Paramagnetic probes attached to a light chain on the myosin head are highly disordered in active muscle fibers

Myosin and Other Energy-Transducing ATPases: Structural Dynamics Studied by Electron Paramagnetic Resonance

The objective of this article was to document the energy-transducing and regulatory interactions in supramolecular complexes such as motor, pump, and clock ATPases. The dynamics and structural features were characterized by motion and distance measurements using spin-labeling electron paramagnetic resonance (EPR) spectroscopy. In particular, we focused on myosin ATPase with actin-troponin-tropomyosin, neural kinesin ATPase with microtubule, P-type ion-motive ATPase, and cyanobacterial clock ATPase. Finally, we have described the relationships or common principles among the molecular mechanisms of various energy-transducing systems and how the large-scale thermal structural transition of flexible elements from one state to the other precedes the subsequent irreversible chemical reactions.

Myosin lever arm orientation in muscle determined with high angular resolution using bifunctional spin labels

High-resolution structural information is invaluable for understanding muscle function. Savich et al. use bifunctional spin labeling to determine the orientation of the myosin lever arm in muscle fibers at high resolution under ambient conditions, augmenting previous insights obtained from fluorescence and EM.

Despite advances in x-ray crystallography, cryo-electron microscopy (cryo-EM), and fluorescence polarization, none of these techniques provide high-resolution structural information about the myosin light chain domain (LCD; lever arm) under ambient conditions in vertebrate muscle. Here, we measure the orientation of LCD elements in demembranated muscle fibers by electron paramagnetic resonance (EPR) using a bifunctional spin label (BSL) with an angular resolution of 4°. To achieve stereoselective site-directed labeling with BSL, we engineered a pair of cysteines in the myosin regulatory light chain (RLC), either on helix E or helix B, which are roughly parallel or perpendicular to the myosin lever arm, respectively. By exchanging BSL-labeled RLC onto oriented muscle fibers, we obtain EPR spectra from which the angular distributions of BSL, and thus the lever arm, can be determined with high resolution relative to the muscle fiber axis. In the absence of ATP (rigor), each of the two labeled helices exhibits both ordered (σ ∼9–11°) and disordered (σ > 38°) populations. Using these angles to determine the orientation of the lever arm (LCD combined with converter subdomain), we observe that the oriented population corresponds to a lever arm that is perpendicular to the muscle fiber axis and that the addition of ATP in the absence of Ca²⁺ (inducing relaxation) shifts the orientation to a much more disordered orientational distribution. Although the detected orientation of the myosin light chain lever arm is ∼33° different than predicted from a standard “lever arm down” model based on cryo-EM of actin decorated with isolated myosin heads, it is compatible with, and thus augments and clarifies, fluorescence polarization, x-ray interference, and EM data obtained from muscle fibers. These results establish feasibility for high-resolution detection of myosin LCD rotation during muscle contraction.

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.

Myosin regulatory domain orientation in skeletal muscle fibers: application of novel electron paramagnetic resonance spectral decomposition and molecular modeling methods

Reorientation of the regulatory domain of the myosin head is a feature of all current models of force generation in muscle. We have determined the orientation of the myosin regulatory light chain (RLC) using a spin-label bound rigidly and stereospecifically to the single Cys-154 of a mutant skeletal isoform. Labeled RLC was reconstituted into skeletal muscle fibers using a modified method that results in near-stoichiometric levels of RLC and fully functional muscle. Complex electron paramagnetic resonance spectra obtained in rigor necessitated the development of a novel decomposition technique. The strength of this method is that no specific model for a complex orientational distribution was presumed. The global analysis of a series of spectra, from fibers tilted with respect to the magnetic field, revealed two populations: one well-ordered (+/-15 degrees ) with the spin-label z axis parallel to actin, and a second population with a large distribution (+/-60 degrees ). A lack of order in relaxed or nonoverlap fibers demonstrated that regulatory domain ordering was defined by interaction with actin rather than the thick filament surface. No order was observed in the regulatory domain during isometric contraction, consistent with the substantial reorientation that occurs during force generation. For the first time, spin-label orientation has been interpreted in terms of the orientation of a labeled domain. A Monte Carlo conformational search technique was used to determine the orientation of the spin-label with respect to the protein. This in turn allows determination of the absolute orientation of the regulatory domain with respect to the actin axis. The comparison with the electron microscopy reconstructions verified the accuracy of the method; the electron paramagnetic resonance determined that axial orientation was within 10 degrees of the electron microscopy model.

