Probing Muscle Myosin Motor Action: X-Ray (M3 and M6) Interference Measurements Report Motor Domain Not Lever Arm Movement

The key question in understanding how force and movement are produced in muscle concerns the nature of the cyclic interaction of myosin molecules with actin filaments. The lever arm of the globular head of each myosin molecule is thought in some way to swing axially on the actin-attached motor domain, thus propelling the actin filament past the myosin filament. Recent X-ray diffraction studies of vertebrate muscle, especially those involving the analysis of interference effects between myosin head arrays in the two halves of the thick filaments, have been claimed to prove that the lever arm moves at the same time as the sliding of actin and myosin filaments in response to muscle length or force steps. It was suggested that the sliding of myosin and actin filaments, the level of force produced and the lever arm angle are all directly coupled and that other models of lever arm movement will not fit the X-ray data. Here, we show that, in addition to interference across the A-band, which must be occurring, the observed meridional M3 and M6 X-ray intensity changes can all be explained very well by the changing diffraction effects during filament sliding caused by heads stereospecifically attached to actin moving axially relative to a population of detached or non-stereospecifically attached heads that remain fixed in position relative to the myosin filament backbone. Crucially, and contrary to previous interpretations, the X-ray interference results provide little direct information about the position of the myosin head lever arm; they are, in fact, reporting relative motor domain movements. The implications of the new interpretation are briefly assessed.     

Electron Tomography of Cryofixed,Isometrically Contracting Insect Flight Muscle Reveals Novel Actin-Myosin Interactions

Background

Isometric muscle contraction, where force is generated without muscle shortening, is a molecular traffic jam in which the number of actin-attached motors is maximized and all states of motor action are trapped with consequently high heterogeneity. This heterogeneity is a major limitation to deciphering myosin conformational changes in situ.

Methodology

We used multivariate data analysis to group repeat segments in electron tomograms of isometrically contracting insect flight muscle, mechanically monitored, rapidly frozen, freeze substituted, and thin sectioned. Improved resolution reveals the helical arrangement of F-actin subunits in the thin filament enabling an atomic model to be built into the thin filament density independent of the myosin. Actin-myosin attachments can now be assigned as weak or strong by their motor domain orientation relative to actin. Myosin attachments were quantified everywhere along the thin filament including troponin. Strong binding myosin attachments are found on only four F-actin subunits, the “target zone”, situated exactly midway between successive troponin complexes. They show an axial lever arm range of 77°/12.9 nm. The lever arm azimuthal range of strong binding attachments has a highly skewed, 127° range compared with X-ray crystallographic structures. Two types of weak actin attachments are described. One type, found exclusively in the target zone, appears to represent pre-working-stroke intermediates. The other, which contacts tropomyosin rather than actin, is positioned M-ward of the target zone, i.e. the position toward which thin filaments slide during shortening.

Conclusion

We present a model for the weak to strong transition in the myosin ATPase cycle that incorporates azimuthal movements of the motor domain on actin. Stress/strain in the S2 domain may explain azimuthal lever arm changes in the strong binding attachments. The results support previous conclusions that the weak attachments preceding force generation are very different from strong binding attachments.     

Conformation of the troponin core complex in the thin filaments of skeletal muscle during relaxation and active contraction

Contraction of skeletal and cardiac muscles is regulated by Ca(2+) binding to troponin in the actin-containing thin filaments, leading to an azimuthal movement of tropomyosin around the filament that uncovers the myosin binding sites on actin. Here, we use polarized fluorescence to determine the orientation of the C-terminal lobe of troponin C (TnC) in skeletal muscle cells as a step toward elucidating the molecular mechanism of troponin-mediated regulation. Assuming, as shown by X-ray crystallography, that this lobe of TnC is part of a well-defined troponin domain called the IT arm, we show that the coiled coil formed by troponin components I and T makes an angle of about 55° with the thin filament axis in relaxed muscle, in contrast with previous models based on electron microscopy in which this angle is close to 0°. The E helix of TnC makes an angle of about 45° with the thin filament axis. Both the IT coiled coil and the TnC E helix tilt by about 10° on muscle activation. By combining in situ measurements of the orientation of the IT arm and regulatory domain of troponin, which together form the troponin core complex, with published intermolecular distances between thin filament components, we derive models of thin filament structure in which the IT arm of troponin holds its regulatory domain close to the actin surface. Although the structure and function of troponin regions outside the core complex remain to be characterized, the present results provide useful constraints for molecular models of the mechanism of muscle regulation.     

