The quantitative measurement of rotational motion of the subfragment-1 region of myosin by saturation transfer epr spectroscopy.

According to current models of muscle contraction (Huxley, H. E., Science 164: 1356-1366 [1969]), motion of flexible myosin crossbridges is essential to the contractile cycle. Using a spin-label analog of iodoacetamide bound to the subfragment No. 1 (S1) region of myosin, we have obtained rotational correlation times (tau 2) for this region of the molecule with the ultimate goal of making quantitative measurements of the motion of the crossbridges under conditions comparable to those in living, contracting muscle. We used the newly developed technique of saturation transfer electron paramagnetic resonance spectroscopy (Hyde, J.S., and Thomas, D.D., Ann. N.Y. Acad Sci. 22:680-692 [1973]), which is uniquely sensitive to rotational motion in the range of 10(-7)-10(-3) sec. Our results indicate that the spin label is rigidly bound to S1 (tau 2 for isolated S1 is 2 X 10(-7) sec) and that the motion of the label reflects the motion of the S1 region of myosin. the value of tau 2 for the S1 segment of myosin is less than twice that for isolated S1, while the molecular weights differ by a factor of 4, indicating flexibility of myosin in agreement with the conclusions of Mendelson et al. (Biochemistry 12:2250-2255 [1973]). Adding F-actin increses tau 2 in either myosin or isolated S1 by a factor of mearly 103, indicating rigid immobilization of S1 by actin. Formation of myosin filaments (at an ionic strength of 0.15 or less) increses tau 2 by a factor of 10-30, depending on the ionic strength, indicating a decrease of the rotational mobility of S1 in these agregates. The remaining motion is at least a factor of 10 faster than would be expected for the filament itself, suggesting motion of the S1 region independent of the filament backbone but slower than in a single molecule. F-actin has a strong immobilizing effect on labeled S1 in myosin filaments (in 0.137 M KC1), but the immobilization is less complete than that observed when F-actin is added to labeled myosin monomers (in 0.5 M KC1). A spin-label analog of maleimide, attached to the SH-2 thiol groups of S1, is immobilized to a much lesser extent by F-actin than is the label on SH-1 groups. The maleimide label also was attached directly to F-actin and was sufficiently immobilized to suggest rigid binding to actin.     

The bound nucleotide of actin.

The extent of actin polymerization has been studied for samples in which the bound nucleotide of the actin was ATP, ADP, or an analog of ATP that was not split (AMPPNP). The equilibrium constants for the addition of a monomer to a polymer end were determined from the concentration of monomer coexisting with the polymer. An analysis of these results concludes that the bound ATP on G-actin provides little energy to promote the polymerization of the actin. AMPPNP was incorporated into F-actin and the interaction of F-actin - AMPPNP with myosin was studied. F-actin - AMPPNP activated the ATPase of myosin to the same extent as did F-actin - ADP. However, the rate of superprecipitation was slower in the case of F-actin - AMPPNP than in the control.     

Vanadate (Vi) and ADP induced domain motions in myosin head by DSC and EPR

Thermal stability and internal dynamics of myosin head in psoas muscle fibres of rabbit in the intermediate state AM.ADP.P_i - mimicked by AM.ADP.V_i - of the ATP hydrolysis cycle was studied by differential scanning calorimetry and spin label electron paramagnetic resonance spectroscopy. Three overlapping endotherms were detected in rigor, in strongly binding ADP and weakly binding AM.ADP.V_i state of myosin to actin. The transition at 54.0°C can be assigned to the 50 k actin-binding domain. The transition at highest temperature (67.3°C) represents the unfolding of actin and the contributions arising from the nucleotide-myosin head interaction. The transition at 58.4°C reflects the melting of the large rod part of myosin. Nucleotide binding (ADP, ATP plus orthovanadate) induced shifts of the melting temperatures and produced changes in the calorimetric enthalpies. The changes of the EPR parameters indicated local rearrangements of the internal structure in myosin heads in agreement with DSC findings. This revised version was published online in July 2006 with corrections to the Cover Date.     

