Evidence for cooperative interactions of myosin heads with thin filament in the force generation of vertebrate skeletal muscle fibers

X-ray Diffraction Studies on the Structural Origin of Dynamic Tension Recovery Following Ramp-Shaped Releases in High-Ca Rigor Muscle Fibers

It is generally believed that during muscle contraction, myosin heads (M) extending from myosin filament attaches to actin filaments (A) to perform power stroke, associated with the reaction, A-M-ADP-Pi → A-M + ADP + Pi, so that myosin heads pass through the state of A-M, i.e., rigor A-M complex. We have, however, recently found that: (1) an antibody to myosin head, completely covering actin-binding sites in myosin head, has no effect on Ca²⁺-activated tension in skinned muscle fibers; (2) skinned fibers exhibit distinct tension recovery following ramp-shaped releases (amplitude, 0.5% of Lo; complete in 5 ms); and (3) EDTA, chelating Mg ions, eliminate the tension recovery in low-Ca rigor fibers but not in high-Ca rigor fibers. These results suggest that A-M-ADP myosin heads in high-Ca rigor fibers have dynamic properties to produce the tension recovery following ramp-shaped releases, and that myosin heads do not pass through rigor A-M complex configuration during muscle contraction. To obtain information about the structural changes in A-M-ADP myosin heads during the tension recovery, we performed X-ray diffraction studies on high-Ca rigor skinned fibers subjected to ramp-shaped releases. X-ray diffraction patterns of the fibers were recorded before and after application of ramp-shaped releases. The results obtained indicate that during the initial drop in rigor tension coincident with the applied release, rigor myosin heads take up applied displacement by tilting from oblique to perpendicular configuration to myofilaments, and after the release myosin heads appear to rotate around the helical structure of actin filaments to produce the tension recovery.

Definite differences between in vitro actin-myosin sliding and muscle contraction as revealed using antibodies to myosin head

Muscle contraction results from attachment-detachment cycles between myosin heads extending from myosin filaments and actin filaments. It is generally believed that a myosin head first attaches to actin, undergoes conformational changes to produce force and motion in muscle, and then detaches from actin. Despite extensive studies, the molecular mechanism of myosin head conformational changes still remains to be a matter for debate and speculation. The myosin head consists of catalytic (CAD), converter (CVD) and lever arm (LD) domains. To give information about the role of these domains in the myosin head performance, we have examined the effect of three site-directed antibodies to the myosin head on in vitro ATP-dependent actin-myosin sliding and Ca2+-activated contraction of muscle fibers. Antibody 1, attaching to junctional peptide between 50K and 20K heavy chain segments in the CAD, exhibited appreciable effects neither on in vitro actin-myosin sliding nor muscle fiber contraction. Since antibody 1 covers actin-binding sites of the CAD, one interpretation of this result is that rigor actin-myosin linkage is absent or at most a transient intermediate in physiological actin-myosin cycling. Antibody 2, attaching to reactive lysine residue in the CVD, showed a marked inhibitory effect on in vitro actin-myosin sliding without changing actin-activated myosin head (S1) ATPase activity, while it showed no appreciable effect on muscle contraction. Antibody 3, attaching to two peptides of regulatory light chains in the LD, had no significant effect on in vitro actin-myosin sliding, while it reduced force development in muscle fibers without changing MgATPase activity. The above definite differences in the effect of antibodies 2 and 3 between in vitro actin-myosin sliding and muscle contraction can be explained by difference in experimental conditions; in the former, myosin heads are randomly oriented on a glass surface, while in the latter myosin heads are regularly arranged within filament-lattice structures.

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.