Polarized fluorescence depletion reports orientation distribution and rotational dynamics of muscle cross-bridges

The method of polarized fluorescence depletion (PFD) has been applied to enhance the resolution of orientational distributions and dynamics obtained from fluorescence polarization (FP) experiments on ordered systems, particularly in muscle fibers. Previous FP data from single fluorescent probes were limited to the 2(nd)- and 4(th)-rank order parameters, <P(2)(cos beta)> and <P(4)(cos beta)>, of the probe angular distribution (beta) relative to the fiber axis and <P(2d)>, a coefficient describing the extent of rapid probe motions. We applied intense 12-micros polarized photoselection pulses to transiently populate the triplet state of rhodamine probes and measured the polarization of the ground-state depletion using a weak interrogation beam. PFD provides dynamic information describing the extent of motions on the time scale between the fluorescence lifetime (e.g., 4 ns) and the duration of the photoselection pulse and it potentially supplies information about the probe angular distribution corresponding to order parameters above rank 4. Gizzard myosin regulatory light chain (RLC) was labeled with the 6-isomer of iodoacetamidotetramethylrhodamine and exchanged into rabbit psoas muscle fibers. In active contraction, dynamic motions of the RLC on the PFD time scale were intermediate between those observed in relaxation and rigor. The results indicate that previously observed disorder of the light chain region in contraction can be ascribed principally to dynamic motions on the microsecond time scale.

Effect of adenosine 5'-[beta,gamma-imido]triphosphate on myosin head domain movements

Conventional and saturation transfer electron paramagnetic resonance spectroscopy (EPR and ST EPR) was used to study the orientation of probe molecules in muscle fibers in different intermediate states of the ATP hydrolysis cycle. A separate procedure was used to obtain ST EPR spectra with precise phase settings even in the case of samples with low spectral intensity. Fibers prepared from rabbit psoas muscle were labeled with isothiocyanate spin labels at the reactive thiol sites of the catalytic domain of myosin. In comparison with rigor, a significant difference was detected in the orientation-dependence of spin labels in the ADP and adenosine 5'-[beta,gamma-imido]triphosphate (AdoPP[CH2]P) states, indicating changes in the internal dynamics and domain orientation of myosin. In the AdoPP[CH2]P state, approximately half of the myosin heads reflected the motional state of ADP-myosin, and the other half showed a different dynamic state with greater mobility.

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 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.

Rotational dynamics of the regulatory light chain in scallop muscle detected by time-resolved phosphorescence anisotropy

We have used time-resolved phosphorescence anisotropy (TPA) to study the rotational dynamics of chicken gizzard regulatory light chain (RLC) bound to scallop adductor muscle myofibrils in key physiological states. Native RLC from scallop myofibrils was extracted and replaced completely with gizzard RLC labeled specifically at Cys 108 with erythrosin iodoacetamide (ErIA). The calcium sensitivity of the ATPase activity of the labeled myofibril preparation was quite similar to that of the native sample, indicating that the ErIA-labeled RLC is functionally bound to the myosin head. In rigor (in the absence of ATP, when all the myosin heads are rigidly bound to the thin filament), a slight decay was observed in the first few microseconds, followed by no change in the anisotropy. This indicates small-amplitude restricted motions of the RLC or the entire LC domain of myosin. Addition of calcium to rigor restricts these motions further. Relaxation with ATP (no Ca) causes a large decay in the anisotropy, indicating large-amplitude rotational motion with correlation times of 5-50 micros. Further addition of calcium, to induce contraction, resulted in a decrease in the rate and amplitude of anisotropy decay. In particular, there is clear evidence for a slow rotational motion with a correlation time of approximately 300 micros, which is not present either in rigor or relaxation. This indicates rotational motion that specifically correlates with force generation. The changes in the rotational dynamics of the light-chain domain in rigor, relaxation, and contraction support earlier work based on probes of the catalytic domain that muscle contraction is accompanied by a disorder-to-order transition of the myosin head. However, the motions of the LC domain are different from those of the catalytic domain, which indicates rotation of the two domains relative to each other.