The regional association of actin and myosin with sites of particle phagocytosis.

Contractile proteins are thought to play a causative role in motile processes such as phagocytosis. In order to investigate their role in phagocytosis further, simultaneous immunofluorescence localization of F-actin and myosin was carried out in resident mouse peritoneal macrophages after phagocytosis of opsonized zymosan particles. Both actin and myosin appeared to concentrate rapidly at sites of particle phagocytosis. The observed concentration of both proteins at such sites preceded ultimate particle engulfment. Cytochalasin B, a drug which was shown to block pseudopod extensions around the particle, did not prevent the concentration of the two congth effects as an explanation for the observed concentration of actin and myosin at phagocytic sites. Kinetic analysis showed that actin rapidly concentrates at particle-cell binding sites within minutes (or less) of contact with cell surface. The two proteins are present throughout the engulfment phase until and after ingestion is complete. Finally, at later times the particles become clustered over the cell nucleus and the particle-associated actin-myosin seen earlier is no longer evident.     

Purification and characterization of an actin-, calmodulin- and tropomyosin-binding protein from chicken gizzard smooth muscle.

An actin-binding protein (p33) has been purified from chicken gizzard smooth muscle. The homogenous protein has a molecular weight near 33000 as determined by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and size exclusion chromatography. Its binding ability to F-actin remained after heating at 95 degrees C for 4 min. Immunoblot analyses indicated that p33 was not a degradation product from higher molecular components. The binding of p33 to F-actin was saturable in a molar ratio of about one p33 to 2-3 actin molecules with an apparent binding constant of 6.6 x 10(7) M-1. p33 also bound to calmodulin and tropomyosin. The bindings of p33 to F-actin and tropomyosin were regulated by calmodulin in a Ca(2+)-dependent fashion. In addition to actin, caldesmon and tropomyosin, p33 was contained in the native thin filaments prepared from smooth muscle. Other actin-binding proteins, including alpha-actinin, caldesmon and filamin, had little effect on p33 binding to actin filaments. These results demonstrate that p33 may function in actin-based cellular processes which are mediated by Ca2+ and calmodulin.     

Interaction of myosin and paramyosin.

The interaction of myosin and paramyosin was investigated by enzymological and ultrastructural techniques. The actin-activated Mg+2 ATPase of rabbit skeletal muscle myosin can be inhibited by clam adductor paramyosin. Both proteins must be rapidly coprecipitated to form filaments for this inhibition. Slowly formed cofilaments are fully activatable by F-actin. In both cases, the cofilaments possess unique structural characteristics when compared to homofilaments. The mode of inhibition appears to be competitive when different concentrations of paramyosin and F-actin are compared. The apparent affinity of the myosin heads for actin is reduced by the presence of paramyosin within rapidly reconstituted thick filaments. These results suggest that paramyosin may serve as part of a relaxing mechanism within invertebrate muscles. It is unlikely that paramyosin plays a role in the initiation and maintenance of catch within specialized molluscan muscles.     

DSC study on the motor protein myosin in fibre system

We have examined by DSC the complexes of myosin with actin in fibre system in the absence of nucleotides and the intermediate state of ATP hydrolysis by mimicking stable complex with myosin and ADP and beryllium fluoride in muscle fibres. Comparing the DSC results with other structural analogues of phosphate P_i leads the conclusion that the AM.ADP.BeF_x complex favours the AM.ADP.P_i complex in fibre system. The deconvolution of DSC scans resulted in four transitions, the first three transition temperatures were almost independent of the intermediate state of the muscle, the last transition temperature was shifted to higher temperature, depending on the actual intermediate states of ATP hydrolysis. In AM.ADP.V_i state the transition temperature at the second and third transitions (actin binding domain and myosin rod) varied only slightly, whereas the last one (the fourth transition) shifted markedly to higher temperature depending on the ternary complex, e.g. in case of ADP plus BeF_x it was 77.7 °C, the highest value in weakly binding state of myosin to actin. The sum of calorimetric enthalpies of the first and last curves was practically constant, but their fractions depended on the state of the muscle. In strongly binding state of myosin to actin (rigor, ADP state) the fraction of the first transition was much larger, than the last one, whereas in weakly binding state of myosin to actin, the fraction of the first transition decreased at the expense of the last one. It supports also the view that these transitions are parts of the same portion of the myosin molecule.     

