Energetics and kinetics of interconversion of two myosin subfragment-1.adenosine 5'-diphosphate complexes as viewed by phosphorus-31 nuclear magnetic resonance

Bringing dynamic molecular machines into focus by methyl-TROSY NMR

Large macromolecular assemblies, so-called molecular machines, are critical to ensuring proper cellular function. Understanding how proper function is achieved at the atomic level is crucial to advancing multiple avenues of biomedical research. Biophysical studies often include X-ray diffraction and cryo-electron microscopy, providing detailed structural descriptions of these machines. However, their inherent flexibility has complicated an understanding of the relation between structure and function. Solution NMR spectroscopy is well suited to the study of such dynamic complexes, and continued developments have increased size boundaries; insights into function have been obtained for complexes with masses as large as 1 MDa. We highlight methyl-TROSY (transverse relaxation optimized spectroscopy) NMR, which enables the study of such large systems, and include examples of applications to several cellular machines. We show how this emerging technique contributes to an understanding of cellular function and the role of molecular plasticity in regulating an array of biochemical activities.

Accuracy and precision of protein-ligand interaction kinetics determined from chemical shift titrations

NMR-monitored chemical shift titrations for the study of weak protein-ligand interactions represent a rich source of information regarding thermodynamic parameters such as dissociation constants (K ( D )) in the micro- to millimolar range, populations for the free and ligand-bound states, and the kinetics of interconversion between states, which are typically within the fast exchange regime on the NMR timescale. We recently developed two chemical shift titration methods wherein co-variation of the total protein and ligand concentrations gives increased precision for the K ( D ) value of a 1:1 protein-ligand interaction (Markin and Spyracopoulos in J Biomol NMR 53: 125-138, 2012). In this study, we demonstrate that classical line shape analysis applied to a single set of (1)H-(15)N 2D HSQC NMR spectra acquired using precise protein-ligand chemical shift titration methods we developed, produces accurate and precise kinetic parameters such as the off-rate (k ( off )). For experimentally determined kinetics in the fast exchange regime on the NMR timescale, k ( off ) ~ 3,000 s(-1) in this work, the accuracy of classical line shape analysis was determined to be better than 5 % by conducting quantum mechanical NMR simulations of the chemical shift titration methods with the magnetic resonance toolkit GAMMA. Using Monte Carlo simulations, the experimental precision for k ( off ) from line shape analysis of NMR spectra was determined to be 13 %, in agreement with the theoretical precision of 12 % from line shape analysis of the GAMMA simulations in the presence of noise and protein concentration errors. In addition, GAMMA simulations were employed to demonstrate that line shape analysis has the potential to provide reasonably accurate and precise k ( off ) values over a wide range, from 100 to 15,000 s(-1). The validity of line shape analysis for k ( off ) values approaching intermediate exchange (~100 s(-1)), may be facilitated by more accurate K ( D ) measurements from NMR-monitored chemical shift titrations, for which the dependence of K ( D ) on the chemical shift difference (Δω) between free and bound states is extrapolated to Δω = 0. The demonstrated accuracy and precision for k ( off ) will be valuable for the interpretation of biological kinetics in weakly interacting protein-protein networks, where a small change in the magnitude of the underlying kinetics of a given pathway may lead to large changes in the associated downstream signaling cascade.

GFP-tagged regulatory light chain monitors single myosin lever-arm orientation in a muscle fiber