Microsecond rotational dynamics of spin-labeled myosin regulatory light chain induced by relaxation and contraction of scallop muscle

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to study the rotational dynamics of spin-labeled regulatory light chain (RLC) in scallop (Placopecten magellanicus) muscle fibers. The single cysteine (Cys 51) in isolated clam (Mercenaria) RLC was labeled with an indanedione spin label (InVSL). RLC was completely and specifically extracted from scallop striated muscle fibers, eliminating the Ca sensitivity of ATPase activity and isometric force, which were both completely restored by stoichiometric incorporation of labeled RLC. The EPR spectrum of the isolated RLC revealed nanosecond rotational motions within the RLC, which were completely eliminated when the labeled RLC was bound to myosin heads in myofibrils or fibers in rigor. This is the most strongly immobilized RLC-bound probe reported to date and thus offers the most reliable detection of the overall rotational motion of the LC domain. Conventional EPR spectra of oriented fibers indicated essentially complete probe disorder, independent of ATP and Ca, eliminating orientational dependence and thus making this probe ideal for unambiguous measurement of microsecond rotational motions of the LC domain by ST-EPR. ST-EPR spectra of fibers in rigor indicated an effective rotational correlation time (taureff) of 140 +/- 5 microseconds, similar to that observed for the same spin label bound to the catalytic domain. Relaxation by ATP induced microsecond rotational motion (taureff = 70 +/- 4 microseconds), and this motion was slightly slower upon Ca activation of isometric contraction (taureff = 100 +/- 5 microseconds). These motions in relaxation and contraction are similar to, but slower than, the motions previously reported for the same spin label bound to the catalytic domain. These results support a model for force generation involving rotational motion of the LC domain relative to the catalytic domain and dynamic disorder-to-order transitions in both domains.

Domain motion between the regulatory light chain and the nucleotide site in skeletal myosin

Resonance energy transfer probes were attached to skeletal myosin's nucleotide site and regulatory light chain (RLC) to examine nucleotide analog-induced structural transitions. A novel chemical modification of the RLC was developed for specific labeling of the basic N-terminus without affecting myosin ATPase activity. The modification allows attachment of a terbium chelate to rabbit skeletal RLC and was mapped by tryptic digestion to an amino group on the six N-terminal RLC residues. The use of terbium as a resonance energy transfer donor allowed the determination of the efficiency of energy transfer by sensitized emission lifetime measurements that practically eliminate background from unlabeled donor and acceptor sites as well as potential orientation factor artifacts in the calculation of the critical transfer distance. The nucleotide site was labeled with a functional CY3-labeled nucleotide as an energy transfer acceptor. Of the nucleotide states examined, ADP, ADP. vanadate, ADP. A1F4, and ADP. BeFx, the difference between the ADP and ADP. vanadate states was greatest (0.4-nm change), but was not considered to be statistically significant. The binding of actin to ADP-myosin also failed to produce a statistically significant change (0.3-nm change). These results are not consistent with a number of versions of the swinging lever arm hypothesis.