Thermal transitions of actin

Actin is one of the main components in the eukaryote cells which plays significant role in many cellular processes, like force-generation, maintenance of the shape of cells, cell-division cycle and transport processes. In this study the thermal transitions of monomer and polymerized actins were studied to get information about the changes induced by polymerization and binding of myosin to actin using DSC and EPR techniques. The main thermal transition of F-actin was at 67.5°C by EPR using spin-labeled actin (the relative viscosity change was around 62°C), while the DSC denaturation T _ms were at 60.3d°C for G-actin and at 70.5°C for F-actin. Applying the Lumry-Eyring model to obtain the parameters of the kinetic process and calculate the activation energy, a ‘break’ was found for F-actin in the function of first-order kinetic constant vs. 1/T. This indicates that an altered interdomain interaction is present in F-actin. The addition of myosin or heavy meromyosin (HMM) in different molar ratio of myosin to actin has changed significantly the EPR spectrum of spin-labeled F-actin, indicating the presence of the supramolecular complex. Analyzing the DSC traces of the actomyosin complex it was possible to identify the different structural domains of myosin and actin.     

Nano-biomachine from actin and myosin gels

We introduce here an ATP (adenosine triphosphate)-fueled nano-biomachine constructed from actin and myosin gels. Various types of chemically cross-linked actin gel, which are tens of times larger in size than native actin filaments (F-actin), were formed by complexing with cation-polymers and placed on a chemically cross-linked myosin gel. By adding dilute solution of ATP, they moved along the myosin gel with a velocity as high as that of native F-actin by coupling to ATP hydrolysis. Formation mechanism and structure of actin complexes as well as those of myosin gels were studied in detail and elucidated with the specific characteristics of the motility. These results demonstrate that one can construct nano-biomachines fueled by chemical energy of ATP with controlled motility. The text was submitted by the authors in English.     

A possible modulation of acetylcholine receptors of embryonic chick muscle cells by alpha-bungarotoxin

Acetylcholine receptors were assayed with alpha-bugarotoxin on embryonic chick skeletal muscle growing in primary cell culture. Toxin was bound specifically to muscle cells and could be competed with D-tubocurarine. Two dissociation constants were obtained by equilibrium binding: 7.2 x 10(-9)M and 2.7 x 10(-7)M at 25 degrees C. Two sets of rate constants were also obtained from dissociation kinetics. There are five times more low affinity sites on cells than high affinity sites. The average density of high-affinity receptors is about 200/micrometers2. A time course of toxin binding to receptors at 37 degrees C in growth medium revealed that under conditions permitting growth and metabolism, toxin bound to cells was lost. The possibility that the growth medium was inactivating toxin molecules was ruled out by showing that unbound toxin molecules in the medium were fully capable of binding to fresh cultures.     

Coordination environment of a site-bound metal ion in the hammerhead ribozyme determined by 15N and 2H ESEEM spectroscopy

Although site-bound Mg2+ ions have been proposed to influence RNA structure and function, establishing the molecular properties of such sites has been challenging due largely to the unique electrostatic properties of the RNA biopolymer. We have previously determined that, in solution, the hammerhead ribozyme (a self-cleaving RNA) has a high-affinity metal ion binding site characterized by a K(d,app) < 10 microM for Mn2+ in 1 M NaCl and speculated that this site has functional importance in the ribozyme cleavage reaction. Here we determine both the precise location and the hydration level of Mn2+ in this site using ESEEM (electron spin-echo envelope modulation) spectroscopy. Definitive assignment of the high-affinity site to the activity-sensitive A9/G10.1 region is achieved by site-specific labeling of G10.1 with 15N guanine. The coordinated metal ion retains four water ligands as measured by 2H ESEEM spectroscopy. The results presented here show that a functionally important, specific metal binding site is uniquely populated in the hammerhead ribozyme even in a background of high ionic strength. Although it has a relatively high thermodynamic affinity, this ion remains partially hydrated and is chelated to the RNA by just two ligands.     