Myosin is the molecular motor in muscle-binding actin and executing a power stroke by rotating its lever arm through an angle of approximately 70 degrees to translate actin against resistive force. A green fluorescent protein (GFP)-tagged human cardiac myosin regulatory light chain (HCRLC) was constructed to study in situ lever arm orientation one molecule at a time by polarized fluorescence emitted from the GFP probe. The recombinant protein physically and functionally replaced the native RLC on myosin lever arms in the thick filaments of permeabilized skeletal muscle fibers. Detecting single molecules in fibers where myosin concentration reaches 300 microM is accomplished using total internal reflection fluorescence microscopy. With total internal reflection fluorescence, evanescent field excitation, supercritical angle fluorescence detection, and CCD detector pixel size limits detection volume to just a few attoliters. Data analysis manages both the perturbing effect of the TIR interface on probe emission and the effect of high numerical aperture collection of light. The natural myosin concentration gradient in a muscle fiber allows observation of fluorescence polarization from C-term GFP-tagged HCRLC exchanged myosin from regions in the thick filament containing low and high myosin concentrations. In rigor, cross-bridges at low concentration at the end of the thick filament maintain GFP dipole moments at two distinct polar angles relative to the fiber symmetry axis. The lower angle, where the dipole is nearly parallel to fiber axis, is more highly populated than the alternative, larger angle. Cross-bridges at higher concentration in the center of the thick filament are oriented in a homogeneous band at approximately 45 degrees to the fiber axis. The data suggests molecular crowding impacts myosin conformation, implying mutual interactions between cross-bridges alter how the muscle generates force. The GFP-tagged RLC is a novel probe to assess single-lever-arm orientation characteristics in situ.

Myosin II from rabbit skeletal muscle and Dictyostelium discoideum and its interaction with F-actin studied by (1)H NMR spectroscopy

Mg-F-actin occurs in two conformational states, I and M, where the N-terminal amino acids are either immobile or highly mobile. In the rigor or ADP complex of rabbit myosin S1 with Mg-F-actin the N-terminal acetyl group of actin stays in its highly mobile state. The same is true for the complexes with the myosin motor domain from Dictyostelium discoideum. This excludes a direct strong interaction of the N-terminal amino acids with myosin in the rigor state as suggested. An interaction of the N-terminus of F-actin with myosin is also not promoted by occupying its low-affinity binding site(s) with divalent ions. The N-terminal high-mobility region may be part of a structural system which has evolved for releasing inadequate stress applied to the actin filaments.

Changes in the 31P NMR spectrum of rabbit muscle myosin subfragment 1. MgADP with temperature

In pioneering studies on the 31P NMR spectra of MgADP bound to the "molecular motor" myosin subfragment 1 (S1) in the temperature range of 0 to 25 degrees C, Shriver and Sykes [Biochemistry 20 (1981) 2004-2012/6357-6362; Biochemistry 21 (1982) 3022-3028], proposed that MgADP binds to myosin S1 as a mixture of two interconvertible conformers with different chemical shifts for the beta-P resonance of the S1-bound MgADP and that the concentrations of these conformers are related by an equilibrium constant K(T). Their model implied that the weighted average of the chemical shifts of the beta-P(MgADP) for S1-bound MgADP asymptotically approaches a high temperature limit. Here, and in our earlier paper [K. Konno, K. Ue, M. Khoroshev, H., Martinez, B.D. Ray, M.F. Morales, Proc. Natl. Acad. Sci. USA 97 (2000) 1461-1466], we report experimental similarities to Shriver and Sykes, but diverge from them (especially at 0 degrees C) in not finding two distinct peaks and in finding that the average chemical shift does not change with temperature. Our observations can be explained by chemical exchange of beta-P(MgADP) of S1-bound MgADP between two nearly energetically equivalent environments.

The three-dimensional structure of septum site-determining protein MinD from Pyrococcus horikoshii OT3 in complex with Mg-ADP

BACKGROUND

In Escherichia coli, the cell division site is determined by the cooperative activity of min operon products MinC, MinD, and MinE. MinC is a nonspecific inhibitor of the septum protein FtsZ, and MinE is the supressor of MinC. MinD plays a multifunctional role. It is a membrane-associated ATPase and is a septum site-determining factor through the activation and regulation of MinC and MinE. MinD is also known to undergo a rapid pole-to-pole oscillation movement in vivo as observed by fluorescent microscopy.

RESULTS

The three-dimensional structure of the MinD-2 from Pyrococcus horikoshii OT3 (PH0612) has been determined at 2.3 A resolution by X-ray crystallography using the Se-Met MAD method. The molecule consists of a beta sheet with 7 parallel and 1 antiparallel strands and 11 peripheral alpha helices. It contains the classical mononucleotide binding loop with bound ADP and magnesium ion, which is consistent with the suggested ATPase activity.