Steady-state fluorescence polarization studies of the orientation of myosin regulatory light chains in single skeletal muscle fibers using pure isomers of iodoacetamidotetramethylrhodamine

The regulatory light chain (RLC) from chicken gizzard myosin was covalently modified on cysteine 108 with either the 5- or 6-isomer of iodoacetamidotetramethylrhodamine (IATR). Labeled RLCs were purified by fast protein liquid chromatography and characterized by reverse-phase high-performance liquid chromatography (HPLC), tryptic digestion, and electrospray mass spectrometry. Labeled RLCs were exchanged into the native myosin heads of single skinned fibers from rabbit psoas muscle, and the ATR dipole orientations were determined by fluorescence polarization. The 5- and 6-ATR dipoles had distinct orientations, and model orientational distributions suggest that they are more than 20 degrees apart in rigor. In the rigor-to-relaxed transition (sarcomere length 2.4 microm, 10 degrees C), the 5-ATR dipole became more perpendicular to the fiber axis, but the 6-ATR dipole became more parallel. This orientation change was absent at sarcomere length 4.0 microm, where overlap between myosin and actin filaments is abolished. When the temperature of relaxed fibers was raised to 30 degrees C, the 6-ATR dipoles became more parallel to the fiber axis and less ordered; when ionic strength was lowered from 160 mM to 20 mM (5 degrees C), the 6-ATR dipoles became more perpendicular to the fiber axis and more ordered. In active contraction (10 degrees C), the orientational distribution of the probe dipoles was similar but not identical to that in relaxation, and was not a linear combination of the orientational distributions in relaxation and rigor.

Luminescence resonance energy transfer measurements in myosin

Myosin is thought to generate force by a rotation between the relative orientations of two domains. Direct measurements of distances between the domains could potentially confirm and quantify these conformational changes, but efforts have been hampered by the large distances involved. Here we show that luminescence resonance energy transfer (LRET), which uses a luminescent lanthanide as the energy-transfer donor, is capable of measuring these long distances. Specifically, we measure distances between the catalytic domain (Cys707) and regulatory light chain domain (Cys108) of the myosin head. An energy transfer efficiency of 21.2 +/- 1.9% is measured in the myosin complex without nucleotide or actin, corresponding to a distance of 73 A, consistent with the crystal structure of Rayment et al. Upon binding to actin, the energy transfer efficiency decreases by 4.5 +/- 1.0%, indicating a conformational change in myosin that involves a relative rotation and/or translation of Cys707 relative to the light chain domain. Addition of ADP also alters the energy transfer efficiency, likely through a rotation of the probe attached to Cys707. These results demonstrate that LRET is capable of making accurate measurements on the relatively large actomyosin complex, and is capable of detecting conformational changes between the catalytic and light chain domains of myosin.

A large and distinct rotation of the myosin light chain domain occurs upon muscle contraction

For more than 30 years, the fundamental goal in molecular motility has been to resolve force-generating motor protein structural changes. Although low-resolution structural studies have provided evidence for force-generating myosin rotations upon muscle activation, these studies did not resolve structural states of myosin in contracting muscle. Using electron paramagnetic resonance, we observed two distinct orientations of a spin label attached specifically to a single site on the light chain domain of myosin in relaxed scallop muscle fibers. The two probe orientations, separated by a 36 degrees +/- 5 degrees axial rotation, did not change upon muscle activation, but the distribution between them changed substantially, indicating that a fraction (17% +/- 2%) of myosin heads undergoes a large (at least 30 degrees) axial rotation of the myosin light chain domain upon force generation and muscle contraction. The resulting model helps explain why this observation has remained so elusive and provides insight into the mechanisms by which motor protein structural transitions drive molecular motility.

Independent mobility of catalytic and regulatory domains of myosin heads

The recent determination of the myosin head atomic structure has led to a new model of muscle contraction, according to which mechanical torque is generated in the catalytic domain and amplified by the lever arm made of the regulatory domain [Fisher, A. J., Smith, C. A., Thoden, J., Smith, R., Sutoh, K., Holden, H. M. & Rayment, I. (1995) Biochemistry 34, 8960-8972]. A crucial aspect of this model is the ability of the regulatory domain to move independently of the catalytic domain. Saturation transfer-EPR measurements of mobility of these two domains in myosin filaments give strong support for this notion. The catalytic domain of the myosin head was labeled at Cys-707 with indane dione spin label; the regulatory domain was labeled at the single cysteine residue of the essential light chain and exchanged into myosin. The mobility of the regulatory domain in myosin filaments was characterized by an effective rotational correlation time (tauR) between 24 and 48 micros. In contrast, the mobility of the catalytic domain was found to be tauR = 5-9 micros. This difference in mobility between the two domains existed only in the filament form of myosin. In the monomeric form, or when bound to actin, the mobility of the two domains in myosin was indistinguishable, with tauR = 1-4 micros and >1,000 micros, respectively. Therefore, the observed difference in filaments cannot be ascribed to differences in local conformations of the spin-labeled sites. The most straightforward interpretation suggests a flexible hinge between the two domains, which would have to stiffen before force could be generated.