Effect of deuterium oxide on contraction characteristics and ATPase activity in glycerinated single rabbit skeletal muscle fibers

We studied the effect of deuterium oxide (D(2)O) on contraction characteristics and ATPase activity of single glycerinated muscle fibers of rabbit psoas. D(2)O increased the maximum isometric force P(0) by about 20%, while the force versus stiffness relation did not change appreciably. The maximum shortening velocity under zero load V(max) did not change appreciably in D(2)O, so that the force-velocity (P-V) curve was scaled depending on the value of P(0). The Mg-ATPase activity of the fibers during generation of steady isometric force P(0) was reduced by about 50% in D(2)O. Based on the Huxley contraction model, these results can be accounted for in terms of D(2)O-induced changes in the rate constants f(1) and g(1) for making and breaking actin-myosin linkages in the isometric condition, in such a way that f(1)/(f(1)+g(1)) increases by about 20%, while (f(1)+g(1)) remains unchanged. The D(2)O effect at the molecular level is discussed in connection with biochemical studies on actomyosin ATPase.

Ca2+ regulation of rabbit skeletal muscle thin filament sliding: role of cross-bridge number

We investigated how strong cross-bridge number affects sliding speed of regulated Ca(2+)-activated, thin filaments. First, using in vitro motility assays, sliding speed decreased nonlinearly with reduced density of heavy meromyosin (HMM) for regulated (and unregulated) F-actin at maximal Ca(2+). Second, we varied the number of Ca(2+)-activatable troponin complexes at maximal Ca(2+) using mixtures of recombinant rabbit skeletal troponin (WT sTn) and sTn containing sTnC(D27A,D63A), a mutant deficient in Ca(2+) binding at both N-terminal, low affinity Ca(2+)-binding sites (xxsTnC-sTn). Sliding speed decreased nonlinearly as the proportion of WT sTn decreased. Speed of regulated thin filaments varied with pCa when filaments contained WT sTn but filaments containing only xxsTnC-sTn did not move. pCa(50) decreased by 0.12-0.18 when either heavy meromyosin density was reduced to approximately 60% or the fraction of Ca(2+)-activatable regulatory units was reduced to approximately 33%. Third, we exchanged mixtures of sTnC and xxsTnC into single, permeabilized fibers from rabbit psoas. As the proportion of xxsTnC increased, unloaded shortening velocity decreased nonlinearly at maximal Ca(2+). These data are consistent with unloaded filament sliding speed being limited by the number of cycling cross-bridges so that maximal speed is attained with a critical, low level of actomyosin interactions.

Myosin light chain 2 modulates MgADP-induced contraction in rabbit skeletal and bovine cardiac skinned muscle

Skinned skeletal and cardiac muscle fibres can be activated by MgADP in the presence of MgATP without Ca2+; the isometric tension is developed in a sigmoidal manner with the addition of MgADP under relaxing conditions. The critical concentrations of MgADP for this MgADP-induced contraction are about 7.5 and 2.6 mM for skeletal and cardiac muscle fibres, respectively. To investigate whether muscle regulatory proteins, myosin light chain 2 (LC2) and troponin C (TnC), play a part in the MgADP-induced contraction, these proteins were partly extracted by treatment with trans-1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid (CDTA), a chelater of divalent cations, and the MgADP-tension relationship was examined in rabbit psoas and bovine cardiac skinned fibres. We found that the sigmoidal MgADP-tension relationship became hyperbolic after a partial extraction of LC2 (about 30 %) and TnC (about 70 %). Reconstitution with LC2 restored the sigmoidal MgADP-tension relationship of control fibres almost fully in both skeletal and cardiac fibres, whereas reconstitution with TnC alone had no effect. Furthermore, cardiac fibres reconstituted with skeletal LC2 exhibited an MgADP-tension relationship intermediate between skeletal and cardiac fibres. The partial extraction of LC2 and TnC resulted in a reduction of the inhibitory effect of inorganic phosphate (P(i)) on the MgADP-activated tension. Reconstitution with LC2 restored the original P(i)-tension relationship, whereas reconstitution with TnC had no effect. In other words, extraction of LC2 apparently increased the affinity of myosin for MgADP but decreased the affinity for P(i). These results demonstrate that LC2 modulates MgADP-induced activation of actomyosin interaction.