The potent anti-HIV protein cyanovirin-N contains two novel carbohydrate binding sites that selectively bind to Man(8) D1D3 and Man(9) with nanomolar affinity: implications for binding to the HIV envelope protein gp120.

Cyanovirin-N (CVN) is a monomeric 11 kDa cyanobacterial protein that potently inactivates diverse strains of human immunodeficiency virus (HIV) at the level of cell fusion by virtue of high affinity interactions with the surface envelope glycoprotein gp120. Several lines of evidence have suggested that CVN-gp120 interactions are in part mediated by N-linked complex carbohydrates present on gp120, but experimental evidence has been lacking. To this end we screened a comprehensive panel of carbohydrates which represent structurally the N-linked carbohydrates found on gp120 for their ability to inhibit the fusion-blocking activity of CVN in a quantitative HIV-1 envelope-mediated cell fusion assay. Our results show that CVN specifically recognizes with nanomolar affinity Man(9)GlcNAc(2) and the D1D3 isomer of Man(8)GlcNAc(2). Nonlinear least squares best fitting of titration data generated using the cell fusion assay show that CVN binds to gp120 with an equilibrium association constant (K(a)) of 2.4 (+/- 0.1) x 10(7) M(-1) and an apparent stoichiometry of 2 equiv of CVN per gp120, Man(8)GlcNAc(2) D1D3 acts as a divalent ligand (2 CVN:1 Man(8)) with a K(a) of 5.4 (+/- 0.5) x 10(7) M(-1), and Man(9)GlcNAc(2) functions as a trivalent ligand (3 CVN:1 Man(9)) with a K(a) of 1.3 (+/- 0.3) x 10(8) M(-1). Isothermal titration calorimetry experiments of CVN binding to Man(9)GlcNAc(2) at micromolar concentrations confirmed the nanomolar affinity (K(a) = 1.5 (+/- 0.9) x 10(8) M(-1)), and the fitted data indicated a stoichiometry equal to approximately one (1 Man(9):1 CVN). The 1:1 stoichiometry at micromolar concentrations suggested that CVN has not only a high affinity binding site-relevant to the studies at nM concentrations-but a lower affinity site as well that facilitates cross-linking of CVN-oligomannose at micromolar concentrations or higher. The specificity of CVN for Man(8) D1D3 and Man(9) over the D1D2 isomer of Man(8) indicated that the minimum structure required for high affinity binding comprises Manalpha1 --> 2Manalpha. By following the (1)H-(15)N correlation spectrum of (15)N-labeled CVN upon titration with this disaccharide, we unambiguously demonstrate that CVN recognizes and binds to the disaccharide Manalpha1 --> 2Manalpha via two distinct binding sites of differing affinities located on opposite ends of the protein. The high affinity site has a K(a) of 7.2 (+/- 4) x 10(6) M(-1) and the low affinity site a K(a) of 6.8 (+/- 4) x 10(5) M(-1) as determined by isothermal titration calorimetry. Mapped surfaces of the carbohydrate binding sites are presented, and implications for binding to gp120 are discussed.     

Single crystal 55Mn ENDOR of concanavalin a: detection of two Mn2+ sites with different 55Mn quadrupole tensors

