On the mechanism of energy transduction in myosin subfragment 1

Natural variant frequencies across domains from different sarcomere proteins cross-correlate to identify inter-protein contacts associated with cardiac muscle function and disease

Coordinated sarcomere proteins produce contraction force for muscle shortening. In human ventriculum they include the cardiac myosin motor (βmys), repetitively converting ATP free energy into work, and myosin binding protein C (MYBPC3) that in complex with βmys is regulatory. Single nucleotide variants (SNVs) causing hereditary heart diseases frequently target this protein pair. The βmys/MYBPC3 complex models a regulated motor and is used here to study how the proteins couple. SNVs in βmys or MYBPC3 survey human populations worldwide. Their protein expression modifies domain structure affecting phenotype and pathogenicity outcomes. When the SNV modified domain locates to inter-protein contacts it could affect complex coordination. Domains involved, one in βmys the other in MYBPC3, form coordinated domains (co-domains). Co-domain bilateral structure implies the possibility for a shared impact from SNV modification in either domain suggesting a correlated response to a common perturbation could identify their location. Genetic divergence over human populations is proposed to perturb SNV probability coupling that is detected by cross-correlation in 2D correlation genetics (2D-CG). SNV probability data and 2D-CG identify three critical sites, two in MYBPC3 with links to several domains across the βmys motor, and, one in βmys with links to the MYBPC3 regulatory domain. MYBPC3 sites are hinges sterically enabling regulatory interactions with βmys. The βmys site is the actin binding C-loop (residues 359-377). The C-loop is a trigger for actin-activated myosin ATPase and a contraction velocity modulator. Co-domain identification implies their spatial proximity suggesting a novel approach for in vivo protein complex structure determination.

FRET characterisation for cross-bridge dynamics in single-skinned rigor muscle fibres

In this work we demonstrate for the first time the use of Förster resonance energy transfer (FRET) as an assay to monitor the dynamics of cross-bridge conformational changes directly in single muscle fibres. The advantage of FRET imaging is its ability to measure distances in the nanometre range, relevant for structural changes in actomyosin cross-bridges. To reach this goal we have used several FRET couples to investigate different locations in the actomyosin complex. We exchanged the native essential light chain of myosin with a recombinant essential light chain labelled with various thiol-reactive chromophores. The second fluorophore of the FRET couple was introduced by three approaches: labelling actin, labelling SH1 cysteine and binding an adenosine triphosphate (ATP) analogue. We characterise FRET in rigor cross-bridges: in this condition muscle fibres are well described by a single FRET population model which allows us to evaluate the true FRET efficiency for a single couple and the consequent donor–acceptor distance. The results obtained are in good agreement with the distances expected from crystallographic data. The FRET characterisation presented herein is essential before moving onto dynamic measurements, as the FRET efficiency differences to be detected in an active muscle fibre are on the order of 10–15% of the FRET efficiencies evaluated here. This means that, to obtain reliable results to monitor the dynamics of cross-bridge conformational changes, we had to fully characterise the system in a steady-state condition, demonstrating firstly the possibility to detect FRET and secondly the viability of the present approach to distinguish small FRET variations.

An unusual transduction pathway in human tonic smooth muscle myosin

The motor protein myosin binds actin and ATP, producing work by causing relative translation of the proteins while transducing ATP free energy. Smooth muscle myosin has one of four heavy chains encoded by the MYH11 gene that differ at the C-terminus and in the active site for ATPase due to alternate splicing. A seven-amino-acid active site insert in phasic muscle myosin is absent from the tonic isoform. Fluorescence increase in the nucleotide sensitive tryptophan (NST) accompanies nucleotide binding and hydrolysis in several myosin isoforms implying it results from a common origin within the motor. A wild-type tonic myosin (smA) construct of the enzymatic head domain (subfragment 1 or S1) has seven tryptophan residues and nucleotide-induced fluorescence enhancement like other myosins. Three smA mutants probe the molecular basis for the fluorescence enhancement. W506+ contains one tryptophan at position 506 homologous to the NST in other myosins. W506F has the native tryptophans except phenylalanine replaces W506, and W506+(Y499F) is W506+ with phenylalanine replacing Y499. W506+ lacks nucleotide-induced fluorescence enhancement probably eliminating W506 as the NST. W506F has impaired ATPase activity but retains nucleotide-induced fluorescence enhancement. Y499F replacement in W506+ partially rescues nucleotide sensitivity demonstrating the role of Y499 as an NST facilitator. The exceptional response of W506 to active site conformation opens the possibility that phasic and tonic isoforms differ in how influences from active site ATPase propagate through the protein network.

On the tryptophan residue of smooth muscle myosin that responds to binding of nucleotide

Initially, we asked which (of 10) smooth muscle myosin head residues responds to MgADP or MgATP binding with enhanced fluorescence emission (Trp-441 and Trp-512 were leading candidates)? To decide, we prepared sham-mutated smooth muscle heavy meromyosin (HMM), W441F HMM, and W512F HMM. On adding MgATP, emission of wild-type and W441F HMMs increased by 25-27%, but that of W512F HMM by 5%. So, in myosin, 512 is the "sensitive Trp." Unexpectedly, properties of W512F HMM [elevated Ca(2+)-ATPase, depressed EDTA (K(+))-ATPase, no regulation of its basal or actin-activated Mg(2+)-ATPase by phosphorylation of its "regulatory" light chain, limited actin activation, and inability to move actin filaments in a motility assay] are strikingly like those of smooth muscle myosin reacted at Cys-717 with thiol reagent. From crystallography-based [Houdusse, A., Kalabakis, V. N., Himmel, D., Szent-Györgyi, A. G. & Cohen, C. (1999) Cell 97, 459-470] simulations, we found that in wild-type HMM with MgADP added, Trp-512 is in a "hydrophobic pocket," but that pocket becomes distorted in W512F HMM. We think that there is a "path of influence" from 512 to 717 to the active site. We suggest that the mutational changes at 512 are transmitted along this path to Cys-717, where they induce changes similar to those caused by reacting wild-type HMM with thiol reagent.