CONCLUSIONS

Structure analysis shows that MinD is most similar to nitrogenase iron protein, which is a member of the P loop-containing nucleotide triphosphate hydrolase superfamily of proteins. Unlike nitrogenase or other member proteins that normally work as a dimer, MinD was present as a monomer in the crystal. Both the 31P NMR and Malachite Green method exhibited relatively low levels of ATPase activity. These facts suggest that MinD may work as a molecular switch in the multiprotein complex in bacterial cell division.

Steady-state kinetics of MgATP splitting by native myosin RLC-free subfragment 1

MgATP positively regulates the dimerisation reaction, resulting in an increase in the rate of MgATP splitting with increasing MgATP concentration. We investigated the stoichiometry of this dimerisation reaction and found that each subunit in the dimer bound one molecule of MgATP at the dimerisation site. We studied changes with temperature in the MgATPase activity of S1 in the dimeric form for temperatures of 18-25 degrees C. Between 18.0 and 21.2 degrees C, kcat increased steadily with temperature. Between 21.2 and 21.8 degrees C, there was a large decrease in kcat. A strong increase in kcat occurred at temperatures above 21.8 degrees C, corresponding to a new reversible conformation of S1, unable to dimerise. The steep decrease in kcat between 21.2 and 21.8 degrees C is due to a temperature transition in the monomer-dimer equilibrium.

Consequences of placing an intramolecular crosslink in myosin S1

This paper describes the placement of a crosslinking agent (dibromobimane) between two thiols (Cys-522 and Cys-707) of a fragment, "S1," of the motor protein, myosin. It turns out that fastening the first anchor of the crosslinker is easy and rapid, but fastening the second anchor (Cys-522) is very temperature dependent, taking 30 min at room temperature but about a week on ice. Moreover, crystallography taken at 4 degrees C would seem to predict that the linkage is impossible, because the span of the crosslinking agent is much less than the interthiol distance. The simplest resolution of this seeming paradox is that structural fluctuations of the protein render the linkage increasingly likely as the temperature increases. Also, measurements of the affinity of MgADP for the protein, as well as the magnetic resonance of the P-atoms of the ADP once emplaced, suggest that binding the first reagent anchor to Cys-707 initiates an influence that travels to the rather distant ADP-binding site, and it is speculated what this "path of influence" might be.

Structural studies of kinesin-nucleotide intermediates

We have investigated the structural changes that occur in the molecular motor kinesin during its ATPase cycle, utilizing two bacterially expressed constructs. The structure of both constructs has been examined as a function of the nature of the nucleotide intermediate occupying the active site by means of sedimentation velocity, sedimentation equilibrium, fluorescence solute quenching, fluorescence anisotropy decay, and limited proteolysis. While the molecular weight of monomeric and dimeric human kinesin constructs, as measured by sedimentation velocity and sedimentation equilibrium, and the tryptic cleavage pattern are unaffected by the nucleotide intermediate occupying the active site, significant changes in the rotational correlation time of fluorescently labeled kinesin-nucleotide intermediates can be detected. These results suggest that kinesin contains an internal "hinge" whose flexibility varies through the course of the ATPase cycle. In prehydrolytic, "strong" binding states, this hinge is relatively rigid, while in posthydrolytic, "weak" binding states, it is more flexible. Our results, in conjunction with anisotropy decay studies of myosin, suggest that these two molecular motors may share a common structural feature; viz. weak binding states are characterized by segmental flexibility, which is lost upon assumption of a strong binding conformation.

Myosin catalytic domain flexibility in MgADP

Conventional EPR studies of muscle fibers labeled with a novel alpha-iodoketo spin label at Cys-707 of the myosin head revealed substantial internal domain reorganization on the addition of ADP to rigor fibers. The spin probes that are well-ordered in the rigor state become disordered and form two distinct populations. These orientational changes do not correspond to rotation of the myosin catalytic domain as a whole because other probes (maleimide and iodoacetamide nitroxides attached to the same Cys-707 of myosin head) report only a small (5-10 degrees) torsional rotation and little or no change in the tilt angle [Ajtai et al. (1992) Biochemistry 31, 207-17; Fajer (1994) Biophys. J. 66, 2039-50]. In the presence of ADP, the labeled domain becomes more flexible and executes large-amplitude microsecond motions, as measured by saturation-transfer EPR with rates (tau r = 150 microseconds) intermediate between the rotations of detached (tau r = 7 microseconds) and rigor heads (tau r = 2500 microseconds). This finding contrasts with an absence of global motion of the myosin head in ADP (tau r = 2200 microseconds) as reported by the maleimide spin label. Our results imply that the myosin head in a single chemical state (AM.ADP) is capable of attaining many internal configurations, some of which are dynamic. The presence of these slow structural fluctuations might be related to the slow release of the hydrolysis products of actomyosin ATPase.