ADP release produces a rotation of the neck region of smooth myosin but not skeletal myosin

Current theories of muscle cross-bridge function suggest that force is generated by a change in the orientation of the myosin neck region. We attached a paramagnetic probe to a subunit in the neck region and measured the orientation of the probe using electron paramagnetic resonance spectroscopy. The angle of the probes on smooth myosin S1 were changed by 20 degrees +/- 4 degrees on addition of ADP (50% effect at 5 +/- 2 microM), but ADP produced little effect on skeletal S1. The orientation of smooth myosin, +ADP, resembled that of skeletal myosin, +/- ADP, suggesting that the release of ADP generates an extra rotation of the neck region in smooth muscle at the end of its power stroke.

Orientation of paramagnetic probes attached to gizzard regulatory light chain bound to myosin heads in rabbit skeletal muscle

The orientation of the myosin neck was monitored using electron paramagnetic resonance (EPR) spectroscopy. Gizzard regulatory light chain was labeled with a nitroxide spin probe and exchanged for the native subunit, located in the myosin neck, in rabbit psoas muscle fibers. The EPR spectra of rigor fibers indicated a substantial degree of probe immobilization and showed a strong dependence on the orientation of the fiber axis relative to the magnetic field, indicating that the neck was ordered in this state. Spectra of relaxed fibers at 24 degrees C showed that the neck was disordered, but the spectra of relaxed fibers at 4 degrees C indicated that the neck was partially ordered. Active fibers at the two temperatures produced spectra identical to relaxed fibers, indicating that no novel angles could be seen in the neck during the powerstroke. Proteolytic fragments of myosin, S1 and HMM, were exchanged with labeled light chains and bound to thin filaments in unlabeled fibers. The distribution of probe orientations for HMM was identical to that of labeled rigor fibers, while S1 showed a slightly different distribution, suggesting that the neck is distorted (by a few degrees) by the interactions of the two heads of myosin when bound to actin.

Distance measurements near the myosin head-rod junction using fluorescence spectroscopy

We reacted a fluorescent probe, N-methyl-2-anilino-6-naphthalenesulfonyl chloride (MNS-Ci), with a specific lysine residue of porcine cardiac myosin located in the S-2 region of myosin. We performed fluorescence resonance energy transfer (FRET) spectroscopy measurements between this site and three loci (Cys109, Cys125, and Cys154) located within different myosin light-chain 2s (LC2) bound to the myosin "head". We used LC2s from rabbit skeletal muscle myosin (Cys125), chicken gizzard smooth muscle myosin (Cys109), or a genetically engineered mutant of chicken skeletal muscle myosin (Cys154). The atomic coordinates of these LC2 loci can be closely approximated, and the FRET measurements were used to determine the position of the MNS-labeled lysine with respect to the myosin head. The C-terminus of myosin subfragment-1 determined by Rayment et al. ends abruptly after a sharp turn of its predominantly alpha-helical structure. We have constructed a model based on our FRET distance data combined with the known structure of chicken skeletal muscle myosin subfragment-1. This model suggests that the loci that bracket the head-rod junction will be useful for evaluating dynamic changes in this region.