Computer simulation of flagellar movement: VII. Conventional but functionally different cross-bridge models for inner and outer arm dyneins can explain the effects of outer arm dynein removal

Outer arm dynein removal from flagella by genetic or chemical methods causes decreased frequency and power, but little change in bending pattern. These results suggest that outer arm dynein operates within bends to increase the speed of bend propagation, but does not produce forces that alter the bending pattern established by inner arm dyneins. A flagellar model incorporating different cross-bridge models for inner and outer arm dyneins has been examined. The inner arm dynein model has a hyperbolic force-velocity curve, with a maximum average force at 0 sliding velocity of about 14 pN for each 96 nm group of inner arm dyneins. The outer arm dynein model has a very different force-velocity curve, with a maximum force at about 10-15% of V(max). The outer arm dynein model is adjusted so that the unloaded sliding velocity for a realistic mixture of inner and outer arm dyneins is twice the unloaded sliding velocity for the inner arm dynein model alone. With these cross-bridge models, a flagellar model can be obtained that reduces its sliding velocity and frequency by approximately 50% when outer arm dyneins are removed, with little change in bending pattern. The addition of outer arm dyneins, therefore, gives an approximately 4-fold increase in power output against viscous resistances, and outer arm dyneins may generate 90% or more of the power output. Cell Motil.

Consequences of placing an intramolecular crosslink in myosin S1

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

Computer simulation of flagellar movement: VII. Conventional but functionally different cross-bridge models for inner and outer arm dyneins can explain the effects of outer arm dynein removal

Outer arm dynein removal from flagella by genetic or chemical methods causes decreased frequency and power, but little change in bending pattern. These results suggest that outer arm dynein operates within bends to increase the speed of bend propagation, but does not produce forces that alter the bending pattern established by inner arm dyneins. A flagellar model incorporating different cross-bridge models for inner and outer arm dyneins has been examined. The inner arm dynein model has a hyperbolic force-velocity curve, with a maximum average force at 0 sliding velocity of about 14 pN for each 96 nm group of inner arm dyneins. The outer arm dynein model has a very different force-velocity curve, with a maximum force at about 10-15% of V(max). The outer arm dynein model is adjusted so that the unloaded sliding velocity for a realistic mixture of inner and outer arm dyneins is twice the unloaded sliding velocity for the inner arm dynein model alone. With these cross-bridge models, a flagellar model can be obtained that reduces its sliding velocity and frequency by approximately 50% when outer arm dyneins are removed, with little change in bending pattern. The addition of outer arm dyneins, therefore, gives an approximately 4-fold increase in power output against viscous resistances, and outer arm dyneins may generate 90% or more of the power output. Cell Motil.

Critical review of the swinging crossbridge theory and of the cardinal active role of water in muscle contraction

A critical analysis is presented of the experimental findings that led to the sliding filament model and to its offspring--the swinging (by rotating or tilting) crossbridge theory of muscle contraction (SCBT). Several principles that have been taken for granted implicitly and explicitly by the creators of these dogmas are discussed. The failure of numerous efforts to verify predictions of the SCBT, particularly the idea that the myosin molecules undergo a major conformational change, is critically reviewed. Analysis of various experimental data suggests that water may play an active role in muscular contraction. Examination of both the experiments that do not fulfill the expectations of the SCBT and the measurements of water liberation during the "contractile" process suggests a new outlook according to which tension development and movement are not due to major conformational changes but rather to restructuring of the hydration shells of actin and myosin.

Probes bound to myosin Cys-707 rotate during length transients in contraction

It is widely conjectured that muscle shortens because portions of myosin molecules (the "cross-bridges") impel the actin filament to which they transiently attach and that the impulses result from rotation of the cross-bridges. Crystallography indicates that a cross-bridge is articulated-consisting of a globular catalytic/actin-binding domain and a long lever arm that may rotate. Conveniently, a rhodamine probe with detectable attitude can be attached between the globular domain and the lever arm, enabling the observer to tell whether the anchoring region rotates. Well-established signature effects observed in shortening are tension changes resulting from the sudden release or quick stretch of active muscle fibers. In this investigation we found that closely correlated with such tension changes are changes in the attitude of the rhodamine probes. This correlation strongly supports the conjecture about how shortening is achieved.

Immunochemical probing of the functional role of the 238-246 and 567-574 sequences of myosin heavy chain

Polyclonal site-directed peptide antibodies were raised against the 567-574 and 238-246 sequences of the rabbit skeletal muscle myosin heavy chain. These sequences, which are located in the subfragment 1 (S1) segment of myosin, have been implicated by former studies in actin and nucleotide binding of the molecule and in the communication between the two binding sites. The antibodies obtained from rabbit sera were found to be conformation-sensitive since they specifically reacted with S1 in solid-phase binding assay but not in Western blot. The binding of both antibodies to S1 was strongly inhibited by actin. The antibody against the 567-574 sequence, Ab567-574, moderately decreased the binding of S1 to actin filaments in rigor but not in the weakly-attached state, while Ab238-246 did not influence the binding of S1 to actin under either conditions. Both antibodies inhibited the actin activation of the MgATPase of S1 but did not affect MgATPase without actin or the Ca- and K(EDTA)-activated ATPase activities of S1. The sliding velocity of actin filaments in the in vitro motility assays were also reduced in the presence of the antibodies. Ab567-574 had especially strong inhibitory effect on the movement of actin filaments. The results indicate that the binding of antibodies may induce conformational changes, which propagate in the S1 structure, perturb the coupling between the binding sites and impair the motor function of myosin.

