Rotational dynamics of actin-bound intermediates in the myosin ATPase cycle

The structural dynamics of actin during active interaction with myosin depends on the isoform of the essential light chain

We have used time-resolved phosphorescence anisotropy to investigate the effects of essential light chain (ELC) isoforms (A1 and A2) on the interaction of skeletal muscle myosin with actin, to relate structural dynamics to previously reported functional effects. Actin was labeled with a phosphorescent probe at C374, and the myosin head (S1) was separated into isoenzymes S1A1 and S1A2 by ion-exchange chromatography. As previously reported, S1A1 exhibited substantially lower ATPase activity at saturating actin concentrations but substantially higher apparent actin affinity, resulting in a higher catalytic efficiency. In the absence of ATP, each isoenzyme increased actin's final anisotropy cooperatively and to a similar extent, indicating a similar restriction of the amplitude of intrafilament rotational motions in the strong-binding (S) state of actomyosin. In contrast, in the presence of a saturating level of ATP, S1A1 increased actin anisotropy much more than S1A2 and with greater cooperativity, indicating that S1A1 was more effective in restricting actin dynamics during the active interaction of actin and myosin. We conclude that during the active interaction of actin and ATP with myosin, S1A1 is more effective at stabilizing the S state (probably the force-generating state) of actomyosin, while S1A2 tends to stabilize the weak-binding (non-force-generating) W state. When a mixture of isoenzymes is present, S1A1 is dominant in its effects on actin dynamics. We conclude that ELC of skeletal muscle myosin modulates strong-to-weak structural transitions during the actomyosin ATPase cycle in an isoform-dependent manner, with significant implications for the contractile function of actomyosin.

Conformationally trapping the actin-binding cleft of myosin with a bifunctional spin label

We have trapped the catalytic domain of Dictyostelium (Dicty) myosin II in a weak actin-binding conformation by chemically crosslinking two engineered cysteines across the actin-binding cleft, using a bifunctional spin label (BSL). By connecting the lower and upper 50 kDa domains of myosin, the crosslink restricts the conformation of the actin-binding cleft. Crosslinking has no effect on the basal ATPase activity of isolated myosin, but it impairs rigor actin binding and actin-activation of myosin ATPase. EPR spectra of BSL provide insight into actomyosin structural dynamics. BSL is highly immobilized within the actin-binding cleft and is thus exquisitely sensitive to the global orientation and rotational motions of the myosin head. Conventional EPR shows that myosin heads bound to oriented actin filaments are highly disordered with respect to the actin filament axis, in contrast to the nearly crystalline order of myosin heads in rigor. This disorder is similar to that of weakly bound heads induced by ATP, but saturation transfer EPR shows that the disorder of crosslinked myosin is at least 100 times slower. Thus this cleft-crosslinked myosin is remarkably similar, in both actin affinity and rotational dynamics, to SH1-SH2 crosslinked BSL-myosin S1. We conclude that, whether myosin is trapped at the actin-myosin interface or in the force-generating region between the active site and lever arm, the structural state of myosin is intermediate between the weak-binding state preceding phosphate release and the strong-binding state that succeeds it. We propose that it represents the threshold of force generation.

Three distinct actin-attached structural states of myosin in muscle fibers

We have used thiol cross-linking and electron paramagnetic resonance (EPR) to resolve structural transitions of myosin's light chain domain (LCD) and catalytic domain (CD) that are associated with force generation. Spin labels were incorporated into the LCD of muscle fibers by exchanging spin-labeled regulatory light chain for endogenous regulatory light chain, with full retention of function. To trap myosin in a structural state analogous to the elusive posthydrolysis ternary complex A.M'.D.P, we used pPDM to cross-link SH1 (Cys(707)) to SH2 (Cys(697)) on the CD. LCD orientation and dynamics were measured in three biochemical states: relaxation (A.M.T), SH1-SH2 cross-linked (A.M'.D.P analog), and rigor (A.M.D). EPR showed that the LCD of cross-linked fibers has an orientational distribution intermediate between relaxation and rigor, and saturation transfer EPR revealed slow rotational dynamics indistinguishable from that of rigor. Similar results were obtained for the CD using a bifunctional spin label to cross-link SH1-SH2, but the CD was more disordered than the LCD. We conclude that SH1-SH2 cross-linking traps a state in which both the CD and LCD are intermediate between relaxation (highly disordered and microsecond dynamics) and rigor (highly ordered and rigid), supporting the hypothesis that the cross-linked state is an A.M'D.P analog on the force generation pathway.