ATP-induced axial movement of myosin heads in living thick filaments recorded with a gas environmental chamber attached to the electron microscope

Using a gas environmental (hydration) chamber, in which biological specimens can be kept in wet state, we succeeded in recording images of 'living' muscle thick filaments with gold position markers attached to the myosin heads. The position of individual myosin heads did not change appreciably with time in the absence of ATP, indicating stability of the myosin head mean position. On application of ATP, the position of individual myosin heads was found to move by approximately 20 nm along the filament axis, while no appreciable movement of the filaments was detected. The ATP-induced myosin head movement was not observed in filaments in which ATPase activity of the myosin heads was eliminated. Application of ADP produced no appreciable myosin head movement. These results show that the ATP-induced myosin head movement takes place in the absence of the thin filaments. Since ATP reacts rapidly with the myosin head (M) to form the complex (M.ADP.Pi) having average lifetime of > 10 s, the observed myosin head movement may be mostly associated with reaction, M + ATP-->M.ADP.Pi. This work will open a new research field to study dynamic structural changes of individual biomolecules which are kept in 'living' state in an electron microscope.

Behavior of N-phenylmaleimide- and p-phenylenedimaleimide-reacted muscle crossbridge heads

The finding of Barnett et al. (Biophys. J. 61 (1992) 358) that NPM-reacted crossbridge heads do not bind strongly to actin in rigor solution is not easily interpreted in terms of the solution studies of Xie and Schoenberg (Biochemistry 37 (1998) 8048) who found strong binding of NPM-reacted myosin subfragment-1 to actin in solutions devoid of MgATP. For this reason, the current work uses stiffness measurement to re-investigate the binding of rabbit skeletal muscle crossbridges to actin in rigor solution. It is found that NPM-reacted crossbridge heads bind strongly to actin in rigor solution providing one is extremely careful to reduce MgATP contamination to levels well below those that would have a detectable effect on unmodified fibers. The reason for this is that NPM-reacted crossbridge heads, which hydrolyze MgATP extremely slowly, are especially susceptible to contaminant MgATP. The new fiber results show a strong correlation with the solution results. A further manifestation of this correlation is that pPDM-reacted crossbridge heads are different from NPM-reacted ones in that, like in solution, they remain weakly binding to actin even at extremely low MgATP levels. The findings suggest that the covalent crosslinking of SH1 and SH2 by pPDM is likely playing a significant role in locking pPDM-reacted crossbridge heads in a weakly binding conformation.

Binding of SH1-SH2-modified myosin subfragment-1 to actin

Myosin subfragment-1 (S1) was labeled with NPM in the presence of ATP or with pPDM in the presence of ADP at 0 degreesC, conditions which favor linking of maleimide groups to both Cys-707 (SH1) and Cys-697 (SH2). Unmodified S1 was removed by sedimentation with a small amount of F-actin, and the modified protein in the supernatant thoroughly dialyzed. The myosin high-salt EDTA and calcium ATPase activities of the isolated modified S1 were close to zero, suggesting nearly complete modification of SH1 and SH2. The binding of control and these modified myosins to actin was measured at 100 mM ionic strength using a co-sedimentation assay. In the presence of high MgATP concentration, control and NPM- and pPDM-reacted S1 all bind weakly to actin, with binding constants K3 of 4.9, 2.2, and 1.9 x 10(4) M-1, respectively. In the absence of MgATP, the binding constant K2 of pPDM-reacted S1 remains weak, 4.6 x 10(4) M-1,while that of NPM-reacted and control S1 becomes strong, 4.7 and 31 x 10(6) M-1, respectively. The binding constant for ATP to acto-NPM-reacted-S1 is approximately 2 x 10(4) M-1. Our data suggest that the binding of NPM-S1 to F-actin, in contrast to that of pPDM-S1, is ATP sensitive and can be quite strong at very low ATP concentration. They also suggest that while simple alkylation of SH1 and SH2 may be sufficient to inhibit myosin's ability to hydrolyze ATP, actual covalent linkage of SH1 and SH2 may be necessary to inhibit the weakly to strongly binding conformational change.