Concanavalin A is a member of the plant hemeagglutinin (or plant lectin) family that contains two metal binding sites; one, called S1, is occupied by Mn2+ and the other, S2, by Ca2+. 55Mn electron-nuclear double resonance (ENDOR) measurements were performed on a single crystal of concanavalin A at W-band (95 GHz, ~3.5 T) to determine the 55Mn nuclear quadrupole interaction in a protein binding site and its relation to structural parameters. Such measurements are easier at a high field because of the high sensitivity for size-limited samples and the reduction of second-order effects on the spectrum which simplifies spectral analysis. The analysis of the 55Mn ENDOR rotation patterns showed that two chemically inequivalent Mn2+ types are present at low temperatures, although the high-resolution X-ray structure reported only one site. Their quadrupole coupling constants, e2Qq/h, are significantly different; 10.7 +/- 0.6 MHz for Mand only -2.7 +/-0.6 MHz for M. The ENDOR data also refined the hyperfine coupling determined earlier by single-crystal EPR measurements, yielding a small but significant difference between the two: -262.5 MHz for M and -263.5 MHz for M. The principal z-axis for M is not aligned with any of the Mn-ligand directions, but is 25 off the Mn-asp10 direction, and its orientation is different than that of the zero-field splitting (ZFS) interaction. Because of the small quadrupole interaction of M the orientation dependence was very mild, leading to larger uncertainties in the asymmetry parameter. Nonetheless, there too z is not along the Mn-ligand bonds and is rotated 90 with respect to MnA. These results show, that similar to the ZFS, the quadrupolar interaction is highly sensitive to small differences in the coordination sphere of the Mn2+, and the resolution of the two types is in agreement with the earlier observation of a two-site conformational dynamic detected through the ZFS interaction, which is frozen out at low temperatures and averaged at room temperature. To account for the structural origin of the different e2Qq/h values, the electric field gradient tensor was calculated using the point-charge model. The calculations showed that a relatively small displacement of the oxygen ligand of asp10 can lead to differences on the order observed experimentally.     

Specific interaction of cytochalasins with muscle and platelet actin filaments in vitro

The cytochalasins (CE, CD, CB and H2CB) inhibit numerous cellular processes which require the interaction of actin with other structural and contractile proteins. In this report we describe the effects of the cytochalasins on the viscosity and morphology of muscle and platelet actin. The cytochalasins decreased the viscosity of F-actin solutions. The effect of H2CB, CB and CD ON F-actin viscosity was maximal at concentrations of 20-50 micro M and did not increase with time. In contrast, CE caused a progressive decrease in the viscosity of F-actin solutions which was dependent upon the concentration of CE and the duration of incubation of the CE-actin mixture. After two hours of incubation of drug-actin mixtures, the relative effectiveness of the cytochalasins in reducing the viscosity of F-actin was CE greater than CD greater than CB=H2CB. The effects of CD and CE were paralleled by morphologic changes in negatively stained actin filaments. The effects of the cytochalasins on the viscosity and morphology of muscle and platelet actin were the same whether the drugs were added before or after the polymerization of the protein. These studies show that the interaction of the cytochalasins with actin is highly specific. Because the relative potencies of these drugs for affecting motile processes and the relative affinities of the drugs for binding sites within a variety of cells are CE greater than CD greater than CB=H2CB, the effects of cytochalasins on actin described here may contribute to some of the biological effects of the drugs on motile processes.     

Evaluation of alternatives to warfarin as probes for Sudlow site I of human serum albumin: characterization by high-performance affinity chromatography

Warfarin is often used as a site-specific probe for examining the binding of drugs and other solutes to Sudlow site I of human serum albumin (HSA). However, warfarin has strong binding to HSA and the two chiral forms of warfarin have slightly different binding affinities for this protein. Warfarin also undergoes a slow change in structure when present in common buffers used for binding studies. This report examined the use of four related, achiral compounds (i.e., coumarin, 7-hydroxycoumarin, 7-hydroxy-4-methylcoumarin, and 4-hydroxycoumarin) as possible alternative probes for Sudlow site I in drug binding studies. High-performance affinity chromatography and immobilized HSA columns were used to compare and evaluate the binding properties of these probe candidates. Binding for each of the tested probe candidates to HSA was found to give a good fit to a two-site model. The first group of sites had moderate-to-high affinities for the probe candidates with association equilibrium constants that ranged from 6.4 x 10(3)M(-1) (coumarin) to 5.5 x 10(4)M(-1) (4-hydroxycoumarin) at pH 7.4 and 37 degrees C. The second group of weaker, and probably non-specific, binding regions, had association equilibrium constants that ranged from 3.8 x 10(1)M(-1) (7-hydroxy-4-methylcoumarin) to 7.3 x 10(2)M(-1) (coumarin). Competition experiments based on zonal elution indicated that all of these probe candidates competed with warfarin at their high affinity regions. Warfarin also showed competition with coumarin, 7-hydroxycoumarin and 7-hydroxy-4-methycoumarin for their weak affinity sites but appeared to not bind and/or compete for all of the weak sites of 4-hydroxycoumarin. It was found from this group that 4-hydroxycoumarin was the best alternative to warfarin for examining the interactions of drugs at Sudlow site I on HSA. These results also provided information on how the major structural components of warfarin contribute to the binding of this drug at Sudlow site I.     