Structural dynamics of the actomyosin complex probed by a bifunctional spin label that cross-links SH1 and SH2

We have used a bifunctional spin label (BSL) to cross-link Cys(707) (SH1) and Cys(697) (SH2) in the catalytic domain of myosin subfragment 1 (S1). BSL induces the same weakened ATPase activity and actin-binding affinity that is observed when SH1 and SH2 are cross-linked with pPDM, which traps an analog of the post-hydrolysis state A.M.ADP.P. Electron paramagnetic resonance showed that BSL reports the global orientation and dynamics of S1. When bound to actin in oriented muscle fibers in the absence of ATP, BSL-S1 showed almost complete orientational disorder, as reported previously for the weakly bound A.M.ADP. In contrast, helical order is observed for the strongly bound state A.M. Saturation transfer electron paramagnetic resonance showed that the disorder of cross-linked S1 on actin is nearly static on the microsecond timescale, at least 30 times slower than that of A.M.ADP. We conclude that cross-linked S1 exhibits rotational disorder comparable to that of A.M.ADP, slow rotational mobility comparable to that of A.M, and intermediate actin affinity. These results support the hypothesis that the catalytic domain of myosin is orientationally disordered on actin in a post-hydrolysis state in the early stages of force generation.

Regulation and recycling of myosin V

Recently there has been considerable progress in our understanding of regulation for unconventional myosin-V through elucidation of the structure of its inactive conformation and the factors that affect stability of this conformation. The inactive conformation is a folded compact structure characterized by interactions between the myosin head and the C-terminal cargo binding domain. Concentrations of Ca2+ greater than 10 microM disrupt folding. The 3-D structure determined by cryoelectron tomography of 2-D arrays in one study and electron micrographs of isolated molecules reported in another reveal similar features, but suggest different F-actin affinities for the inactive conformation. This has raised the question of how inactive myosin-V is recycled to other sites for additional rounds of cargo transport.

A unique ATP hydrolysis mechanism of single-headed processive myosin, myosin IX

Recent studies have revealed that myosin IX is a single-headed processive myosin, yet it is unclear how myosin IX can achieve the processive movement. Here we studied the mechanism of ATP hydrolysis cycle of actomyosin IXb. We found that myosin IXb has a rate-limiting ATP hydrolysis step unlike other known myosins, thus populating the prehydrolysis intermediate (M.ATP). M.ATP has a high affinity for actin, and, unlike other myosins, the dissociation of M.ATP from actin was extremely slow, thus preventing myosin from dissociating away from actin. The ADP dissociation step was 10-fold faster than the overall ATP hydrolysis cycle rate and thus not rate-limiting. We propose the following model for single-headed processive myosin. Upon the formation of the M.ATP intermediate, the tight binding of actomyosin IX at the interface is weakened. However, the head is kept in close proximity to actin due to the tethering role of loop 2/large unique insertion of myosin IX. There is enough freedom for the myosin head to find the next location of the binding site along with the actin filament before complete dissociation from the filament. After ATP hydrolysis, Pi is quickly released to form a strong actin binding form, and a power stroke takes place.

Measuring kinesin's first step

A variety of models have recently emerged to explain how the molecular motor kinesin is able to maintain processive movement for over 100 steps. Although these models differ in significant features, they all predict that kinesin's catalytic domains intermittently separate from each other as the motor takes 8-nm steps along the microtubule. Furthermore, at some point in this process, one molecule of ATP is hydrolyzed per step. However, exactly when hydrolysis and product release occur in relation to this forward step have not been established. Furthermore, the rate at which this separation occurs as well as the speed of motor stepping onto and release from the microtubule have not been measured. In the absence of this information, it is difficult to critically evaluate competing models of kinesin function. We have addressed this issue by developing spectroscopic probes whose fluorescence is sensitive to motor-motor separation or microtubule binding. The kinetics of these fluorescence changes allow us to directly measure how fast kinesin steps onto and releases from the microtubule and provide insight into how processive movement is maintained by this motor.