Orientation of intermediate nucleotide states of indane dione spin-labeled myosin heads in muscle fibers

We have used electron paramagnetic resonance to study the orientation of myosin heads in the presence of nucleotides and nucleotide analogs, to induce equilibrium states that mimic intermediates in the actomyosin ATPase cycle. We obtained electron paramagnetic resonance spectra of an indane dione spin label (InVSL) bound to Cys 707 (SH1) of the myosin head, in skinned rabbit psoas muscle fibers. This probe is rigidly immobilized on the catalytic domain of the head, and the principal axis of the probe is aligned nearly parallel to the fiber axis in rigor (no nucleotide), making it directly sensitive to axial rotation of the head. On ADP addition, all of the heads remained strongly bound to actin, but the spectral hyperfine splitting increased by 0.55 +/- 0.02 G, corresponding to a small but significant axial rotation of 7 degrees. Adenosine 5'-(adenylylim-idodiphosphate) (AMPPNP) or pyrophosphate reduced the actomyosin affinity and introduced a highly disordered population of heads similar to that observed in relaxation. For the remaining oriented population, pyrophosphate induced no significant change relative to rigor, but AMPPNP induced a slight but probably significant rotation (2.2 degrees +/- 1.6 degrees), in the direction opposite that induced by ADP. Adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) relaxed the muscle fiber, completely dissociated the heads from actin, and produced disorder similar to that in relaxation by ATP. ATP gamma S plus Ca induced a weak-binding state with most of the actin-bound heads disordered. Vanadate had negligible effect in the presence of ADP, but in isometric contraction vanadate substantially reduced both force and the fraction of oriented heads. These results are consistent with a model in which myosin heads are disordered early in the power stroke (weak-binding states) and rigidly oriented later in the power stroke (strong-binding states), whereas transitions among the strong-binding states induce only slight changes in the axial orientation of the catalytic domain.

Orientation changes in myosin regulatory light chains following photorelease of ATP in skinned muscle fibers

The orientation of the light-chain region of myosin heads in muscle fibers was followed by polarized fluorescence from an extrinsic probe during tension transients elicited by photolysis of caged ATP. Regulatory light chain from chicken gizzard myosin was covalently modified with iodoacetamidotetramethylrhodamine and exchanged into skinned fibers from rabbit psoas muscle without significant effect of the tension transients. Fluorescence polarization ratios Q parallel = (parallel I parallel-perpendicular I parallel)/ (parallel I parallel+perpendicular I parallel) and Q perpendicular = perpendicular I perpendicular - parallel I perpendicular)/ (perpendicular I perpendicular + parallel I perpendicular), where mIn denote fluorescence intensities for excitation (pre-subscript) and emission (post-subscript) parallel or perpendicular to the fiber axis, were simultaneously measured at 0.5 ms time resolution. Q perpendicular decreased and Q parallel increased promptly after ATP release in the presence or absence of CA2+, indicating changes in orientation of the light-chain region associated with ATP binding or cross-bridge detachment. Little further change in the Q signals accompanied either active tension development (+Ca2+) or the final relaxation (-Ca2+). The Q and tension transients slowed when liberated ATP concentration was reduced. Assuming that ATP is released at 118 s-1 (20 degrees C), the apparent second-order rate constants were 3-10 x 10(5) M-1 s-1 for Q parallel, 1-5 x 10(5) M-1 s-1 for Q perpendicular, and 0.5-2 x 10(5) M-1 s-1 for the convergence of tension traces starting from different rigor values. Fitting of model orientation distributions to the Q signals indicated that the angular disorder increases after ATP binding. This orientation change is specific to ATP because photo release of ADP caused much smaller changes in the Q signals.

Fluorescent probes of the orientation of myosin regulatory light chains in relaxed, rigor, and contracting muscle