Electron microscopic evidence for the myosin head lever arm mechanism in hydrated myosin filaments using the gas environmental chamber

Muscle contraction results from an attachment–detachment cycle between the myosin heads extending from myosin filaments and the sites on actin filaments. The myosin head first attaches to actin together with the products of ATP hydrolysis, performs a power stroke associated with release of hydrolysis products, and detaches from actin upon binding with new ATP. The detached myosin head then hydrolyses ATP, and performs a recovery stroke to restore its initial position. The strokes have been suggested to result from rotation of the lever arm domain around the converter domain, while the catalytic domain remains rigid. To ascertain the validity of the lever arm hypothesis in muscle, we recorded ATP-induced movement at different regions within individual myosin heads in hydrated myosin filaments, using the gas environmental chamber attached to the electron microscope. The myosin head were position-marked with gold particles using three different site-directed antibodies. The amplitude of ATP-induced movement at the actin binding site in the catalytic domain was similar to that at the boundary between the catalytic and converter domains, but was definitely larger than that at the regulatory light chain in the lever arm domain. These results are consistent with the myosin head lever arm mechanism in muscle contraction if some assumptions are made.     

Cross-bridge number, position, and angle in target zones of cryofixed isometrically active insect flight muscle

Electron micrographic tomograms of isometrically active insect flight muscle, freeze substituted after rapid freezing, show binding of single myosin heads at varying angles that is largely restricted to actin target zones every 38.7 nm. To quantify the parameters that govern this pattern, we measured the number and position of attached myosin heads by tracing cross-bridges through the three-dimensional tomogram from their origins on 14.5-nm-spaced shelves along the thick filament to their thin filament attachments in the target zones. The relationship between the probability of cross-bridge formation and axial offset between the shelf and target zone center was well fitted by a Gaussian distribution. One head of each myosin whose origin is close to an actin target zone forms a cross-bridge most of the time. The probability of cross-bridge formation remains high for myosin heads originating within 8 nm axially of the target zone center and is low outside 12 nm. We infer that most target zone cross-bridges are nearly perpendicular to the filaments (60% within 11 degrees ). The results suggest that in isometric contraction, most cross-bridges maintain tension near the beginning of their working stroke at angles near perpendicular to the filament axis. Moreover, in the absence of filament sliding, cross-bridges cannot change tilt angle while attached nor reach other target zones while detached, so may cycle repeatedly on and off the same actin target monomer.     

Modeling rigor cross-bridge patterns in muscle I. Initial studies of the rigor lattice of insect flight muscle.

We have undertaken some computer modeling studies of the cross-bridge observed by Reedy in insect flight muscle so that we investigate the geometric parameters that influence the attachment patterns of cross-bridges to actin filaments. We find that the appearance of double chevrons along an actin filament indicates that the cross-bridges are able to reach 10--14 nm axially, and about 90 degrees around the actin filament. Between three and five actin monomers are therefore available along each turn of one strand of actin helix for labeling by cross-bridges from an adjacent myosin filament. Reedy's flared X of four bridges, which appears rotated 60 degrees at successive levels on the thick filament, depends on the orientation of the actin filaments in the whole lattice as well as on the range of movement in each cross-bridge. Fairly accurate chevrons and flared X groupings can be modeled with a six-stranded myosin surface lattice. The 116-nm long repeat appears in our models as "beating" of the 14.5-nm myosin repeat and the 38.5-nm actin period. Fourier transforms of the labeled actin filaments indicate that the cross-bridges attach to each actin filament on average of 14.5 nm apart. The transform is sensitive to changes in the ease with which the cross-bridge can be distorted in different directions.     

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.     

Mechanics and models of the myosin motor

In striated muscles, shortening comes about by the sliding movement of thick filaments, composed mostly of myosin, relative to thin filaments, composed mostly of actin. This is brought about by cyclic action of 'cross-bridges' composed of the heads of myosin molecules projecting from a thick filament, which attach to an adjacent thin filament, exert force for a limited time and detach, and then repeat this cycle further along the filament. The requisite energy is provided by the hydrolysis of a molecule of adenosine triphosphate to the diphosphate and inorganic phosphate, the steps of this reaction being coupled to mechanical events within the cross-bridge. The nature of these events is discussed. There is good evidence that one of them is a change in the angle of tilt of a 'lever arm' relative to the 'catalytic domain' of the myosin head which binds to the actin filament. It is suggested here that this event is superposed on a slower, temperature-sensitive change in the orientation of the catalytic domain on the actin filament. Many uncertainties remain.     