Changes in actin and myosin structural dynamics due to their weak and strong interactions

Figure 3 summarizes the effects of actomyosin binding on the internal and global dynamics of either protein, as discussed in this chapter. These effects depend primarily on the strength of the interaction; which in turn depends on the state of the nucleotide at the myosin active site. When either no nucleotide or ADP is bound, the interaction is strong and the effect on each protein is maximal. When the nucleotide is ATP or ADP.Pi, or the equivalent nonhydrolyzable analogs, the interaction is weak and the effect on molecular dynamics of each protein is minimal. The weaker effects in weak-binding states are not simply the reflection of lower occupancy of binding sites--the molecular models in Fig. 3 illustrate the effects of the formation of the ternary complex, after correction for the free actin and myosin in the system. Thus EPR on myosin (Berger and Thomas 1991; Thomas et al. 1995) and pyrene fluorescence studies on actin (Geeves 1991) have shown that the formation of a ternary complex has a negligible effect on the internal dynamics of both [figure: see text] proteins (left side of Fig. 3, white arrows). As shown by both EPR (Baker et al. 1998; Roopnarine et al. 1998) and phosphorescence (Ramachandran and Thomas 1999), both domains of myosin are dynamically disordered in weak-binding states, and this is essentially unaffected by the formation of the ternary complex (left side of Fig. 3, indicated by disordered myosin domains). The only substantial effect of the formation of the weak interaction that has been reported is the EPR-detected (Ostap and Thomas 1991) restriction of the global dynamics of actin upon weak myosin binding (left column of Fig. 3, gray arrow). The effects of strong actomyosin formation are much more dramatic. While substantial rotational dynamics, both internal and global, exist in both myosin and actin in the presence of ADP or the absence of nucleotides, spin label EPR, pyrene fluorescence, and phosphorescence all show dramatic restrictions in these motions upon formation of the strong ternary complex (right column of Fig. 3). One implication of this is that the weak-to-strong transition is accompanied by a disorder-to-order transition in both actin and myosin, and this is itself an excellent candidate for the structural change that produces force (Thomas et al. 1995). Another clear implication is that the crystal structures obtained for isolated myosin and actin are not likely to be reliable representations of structures that exist in ternary complexes of these proteins (Rayment et al. 1993a and 1993b; Dominguez et al. 1998; Houdusse et al. 1999). This is clearly true of the strong-binding states, since the spectroscopic studies indicate consistently that substantial changes occur in both proteins upon strong complex formation. For the weak complexes, the problem is not that complex formation induces large structural changes, but that the structures themselves are dynamically disordered. This is probably why so many different structures have been obtained for myosin S1 with nucleotides bound--each crystal is selecting one of the many different substates represented by the dynamic ensemble. Finally, there is the problem that the structures of actomyosin complexes are probably influenced strongly by their mechanical coupling to muscle protein lattice (Baker at al. 2000). Thus, even if co-crystals of actin and myosin are obtained in the future, an accurate description of the structural changes involved in force generation will require further experiments using site-directed spectroscopic probes of both actin and myosin, in order to detect the structural dynamics of these ternary complexes under physiological conditions.

Myosin head orientation and mobility during isometric contraction: effects of osmotic compression

We have correlated the mobility and the generation of force of myosin heads by applying radial compression to isometrically contracting muscle fibers. Osmotic pressure was produced by dextran T-500, and its effect on the orientation and mobility of myosin heads labeled with N-(1-oxy-2,2,5,5-tetramethyl-4-pyperidinyl)maleimide was observed by conventional and saturation-transfer electron paramagnetic resonance methods. A biphasic behavior is spectral changes coinciding with the tension dependence was observed as the fibers were compressed. At diameters above the equilibrium spacing, the large myosin head disorder characteristic during contraction in the absence of compression was largely maintained, whereas the mobility decreased threefold, from tauR approximately 25 microseconds to approximately 80-90 microseconds. The inhibition of fast microsecond motions was not accompanied by tension loss, implying that these motions are not necessary for force generation. At diameters below the equilibrium spacing, both the disorder and the mobility decreased dramatically in parallel with the tension inhibition, suggesting that slower microsecond motions and the disorder of the myosin head are necessary for muscle function.