Behavior of N-phenylmaleimide-reacted muscle fibers in magnesium-free rigor solution

Using x-ray diffraction and mechanical stiffness, the response of N-phenylmaleimide (NPM)-reacted cross-bridges to solutions containing different amounts of ATP and Mg2+ has been studied. In relaxing solution containing greater than millimolar amounts of ATP and Mg2+, NPM-treated muscle fibers give x-ray diffraction patterns and stiffness records, which are nearly indistinguishable from those of untreated relaxed fibers. In a solution devoid of added ATP, but with Mg2+ (rigor(+Mg) solution), the muscle fibers still give x-ray diffraction patterns and mechanical responses characteristic of relaxed muscle. The new finding reported here is that in a solution devoid of both ATP and Mg2+ (rigor(-Mg) solution containing EDTA with no added ATP), NPM-reacted cross-bridges do give rigor-like behavior. This is the first report that NPM-reacted cross-bridges, at least in the presence of EDTA, are capable of going into a strongly binding conformation.

Dynamic electron microscopy of ATP-induced myosin head movement in living muscle thick filaments

Although muscle contraction is known to result from movement of the myosin heads on the thick filaments while attached to the thin filaments, the myosin head movement coupled with ATP hydrolysis still remains to be investigated. Using a gas environmental (hydration) chamber, in which biological specimens can be kept in wet state, we succeeded in recording images of living muscle thick filaments with gold position markers attached to the myosin heads. The position of individual myosin heads did not change appreciably with time in the absence of ATP, indicating stability of the myosin head mean position. On application of ATP, the position of individual myosin heads was found to move by approximately 20 nm along the filament axis, whereas no appreciable movement of the filaments was detected. The ATP-induced myosin head movement was not observed in filaments in which ATPase activity of the myosin heads was eliminated. Application of ADP produced no appreciable myosin head movement. These results show that the ATP-induced myosin head movement takes place in the absence of the thin filaments. Because ATP reacts rapidly with the myosin head (M) to form the complex (M. ADP.Pi) with an average lifetime of >10 s, the observed myosin head movement may be mostly associated with reaction, M + ATP --> M.ADP. Pi. This work will open a new research field to study dynamic structural changes of individual biomolecules, which are kept in a living state in an electron microscope.

The strength of binding of the weakly-binding crossbridge created by sulfhydryl modification has very low calcium sensitivity

The acto-subfragment-1.ATP state is an important intermediate in the Ca-activated acto-S1 ATPase reaction, suggesting that the myosin.ATP crossbridge seen in muscle fibers similarly may be an important intermediate in the contractile cycle. Treatment of muscle fibers with either para-phenylenedimaleimide (pPDM) or N-phenylmaleimide (NPM) alters the myosin crossbridges so that they bind to the actin filament with about the same affinity as the myosin.ATP crossbridge. Additionally, the treated crossbridges and the myosin.ATP crossbridge have virtually identical attachment and detachment rate constants. Thus the treated crossbridges appear to be reasonable analogues of the weakly-binding myosin.ATP crossbridges of relaxed fibers and studies of the treated fibers may shed some light on the behavior of the physiologically important myosin.ATP crossbridge. We have examined the influence of Ca2+ on the binding and rate constants of pPDM- and NPM-treated weakly-binding crossbridges. In agreement with earlier solution studies, we found almost no Ca-sensitivity of the binding of pPDM- or NPM-treated crossbridges.

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)

Time-resolved synchrotron X-ray diffraction studies of a single frog skeletal muscle fiber. Time courses of intensity changes of the equatorial reflections and intracellular Ca2+ transients