Disorder induced in nonoverlap myosin cross-bridges by loss of adenosine triphosphate.

Adenosine triphosphate-dependent changes in myosin filament structure have been directly observed in whole muscle by electron microscopy of thin sections of rapidly frozen, demembranated frog sartorius specimens. In the presence of ATP the thick filaments show an ordered, helical array of cross-bridges except in the bare zone. In the absence of ATP they show two distinct appearances: in the region of overlap with actin, there is an ordered, rigorlike array of cross-bridges between the thick and thin filaments, whereas in the nonoverlap region (H-zone) the myosin heads move away from the thick filament backbone and lose their helical order. This result suggests that the presence of ATP is necessary for maintenance of the helical array of cross-bridges characteristic of the relaxed state. The primary effect of ATP removal on the myosin heads appears to be weaken their binding to the thick filament backbone; released heads that are close to an actin filament subsequently form a new actin-based, ordered array.     

Independent movement of the regulatory and catalytic domains of myosin heads revealed by phosphorescence anisotropy.

Inter- and intradomain flexibility of the myosin head was measured using phosphorescence anisotropy of selectively labeled parts of the molecule. Whole myosin and the myosin head, subfragment-1 (S1), were labeled with eosin-5-iodoacetamide on the catalytic domain (Cys 707) and on two sites on the regulatory domain (Cys 177 on the essential light chain and Cys 154 on the regulatory light chain). Phosphorescence anisotropy was measured in soluble S1 and myosin, with and without F-actin, as well as in synthetic myosin filaments. The anisotropy of the former were too low to observe differences in the domain mobilities, including when bound to actin. However, this was not the case in the myosin filament. The final anisotropy of the probe on the catalytic domain was 0.051, which increased for probes bound to the essential and regulatory light chains to 0.085 and 0.089, respectively. These differences can be expressed in terms of a "wobble in a cone" model, suggesting various amplitudes. The catalytic domain was least restricted, with a 51 +/- 5 degrees half-cone angle, whereas the essential and regulatory light chain amplitude was less than 29 degrees. These data demonstrate the presence of a point of flexibility between the catalytic and regulatory domains. The presence of the "hinge" between the catalytic and regulatory domains, with a rigid regulatory domain, is consistent with both the "swinging lever arm" and "Brownian ratchet" models of force generation. However, in the former case there is a postulated requirement for the hinge to stiffen to transmit the generated torque associated by nucleotide hydrolysis and actin binding.     

The "roll and lock" mechanism of force generation in muscle

Muscle force results from the interaction of the globular heads of myosin-II with actin filaments. We studied the structure-function relationship in the myosin motor in contracting muscle fibers by using temperature jumps or length steps combined with time-resolved, low-angle X-ray diffraction. Both perturbations induced simultaneous changes in the active muscle force and in the extent of labeling of the actin helix by stereo-specifically bound myosin heads at a constant total number of attached heads. The generally accepted hypothesis assumes that muscle force is generated solely by tilting of the lever arm, or the light chain domain of the myosin head, about its catalytic domain firmly bound to actin. Data obtained suggest an additional force-generating step: the "roll and lock" transition of catalytic domains of non-stereo-specifically attached heads to a stereo-specifically bound state. A model based on this scheme is described to quantitatively explain the data.     

Cooperativity of myosin molecules through strain-dependent chemistry

There is mounting evidence that the myosin head domain contains a lever arm which amplifies small structural changes that occur at the nucleotide-binding site. The mechanical work associated with movement of the lever affects the rates at which the products of ATP hydrolysis are released. During muscle contraction, this strain-dependent chemistry leads to cooperativity of the myosin molecules within a thick filament. Two aspects of cooperative action are discussed, in the context of a simple stochastic model. (i) A modest motion of the lever arm on ADP release can serve to regulate the fraction of myosin bound to the thin filament, in order to recruit more heads at higher loads. (ii) If the lever swings through a large angle when phosphate is released, the chemical cycles of the myosin molecules can be synchronized at high loads. This leads to stepwise sliding of the filaments and suggests that the isometric condition is not a steady state.     