Domain motion in actin observed by fluorescence resonance energy transfer

Actin is composed of two well-separated globular domains which are further subdivided into two subdomains [Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F., & Holmes, K. C. (1990) Nature 347, 37-44]. Subdomains 1 and 2 constitute the small domain, and subdomains 3 and 4 comprise the large domain. In order to test a hinge bending domain motion in actin such as observed in many kinases, fluorescence resonance energy transfer between two probes attached to each of the two domains was measured by steady-state and time-resolved fluorometers. The adenine base is bound in a hydrophobic pocket between subdomains 3 and 4, and Tyr-69 is located at subdomain 2. In the present study, the adenine moiety was labeled with a fluorescence donor, epsilon ATP, and tyrosine-69 was labeled with the energy acceptor, dansyl chloride. Assuming the random orientation factor k2 = 2/3, the distance between epsilon-adenine moiety and dansyl chloride attached to Tyr-69 in G-actin was determined to be 2.46 nm from steady-state fluorescence measurements. The addition of DNase I did not appreciably change the distance (less than 0.1 nm). The distance decreased to 2.27 nm during polymerization by the addition of phalloidin under physiological salt conditions. On the other hand, time-resolved fluorescence energy transfer measurements have been used to investigate a distribution of distances for a donor-acceptor pair. In G-actin, the mean distance between probes was 2.79 nm with a full width at half-maximum of 3.91 nm, indicating a large number of conformational substates in solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross-linking myosin subfragment 1 Cys-697 and Cys-707 modifies ATP and actin binding site interactions

Skeletal muscle myosin is an enzyme that interacts allosterically with MgATP and actin to transduce the chemical energy from ATP hydrolysis into work. By modifying myosin structure, one can change this allosteric interaction and gain insight into its mechanism. Chemical cross-linking with N,N'-p-phenylenedimaleimide (pPDM) of Cys-697 to Cys-707 of the myosin-ADP complex eliminates activity and produces a species that resembles myosin with ATP bound (Burke et al., 1976). Nucleotide-free pPDM-modified myosin subfragment 1 (S1) was prepared, and its structural and allosteric properties were investigated by comparing the nucleotide and actin interactions of S1 to those of pPDM-S1. The structural properties of the nucleotide-free pPDM-S1 are different from those of S1 in several respects. pPDM-S1 intrinsic tryptophan fluorescence intensity is reduced 28%, indicating a large increase of an internal quenching reaction (the fluorescence intensity of the related vanadate complex of S1, S1-MgADP-Vi, is reduced by a similar degree). Tryptophan fluorescence anisotropy increases from 0.168 for S1 to 0.192 for pPDM-S1, indicating that the unquenched tryptophan population in pPDM-S1 has reduced local freedom of motion. The actin affinity of pPDM-S1 is over 6,000-fold lower than that of S1, and the absolute value of the product of the net effective electric charges at the acto-S1 interface is reduced from 8.1 esu2 for S1 to 1.6 esu2 for pPDM-S1. In spite of these changes, the structural response of pPDM-S1 to nucleotide and the allosteric communication between its ATP and actin sites remain intact. Compared to pPDM-S1, the fluorescence intensity of pPDM-S1 *MgADP is increased 50%(compared to 8 and 31% increases, respectively, for MgADP and MgATP binding to S1). Compared to acto-pPDM-S1, the absolute value of the product of the net effective electric charge at the actin binding interface of acto-pPDM-S1 *MgADP increases 7.3 esu2 (compared to a 0.9 esu2 decrease and an 11.0 esu2 increase, respectively, for MgADP and MgATP binding to acto-Sl).The interaction free energy for the ligands MgADP and actin, is -2.0 kcal/mol for pPDM-S1, compared to -1.2 kcal/mol for unmodified S1.

Photochemical cross-linking of the skeletal myosin head heavy chain to actin subdomain-1 at Arg95 and Arg28

F-actin specifically substituted with the photocross-linker, p-azidophenylglyoxal, at Arg95 and Arg28 was isolated and characterized. Upon complexation with myosin subfragment-1 (S1) and photolysis at 365 nm, it was readily cross-linked to the S1 heavy chain with a yield of about 13-25%, generating four major actin-heavy-chain adducts with molecular masses in the range 165-240 kDa. The elevated Mg(2+)-ATPase of the covalent complexes displayed a turnover rate of 33 +/- 8 s-1 which is similar to the values reported earlier for other acto-S1 conjugates. The cross-linking between various proteolytic S1 and actin derivatives, combined with the fluorescent and immunochemical detection of the photocross-linked products, indicated that the arylnitrene group on Arg95 was inserted predominantly in the central 50-kDa region, whereas that attached to Arg28 mediated the selective cross-linking of the COOH-terminal 22-21-kDa fragments of the heavy chain, most probably by reacting at or near the connector segment between the 50-kDa and 20-kDa fragments. The rapid photoactivation and cross-linking to S1 of the substituted F-actin, which can be accomplished on a millisecond time scale, may serve to probe the structural dynamics of the interaction of the S1 heavy chain with subdomain-1 of actin during the ATPase cycle.