Transient kinetics of the interaction of actin with myosin subfragment-1 in the absence of nucleotide

The kinetics of the association of actin with myosin subfragment-1 (S1) has been studied by using S1 labeled at the sulfhydryl group SH1 with 5-(iodoacetamido)fluorescein (S1-AF). Upon rapid mixing in a stopped-flow apparatus, the fluorescence intensity of the fluorescein moiety increased by 50%, followed by a slower increase that was well resolved. This slow phase of the fluorescence change could not be fitted to either a monoexponential or a biexponential function, but it could be fitted to a sum of three exponential terms yielding three observed first-order rate constants (lambda i). The dissociation of acto.-(S1-AF) was studied by displacement of S1-AF from the complex with native S1. The dissociation kinetics was characterized by a single rate constant (approximately 0.012 s-1 at 20 degrees C), and this constant was independent of S1 concentration. Together with previous equilibrium data that were obtained under identified conditions for formation of acto-subfragment-1 (Lin, S.-H., and H. C. Cheung. 1991. Biochemistry. 30:4317-4323), a six-state two-pathway model is proposed as a minimum kinetic scheme for formation of rigor acto.S1. In this model, unbound subfragment-1 exists in two conformational states (S1' and S1) which are in equilibrium with each other, one corresponding to the previously established low-temperature state and the other to the high-temperature state. Each subfragment-1 state can interact with actin to form a collision complex, followed by two isomerizations to form two acto-subfragment-1 states (A.S1' and A.S1). Both isomerizations were visible in stopped-flow experiments. Two special cases of the model were considered: 1) a rapid pre-equilibration of the initial collision complex with actin and S1, and 2) trace accumulation of the collision complex. The first case required that the three combinations of the three observed rate constants be independent of actin concentration. The data were incompatible with this approximation. The other special case required that the sum of the lambda i vary linearly with actin concentration and the other two combinations of lambda i vary with actin concentration in a quadratic fashion. The present data were in agreement with the second case. At 20 degrees C and in 60 mM KCl, 2 mM MgCl2, 30 mM 2-([-hydroxy-1,1-bis(hydroxymethyl)ethyl]amino)ethanesulfonic acid, and pH 7.5, the biomolecular association rate constants for the interaction of actin with S1' and S1 were 8.58 x 10(5) and 1.11 x 10(6) M-1 s-1, respectively.

Nucleotide- and temperature-induced changes in myosin subfragment-1 structure

The effects of nucleotide binding and temperature on the internal structural dynamics of myosin subfragment 1 (S1) were monitored by intrinsic tryptophan phosphorescence lifetime and fluorescence anisotropy measurements. Changes in the global conformation of S1 were monitored by measuring its rate of rotational diffusion using transient electric birefringence techniques. At 5 degrees C, the binding of MgADP, MgADP,P and MgADP,V (vanadate) progressively reduce the rotational freedom of S1 tryptophans, producing what appear to be increasingly more rigidified S1-nucleotide structures. The changes in the luminescence properties of the tryptophans suggest that at least one is located at the interface of two S1 subdomains. Increasing the temperature from 0 to 25 degrees C increases the apparent internal mobility of S1 tryptophans in all cases and, in addition, a reversible temperature-dependent transition centered near 15 degrees C was observed for S1, S1-MgADP and S1-MgADP,P, but not for S1-MgADP,V. The rotational diffusion constants of S1 and S1-MgADP were measured at temperatures between 0 and 25 degrees C. After adjusting for the temperature and viscosity of the solvent, the data indicate that the thermally induced transition at 15 degrees C comprises local conformational changes, but no global conformational change. Structural features of S1-MgADP,P, which may relate to its role in force generation while bound to actin, are presented.