Recent X-ray Diffraction and Electron Microscope Studies of Striated Muscle

The sliding filament model for muscular contraction supposes that an appropriately directed force is developed between the actin and myosin filaments by some process in which the cross-bridges are involved. The cross-bridges between the filaments are believed to represent the parts of the myosin molecules which possess the active sites for ATPase activity and actin-binding ability, and project out sidewise from the backbone of the thick filaments. The arrangement of the cross-bridges is now being studied by improved low-angle X-ray diffraction techniques, which show that in a resting muscle, they are arranged approximately but not exactly in a helical pattern, and that there are other structural features of the thick filaments which give rise to additional long periodicities shown up by the X-ray diagram. The actin filaments also contain helically arranged subunits, and both the subunit repeat and the helical repeat are different from those in the myosin filaments. Diffraction diagrams can be obtained from muscles in rigor (when permanent attachment of the cross-bridges to the actin subunits takes place) and now, taking advantage of the great increase in the speed of recording, from actively contracting muscles. These show that changes in the arrangement of the cross-bridges are produced under both these conditions and are no doubt associated in contraction with the development of force. Thus configurational changes of the myosin component in muscle have been demonstrated: these take place without any significant over-all change in the length of the filaments.     

Myosin S2 Origins Track Evolution of Strong Binding on Actin by Azimuthal Rolling of Motor Domain

Myosin crystal structures have given rise to the swinging lever arm hypothesis, which predicts a large axial tilt of the lever arm domain during the actin-attached working stroke. Previous work imaging the working stroke in actively contracting, fast-frozen Lethocerus muscle confirmed the axial tilt; but strongly bound myosin heads also showed an unexpected azimuthal slew of the lever arm around the thin filament axis, which was not predicted from known crystal structures. We hypothesized that an azimuthal reorientation of the myosin motor domain on actin during the weak-binding to strong-binding transition could explain the lever arm slew provided that myosin’s α-helical coiled-coil subfragment 2 (S2) domain emerged from the thick filament backbone at a particular location. However, previous studies did not adequately resolve the S2 domain. Here we used electron tomography of rigor muscle swollen by low ionic strength to pull S2 clear of the thick filament backbone, thereby revealing the azimuth of its point of origin. The results show that the azimuth of S2 origins of those rigor myosin heads, bound to the actin target zone of actively contracting muscle, originate from a restricted region of the thick filament. This requires an azimuthal reorientation of the motor domain on actin during the weak to strong transition.     

Interactions between actin and myosin filaments in skeletal muscle visualized in frozen-hydrated thin sections.

For the purpose of determining net interactions between actin and myosin filaments in muscle cells, perhaps the single most informative view of the myofilament lattice is its averaged axial projection. We have studied frozen-hydrated transverse thin sections with the goal of obtaining axial projections that are not subject to the limitations of conventional thin sectioning (suspect preservation of native structure) or of equatorial x-ray diffraction analysis (lack of experimental phases). In principle, good preservation of native structure may be achieved with fast freezing, followed by low-dose electron imaging of unstained vitrified cryosections. In practice, however, cryosections undergo large-scale distortions, including irreversible compression; furthermore, phase contrast imaging results in a nonlinear relationship between the projected density of the specimen and the optical density of the micrograph. To overcome these limitations, we have devised methods of image restoration and generalized correlation averaging, and applied them to cryosections of rabbit psoas fibers in both the relaxed and rigor states. Thus visualized, myosin filaments appear thicker than actin filaments by a much smaller margin than in conventional thin sections, and particularly so for rigor muscle. This may result from a significant fraction of the myosin S1-cross-bridges averaging out in projection and thus contributing only to the baseline of projected density. Entering rigor incurs a loss of density from an annulus around the myosin filament, with a compensating accumulation of density around the actin filament. This redistribution of mass represents attachment of the fraction of cross-bridges that are visible above background. Myosin filaments in the "nonoverlap" zone appear to broaden on entering rigor, suggesting that on deprivation of ATP, cross-bridges in situ move outwards even without actin in their immediate proximity.     

Myosin VI walks "wiggly" on actin with large and variable tilting

Myosin VI is an unconventional motor protein with unusual motility properties such as its direction of motion and path on actin and a large stride relative to its short lever arms. To understand these features, the rotational dynamics of the lever arm were studied by single-molecule polarized total internal reflection fluorescence (polTIRF) microscopy during processive motility of myosin VI along actin. The axial angle is distributed in two peaks, consistent with the hand-over-hand model. The changes in lever arm angles during discrete steps suggest that it exhibits large and variable tilting in the plane of actin and to the sides. These motions imply that, in addition to the previously suggested flexible tail domain, there is a compliant region between the motor domain and lever arm that allows myosin VI to accommodate the helical position of binding sites while taking variable step sizes along the actin filament.     