Time-resolved X-ray equatorial diffraction studies on a single frog skeletal muscle fiber were performed with a 10 ms time resolution using synchrotron radiation in order to compare the time courses of the molecular changes of contractile proteins and the intracellular Ca2+ transient during an isometric twitch contraction at 2.7 degrees C. Measurements of the Ca2+ transient using aequorin as an intracellular Ca2+ indicator were conducted separately just before and after the X-ray experiments under very similar experimental conditions. The results, which showed a similar time course of tension to that observed in the X-ray experiment, were compared with the aequorin light signal, tension and the intensity changes of the 1,0 and 1,1 equatorial reflections. No appreciable change in both reflection spacings indicated that the effect of internal shortening of the muscle was minimized during contraction. The intensity change of the equatorial reflections generally occurred after the aequorin light signal. In the rising phase, the time course of increase in the 1,1 intensity paralleled that of the rise of the light signal and the intensity peak occurred 20-30 ms after the peak of the light signal. The decrease in the 1,0 intensity showed a time course similar to that of tension and the intensity minimum roughly coincided with the tension peak, coming at 80-90 ms and about 60 ms after the peaks of the light signal and the 1,1 intensity change, respectively. In the relaxation phase, the 1,1 intensity seemed to fall rapidly just before the tension peak and then returned to the original level in parallel with the decay of tension. The 1,0 intensity returned more slowly than the tension relaxation. Thus, the change of the 1,1 intensity was faster than that of the 1,0 intensity in both the rising and relaxation phases. When the measured aequorin light signal was corrected for the kinetic delay of the aequorin reaction with a first-order rate constant of either 50 or 17 s-1, the peak of the corrected light signal preceded that of the measured one by approx. 30 ms. Thus, the peak of the Ca2+ transient appeared earlier than the peaks of the 1,1 and 1,0 intensity changes by 50-60 and 110-120 ms, respectively. The time lag between the extent of structural change and the Ca2+ transient is discussed in relation to the double-headed attachment of a cross-bridge to actin.

X-ray diffraction and electron microscopy from Lethocerus flight muscle partially relaxed by adenylylimidodiphosphate and ethylene glycol

The low-angle X-ray diffraction pattern from Lethocerus flight muscle fibres was recorded in rigor or under two conditions that modify crossbridge structure and behaviour, aqueous adenylylimidodiphosphate (AMPPNP) and AMPPNP + calcium in an ethylene glycol-water mixture. The effects on the 38.7 nm layer-line peaks (hk.6) of the diffraction patterns were studied in detail. In aqueous AMPPNP at room temperature, a condition in which rigor tension drops to half without loss of stiffness, the peaks remained nearly as intense as in rigor except for the 10.6, which dropped to half. In 20% (v/v) ethylene glycol-AMPPNP + 100 microM-Ca2+ at 23 degrees C (gly + pnp + Ca), a condition which removed muscle tension but left stiffness close to the rigor value, the 10.6 and 11.6 peaks greatly decreased but the 31.6 remained relatively high. The 14.5 nm meridional peak (00.16) became stronger on addition of AMPPNP and again on adding glycol + calcium. Considered in terms of constructively interfering filaments and crossbridges, the X-ray data indicated a transfer of diffracting crossbridge mass towards the thick filament as relaxation proceeds. We compared the X-ray diffraction patterns and crossbridge structure seen with electron microscopy (EM) under the same chemical conditions. EM and X-ray observations were mutually quite consistent overall. However, X-ray data indicated that more crossbridge mass was stereospecifically related to actin before fixation in the partially relaxed state (gly + pnp + Ca) than was suggested by the disordered crossbridge profiles seen by EM. We conclude that myosin heads at the start of the power stroke may both be closely related to their thick filament origins and form actin-determined attachments to the thin filament.