The kinetics of a two-state transition of myosin subfragment 1. A temperature-jump relaxation study

Temperature-jump measurements were carried out on myosin subfragment 1 (S1) labeled at Cys-707 with 5-(iodoacetamido)fluorescein (S1-AF). The relaxation was monitored by following the increase in the fluorescence intensity of the attached probe after a jump of 5.8 degrees C. A single relaxation process was observed over a range of final temperatures, and the relaxation time decreased from 16.69 ms at 15 degrees C to 3.91 ms at 27 degrees C. The relaxation results are interpreted in terms of a two-state transition: (S1-AF)L K+ in equilibrium with K- (S1-AF)H, and the observed single relaxation time (tau) equals l/(k(+) + k-). The individual first-order rate constants, k+ and k-, were calculated from tau and the equilibrium constant previously determined. The activation energy was 21.9 kcal/mol for the forward reaction and 9.3 kcal/mol for the reverse reaction, corresponding to an enthalpy value of 12.6 kcal/mol for the two-state transition. The results provide, for the first time, direct kinetic evidence of a two-state transition of S1 in the absence of bound nucleotide, and support a two-state model of unliganded myosin subfragment 1.

Temperature-induced changes in the flexibility of the loop between SH1 (Cys-707) and SH3 (Cys-522) in myosin subfragment 1 detected by cross-linking

The ability of dibromobimane to cross-link SH1 (Cys-707) in the 21-kDa C-terminal segment to SH3 (Cys-522) in the 50-kDa middle segment of the myosin S1 heavy chain has been examined as a function of nucleotide binding and temperature. The results obtained indicate that, while the reagent rapidly reacts with SH1 at both 25 and 4 degrees C, its ability to cross-link to SH3 is highly dependent on temperature. At 25 degrees C, substantial cross-linking from monofunctionally labeled SH1 to SH3 occurs, in agreement with recent work of Mornet, Ue, and Morales (1985, Proc. Natl. Acad. Sci, USA 82, 1658-1662) and of Ue (1987, Biochemistry 26, 1889-1894) and with their conclusion that a loop, allowing SH1 and SH3 to reside at the cross-linking span of dibromobimane, preexists in the protein. At 4 degrees C, however, negligible amounts of cross-linking are observed whether or not a nucleotide is present, despite indications that SH1 is labeled rapidly by the reagent at this temperature. The inability to form this cross-link is not due to an alternate cross-link between monofunctionally labeled SH1 and another thiol in the 21-kDa segment. These results indicate that this loop exists at 25 degrees C and does not exist (or exists only transiently) at the lower temperature.

Differential scanning calorimetry of a conformational transition in heavy meromyosin

We have investigated the potential use of differential scanning calorimetry (DSC) to characterize conformational changes in proteins with emphasis on a conformational change in the myosin head which may be related to the power-stroke providing force production in muscle contraction. Simulations indicate that two-state conformational transitions with enthalpy changes greater than approximately 30 kcal/mol should be observable by DSC. We present here differential scanning calorimetric studies of a predenaturation structural change in heavy meromyosin. The high concentration of protein required for these experiments leads to potential contributions from intermolecular interactions. The technical difficulties associated with studying conformational transitions by DSC are discussed.

Conformational transitions in the myosin head induced by temperature, nucleotide and actin. Studies on subfragment-1 of myosins from rabbit and frog fast skeletal muscle with a limited proteolysis method

Tryptic digestion patterns reveal a close similarity of the substructure of frog subfragment-1 (S1) to that established for rabbit S1. The 97-kDa heavy chain of chymotryptic S1 of frog myosin is preferentially cleaved into three fragments with apparent molecular masses of 29 kDa, 49 kDa and 20 kDa. These fragments correspond to the 27-kDa, 50-kDa and 20-kDa fragments of rabbit S1, respectively; this is indicated by the sequence of their appearance during digestion, by the suppression by actin of the generation of the 49-kDa and 20-kDa peptides, and by a nucleotide-promoted cleavage of the 29-kDa peptide to a 24-kDa fragment and the 49-kDa peptide to a 44-kDa fragment, analogous to the nucleotide-promoted cleavage of the 27-kDa and 50-kDa fragments of rabbit S1 to the 22-kDa and 45-kDa peptides. The same changes in the digestion patterns as those produced by the presence of nucleotide (ATP or its beta,gamma-imido analog AdoP P[NH]P) at 25 degrees C were observed when the digestion was carried out at 0 degrees C in the absence of nucleotide. The low-temperature-induced changes were particularly well seen in the preparations from frog myosin. The presence of ATP or AdoP P[NH]P at 0 degrees C enhanced, whereas the complex formation with actin prevented, the low-temperature-induced changes. The results are consistent with there being two fundamental conformational states of the myosin head in an equilibrium that is dependent on the temperature, the nucleotide bound at the active site, and the presence or absence of actin.