The orientation of the light-chain region of myosin heads in relaxed, rigor, and isometrically contracting fibers from rabbit psoas muscle was studied by fluorescence polarization. Cysteine 108 of chicken gizzard myosin regulatory light chain (cgRLC) was covalently modified with iodoacetamidotetramethylrhodamine (iodo-ATR). Native RLC of single glycerinated muscle fibers was exchanged for labeled cgRLC in a low [Mg2+] rigor solution at 30 degrees C. Troponin and troponin C removed in this procedure were replaced. RLC exchange had little effect on active force production. X-ray diffraction showed normal structure in rigor after RLC exchange, but loss of axial and helical order in relaxation. In isolated myofibrils labeled cgRLC was confined to the regions of the sarcomere containing myosin heads. The ATR dipoles showed a preference for orientations perpendicular to the fiber axis, combined with limited nanosecond rotational motion, in all conditions studied. The perpendicular orientation preference was more marked in rigor than in either relaxation or active contraction. Stretching relaxed fibers to sarcomere length 4 microns to eliminate overlap between actin- and myosin-containing filaments had little effect on the orientation preference. There was no change in orientation preference when fibers were put into rigor at sarcomere length 4.0 microns. Qualitatively similar results were obtained with ATR-labeled rabbit skeletal RLC.

A 35-A movement of smooth muscle myosin on ADP release

Myosin II crossbridges interact with F-actin producing powerstrokes of around 100 A (refs 1, 2), during which the products of ATP hydrolysis are released. This has been postulated to involve an articulation of the myosin head (S1) on actin, or substantial conformational changes in S1 itself. Small movements of the regulatory light chain have been detected (see, for example, refs 9, 10), but most data suggest that the bulk of S1 does not move on actin during crossbridge cycling. Here we present three-dimensional maps of S1-decorated F-actin in the presence and absence of MgADP. The myosin motor domain is similar in both states but there are major orientational differences in the light-chain-binding domain. This domain acts as a rigid level arm pivoting about the end of the motor domain and swinging approximately 23 degrees, resulting in a approximately 35-A step. Small, nucleotide-mediated conformational changes in the motor domain may thus be converted by the light-chain domain into large movement steps.

The myosin catalytic domain does not rotate during the working power stroke

Electron paramagnetic resonance spectroscopy of a spin probe attached to cys-707 on myosin cross-bridges was used to monitor the orientation of the myosin catalytic domain at the beginning and end of the working power stroke in active muscle. Elevated concentrations of orthophosphate and decreased pH were used to shift the population of cross-bridges from force-producing states into low force, pre-power-stroke states. The spectrum of probes in active fibers was not changed by conditions that reduced tension by 70%, indicating that the orientation of the catalytic domain was the same at the beginning and end of the power stroke. Thus the data show that the catalytic domain remains rigidly oriented on the actin filament during the power stroke.

Fluorescence polarization study of the rigor complexes formed at different degrees of saturation of actin filaments with myosin subfragment-1

A serine residue located in the active site of myosin head (S1) was labelled by 9-anthroylnitrile, an amino group located in the central domain of S1 was labelled by 7-diethylamino-3-(4'-isothio-cyanato-phenyl)-4-methylcoumari n, a cysteine residue located near the C-terminus of S1 was labelled by 5-[2-((iodoacetyl)-amino)ethyl]-amino-naphthalene-1-sulfonic acid (1,5-IAEDANS) and a cysteine residue located near the C-terminus of the alkali light chain 1 was labelled with iodoacetamido-tetramethyl-rhodamine. Polarization of fluorescence of S1 was measured in solution (where it indicated the mobility of actin-bound S1) and in myofibrils (where it indicated orientation of probes) to check whether the anisotropy of S1 labelled at different positions depended on the molar ratio S1:actin. In solution, when increasing amounts of actin were added to a fixed amount of labelled S1 (i.e. when myosin heads were initially in excess over actin), anisotropy saturated at 1 mol of S1 per 1 mol of actin. When increasing amounts of S1 were added to a fixed amount of F-actin (i.e. when actin was initially in excess over S1), the anisotropy saturated at 1 mol of S1 per 2 mols of actin. In myofibrils, orientation of S1 was different when S1 was added at nanomolar concentration (intrinsic actin was in excess over extrinsic S1) then when it was added at micromolar concentration (excess of S1 over actin). The fact that the anisotropy of S1 labelled at different positions depended on the molar ratio excluded the possibility that changes were confined to one part of the cross-bridge and supports our earlier proposal that the two rigor complexes which S1 can form with F-actin differ globally in conformation.