The mitochondrial protein import motor: dissociation of mitochondrial hsp70 from its membrane anchor requires ATP binding rather than ATP hydrolysis

During protein import into mitochondria, matrix-localized mitochondrial hsp70 (mhsp70) interacts with the inner membrane protein Tim44 to pull a precursor across the inner membrane. We have proposed that the Tim44-mhsp70 complex functions as an ATP-dependent "translocation motor" that exerts an inward force on the precursor chain. To clarify the role of ATP in mhsp70-driven translocation, we tested the effect of the purified ATP analogues AMP-PNP and ATP gamma S on the Tim44-mhsp70 interaction. Both analogues mimicked ATP by causing dissociation of mhsp70 from Tim44. ADP did not disrupt the Tim44-mhsp70 complex, but did block the ATP-induced dissociation of this complex. In the presence of ADP, mhsp70 can bind simultaneously to Tim44 and to a peptide substrate. These data are consistent with a model in which mhsp70 first hydrolyzes ATP, then associates tightly with Tim44 and a precursor protein, and finally undergoes a conformational change to drive translocation.

Resolution of three structural states of spin-labeled myosin in contracting muscle

We have used electron paramagnetic resonance (EPR) spectroscopy to detect ATP- and calcium-induced changes in the structure of spin-labeled myosin heads in glycerinated rabbit psoas muscle fibers in key physiological states. The probe was a nitroxide iodoacetamide derivative attached selectively to myosin SH1 (Cys 707), the conventional EPR spectra of which have been shown to resolve several conformational states of the myosin ATPase cycle, on the basis of nanosecond rotational motion within the protein. Spectra were acquired in rigor and during the steady-state phases of relaxation and isometric contraction. Spectral components corresponding to specific conformational states and biochemical intermediates were detected and assigned by reference to EPR spectra of trapped kinetic intermediates. In the absence of ATP, all of the myosin heads were rigidly attached to the thin filament, and only a single conformation was detected, in which there was no sub-microsecond probe motion. In relaxation, the EPR spectrum resolved two conformations of the myosin head that are distinct from rigor. These structural states were virtually identical to those observed previously for isolated myosin and were assigned to the populations of the M*.ATP and M**.ADP.Pi states. During isometric contraction, the EPR spectrum resolves the same two conformations observed in relaxation, plus a small fraction (20-30%) of heads in the oriented actin-bound conformation that is observed in rigor. This rigor-like component is a calcium-dependent, actin-bound state that may represent force-generating cross-bridges. As the spin label is located near the nucleotide-binding pocket in a region proposed to be pivotal for large-scale force-generating structural changes in myosin, we propose that the observed spectroscopic changes indicate directly the key steps in energy transduction in the molecular motor of contracting muscle.

Orientational dynamics of indane dione spin-labeled myosin heads in relaxed and contracting skeletal muscle fibers

We have used electron paramagnetic resonance (EPR) spectroscopy to study the orientation and rotational motions of spin-labeled myosin heads during steady-state relaxation and contraction of skinned rabbit psoas muscle fibers. Using an indane-dione spin label, we obtained EPR spectra corresponding specifically to probes attached to Cys 707 (SH1) on the catalytic domain of myosin heads. The probe is rigidly immobilized, so that it reports the global rotation of the myosin head, and the probe's principal axis is aligned almost parallel with the fiber axis in rigor, making it directly sensitive to axial rotation of the head. Numerical simulations of EPR spectra showed that the labeled heads are highly oriented in rigor, but in relaxation they have at least 90 degrees (Gaussian full width) of axial disorder, centered at an angle approximately equal to that in rigor. Spectra obtained in isometric contraction are fit quite well by assuming that 79 +/- 2% of the myosin heads are disordered as in relaxation, whereas the remaining 21 +/- 2% have the same orientation as in rigor. Computer-simulated spectra confirm that there is no significant population (> 5%) of heads having a distinct orientation substantially different (> 10 degrees) from that in rigor, and even the large disordered population of heads has a mean orientation that is similar to that in rigor. Because this spin label reports axial head rotations directly, these results suggest strongly that the catalytic domain of myosin does not undergo a transition between two distinct axial orientations during force generation. Saturation transfer EPR shows that the rotational disorder is dynamic on the microsecond time scale in both relaxation and contraction. These results are consistent with models of contraction involving 1) a transition from a dynamically disordered preforce state to an ordered (rigorlike) force-generating state and/or 2) domain movements within the myosin head that do not change the axial orientation of the SH1-containing catalytic domain relative to actin.