ATP-induced dissociation of rabbit skeletal actomyosin subfragment 1. Characterization of an isomerization of the ternary acto-S1-ATP complex

The adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) induced dissociation of actomyosin subfragment 1 (S1) has been investigated by monitoring the light scattering changes that occur on dissociation. We have shown that ATP gamma S dissociates acto-S1 by a mechanism similar to that of ATP but at a rate 10 times slower. The maximum rate of dissociation is limited by an isomerization of the ternary actin-S1-nucleotide complex, which has a rate of 500 s-1 for ATP gamma S and an estimated rate of 5000 s-1 for ATP (20 degrees C, 0.1 M KCl, pH 7.0). The activation energy for the isomerization is the same for ATP and ATP gamma S, and both show a break in the Arrhenius plot at 5 degrees C. The reaction between acto-S1 and ATP was also followed by the fluorescence of a pyrene group covalently attached to Cys-374. We show that the fluorescence of the pyrene group reports the isomerization step and not actin dissociation. The characterization of this isomerization is discussed in relation to force-generating models of the actomyosin cross-bridge cycle.

Observation of two orientations from rigor cross-bridges in glycerinated muscle fibers

The fluorescence polarization from rhodamine labels specifically attached to the fast-reacting thiol of the myosin cross-bridge in glycerinated muscle fibers has been measured to determine the angular distribution of the cross-bridges in different physiological states of the fibers as a function of temperature. To investigate the fibers at temperatures below 0 degree C, we have added glycerol to the bathing solution as an anti-freezing agent. We find that the fluorescence polarization from the rhodamine probe detects distinct angular distributions of the cross-bridges in isometric-active, rigor, MgADP, and low ionic strength relaxed fibers at 4 degrees C. We also find that the rigor cross-bridges in the presence of glycerol can maintain at least two distinct orientations relative to the actin filament, one dominant at temperatures T greater than 2 degrees C and another dominant at T less than -10 degrees C. MgADP cross-bridges in the presence of glycerol maintain approximately the same orientation for all temperatures investigated. The rigor cross-bridge orientation at T less than -10 degrees C is similar to both the MgADP cross-bridge orientation in the presence of glycerol and the active muscle cross-bridge orientation at 4 degrees C. These findings show that the rigor cross-bridge in the presence of glycerol has at least two distinct orientations while attached to actin: one of them dominant at high temperature, the other dominant at low temperature or when MgADP is present. The latter orientation resembles that present in isometric-active fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism of muscle contraction

Knowledge of the mechanism of contraction has been obtained from studies of the interaction of actin and myosin in solution, from an elucidation of the structure of muscle fibers, and from measurements of the mechanics and energetics of fiber contraction. Many of the states and the transition rates between them have been established for the hydrolysis of ATP by actin and myosin subfragments in solution. A major goal is to now understand how the kinetics of this interaction are altered when it occurs in the organized array of the myofibril. Early work on the structure of muscle suggested that changes in the orientation of myosin cross-bridges were responsible for the generation of force. More recently, fluorescent and paramagnetic probes attached to the cross-bridges have suggested that at least some domains of the cross-bridges do not change orientation during force generation. A number of properties of active cross-bridges have been defined by measurements of steady state contractions of fibers and by the transients which follow step changes in fiber length or tension. Taken together these studies have provided firm evidence that force is generated by a cyclic interaction in which a myosin cross-bridge attaches to actin, exerts force through a "powerstroke" of 12 nm, and is then released by the binding of ATP. The mechanism of this interaction at the molecular level remains unknown.