Two attached non-rigor crossbridge forms in insect flight muscle

We have performed thin-section electron microscopy on muscle fibers fixed in different mechanically monitored states, in order to identify structural changes in myosin crossbridges associated with force production and maintenance. Tension and stiffness of fibers from glycerinated Lethocerus flight muscle were monitored during a sequence of conditions using AMPPNP and then AMPPNP plus increasing concentrations of ethylene glycol, which brought fibers through a graded sequence from rigor relaxation. Two intermediate crossbridge forms distinct from the rigor or relaxed forms were observed. The first was produced by AMPPNP at 20 degrees C, which reduced isometric tension 60 to 70% below rigor level without reducing rigor stiffness. Electron microscopy of these fibers showed that, in spite of the drop in tension, no obvious change from the 45 degrees crossbridge angle characteristic of rigor occurred. However, the thick filament ends of the crossbridges were altered from their rigor positions, so that they now marked a 14.5 nm repeat, and formed four separate origins at each crossbridge level. The bridges were also less slewed and bent than rigor bridges, as seen in transverse sections. The second crossbridge form was seen in glycol-AMPPNP at 4 degrees C, just below the glycol concentration that produced mechanical relaxation. These fibers retained 90% of rigor stiffness at 40 Hz oscillation, but would not bear sustained tension. Stiffness was also high in the presence of calcium at room temperature under similar conditions. Electron microscopy showed crossbridges projecting from the thick filaments at an angle that centered around 90 degrees, rather than the 45 degree angle familiar from rigor. This coupling of relaxed appearance with persistent stiffness suggests that the 90 degree form may represent a weakly attached crossbridge state like that proposed to precede force development in current models of the crossbridge power stroke.

A self-induced translation model of myosin head motion in contracting muscle. I. Force-velocity relation and energy liberation

In our previous model, it was assumed that the two heads of myosin act co-operatively in producing force for the sliding of actin filaments relative to myosin filaments. We eliminate the assumption of co-operativity in the present model, following the conclusion by Harada and co-workers that a co-operative interaction between the two heads of myosin is not essential in producing actin filament movement. We assume that (1) a myosin head activated by ATP hydrolysis binds to the thin filament at a definite angle and does not do the power stroke, i.e. does not change its orientation during attachment, (2) a potential of force acting on the myosin head is induced around the thin filament when an ATP-activated myosin head binds to an actin molecule in the thin filament, and (3) the potential remains for a while after detachment of the myosin head and statistically controls the direction of thermal motion of the myosin head, so that the myosin head translates toward the Z-line as a statistical average. We did calculations on these assumptions with a mean tension approximation and got the following results. (a) The calculated force-velocity relation in muscle contraction is in fairly good agreement with experimental observation, including the give phenomenon that lengthening velocity becomes very large for a force about twice the isometric tension. (b) The calculated rate of energy liberation during muscle contraction as a function of load on muscle is in good agreement with experimental results. (c) The calculated distance over which a myosin molecule moves along the thin filament during one ATP hydrolysis can be more than 60 nm under unloaded conditions.

Direct observation of molecular motility by light microscopy

We used video-fluorescence microscopy to directly observe the sliding movement of single fluorescently labeled actin filaments along myosin fixed on a glass surface. Single actin filaments labeled with phalloidin-tetramethyl-rhodamine, which stabilizes the filament structure of actin, could be seen very clearly and continuously for at least 60 min in 02-free solution, and the sensitivity was high enough to see very short actin filaments less than 40 nm long that contained less than eight dye molecules. The actin filaments were observed to move along double-headed and, similarly, single-headed myosin filaments on which the density of the heads varied widely in the presence of ATP, showing that the cooperative interaction between the two heads of the myosin molecule is not essential to produce the sliding movement. The velocity of actin filament independent of filament length (greater than 1 micron) was almost unchanged until the density of myosin heads along the thick filament was decreased from six heads/14.3 nm to 1 head/34 nm. This result suggests that five to ten heads are sufficient to support the maximum sliding velocity of actin filaments (5 micron/s) under unloaded conditions. In order for five to ten myosin heads to achieve the observed maximum velocity, the sliding distance of actin filaments during one ATP cycle must be more than 60 nm.

Sliding movement of single actin filaments on one-headed myosin filaments

The myosin molecule consists of two heads, each of which contains an enzymatic active site and an actin-binding site. The fundamental problem of whether the two heads function independently or cooperatively during muscle contraction has been studied by methods using an actomyosin thread, superprecipitation and chemical modification of muscle fibres. No clear conclusion has yet been reached. We have approached this question using an assay system in which sliding movements of fluorescently labelled single actin filaments along myosin filaments can be observed directly. Here, we report direct measurement of the sliding of single actin filaments along one-headed myosin filaments in which the density of heads was varied over a wide range. Our results show that cooperative interaction between the two heads of myosin is not essential for inducing the sliding movement of actin filaments.