The region in myosin S-1 that may be involved in energy transduction

Newly-reported structural information about certain proximities between points on bound nucleotide and points on the heavy chain of myosin S-1 are incorporated into a previously-reported [Botts, J. Thomason, J.F. & Morales, M.F. Proc. Nat. Acad. Sci. USA, 86, 2204-2208 (1989)] structure of S-1. The resulting, enhanced structure is then used to identify some functionalities (e.g., the ATP-perturbable tryptophans), and to explain certain observations (e.g., some concerning the role of bound Mg2+ in the spectral response of TNBS-labelled Lys-83, and some concerning the response of the S-1 CD signal to nucleotide binding and to temperature change). These considerations lead to the suggestion that a strand of the 50 kDa "domain" (residues 510 to 540), and a strand of the 20 kDa 'domain' (residues 697-719) are involved in transmitting the effects of nucleotide binding and hydrolysis to the loop (constituted from the same "domain") that reaches a major (S-1)-actin interface.

Effect of actin on the tryptic digestion of myosin subfragment 1 in the weakly attached state

The structure of myosin subfragment 1 (S1) in the weakly attached complex with actin was studied at three specific sites, at the 50-kDa/20-kDa and 27-kDa/50-kDa junctions, and at the N-terminal region, using tryptic digestion as a structure-exploring tool. The structure of S1 at the vicinity of the 50-kDa/20-kDa junction is pH dependent in the weakly attached state because the tryptic cleavage at this site was fully protected by actin at pH 6.2, but the protection was only partial at pH 8.0. Since the actin protection is complete in rigor at both pH values, the results indicate that the structure of S1 at the 50-kDa/20-kDa junction differs in the two states at pH 8.0, but not at pH 6.2. Actin restores the ADP-suppressed tryptic cleavage after Lys213 at the 27-kDa/50-kDa junction in the strongly attached state, but not in the weakly attached state, which indicates structural difference between the two states at this site. ATP and ADP open a new site for tryptic cleavage in the N-terminal region of the S1 heavy chain between Arg23 and Ile24. Actin was found to suppress this cleavage in both weakly and strongly attached states, which shows that, in the vicinity of this site, the structure of S1 is similar in both states. The results indicate that the binding of S1 to actin induces localized changes in the S1 structure, and the extent of these changes is different in the various actin-S1 complexes.

Molecular movements in the actomyosin complex: F-actin-promoted internal cross-linking of the 25- and 20-kDa heavy chain fragments of skeletal myosin subfragment

We describe, for the first time, the F-actin-promoted changes in the spatial relationship of strands in the NH2-terminal 25-kDa and COOH-terminal 20-kDa heavy chain fragments of the skeletal myosin subfragment 1 (S-1), detected by their exclusive chemical cross-linking in the rigor F-actin-S-1 complex with m-maleimidobenzoic acid N-hydroxysuccinimide ester (MBS). Quantitative electrophoretic analysis of the reaction products showed extensive conversion of the 95-kDa heavy chain of the actin-bound S-1 into a new species with an apparent mass of 135 kDa (yield = 50-60%), whereas the heavy chain mobility remained unaffected when actin was omitted. The 135-kDa entity retained the fluorescence of AEDANS-S-1 but not of AEDANS-actin, indicating that it was not a cross-linked acto-heavy chain adduct. Its extent of production depended markedly on the S-1: actin molar ratio and was maximum near a ratio of 1:4. The MBS treatment of acto-S-1 led also to some covalent actin-actin oligomers which could be suppressed by using trypsin-truncated F-actin lacking Cys-374, without altering the generation of the 135-kDa heavy chain derivative.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibody and peptide probes of interactions between the SH1-SH2 region of myosin subfragment 1 and actin's N-terminus

The negatively charged residues in the N-terminus of actin and the 697-707 region on myosin subfragment 1 (S-1), containing the reactive cysteines SH1 and SH2, are known to be important for actin-activated myosin ATPase activity. The relationship between these two sites was first examined by monitoring the rates of SH1 and SH2 modification with N-ethylmaleimide in the presence of actin and, secondly, by testing for direct binding of SH1 peptides to the N-terminal segment on actin. While actin alone protected SH1 from N-ethylmaleimide modification, this effect was abolished by an antibody against the seven N-terminal amino acids on actin, F(ab)(1-7), and was greatly reduced when the charge of acidic residues at actin's N-terminus was altered by carbodiimide coupling of ethylenediamine. Neither F(ab)(1-7) nor ethylenediamine treatment reversed the effect of F-actin on SH2 reactivity in SH1-modified S-1. These results show a communication between the SH1 region on S-1 and actin's N-terminus in the acto-S-1 complex. To test whether such a communication involves the binding of the SH1 site on S-1 to the N-terminal segment of actin, the SH1 peptide IRICRKG-NH2(4+) was used. Cosedimentation experiments revealed the binding of three to six peptides per actin monomer. Peptide binding to actin was affected slightly, if at all, by F(ab)(1-7). The antibody also did not change the polymerization of G-actin by the peptides. The peptides caused a small reduction in the binding of S-1 to actin and did not change the binding of F(ab)(1-7).(ABSTRACT TRUNCATED AT 250 WORDS)

Models of the actin monomer and filament from fluorescence resonance-energy transfer