Resonance energy transfer evidence for two attached states of the actomyosin complex

Resonance energy transfer measurements were made between a donor fluorophore, N-(bromacetyl)-N'-(1-sulpho-5-naphthyl)ethylenediamine, located on the single cysteine of the Al light chain of myosin (S1(A1), and an acceptor fluorophore, 5-(iodoacetamido)fluorescein, sited on Cys-374 of actin. In the binary rigor complex a transfer efficiency of 24% was noted, representing a spatial separation of about 6 nm. When the same measurements were made using a stable analogue of S1 X ATP, in which the fast reacting SH1 thiol group is crosslinked to another thiol group in the 20 kDa domain of S1, the 2 fluorophores were found to have moved closer together by greater than or equal to 3 nm. This provides, for the first time, direct experimental evidence for a change in structure of the myosin crossbridge that could account for tension generation.

Kinetics of acto-S1 interaction as a guide to a model for the crossbridge cycle

Recent experiments on the kinetics of the interaction between myosin subfragment 1 (S1) and F-actin in solution are summarized. It is concluded that, at every step of the ATPase cycle, the association between the two proteins takes place in two stages. The equilibrium constant of the second step and thus the affinity of S1 for actin changes from step to step during the enzymatic reaction. It is proposed that the transient kinetic evidence can be interpreted in terms of two different classes of contraction models. The first one, which is widely used at present, identifies particular steps in the enzymatic reaction as directly responsible for the conformational change which represents the power stroke of muscle contraction (direct coupling model). In the second class of model, to which we wish to draw attention, changes in affinity modulated by the enzymatic reaction result in changes in the relative amounts of time spent by parts of the myosin molecule in two different environments. These environments determine whether the molecule exists in the 'long' or 'short' state, and it is the transition between these two which constitutes the power stroke (indirect coupling model).

Magnesium ion-dependent contraction of skinned frog muscle fibers in calcium-free solution

Skinned frog fibers were reversibly activated in Ca-free solutions containing 0 mM KCl, 23 microM free Mg, and having an ionic strength of approximately 50 mM. Contractile force was nearly maximal at 22 degrees - 25 degrees C and decreased at lower temperatures. Maximal force in Ca-free solution at 50 mM ionic strength was close to twice the calcium-activated force with pCa 5 and 190 mM ionic strength. The force in Ca-free solution could be reduced to zero by raising the concentration of free Mg from 23 microM to 1.0 mM at the same ionic strength (50 mM). On stretching the fiber from 2.0 to 3.2 micron the force decreased; this effect was similar to that seen with Ca-activated fiber and the data support the idea that Ca-free tension is made at the cross-bridge level. Isotonic contraction during Ca-free activation showed a velocity transient as in Ca-activated fiber at 190 mM ionic strength, but the transient in the present case was very much prolonged. This finding suggests that contraction mechanisms for force generation and for shortening are essentially the same in the two conditions, but that certain rate constants of cross-bridge turnover are slower for the Ca-free contraction. Also, the results indicate that, in low ionic strength, Ca binding to thin filaments is not essential for unmasking the cross-bridge attachment sites, which suggests that the steric blocking mechanism is modified under these conditions.

Fluorescence energy transfer between the myosin subfragment-1 isoenzymes and F-actin in the absence and presence of nucleotides

