.     

Transient phase of adenosine triphosphate hydrolysis by myosin, heavy meromyosin, and subfragment 1.

The transient phase of adenosine triphosphate (ATP) hydrolysis (early burst) was investigated for myosin, heavy meromyosin (HMM), and subfragment 1 (S-1) over a range of temperatures and pH's. The burst size at pH 8,20 degrees C, is 0.8-0.85, based on steady-state and transient measurements. The equilibrium constant for the enzyme-substrate to enzyme-product transition is 0.85 +/- 0.05. It is concluded that both myosin heads undergo the rapid hydrolysis step and that there are no significant differences for S-1 vs. HMM or myosin. The transient data are fitted reasonably well by a single rate process, but available evidence is consistent with some heterogeneity and a range of rate constants differing by a factor of two. At pH 6.9 and 3 degrees C, the burst size is 0.5 and the hydrolysis is slower than the configuration change measured by fluorescence. The results are consistent with the kinetic scheme (see article). The lower burst at low temperature and pH can be partly explained by a reduction in the equilibrium constant, K3, and ATP can be synthesized on the enzyme by a pH-temperature jump.     

Affinity chromatography of myosin, heavy meromyosin, and heavy meromyosin subfragment one on F-actin columns stabilized by phalloidin.

A method of affinity chromatography based on the trapping of actin filaments within agarose gel beads is described. This method can be used for the purification of myosin and its active proteolytic subfragments, as well as for studies on the interaction between actin and these proteins. Actin columns stabilized by phalloidin bind myosin, heavy meromyosin (HMM), and heavy meromyosin subfragment 1 (HMM-S1) specifically and reversibly. The effect of pyrophosphate and KCl on the dissociation of actomyosin, acto-HMM, or acto-HMM-S1 complex is reported. We also describe the single-step purification of myosin from a crude rabbit psoas muscle extract.     

Calorimetric studies of the ADP binding to myosin subfragment 1, heavy meromyosin, and to myosin filaments.

A calorimetric titration method was used to study the ADP binding to the chymotryptic subfragments of myosin, heavy meromyosin (HMM) and myosin subfragment 1 (S-1), and to myosin aggregated into filaments at low ionic strength. The binding constant (K) and heat of reaction (deltaH, kiloJoules (moles of ADP bound)-1) were determined. For HMM in 0.5 M KCl, 0.01 M MgCl2, 0.02 M Tris (pH 7.8) at 12 degrees, log K = 5.92 +/- 0.13 and deltaH = -70.9 +/- 3.6 kJ mol-1. These results agree with our previous findings for myosin in 0.5 M KCl at 12 degrees. When the KCl concentration was reduced to 0.1 M, the binding constant did not change significantly (log K = 6.09 +/- 0.06) but the binding was more exothermic (deltaH = -90.1 +/- 3.3 kJ mol-1). Similar results were obtained for myosin filaments in 0.1 M KCl and also for both the isoenzymes of S-1(S-1(A1) and S-1(A2) in 0.1 M KCl. In 0.5 M KCl, the binding curves suggest that about one ADP is bound per active site, but as 0.1 M KCl, the apparent stoichiometry drops from 0.7 to 0.75. The most probable explanation is that there is some site heterogeneity which is more evident at lower ionic strength.     

The biological activity of subfragment 1 prepared from heavy meromyosin

1. The action of trypsin, chymotrypsin and subtilisin on the adenosine-triphosphatase and actin-combining activities, as measured by viscometric means, of H-meromyosin were compared. 2. Subfragment 1 produced by prolonged tryptic digestion has a molecular weight of 129000. 3. The preparations isolated by gel filtration and actin combination were shown to be similar. 4. Subfragment-1 preparations possess appreciably higher adenosine-triphosphatase activities than H-meromyosin when related to total nitrogen. 5. Chromatographic and gelfiltration studies indicated that adenosine-triphosphatase activity is not distributed uniformly in all fractions of subfragment 1. 6. The Ca(2+)-activated adenosine triphosphatase of subfragment 1 was stimulated by thiol reagents in a similar fashion to myosin and H-meromyosin. 7. Subfragment 1 differed from myosin and H-meromyosin in that its adenosine triphosphatase was only slightly activated by Mg(2+) in the presence of actin. 8. A subfragment-1-like component was obtained by chymotryptic digestion of H-meromyosin. 9. The results obtained from enzymic and hydrodynamic studies and from amino acid analyses are compatible with the concept of one molecule of H-meromyosin giving rise to one molecule of subfragment 1 on proteolytic digestion.     