Ultra-fastChara myosin: A test case for the swinging lever arm model for force production by myosin

Recent breakthroughs and technological improvements are rapidly generating evidence supporting the “swinging lever arm model” for force production by myosin. Unlike previous models, this model posits that the globular domain of the myosin motor binds to actin with a constant orientation during force generation. Movement of the neck domain of the motor is hypothesized to occur relative to the globular domain much like a lever arm. This intramolecular conformational change drives the movement of the bound actin. The swinging lever arm model is supported by or consistent with a large number of experimental data obtained with skeletal muscle or slime mold myosins, all of which move actin filaments at rates between 1 and 10 μm/sin vitro. Recently myosin was purified, fromChara internodal cells.In vitro the purifiedChara myosin moves actin filaments at rates one order of magnitude faster than the “fast” skeletal muscle myosin. While this ultra fast movement is not necessarily inconsistent with the swinging lever arm model, one or more specific facets of the motor must be altered in theChara motor in order to accommodate such rapid movement. These characteristics are experimentally testable, thus the ultra fast movement byChara myosin represents a powerful and compelling test of the swinging lever arm model.     

X-ray interference studies of crossbridge action in muscle contraction: evidence from muscles during steady shortening

During normal muscle shortening, the myosin heads must undergo many cycles of interaction with the actin filaments sliding past them. It is important to determine what range of configurations is found under these circumstances, and, in terms of the tilting lever arm model, what range of orientations the lever arms undergo. We have studied this using the X-ray interference technique described in the previous article, focusing mainly on the changes in the first order meridional reflection (M3) as compared to isometric. The change in ratio of the heights of the interference peaks indicates how far the mean lever arm angle has moved towards the end of the working stroke; the total intensity change depends on the angle change, on the number of heads now attached at any one time, and on the dispersion of lever arm angles. The latter provides a measure of the distance over which myosin heads remain attached to actin as they go through their working strokes. Surprisingly, the mean position of the attached heads moves only about 1 nm inwards (towards the center of the A-band) at low velocity shortening (around 0.9 T0): their dispersion changes very little. This shows that they must be detaching very early in the working stroke. However, at loads around 0.5 T0, the mean lever arm angle is about half way towards the end of the working stroke, and the dispersion of lever arm angles (with a uniform dispersion) is such as to distribute the heads throughout the whole of the working stroke. At higher velocities of shortening (at 0.3 T0), the mean position shifts further towards the end of the stroke, and the dispersion increases further. The details of the measurements, together with other data on muscle indicate that the force-generating mechanism within the myosin heads must have some unexpected properties.     

The regulatory domain of the myosin head behaves as a rigid lever.

The regulatory domain of the myosin head is believed to serve as a lever arm that amplifies force generated in the catalytic domain and transmits this strain to the thick filament. The lever arm itself either can be passive or may have a more active role storing some of the energy created by hydrolysis of ATP. A structural correlate which might distinguish between these two possibilities (a passive or an active role) is the stiffness of the domain in question. To this effect we have examined the motion of the proximal (ELC) and distal (RLC) subdomains of the regulatory domain in reconstituted myosin filaments. Each subdomain was labeled with a spin label at a unique cysteine residue, Cys-136 of ELC or Cys-154 of mutant RLC, and its mobility was determined using saturation transfer electron paramagnetic resonance spectroscopy. The mobility of the two domains was similar; the effective correlation time (tau(eff)) for ELC was 17 micros and that for RLC was 22 micros. Additionally, following a 2-fold change of the global dynamics of the myosin head, effected by decreasing the interactions with the filament surface (or the other myosin head), the coupling of the intradomain dynamics remained unchanged. These data suggest that the regulatory domain of the myosin head acts as a single mechanically rigid body, consistent with the regulatory domain serving as a passive lever.     

An actin-dependent conformational change in myosin

Conformational changes within myosin lead to its movement relative to an actin filament. Several crystal structures exist for myosin bound to various nucleotides, but none with bound actin. Therefore, the effect of actin on the structure of myosin is poorly understood. Here we show that the swing of smooth muscle myosin lever arm requires both ADP and actin. This is the first direct observation that a conformation of myosin is dependent on actin. Conformational changes within myosin were monitored using fluorescence resonance energy transfer techniques. A cysteine-reactive probe is site-specifically labeled on a 'cysteine-light' myosin variant, in which the native reactive cysteines were removed and a cysteine engineered at a desired position. Using this construct, we show that the actin-dependent ADP swing causes an 18 A change in distance between a probe on the 25/50 kDa loop on the catalytic domain and a probe on the regulatory light chain, corresponding to a 23 degrees swing of the light-chain domain.