Rotational dynamics of actin-bound intermediates of the myosin adenosine triphosphatase cycle in myofibrils

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to measure the microsecond rotational motion of actin-bound myosin heads in spin-labeled myofibrils in the presence of the ATP analogs AMPPNP (5'-adenylylimido-diphosphate) and ATP gamma S (adenosine-5'-O-(3-thiotriphosphate)). AMPPNP and ATP gamma S are believed to trap myosin in two major conformational intermediates of the actomyosin ATPase cycle, respectively known as the weakly bound and strongly bound states. Previous ST-EPR experiments with solutions of acto-S1 have demonstrated that actin-bound myosin heads are rotationally mobile on the microsecond time scale in the presence of ATP gamma S, but not in the presence of AMPPNP. However, it is not clear that results obtained with acto-S1 in solution can be extended to actomyosin constrained within the myofibrillar lattice. Therefore, ST-EPR spectra of spin-labeled myofibrils were analyzed explicitly in terms of the actin-bound component of myosin heads in the presence of AMPPNP and ATP gamma S. The fraction of actin-attached myosin heads was determined biochemically in the spin-labeled myofibrils, using the proteolytic rates actomyosin binding assay. At physiological ionic strength (mu = 165 mM), actin-bound myosin heads were found to be rotationally mobile on the microsecond time scale (tau r = 24 +/- 8 microseconds) in the presence of ATP gamma S, but not AMPPNP. Similar results were obtained at low ionic strength, confirming the acto-S1 solution studies. The microsecond rotational motions of actin-attached myosin heads in the presence of ATP gamma S are similar to those observed for spin-labeled myosin heads during the steady-state cycling of the actomyosin ATPase, both in solution and in an active isometric muscle fiber. These results indicate that weakly bound myosin heads, in the pre-force phase of the ATPase cycle, are rotationally mobile, while strongly bound heads, in the force-generating phase, are rotationally immobile. We propose that force generation involves a transition from a dynamically disordered crossbridge to a rigid and stereospecific one.

Kinetic and equilibrium analysis of the interactions of actomyosin subfragment-1.ADP with beryllium fluoride

The hypothesis that the stable ternary complex formed between myosin subfragment-1, MgADP and beryllium fluoride (BeF3-), denoted S-1 not equal to .ADP.BeF3-, is an analog of the intermediate state S-1**.ADP.P(i) has been tested in this work by examining the interactions of S-1 not equal to .ADP.BeF3- with actin. Equilibrium binding measurements revealed that actin bound weakly to the S-1 not equal to .ADP.BeF3- complex (Ka = 10(4) M-1) in the presence of 40 mM KCl. The stability of this complex was strongly salt-dependent. The association constant of BeF3- to the acto-S-1.ADP complex (KBe approximately 10(3) M-1) was 100-fold weaker than its binding to the S-1.ADP complex. While inhibiting the S-1 ATPase strongly, BeF3- had no effect on the Vmax value (10 +/- 1.0 s-1) of the actin-activated ATPase of S-1. The rates of BeF3- binding and dissociation from the acto-S-1.ADP.BeF3- complex were determined by stopped-flow measurements. The hyperbolic dependence of the rates of BeF3- binding to acto-S-1.ADP (kobs) on BeF3- concentrations suggested that the acto-S-1.ADP.BeF3- complex was formed in at least two steps: binding followed by isomerization. The binding constant was 1.2 x 10(3) M-1, and the maximum kobs was 2.5 s-1. The dissociation of BeF3- from the acto-S-1.ADP.BeF3- complex was monitored via decrease in the fluorescence of 1-N6-ethenoadenosine diphosphate (epsilon ADP). The fluorescence decrease fitted two exponential terms.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient detection of spin-labeled myosin subfragment 1 conformational states during ATP hydrolysis