The catalytic Mn2+ sites in the enolase-inhibitor complex: crystallography, single-crystal EPR, and DFT calculations

Crystals of Zn2+/Mn2+ yeast enolase with the inhibitor PhAH (phosphonoacetohydroxamate) were grown under conditions with a slight preference for binding of Zn2+ at the higher affinity site, site I. The structure of the Zn2+/Mn2+-PhAH complex was solved at a resolution of 1.54 A, and the two catalytic metal binding sites, I and II, show only subtle displacement compared to that of the corresponding complex with the native Mg2+ ions. Low-temperature echo-detected high-field (W-band, 95 GHz) EPR (electron paramagnetic resonance) and 1H ENDOR (electron-nuclear double resonance) were carried out on a single crystal, and rotation patterns were acquired in two perpendicular planes. Analysis of the rotation patterns resolved a total of six Mn2+ sites, four symmetry-related sites of one type and two out of the four of the other type. The observation of two chemically inequivalent Mn2+ sites shows that Mn2+ ions populate both sites I and II and the zero-field splitting (ZFS) tensors of the Mn2+ in the two sites were determined. The Mn2+ site with the larger D value was assigned to site I based on the 1H ENDOR spectra, which identified the relevant water ligands. This assignment is consistent with the seemingly larger deviation of site I from octahedral symmetry, compared to that of site II. The ENDOR results gave the coordinates of the protons of two water ligands, and adding them to the crystal structure revealed their involvement in a network of H bonds stabilizing the binding of the metal ions and PhAH. Although specific hyperfine interactions with the inhibitor were not determined, the spectroscopic properties of the Mn2+ in the two sites were consistent with the crystal structure. Density functional theory (DFT) calculations carried out on a cluster representing the catalytic site, with Mn2+ in site I and Zn2+ in site II, and vice versa, gave overestimated D values on the order of the experimental ones, although the larger D value was found for Mn2+ in site II rather than in site I. This discrepancy was attributed to the high sensitivity of the ZFS parameters to the Mn-O bond lengths and orientations, such that small, but significant, differences between the optimized and crystal structures alter the ZFS considerably, well above the difference between the two sites.     

Interaction of homogeneous mitochondrial ATPase from rat liver with adenine nucleotides and inorganic phosphate.

Mitochondrial ATPase from rat liver mitochondria contains multiple nucleotide binding sites. At low concentrations ADP binds with high affinity (1 mole/mole ATPase, KD = 1-2 muM). At high concentrations, ADP inhibits ATP hydrolysis presumably by competing with ATP for the active site (KI = 240-300 muM). As isolated, mitochondrial ATPase contains between 0.6 and 2.5 moles ATP/mole ATPase. This "tightly bound" ATP can be removed by repeated precipitations with ammonium sulfate without altering hydrolytic activity of the enzyme. However, the ATP-depleted enzyme must be redissolved in high concentrations of phosphate to retain activity. AMP-PNP (adenylyl imidodiphosphate) replaces tightly bound ATP removed from the enzyme and inhibits ATP hydrolysis. AMP-PNP has little effect on high affinity binding of ADP. Kinetics studies of ATP hydrolysis reveal hyperbolic velocity vs. ATP plots, provided assays are done in bicarbonate buffer or buffers containing high concentrations of phosphate. Taken together, these studies indicate that sites on the enzyme not directly associated with ATP hydrolysis bind ATP or ADP, and that in the absence of bound nucleotide, Pi can maintain the active form of the enzyme.     