Microcalorimetric measurement of the enthalpy of binding of rabbit skeletal myosin subfragment 1 and heavy meromyosin to F-actin

The heat of binding of rabbit skeletal myosin subfragment 1 (myosin-S1) and heavy meromyosin (HMM) to F-actin has been measured by batch calorimetry. Proton release measurements in unbuffered solutions indicate that less than 0.1 mol of protons is absorbed or released per mol of myosin head bound to actin. Hence, the measured heats are approximately equal to the enthalpy of myosin-S1 and HMM binding to actin. The enthalpy of binding of myosin-S1 to actin was +22 +/- 3 and +27 +/- 5 kJ/mol of myosin-S1 in two series of experiments at 12 degrees C and +26 +/- 5 kJ/mol of myosin-S1 at 0 degrees C, indicating that delta Cp for this reaction in the range of 0-12 degrees C is small (-80 J/mol/K). The enthalpy of binding of HMM to actin at 12 degrees C was found to be +26 +/- 1 kJ/mol of myosin head. The enthalpies determined here and the equilibrium constants obtained from the literature for measurements at 20 degrees C under identical solvent conditions were used to estimate the entropy of the association of myosin S1 and HMM with F-actin: +235 J/mol/K for myosin-S1 and +190 J/mol of myosin head/K for HMM. Thermodynamic parameters of the interaction of myosin-S1 with actin and ADP or AMP-PNP can be evaluated using the enthalpy of association of myosin-S1 with actin determined here, together with literature values for the equilibrium constants and enthalpies of binding of these nucleotides to myosin-S1. The calculated enthalpies of binding of ADP or AMP-PNP to actomyosin-S1 are small and negative.     

Substructure of skeletal myosin subfragment 1 revealed by thermal denaturation

The stability of myosin subfragment 1 (S1) to thermal denaturation has been followed by limited tryptic proteolysis. Digestions done during the thermal denaturation show that at temperatures at and above 37 degrees C there is a marked increase in the susceptibility of S1 to tryptic degradation, as evidenced by the loss of all bands corresponding to the normally trypsin-resistant fragments of 50, 27, and 21 kDa of the heavy chain and to the light chain. The enhanced digestion of S1 appears to be due to a general unfolding of all segments of S1, although the 50-kDa segment appears to unfold at a lower temperature than the remainder of the S1 structure. Digestions done after 30-min exposure to higher temperatures or after subsequent cooling to 25 degrees C show marked differences in the susceptibility of the S1 to trypsin. This suggests that, on cooling, a substantial portion of the S1, but not the 50-kDa segment, is capable of refolding to a state corresponding closely to that in the native S1. These data indicate that in terms of thermal denaturation the S1 behaves as though it is comprised of two domains--an unstable 50-kDa domain and a more stable domain comprised of the 27- and 21-kDa segments of the heavy chain interacting with the light chain, as proposed recently by Setton and Muhlrad [Setton, A., & Muhlrad, A. (1984) Arch. Biochem. Biophys. 235, 411-417]. The rates of thermal inactivation of the ATPase of S1 are found to correspond closely to the decay rates for the 50-kDa fragment, suggesting that this segment in S1 is closely associated with the ATPase function of the protein.     

Enzymatic properties of the heavy meromyosin subfragment of cardiac myosin from normal and thyrotoxic rabbits.