We have developed algorithms for combining fluorescence resonance-energy transfer (FRET) efficiency measurements into structural models which predict the relative positions of the chemical groups used in FRET. We used these algorithms to construct models of the actin monomer and filament derived solely from FRET measurements based on seven distinct loci. We found a mirror-image pair of monomer models which best fit the FRET data. One of these models agrees well with the atomic-resolution crystal structure recently published by Kabsch et al. in Heidelberg [Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F. & Holmes, K. C. (1990) Nature 347, 37-44]. The root-mean-square deviation between this FRET model and the crystal structure was about 0.9 nm. Other macromolecular models assembled from FRET measurements are likely to have a similar resolution. The largest discrepancy was for the Cys10 locus which deviated 1.44 nm from the crystal position. We discuss the limitations of the FRET method that may have contributed to this discrepancy, and conclude that the Cys10 FRET data have probably located Cys10 incorrectly in the FRET monomer model. Using the FRET monomer models, we found three orientations in the filament which best fit the intermonomer FRET data. These orientations differ substantially from the atomic-resolution filament model proposed by the Heidelberg group [Holmes, K., Popp, D., Gebhard, W. & Kabsch, W. (1990) Nature 347, 44-49], largely because of the discrepancies in the Cys10 data. These data should probably be excluded from the analysis; however, this would leave too few measurements to assemble a filament model. In the near future, we hope to obtain additional FRET measurements to other actin loci so that the filament modelling can be done without the Cys10 data.

Detection of conformational changes in actin by fluorescence resonance energy transfer between tyrosine-69 and cysteine-374

The distance between 5-(dimethylamino)naphthalene-1-sulfonyl chloride (dansyl chloride or DNS-Cl) attached to Tyr-69 and N-[[4-[4-(dimethylamino)phenyl]azo]phenyl]maleimide (DABMI) or N-[4-(dimethylamino)-3,5-dinitrophenyl]maleimide (DDPM) attached to Cys-374 in an actin monomer was measured to be 2.51 nm or 2.27 +/- 0.04 nm, respectively, by fluorescence resonance energy transfer. This distance does not change significantly when the actin monomer binds DNase I, when the monomer is polymerized, when the polymer interacts with myosin subfragment 1, or when it interacts with tropomyosin-troponin in the presence and absence of Ca2+. Changes in the distance were within 0.1 nm. The results indicate that the structure of the region involving Tyr-69 and Cys-374 is substantially rigid. A large blue shift (about 15 nm) of the fluorescence spectrum and a large increase (about 80%) in the fluorescence intensity of DNS-actin were observed when DNS-actin was denatured upon addition of EDTA. On the other hand, a red shift (about 7 nm) of the fluorescence spectrum and a large decrease (about 50%) in the fluorescence intensity were observed when DNS-actin was completely unfolded in 8 M urea. The results indicate that dansyl chromophore becomes less exposed to the aqueous environment by EDTA denaturation in contradiction to the case of intrinsic tryptophan residues in G-actin. Resonance energy transfer measurements showed that the distance between probes attached to Tyr-69 and Cys-374 on an actin monomer changes by 0.37 nm during EDTA denaturation, but that the distance becomes longer than 4.0 nm in 8 M urea in which no energy transfer is observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotide-induced changes in the interaction of myosin subfragment 1 with actin: detection by antibodies against the N-terminal segment of actin

The binding of myosin subfragment 1 (S-1) to actin in the presence and absence of nucleotides was determined under conditions of partial saturation of actin, up to 80%, by Fab(1-7), the antibodies against the first seven N-terminal residues on actin. In the absence of nucleotides, the binding constant of S-1 to actin (2 x 10(7) M-1) was decreased by 1 order of magnitude by Fab(1-7). The binding of S-1 to actin caused only limited displacement of Fab, and between 30 and 50% of actin appeared to bind both proteins. In the presence of MgAMP.PNP, MgADP, and MgPPi and at low S-1 concentrations, the same antibodies caused a large decrease in the binding of S-1 to actin. However, the binding of S-1.nucleotide to actin in the presence of Fab(1-7) increased cooperatively with the increase in S-1 concentration. Also, in contrast to rigor conditions, there was no indication for the binding of Fab(1-7) and S-1.nucleotide to the same actin molecules. These results show a nucleotide-induced transition in the actomyosin interface, most likely related to the different roles of the N-terminal segment of actin in the binding of S-1 and S-1.nucleotide. The possible implications of these findings to the regulation of actomyosin interactions are discussed.

51V NMR study of vanadate binding to myosin and its subfragment 1

The binding of various forms of vanadate to myosin and myosin subfragment 1 (S-1) was studied by 51V NMR at increasing vanadate concentrations between 0.06 and 1.0 mM. The distribution of the various forms of vanadate in the solution depended on the total concentration of vanadate. At low concentrations, the predominant vanadate form was monomeric, while at high concentration, it was tetrameric. The presence of myosin or S-1 in the solution produced a significant broadening of the signal of each form of vanadate, indicating that all of them bind to the protein. Addition of ATP, which does not affect the 51V NMR spectra in the absence of proteins, causes their significant alteration in the presence of myosin or S-1. The changes, which include the broadening of the signal of the monomeric and the narrowing of the signal of the oligomeric vanadate forms, indicate that more monomeric and less oligomeric vanadate binds to the proteins in the presence than in the absence of ATP. Irradiation by near-UV light in the presence of vanadate cleaves S-1 at three specific sites--at 23, 31, and 74 kDa from the N-terminus. The cleavages at 23 and 31 kDa are specifically inhibited by the addition of ATP. The vanadate-associated photocleavage of S-1 also depends on the total concentration of vanadate; it is observed only when the concentration of vanadate is at least 0.2 mM. This was also the lowest concentration at which oligomeric vanadate was detected in the 51V NMR spectra. From the parallel concentration dependence of the photocleavage and the appearance of the tetrameric vanadate, it is concluded that photocleavage occurs only when tetrameric vanadate binds to S-1.