We have used time-resolved electron paramagnetic resonance spectroscopy and caged ATP to detect nucleotide-induced changes in the conformational state of spin-labeled myosin heads (IASL-S1). Changes in the internal rotational dynamics of IASL-S1 were monitored with millisecond time resolution during the pre-steady-state phase of ATP hydrolysis. The changes in the internal protein dynamics were rigorously correlated with specific biochemical kinetic transitions, allowing us to observe directly the dynamic sequence of structural changes in IASL-S1 during the binding and hydrolysis of ATP. When caged ATP was photolyzed (producing 500 microM ATP) in the presence of 100 microM IASL-S1, the EPR signal intensity rose transiently to the steady-state ATPase level, indicating increased rotational motion about the SH1 region of the myosin head. Kinetic and spectral analyses have resolved two phases of this transient, one representing the population of the M*.ATP state and the other representing the population of the M**.ADP.Pi state. We conclude that two motionally distinct states of the myosin head are present during ATP hydrolysis and that these states represent distinct conformational states that can be correlated with specific biochemical intermediates. Since specific labeling of myosin heads with IASL has been achieved in skinned muscle fibers, this study establishes the feasibility for the first direct detection and resolution of myosin's conformational transients during muscle contraction.

Cooperativity in F-actin: chemical modifications of actin monomers affect the functional interactions of myosin with unmodified monomers in the same actin filament

We have chemically modified a fraction of the monomers in actin filaments, and then measured the effects on the functional interaction of myosin with unmodified monomers within the same filament. Two modifications were used: (a) covalent attachment of various amounts of myosin subfragment-1 (S1) with the bifunctional reagent disuccinimidyl suberate and (b) copolymerization of unmodified actin monomers with monomers cross-linked internally with 1-ethyl-3-(dimethylaminopropyl)-carbodiimide. Each of these modifications abolished the interaction of the modified monomers with myosin, so the remaining interactions were exclusively with unmodified monomers. The two modifications had similar effects on the interaction of actin with myosin in solution: decreased affinity of myosin heads for unmodified actin monomers, without a change in the Vmax of actin-activated myosin ATPase activity. However, modification (b) produced much greater inhibition of actin sliding on a myosin-coated surface, as measured by an in vitro motility assay. These results provide insight into the functional consequences of cooperative interactions within the actin filament.

Saturation transfer electron parametric resonance of an indane-dione spin-label. Calibration with hemoglobin and application to myosin rotational dynamics

We have used a recently synthesized indane-dione spin label (2-[-oxyl-2,2,5,5-tetramethyl-3-pyrrolin-3-yl)methenyl]in dane-1,3-dione (InVSL) to study the rotational dynamics of myosin, with saturation-transfer electron paramagnetic resonance (ST-EPR). To determine effective rotational correlation times (tau effr) from InVSL spectra, reference spectra corresponding to known correlation times (tau r) were obtained from InVSL-hemoglobin undergoing isotropic rotational motion in aqueous glycerol solutions. These spectra were used to generate plots of spectral parameters vs. tau r. These plots should be used to analyze ST-EPR spectra of InVSL bound to other proteins, because the spectra are different from those of tempo-maleimide-spin-labeled hemoglobin, which have been used previously as ST-EPR standards. InVSL was covalently attached to the head (subfragment-1; S1) of myosin. EPR spectra and K/EDTA-ATPase activity showed that 70-95% of the heads were labeled, with > or = 90% of the label bound to either cys 707 (SH1) or cys 697 (SH2). ST-EPR spectra of InVSL-S1 attached to glass beads, bound to actin in myofibrils, or precipitated with ammonium sulfate indicated no submillisecond rotational motion. Therefore, InVSL is rigidly immobilized on the protein so that it reports the global rotation of the myosin head. The ST-EPR spectra of InVSL-myosin monomers and filaments indicated tau effr values of 4 and 13 microseconds, respectively, showing that myosin heads undergo microsecond segmental rotations that are more restricted in filaments than in monomers. The observed tau effr values are longer than those previously obtained with other spin labels bound to myosin heads, probably because InVSL binds more rigidly to the protein and/or with a different orientation. Further EPR studies of InVSL-myosin in solution and in muscle fibers should prove complementary to previous work with other labels.