Myosin from the hearts of thyrotoxic animals (myosin-T) exhibits elevated Ca2+-ATPase activity. To clarify the physiological significance of this increased activity, we have investigated the steady state kinetics of the interaction of actin and MgATP with the double-headed heavy meromyosin subfragment of cardiac myosin from thyrotoxic rabbits (HMM-T). The enhanced Ca2+-ATPase activity of myosin-T was completely retained in HMM-T. The Vmax for actin-activated MgATP hydrolysis by HMM-T (1.08 +/- 0.10 mumol of Pi/mg/min). Under physiological ionic conditions, the Vmax was 0.14 +/- 0.02 mumol of Pi/mg/min as compared with the normal value of 0.08 +/- 0.01 mumol of Pi/mg/min. Furthermore, the salt dependence of Vmax and Kapp for the actin-activated ATPase of HMM-T differed markedly from normal and resembled that usually associated with the single-headed (S1) cleavage product of myosin. These results suggest that the changes in enzymatic properties of myosin-T are responsible for the increased speed of contraction observed in the hearts of thyrotoxic animals. Also, the alteration in the interaction of HMM-T with actin suggests that a loss of cooperativity between the myosin heads may occur.     

Preparation and characterization of heavy meromyosin and subfragment 1 from vertebrate cytoplasmic myosins

The soluble fragments of myosin, heavy meromyosin (HMM), and subfragment 1 (S-1) have been instrumental in elucidating the kinetic mechanisms of the actin-activated MgATPase activity of both skeletal and smooth muscle myosin. To date, relatively little has been published on these fragments from vertebrate cytoplasmic myosins. We now describe the preparation and steady-state kinetic characterization of S-1 and HMM from human platelet and avian intestinal epithelial brush border myosin. The HMM prepared from each of these tissues was similar both in their SDS-polyacrylamide gel pattern and in their steady-state kinetic properties. The Vmax of the actin-activated MgATPase activity varied between 0.8 and 2.5 s-1, and the KATPase (the apparent dissociation constant derived from a double-reciprocal plot of the MgATPase activity) was about 1-2 microM. This low value for the apparent dissociation constant was similar to the dissociation constant of HMM for actin directly measured under similar conditions and is about 40 times lower than that determined with avian smooth muscle HMM. The KATPase of the cytoplasmic HMM was only slightly increased when the ionic strength was raised from 12 to 112 mM.     

Thermal denaturation of the alkali light chain-20 kDa fragment complex obtained from myosin subfragment 1.

The thermal denaturation of the myosin subfragment 1 (S1) from rabbit skeletal muscle and of its derivatives obtained by tryptic digestion has been studied by means of differential scanning calorimetry. Two distinct thermal transitions were revealed in the isolated complex of the C-terminal 20 kDa fragment of the S1 heavy chain with the alkali light chain. These transitions were identified by means of a thermal gel analysis method. It has been shown that the thermal denaturation of the 20 kDa fragment of the S1 heavy chain correlates with the melting of the most thermostable domain in the S1 molecule. It is concluded that this domain is located in the C-terminal 20 kDa segment of the S1 heavy chain.     

Equilibrium dialysis binding studies of 1,N6-ethenoadenosine diphosphate (epsilon ADP) to myosin, heavy meromyosin, and subfragment one

The binding of the fluorescent analog of adenosine diphosphate (ADP)¹, 1,N⁶-ethenoadenosine diphosphate (εADP) to myosin and its subfragments, heavy meromyosin (HMM) and subfragment one (S1), has been studied under analagous conditions to those previously used in comparable studies on the binding of ADP to these molecules. The results indicate that there are two binding sites for εADP on myosin and HMM, and one site on S1. The dissociation constants for all had an identical value, within experimental error, of 2.0 (^± .5) × 10^?5 M^?1. This is identical to the values found by Young (J. Biol. Chem., $\text{242}$, 2790 (1967)) for ADP. In addition, the kinetics of hydrolysis of εATP versus ATP by S1 were studied. Values of V_max and K_m were 25 μM phosphate sec^?1 (gm protein)^?1 and 5 × 10^?5 M^?1 for ATP, and 80 μN phosphate sec^?1 (gm protein)^?1 and 45 × 10^?5 M^?1 for εATP. The results indicate that the increased V_max that occurs when εATP is used as a substitute for ATP is not due to either an increased binding affinity of ATP for myosin and its subfragments, nor due to a decreased binding affinity of εATP versus ADP. This in turn suggests that the increase in V_max may be due to an increased hydrolytic rate of εATP vs ATP in the enzyme substrate complex.     