Effect of nucleotides, actin and temperature on thermolysin digestion of myosin subfragment-1

Myosin subfragment-1 from rabbit skeletal muscle was digested by thermolysin at 25 degrees, 12 degrees and 0 degree C. Thermolysin cleaves subfragment-1 heavy chain into two stable fragments, 28 kDa and 70 kDa, aligned in this order from the N-terminus [Applegate, D. & Reisler, E. (1983) Proc. Natl Acad. Sci. USA 80, 7109-7112]. The rate of digestion at 25 degrees C was significantly increased in the presence of MgATP and somewhat less in the presence of MgADP, or magnesium pyrophosphate. This activating effect of the nucleotides was decreased at 12 degrees C and completely eliminated at 0 degrees C. The results can be explained by assuming that there are two subfragment-1 conformers [Shriver, J. W. & Sykes, B. D. (1981) Biochemistry 20, 2004-2012], and that both the addition of ATP or its analogs, and lowering the temperature, shift the conformational equilibrium in the direction that is more susceptible to thermolysin. Actin inhibited thermolysin digestion of subfragment-1 at all three temperatures studied. Actin inhibition can be explained either by shifting the equilibrium of the conformers in the direction of the less susceptible form or by direct interference of actin with the binding of thermolysin to subfragment-1. Actin inhibition of thermolysin digestion also prevailed when subfragment-1 was in a ternary complex with nucleotide and actin, in both the strongly and weakly attached states. Similarly, actin inhibited the digestion of subfragment-1 modified by 4-phenylenedimaleimide [corrected], which also forms a weakly attached complex with actin. No difference could be found in the accessibility of the thermolysin-susceptible site of subfragment-1 at the 28-70 kDa junction in either rigor, strongly or weakly attached states, which indicates the similarity of the structure proximal to this specific site in the three attached states.

Localization of epitopes and functional effects of two novel monoclonal antibodies against skeletal muscle myosin

Two skeletal myosin monoclonal antibodies, raised against human skeletal myosin, were used to study the correlation between function, primary and tertiary structure of S-1 prepared from rabbit skeletal myosin. The heavy chain of S-1 is cleaved into three fragments by trypsin--27 kDa, 50 kDa and 20 kDa--aligned in this order from the N-terminus. The epitope of the first antibody was assigned to the N-terminal 1-23 amino acid stretch of S-1, since it reacted with the 27 kDa N-terminal tryptic fragment of S-1 but not with a derivative of the 27 kDa fragment, which lacks the above amino acid stretch. The epitope of the second antibody was assigned to the 3 kDa N-terminal region of the central 50 kDa domain of S-1. This assignment was based on proteolytic and photochemical cleavage of S-1 and on the labelling of its N-terminus by a specific antibody. The antibodies were visualized binding to the myosin head on electron micrographs of rotary-shadowed complexes of antibodies with myosin. Measurements on the micrographs indicated that the distances between the head-tail junction of myosin and the 'anti-27 K' and 'anti-50 K' epitopes are 14 nm and 17 nm, respectively. Both antibodies have a high affinity to S-1. The affinity of the 'anti-50 K' to S-1 decreased upon actin binding, while that of the 'anti-27 K' was not affected by binding of S-1 to F-actin. The 'anti-50 K' antibody inhibited the K+ (EDTA) and the actin-activated ATPase activity of S-1, while the 'anti-27 K' had no effect. The results indicate that either the epitope of the 'anti-50 K' is near to the actin or to the ATP-binding sites of S-1, or that there is communication, expressed as propagated conformational changes, between these sites and the epitope.

Interaction of myosin subfragment 1 with fluorescent ribose-modified nucleotides. A comparison of vanadate trapping and SH1-SH2 cross-linking

The environment near the ribose binding site of skeletal myosin subfragment 1 (S1) was investigated by use of two adenosine 5'-diphosphate analogues with fluorescent groups attached at the 2'- and 3'-hydroxyls of the ribose ring. We have compared steady-state and time-resolved fluorescent properties of the reversibly bound S1-nucleotide complexes and the complexes generated by N,N'-p-phenylenedimaleimide (pPDM) thiol cross-linking or vanadate (Vi) trapping. A new fluorescent probe, 2'(3')-O-[N-[2-[[[5-(dimethylamino)naphthyl]sulfonyl] amino]ethyl]carbamoyl]adenosine 5'-diphosphate (DEDA-ADP), which contains a base-stable carbamoyl linkage between the ribose ring and the fluorescent dansyl group, was synthesized and characterized. For comparison, we performed parallel experiments with 2'(3')-O-(N-methylanthraniloyl)adenosine 5'-diphosphate (MANT-ADP) [Hiratsuka, T. (1983) Biochim. Biophys. Acta 742, 496-508]. Solute quenching studies indicated that both analogues bound reversibly to a single cleft or pocket near the ribose binding site. However, steady-state polarization measurements indicated that the probes were not rigidly bound to the protein. The quantum yields of both fluorophores were higher for the complexes formed after trapping with pPDM or Vi than for the reversibly bound complexes. Both DEDA-ADP and MANT-ADP, respectively, had nearly homogeneous lifetimes free in solution (3.65 and 4.65 ns), reversibly bound to S1 (12.8 and 8.6 ns), and trapped on S1 by pPDM (12.7 and 8.7 ns) or Vi (12.8 and 8.6 ns). In contrast to the quantum yields, the lifetimes were not increased upon trapping, compared to those of the reversibly bound states. These results suggested that static quenching in the reversibly bound complex was relieved upon trapping. Taken together, the results suggest that there was a conformational change near the ribose binding site upon trapping by either pPDM or Vi. On the basis of the quantum yield, lifetime, polarization, and solute accessibility studies, we could not detect differences between the S1-pPDM-nucleotide analog complex and the S1-Vi-nucleotide analogue complex for either analogue. Thus, previously observed differences with the adenine modified nucleotide analogue 1,N6-ethenoadenosine diphosphate (epsilon ADP) could not be detected with these ribose-modified probes, indicating that structural differences may be localized to the adenine binding site and not transmitted to the region near the ribose ring.