Nucleotides Induced Changes in Skeletal Muscle Myosin by DSC,TMDSC and EPR

Electron paramagnetic resonance (EPR, ST-EPR) and differential scanning calorimetry(DSC) were used in conventional and temperature modulated mode to study internal motions and energetics of myosin in skeletal muscle fibres in different states of the actomyosin ATPase cycle. Psoas muscle fibres from rabbit were spin-labelled with an isothiocyanate-based probe molecule at the reactive sulfhydryl site (Cys-707) of the catalytic domain of myosin. In the presence of nucleotides (ATP, ADP, AMP⋅PNP) and ATP or ADP plus orthovanadate, the conventional EPR spectra showed changes in the ordering of the probe molecules in fibres. In MgADP state a new distribution appeared; ATP plus orthovanadate increased the orientational disorder of myosin heads, a random population of spin labels was superimposed on the ADP-like spectrum. In the complex DSC pattern, higher transition referred to the head region of myosin. The enthalpy of the thermal unfolding depended on the nucleotides, the conversion from a strongly attached state of myosin to actin to a weakly binding state was accompanied with an increase of the transition temperature which was due to the change of the affinity of nucleotide binding to myosin. This was more pronounced in TMDSC mode, indicating that the strong-binding state and rigor state differ energetically from each other. The different transition temperatures indicated alterations in the internal microstructure of myosin head region The monoton decreasing TMDSC heat capacities show that C _p of biological samples should not be temperature independent. This revised version was published online in August 2006 with corrections to the Cover Date.     

CHARACTERIZATION OF SKELETAL MUSCLE ACTIN LABELED WITH THE TRIPLET PROBE ERYTHROSIN-5-IODOACETAMIDE

Abstract We have labeled rabbit skeletal muscle actin with the triplet probe erythrosin-5-iodoacetamide and characterized the labeled protein. Labeling decreased the critical concentration and lowered the intrinsic viscosity of F-actin filaments; labeled filaments were motile in an in vitro motility assay but were less effective than unlabeled F-actin in activating myosin S1 ATPase activity. In unpolymerized globular actin (G-actin), both the prompt and delayed luminescence were red-shifted from the spectra of the free dye in solution and the fluorescence anisotropy of the label was high (0.356); filament formation red shifted all excitation and emission spectra and increased the fluorescence anisotropy to 0.370. The erythrosin phosphorescence decay was at least biexponential in G-actin with an average lifetime of 99 μs while in F-actin the decay was approximately monoexponential with a lifetime of 278 μs. These results suggest that the erythrosin dye was bound at the interface between two actin monomers along the two-start helix. The steady-state phosphorescence anisotropy of F-actin was 0.087 at 20°C and the anisotropy increased to ≈0.16 in immobilized filaments. The phosphorescence anisotropy was also sensitive to binding the physiological ligands phalloidin, cytochalasin B and tropomyosin. This study lays a firm foundation for the use of this triplet probe to study the large-scale molecular dynamics of F-actin.     

Binding of Nucleotides at the Active Site Modulates the Local and Global Conformation of Myosin in Muscle Fibres

Differential scanning calorimetry and electron paramagnetic resonance experiments were performed on glycerinated muscle fibres to study the effect of the binding of nucleotides (ADP and AMP⋅PNP) to myosin. The thermal unfolding of muscle fibres showed three discrete domain regions with thermal stabilities of 52.2, 58.8 and 67.8°C. AMP⋅PNP markedly affected the transitions, implying the strong interaction between AMP⋅PNP and catalytic domain, and partial dissociation of heads from actin. ADP produced only small changes in transition temperatures. Spectrum deconvolution performed on isothiocyanate-labelled fibres in AMP⋅PNP-state resulted in two populations; 50% of labels was highly ordered with respect to fibre axis, whereas the other 50% of labels was randomly oriented. The conformation of the myosin heads which showed high degree of order were in the strongly binding ADP-state, the heads being attached to actin differ from those of heads in rigor. The results support the suggestion that the attached heads in strongly binding state to actin might have different local conformations. This revised version was published online in July 2006 with corrections to the Cover Date.