Reversible phosphorylation of smooth muscle myosin, heavy meromyosin, and platelet myosin

Smooth muscle myosin was purified from turkey gizzards with the 20,000-dalton light chains in the unphosphorylated state. The actin-activated MgATPase activity was 4 nmol/min/mg at 25 degrees C. When the myosin was phosphorylated to 2 mol of Pi/mol of myosin using purified myosin light chain kinase, calmodulin, and ATP, the actin-activated MgATPase activity rose to 51 nmol/min/mg. Complete dephosphorylation of the same myosin by a purified phosphatase lowered the activity to 5 nmol/min/mg, and complete rephosphorylation of the myosin following inhibition of the phosphatase raised it again to 46 nmol/min/mg. Human platelet myosin could be substituted for turkey gizzard myosin, with similar results. A chymotryptic fragment of smooth muscle myosin which retains the phosphorylated site on the 20,000-dalton light chain of myosin was prepared. Using the same scheme for reversible phosphorylation, this smooth muscle heavy meromyosin was found to show the same positive correlation between phosphorylation of the myosin light chain and the actin-activated MgATPase activity. The results with smooth muscle heavy meromyosin show that the effect of phosphorylation on the actin-activated MgATPase activity can be separated from the effects of phosphorylation on myosin filament assembly.     

Cross-linking of myosin subfragment 1 and heavy meromyosin by use of vanadate and a bis(adenosine 5'-triphosphate) analogue

The synthesis of a divalent ATP analogue [3,3'-dithiobis[3'(2')-O-[6-(propionylamino)hexanoyl]adenosine 5'-triphosphate] (bis22ATP)] is described in which two molecules of ATP are linked via esterification of their 3'(2')-hydroxyls to the linear dicarboxylic acid 3,3'-dithiobis[N-(5-carboxypentyl)-propionamide] [[HO2C(CH2)5NHC(O)(CH2)2S-]2]. This linkage introduces 22 atoms (a maximum of approximately 2.8 nm) between the ribose oxygens of two ATP molecules. Myosin subfragment 1 (SF1) or heavy meromyosin (HMM) readily cleave bis22ATP to bis22ADP. Upon subsequent addition of excess vanadate ion, both enzymes are rapidly inactivated by formation of a stable vanadate-bis22ADP complex at the active site. By adjustment of the reaction conditions, dimers of SF1 or HMM, both cross-linked with bis22ADP-vanadate, could be prepared. Dimers of SF1 could be separated from monomers by sucrose gradient centrifugation but not by gel filtration. These observations imply that the average Stokes radius of the dimer approximates that of the monomer, a result predicted only for monomers linked approximately side by side. Conversely, dimers of HMM were separated from HMM monomers by gel filtration, reflecting an increase in their Stokes radii. This increase, however, prevented resolution of HMM dimers from monomers by sucrose gradient centrifugation. These results and the molecular dimensions of bis22ATP suggest that the 3'-(2')-hydroxyl of ATP is no more than 1.3 nm from the surface of myosin and suggest further in the simplest interpretation that the active site is most likely located near the middle of the heads of myosin. Analytical sedimentation velocity experiments were performed in order to compare the sedimentation coefficient (s0(20),w) of the SF1 dimer formed by cross-linking to values predicted from ellipsoidal models of the dimer. The observed s0(20),w of the dimer was much closer to the range predicted for a side-to-side arrangement of SF1 monomers than the range predicted for two monomers linked end to end, a result consistent with the active site location suggested above. During the course of these experiments, unmodified SF1 was used as a control, and its sedimentation behavior was reexamined. We have corroborated the finding that the s0(20),w displayed by SF1 can be affected to a limited extent by the particular experimental parameters employed during centrifugation [Morel, J. E., & Garrigos, M. (1982) Biochemistry 21, 2679-2686].(ABSTRACT TRUNCATED AT 400 WORDS)     

Inactivation, subunit dissociation, aggregation, and unfolding of myosin subfragment 1 during guanidine denaturation.