The mechanism of force generation in myosin: a disorder-to-order transition, coupled to internal structural changes

We propose a molecular mechanism of force generation in muscle, based primarily on site-specific spectroscopic probe studies of myosin heads in contracting muscle fibers and myofibrils. Electron paramagnetic resonance (EPR) and time-resolved phosphorescence anisotropy (TPA) of probes attached to SH1 (Cys 707, in the catalytic domain of the head) have consistently shown that most myosin heads in contracting muscle are dynamically disordered, undergoing large-amplitude rotations in the microsecond time range. Some of these disordered heads are bound to actin, especially in the early (weak-binding, preforce) phase of the ATPase cycle. The small ordered population (10-20%) is rigidly oriented precisely as in rigor, with no other distinct angle observed in contraction or in the presence of intermediate states trapped by nucleotide analogs. These results are not consistent with the classical model in which the entire head undergoes a 45 degree transition between two distinct orientations. Therefore, it has been proposed that the catalytic domain of the myosin head has only one stereospecific (rigor-like) actin-binding angle, and that the head's internal structure changes during force generation, causing the distal light-chain-binding domain to rotate. To test this model, we have performed EPR and TPA studies of probes attached to regulatory light chains (RLCs) in rabbit and scallop myofibrils and fibers. The RLC results confirm the predominance of dynamic (microsecond) rotational disorder in both relaxation and contraction, and show that the different mechanisms of calcium regulation in the two muscles produce different rotational dynamics. In rabbit myofibrils, RLC probes are more dynamically disordered than SH1 probes, especially in rigor and contraction,indicating that the light-chain-binding domain undergoes rotational motions relative to the catalytic domain when myosin heads interact with actin. An SH1-bound spin label, which is sensitive to myosin's internal dynamics, resolves three distinct conformations during contraction, and time-resolved EPR shows that these transitions are coupled to specific steps in the ATPase cycle. We propose that force is generated during contraction by a disorder-to-order transition, in which myosin heads first attach weakly to actin in a nonstereospecific mode characterized by large-scale dynamic disorder, then undergo at least two conformational transitions involving large-scale structural (rotational) changes within the head, culminating in a highly ordered strong-binding state that bears force.

Fluorescence resonance energy transfer within the regulatory light chain of myosin

Rabbit skeletal muscle myosin regulatory light chain-2 (LC2) contains two reactive cysteine residues, Cys125 and Cys154, and one tryptophan at position 137. Using wild-type rabbit LC2 or its genetically engineered mutant with Cys125-->Arg (C125R), these residues can be selectively modified with fluorescent or chromophoric probes for spectroscopic studies. We have bound suitable donor/acceptor probe pairs to the two cysteine residues and Trp137 in LC2 or C125R, and measured the distance in solution between the probes by fluorescence resonance energy transfer spectroscopy. C125R was made to facilitate specific labelling of the less reactive Cys154, thus allowing the distance between Cys154 and Trp137 to be measured. Our measurements show that these residues are in close proximity to each other, the distance between them ranging from 1.7 nm (between Cys125 and Trp137) to 2.7 nm (Cys125 and Cys154). These results suggest that Cys125, Trp137 and Cys154, spanning up to 29 residues in the sequence of LC2, are spatially close, consistent with these residues residing within a C-terminal globular domain. The distances we obtained are in agreement with previous crosslinking studies [Huber, P. A., Brunner, U.T. & Schaub, M. C. (1989) Biochemistry 28, 9116-9123; Saraswat, L. & Lowey, S. (1991) J. Biol. Chem. 266, 19777-19785] and structure predictions of LC2. LC2 is located at the head-rod junction of the myosin crossbridge, and provides the primary regulatory mechanism in molluscan and smooth muscle. In skeletal muscle, its functional role is unclear, although it has been implicated in modulating actomyosin interaction [Metzger, J. M. & Moss, R. L. (1992) Biophys. J. 63, 460-468]. The incorporation of spectroscopic probes onto the light chains of myosin in solution or in fibres has become a valuable tool for evaluating the dynamic properties of the crossbridge during force generation.