MgADP-induced changes in the structure of myosin S1 near the ATPase-related thiol SH1 probed by cross-linking

The structural consequences of MgADP binding at the vicinity of the ATPase-related thiol SH1 (Cys-707) have been examined by subjecting myosin subfragment 1, premodified at SH2 (Cys-697) with N-ethylmaleimide (NEM), to reaction with the bifunctional reagent p-phenylenedimaleimide (pPDM) in the presence and absence of MgADP. By monitoring the changes in the Ca2(+)-ATPase activity as a function of reaction time, it appears that the reagent rapidly modifies SH1 irrespective of whether MgADP is present or not. In the absence of nucleotide, only extremely low levels of cross-linking to the 50-kDa middle segment of S1 can be detected, while in the presence of MgADP substantial cross-linking to this segment is observed. A similar cross-link is also formed if MgADP is added subsequent to the reaction of the SH2-NEM-pre-modified S1 with pPDM in the absence of nucleotide. Isolation of the labeled tryptic peptide from the cross-linked adduct formed with [14C]pPDM, and subsequent partial sequence analyses, indicates that the cross-link is made from SH1 to Cys-522. Moreover, it appears that this cross-link results in the trapping of MgADP in this S1 species. These data suggest that the binding of MgADP results in a change in the structure of S1 in the vicinity of the SH1 thiol relative to the 50-kDa "domain" which enables Cys-522 to adopt the appropriate configuration to enable it to be cross-linked to SH1 by pPDM.

Coupling of nonpolymerizable monomeric actin to the F-actin binding region of the myosin head

Polymerizations of skeletal G-actin induced by salt and myosin subfragment 1 (S-1) were suppressed by reaction of G-actin with m-maleimidobenzoyl-N-hydroxysuccinimide ester. The G-actin derivative, containing few intramolecular crosslinks and a free maleimide group, was covalently coupled in solution to the S-1 heavy chain. The resulting complex could no longer bind to F-actin. The SH-1 and SH-2 thiols of S-1 were not involved in the complexation and the covalent link was shown to be exclusively on the 50-kDa segment of the S-1 heavy chain. The specific conjugation of the two proteins followed formation of a reversibly associated pyrophosphate-sensitive binary complex which was characterized by different approaches. Potentially, these complexes may be useful in developing the crystallography of actin-bound S-1.

On the origin and transmission of force in actomyosin subfragment 1

A proximity map showing the three-dimensional arrangement of 12 chemically defined points in actomyosin subfragment 1 is developed and roughly correlated with published electron microscope reconstruction of others. Several additional points and topological relationships in the primary polypeptide chain folding are assimilated into this model. Certain crosslinkings and distance change observations are interpreted as indicators of transmission of force/displacement between the nucleotide-binding and an actin-binding site--i.e., as indications of how energy is transduced in this system.

Functional sequences of the myosin head

Muscle contraction originates from the sliding of myosin filaments on actin filaments, the energy for which is supplied by the hydrolysis of adenosine-5-triphosphate (ATP) by myosin. The nucleotide first binds to the acto-myosin complex in the myosin head (or subfragment-1), producing a conformational change which induces actin dissociation. The release of phosphate (Pi) then allows a return to the strong actin-myosin association, corresponding to the rigor state. We discuss here certain controversial points arising from current concepts of the actin and nucleotide binding regions at the amino acid sequence level within the subfragment-1 heavy chain. We consider the actin and nucleotide binding regions to be two distinct sites (for each of these regions) one of which is shared competitively between actin and the nucleotide. In our model the cyclical actin-S1 association-dissociation steps correspond to different ATP, actin and ADP affinities for the same amino acid sequence of the S1 heavy chain, contributing alternatively to a single hydrolytic nucleotide site or a strong actin site. We propose the existence of a flexible segment that forms or dismantles the nucleotide or actin sites. The large region (amino acids 540-707) overlapping the actin-myosin interface appears to be the main flexible region of the S1 molecule and we propose that this particular sequence plays a key role in the dissociation pathway of the actin-myosin complex and in the conversion of chemical energy into the mechanical energy of contraction.