The effect of guanidine hydrochloride on ATPase activity, gel filtration, turbidity, exposure of thiol groups, far-UV circular dichroism, and the fluorescence emission intensity of myosin subfragment 1 (S-1) was studied under equilibrium conditions. It was found that the denaturation process involves several intermediate states. The enzymatic activity of S-1 is at first lost at very low concentrations of GdnHCl (lower than 0.5 M). At a slightly higher GdnHCl concentration (about 0.5 M), the light chains dissociate and this dissociation is closely followed by the formation of aggregates between the naked heavy chains of S-1 molecules in the guanidine hydrochloride range of concentrations 0.5-1 M. At GdnHCl concentrations above 1 M, aggregates gradually disappear and S-1 loses its secondary and tertiary structures. These phenomena are partly reversible, and ATPase activity is only partially recovered under highly limited conditions. These results are discussed in relation to the nature of myosin subunit assembly. The head fragment of 20 kDa is thus suggested to be implicated in the binding of light chain to heavy chain and in the self-association of free heavy chains.     

Structural changes in myosin subfragment 1 by mild denaturation and proteolysis probed by antibodies

The perturbations in the structure of myosin subfragment 1 (S1) by mild denaturation or proteolysis were investigated by measuring the inhibition of the binding of antibodies to immobilized S1 by treated S1 in a solution-phase competitive immunochemical assay. The structural changes in S1 were probed by using anti-50-kDa segment, anti-N-terminus, anti-27-kDa segment, and anti-A1 light chain monoclonal antibodies (MAbs). Methanol and heat denaturation increased MAb binding to the 50-kDa segment. MAb binding to regions in the 27-kDa segment was also promoted, slightly by methanol and more drastically by heat. Proteolysis also induced structural alterations in 50- and 27-kDa segments as shown by increased MAb binding to these regions in cleaved S1. These results indicate that mild denaturation and proteolysis induce structural perturbations which alter the epitope accessibility in 50- and 27-kDa segments of S1 and that antibody binding studies afford a sensitive probe to such perturbations.     

Proteolysis of smooth muscle myosin by Staphylococcus aureus protease: preparation of heavy meromyosin and subfragment 1 with intact 20 000-dalton light chains

The proteolysis of gizzard myosin by Staphylococcus aureus protease produces both heavy meromyosin and subfragment 1 in which the 20 000-dalton light chains are intact, and conditions are suggested for the preparation of each. Cleavage of the myosin heavy chain to produce subfragment 1 is dependent on the myosin conformation. Proteolysis of myosin in the 10S conformation yields predominantly heavy meromyosin, and myosin in the 6S conformation yields mostly subfragment 1 and some heavy meromyosin. Two sites are influenced by myosin conformation, and these are located at approximately 68 000 and 94 000 daltons from the N-terminus of the myosin heavy chain. The latter site is thought to be located at the subfragment 1-subfragment 2 junction, and cleavage at this site results in the production of subfragment 1. The time courses of phosphorylation of both heavy meromyosin and subfragment 1 can be fit by a single exponential. The actin-activated Mg2+-ATPase activity of heavy meromyosin is markedly activated by phosphorylation of the 20 000-dalton light chains. From the actin dependence of Mg2+-ATPase activity the following values are obtained: for phosphorylated heavy meromyosin, Vmax approximately 5.6 s-1 and Ka (the apparent dissociation constant for actin) approximately 2 mg/mL; for dephosphorylated heavy meromyosin, Vmax approximately 0.2 s-1 and Ka approximately 7 mg/mL. The actin-activated ATPase activity of subfragment 1 is not influenced by phosphorylation, and Vmax and Ka for both the phosphorylated and dephosphorylated forms are 0.4 s-1 and 5 mg/mL, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)