The unique fast-reacting cysteine residue (SH1) of myosin subfragment 1 (S1), prepared by chymotryptic digestion, and cysteine 373 of actin have been labelled selectively with the fluorescent probes, N-(bromoacetyl)-N'-(1-sulpho-5-naphthyl)ethylenediamine (1,5-BrAEDANS) and 5-(iodoacetamido)fluorescein (5-IAF), whose spectral properties render them a particularly effective donor-acceptor pair in fluorescence energy transfer studies. The transfer efficiency of 40-45% represented a spatial separation of the chromophores of about 5 nm, which is in reasonable agreement with the value of 6 nm reported earlier for similarly labelled S1, prepared by papain digestion, and actin [Takashi, R. (1979) Biochemistry, 18, 5164-5169]. This transfer efficiency did not change when the doubly-labelled binary complex was formed: (1) with acto-S1(A1) or acto-S1(A2) at 10-200 mM KCl, pH 7-8 and different buffer conditions; (2) with either S1 isoenzyme and regulated actin (i.e. actin with tropomyosin and troponin) both in the presence and absence of Ca2+ or when the donor and acceptor attachment sites were reversed. Analysis of donor and acceptor polarized fluorescence showed that the chromophores are not randomly orientated (i.e. chi 2 not equal to 2/3), but they do have some motion relative to either protein. From a knowledge of the limiting values for chi 2, the intersite distance for donor and acceptor chromophores was calculated to be in the range 3.9-6.7 nm. Addition of MgATP to the doubly-labelled acto-S1 complex eliminated energy transfer but this was recovered when ATP hydrolysis was completed. By utilizing the known binding constants between S1, actin and either MgADP or MgAdoPP[NH]P (magnesium adenosine 5'-[beta, gamma-imido]triphosphate) [Konrad, M. and Goody, K. (1982) Eur. J. Biochem. 128, 547-555; Greene, L. E. and Eisenberg, E. (1980) J. Biol. Chem. 255, 543-548], the concentrations of all species present at equilibrium were determined. Experimental conditions were chosen to maximise the amount of ternary acto-S1-nucleotide complex (approximately equal to 50%) and minimise the amount of binary complex (less than or equal to 2%). The spatial separation of the chromophore interaction sites in the ternary complex was found to be the same with both nucleotides and indistinguishable from that found with the binary complex. A similar strategy was employed to compare the conformations of the binary and ternary complexes by 1H-NMR spectroscopy. In these experiments about 90% of the S1 was in the form of the ternary complex. There was no noticeable change in the acto-S1 spectra upon addition of either MgAdoPP[NH]P or MgADP. These observations support the conclusion that there is no large change in structure in the 'rigor' binary acto-S1 complex when it binds either ADP or AdoPP[NH]P.

Kinetic and thermodynamic properties of the ternary complex between F-actin, myosin subfragment 1 and adenosine 5'-[beta, gamma-imido]triphosphate

Equilibrium constants for the formation of a ternary complex between actin, myosin subfragment 1 (S1) and the non-hydrolyzable ATP analog adenosine 5'-[beta, gamma-imido]triphosphate (Ado PP[NH]P) were determined from light-scattering titrations under a variety of conditions. The affinities of S1 (binding constant K1) and acto . S1 (K4) for AdoPP[NH]P have relatively low dependencies on temperature (delta H degrees approximately equal to - 15 - 30 kJ mol-1) and ionic strength, in contrast to the affinities of S1 (K2) and S1 . AdoPP[NH]P (K3) for actin which are influenced quite strongly by temperature (delta H degrees approximately equal to 50 - 65 kJ mol-1) and ionic strength, K2 decreasing by a factor of 10 - 15 between I = 0.05 M and I = 0.2 M and K3 decreasing by a factor of 5.K1, and by detailed balance K2 as well, were found to be about 10-times higher than hitherto reported values (K1 = 3.4 X 10(7) M-1, K2 = 6 X 10(8) M-1, at 24 degrees C,I = 0.09 M, pH 8.0). The binding of ADP to S1 is about 10-fold weaker than that of AdoPP[NH]P, being however much more exothermic (delta H degrees = - 70 kJ mol-1 at I = 0.1 M) and having a negative standard entropy change (delta S = - 125 J mol-1 K-1), in contrast to AdoPP[NH]P binding for which the calculated delta S had positive values. The observed rate constant of dissociation of acto . S1 by AdoPP[NH]P showed an almost hyperbolic dependence on the nucleotide concentration, reaching a maximum of 15 s-1 at I = 0.055 M and 5 s-1 at I = 0.275 M, pH 8.0, 23 degrees C; at 5 degrees C this value was somewhat higher. The rate constant of dissociation of AdoPP[NH]P from its complex with acto . S1 was estimated to exceed 400 s-1 at 23 degrees C, and to be of the order of 150 s-1 at 4 degrees C. The observed rate constant for the association of the S1 . nucleotide complex and actin was proportional to actin concentrations up to 60 microM, thus defining an apparent second-order rate constant of 2 X 10(4) M-1 s-1 at I = 0.125 M and 23 degrees C. A reaction scheme is proposed in which isomerizations of the acto . S1 and acto . S1 . nucleotide complexes can occur.