The effect of nucleotide upon a specific isomerization of actomyosin subfragment 1

The binding of actin to myosin subfragment 1 (S1) has been shown to occur as a two-step reaction [Coates, Criddle & Geeves (1985) Biochem. J. 232, 351-356]. In the first step actin is weakly bound and the second step involves the complex isomerizing to a more tightly bound state. This isomerization can be followed specifically by monitoring the fluorescence of actin that has been covalently labelled with N-(pyren-1-yl)-iodoacetamide at Cys-374 [Geeves, Jeffries & Millar (1986) Biochemistry 25, 8454-8458]. We report here that the presence of nucleotides and nucleotide analogues affects the equilibrium between the strongly bound and weakly bound states (referred to as K2). In the presence of ATP, [gamma-thio]ATP or ADP and vanadate a value of approx. less than 10(-2) was estimated for K2. In the presence of PPi or ADP a value of approx. 2.3 or 10 respectively was obtained. An increase in KCl concentration or the presence of 40% ethylene glycol was found to decrease K2 in the presence of ADP. The data presented here are consistent with the two-step binding model proposed by Geeves, Goody & Gutfreund [(1984) J. Muscle Res. Cell Motil. 5, 351-361], where it was suggested that the transition between weakly bound and strongly bound states is closely associated with the force-generating event in whole muscle.

The effect of pyrophosphate on the reaction of myosin with 2,4,6-trinitrobenzene sulphonate

Myosin was reacted with 2,4,6-trinitrobenzene sulphonate (TNBS) in the presence or absence of Mg-pyrophosphate. The reaction led to trinitrophenylation of lysyl residues which could be divided on the basis of the reaction into three classes: (i) two rapidly reacting lysyl residues (RLR), one residing on each head of myosin, whose rate of reaction depends on the presence of Mg-pyrophosphate; (ii) two lysyl residues which react with intermediate rate (ILR) and reside on the rod segment of myosin; and (iii) the remaining lysyl residues of myosin which react slowly with TNBS. The rate of the trinitrophenylation of RLR was followed spectrophotometrically and enzymatically, measuring an absorbance change at 345 nm, and also changes in K+ (EDTA)-, Mg2+- and Ca2+-activated ATPase activities, respectively. According to analysis of the kinetics of the reaction, Mg-pyrophosphate inhibited the rate of trinitrophenylation in both heads of myosin, not in one head only as was suggested by Miyanishi et al. (J. Biochem Tokyo 85; 1979). Myosin heads (myosin subfragment-1, S-1) were prepared by digesting myosin trinitrophenylated in the absence and presence of Mg-pyrophosphate with chymotrypsin. S-1, with trinitrophenylated RLR, was separated from non-trinitrophenylated S-1 by DEAE cellulose column chromatography. The trinitrophenylated S-1 had a high Mg2+- and a low K+(EDTA)-activated ATPase while the non-trinitrophenylated species had the usual high K+(EDTA)- and low Mg2+-ATPase activity. This results excluded the possibility suggested by Miyanishi et al., that the myosin head, which is resistant to trinitrophenylation in the presence of Mg-pyrophosphate, did not possess K+(EDTA)-activated ATPase activity. The presence of Mg-pyrophosphate during trinitrophenylation substantially affected the enzymic characteristics of the modified myosin. The myosin trinitrophenylated in the presence of Mg-pyrophosphate had a higher K+(EDTA)- and a lower Mg2+-ATPase activity. SH1 (Cys-707) also probably becomes a target of the reaction if myosin is trinitrophenylated in the presence of Mg-pyrophosphate. This is deduced from the following findings: (i) the addition of dithiothreitol after trinitrophenylation partially reversed the loss in the K+(EDTA)-ATPase activity; and (ii) the specific alkylation of the SH1 thiol by 1,5-IAEDANS prior to trinitrophenylation prevented the effect of dithiothreitol on the ATPase activity of myosin. The results indicated that Mg-pyrophosphate induced structural changes in the myosin molecule which influenced the course and possibly the target(s) of trinitrophenylation.

Two kinds of transducer

After recognizing that there are two kinds of biotransducers, attention is focused on the kind that transduces energy, as in muscle or in ion pumps. Using muscle as the example, it is suggested that the essential elements in the transducer (S-1 segment of myosin) are (1) An ATPase site that, in time, is occupied by a succesion of ligands ("intermediates"); (2) A spatially separate site for binding a second ligand (actin, in the case of muscle) that can be held in a variety of spatial relations; and (3) A transmitting mechanism (electric field, propagated structural distortion) that allows the conformation impressed by the ligand of (1) to be conducted over space and impose a specific relation to the ligand of (2).

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

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

Fluorescence resonance energy transfer measurements of distances in actin and myosin. A critical evaluation

The contractile proteins actin and myosin are of considerable biological interest. They are essential for muscle contraction and in eukaryotic cells they play a crucial role in most contractile phenomena. Over the years since the first fluorescence resonance energy transfer (FRET) paper appeared, an extensive body of literature has accumulated on this technique using actin, myosin and the actomyosin complex. These papers are reviewed with several aims in mind: we assess the reliability and consistency of intra- and inter-molecular distances measured between the fluorescent probes attached to specific sites on these proteins; we determine whether the measurements can be assembled into an internally consistent model which can be fitted to the known dimensions of the actomyosin complex; several of the FRET distances are consistent with the available structural data from crystallographic and electron microscopic dimensions; the modelled FRET distances suggest that the assumed value of the orientation factor (k2 = 2/3) is reasonable; we conclude that the model has a predictive value, i.e. it suggests that a small number of the published dimensions may be incorrect and predicts the magnitude of a larger number of measurements which have not yet been reported; and finally (vi) we discuss the contribution of FRET determinations to the current debate on the molecular mechanism of contraction.