The mechanism of muscle contraction

Synthesis of non-nucleotide ATP analogues and characterization of their chemomechanical interaction with muscle fibres

To probe the substrate requirements for the actomyosin chemomechanical interaction, the effects of a series of eight new non-nucleotide ATP analogues on actomyosin-catalysed hydrolysis rates and on fibre mechanics have been investigated. These analogues have substitutions of new functional groups at the 2- and 4-positions of the ATP analogues, 2-[(4-azido-2-nitrophenyl)amino]ethyl triphosphate (NANTP), and 3-[(4-nitrophenyl)amino]propyl triphosphate (PrNANTP). Previous work has shown NANTP but not PrNANTP will support active tension and shortening in skinned muscle fibres in a manner almost identical to ATP. Here all 2- and 4-phenyl substituted analogues had myosin subfragment 1 (S1) NTPase hydrolysis rates higher than ATP and the rates were stimulated by addition of actin. In general, the replacement of the 4-azido group of NANTP with -H, -NO2 or -NH2 had small effects on fibre mechanics while replacement of 2-NO2 group with -H or -NH2 dramatically lowered the ability of the new analogues to support active tension and shortening. All PrNANTP-based analogues were ineffective in supporting active tension or shortening. We found no correlation between S1 or actoS1 NTPase rates and any mechanical parameters. However, for all analogues there was a strong correlation between the maximal velocity of shortening (Vmax) and isometric tension (P0). A three-state, chemomechanical model is proposed in which the analogues effect the transition rate into a strongly-bound, force-producing crossbridge state to account for this correlation. These studies identify 2-[(2-nitrophenyl)amino]ethyl triphosphate as the chemically simplest ATP analogue which closely mimics the effect of ATP in skinned muscle fibres.

Identification of a 120 kd hair-bundle myosin located near stereociliary tips

By adapting to sustained stimuli, hair cells of the internal ear maintain their optimal sensitivity to minute displacements. Biophysical experiments have suggested that adaptation is mediated by a molecular motor, most likely a member of the myosin family. To provide direct evidence for the presence of myosin isozymes in hair bundles, we used photoaffinity labeling with vanadate-trapped uridine and adenine nucleotides to identify proteins of 120, 160, and 230 kd in a preparation of hair bundles purified from the bullfrog's sacculus. The photoaffinity labeling properties of these proteins, particularly the 120 kd protein, resembled those of other well-characterized myosins. A 120 kd hair-bundle protein was also recognized by a monoclonal antibody directed against a vertebrate myosin I isozyme. Immunofluorescence microscopy localized this protein near the beveled top edge of the hair bundle, the site of mechanoelectrical transduction and adaptation.

Interaction and polymerization of the G-actin-myosin head complex: effect of DNase I

The properties of polymerization and interaction of the G-actin-myosin S1 complexes (formed with either the S1(A1) or the S1(A2) isoform) have been studied by light-scattering and fluorescence measurements in the absence and in the presence of DNase I. In the absence of DNase I, the G-actin-S1(A1) and G-actin-S1(A2) complexes were found to be characterized by different limiting concentrations (l.c.), defined as the complex concentrations above which the polymerization occurs spontaneously within 20 h at 20 degrees C in a "no salt" buffer (l.c. = 0.42 and 8.8 microM for G-actin-S1(A1) and G-actin-S1(A2), respectively). The occurrence of a limiting concentration for either complex together with the kinetic properties of the polymerization led us to conclude that the G-actin-S1 polymerization occurs via a nucleation-elongation process. Fluorescence titrations and proteolysis experiments revealed that G-actin interacts with S1 with a 1:1 stoichiometry (independently of the presence of ATP) with dissociation constants, in the absence of nucleotide, of 20 and 50 nM for the G-actin-S1(A1) and G-actin-S1(A2) complexes, respectively. In the presence of at least a 1.5-fold excess of DNase I, the polymerization of the G-actin-S1 complexes was blocked even at high protein concentration or in the presence of salts. In addition, the affinity of either S1 isoform to actin was reduced 4-5-fold by DNase I, while the stoichiometry of the G-actin-S1 complexes was not changed. However, since the dissociation constants remain in the submicromolar range, we could demonstrate the existence of ternary DNase I-G-actin-S1 complexes stable under polymerizing conditions. Finally, the study of the effect of nucleotides and of various salts on the G-actin-S1 interaction further showed significant differences between the G-actin-S1 and F-actin-S1 interactions.

Polarization of fluorescently labeled myosin subfragment-1 fully or partially decorating muscle fibers and myofibrils

Fluorescently labeled myosin heads (S1) were added to muscle fibers and myofibrils at various concentrations. The orientation of the absorption dipole of the dye with respect to the axis of F-actin was calculated from polarization of fluorescence which was measured by a novel method from video images of muscle. In this method light emitted from muscle was split by a birefringent crystal into two nonoverlapping images: the first image was created with light polarized in the direction parallel to muscle axis, and the second image was created with light polarized in the direction perpendicular to muscle axis. Images were recorded by high-sensitivity video camera and polarization was calculated from the relative intensity of both images. The method allows measurement of the fluorescence polarization from single myofibril irrigated with low concentrations of S1 labeled with dye. Orientation was also measured by fluorescence-detected linear dichroism. The orientation was different when muscle was irrigated with high concentration of S1 (molar ratio S1:actin in the I bands equal to 1) then when it was irrigated with low concentration of S1 (molar ratio S1:actin in the I bands equal to 0.32). The results support our earlier proposal that S1 could form two different rigor complexes with F-actin depending on the molar ratio of S1:actin.

Two-dimensional time-resolved X-ray diffraction studies of live isometrically contracting frog sartorius muscle

Results were obtained from contracting frog muscles by collecting high quality time-resolved, two-dimensional, X-ray diffraction patterns at the British Synchrotron Radiation Source (SERC, Daresbury, Laboratory). The structural transitions associated with isometric tension generation were recorded under conditions in which the three-dimensional order characteristic of the rest state is either present or absent. In both cases, new layer lines appear during tension generation, subsequent to changes from activation events in the thin filaments. Compared with the 'decorated' actin layer lines of the rigor state, the spacings of the new layer lines are similar whereas their intensities differ substantially. We conclude that in contracting muscle an actomyosin complex is formed whose structure is not like that in rigor, although it is possible that the interacting sites are the same. Transition from rest to plateau of tension is accompanied by approximately 1.6% increase in the axial spacing of the myosin layer lines. This is explained as arising from axial disposition of the interacting myosin heads in the actomyosin complex. Model calculations are presented which support this view. We argue that in a situation where an actomyosin complex is formed during contraction, one cannot describe the diffraction features as being either thick or thin filament based. Accordingly, the layer lines seen during tension generation are referred to as actomyosin layer lines. It is shown that these layer lines can be indexed as submultiples of a minimum axial repeat of approximately 218.7 nm. After lattice disorder effects are taken into account, the intensity increases on the 15th and 21st AM layer lines at spacings of approximately 14.58 and 10.4 nm respectively, show the same time course as tension rise. However, the time course of the intensity increase of the other actomyosin layer lines and of the spacing change (which is the same for both phenomena) shows a substantial lead over tension rise. These findings suggest that the actomyosin complex formed prior to tension rise is a non-tension-generating state and that this is followed by a transition of the complex to a tension-generating state. The intensity increase in the 15th actomyosin layer line, which parallels tension rise, can be accounted for assuming that in the tension-generating state the attached heads adopt (axially) a more perpendicular orientation with respect to the muscle axis than is seen at rest or in the non-tension-generating state. This suggests the existence of at least two structurally distinct interacting myosin head conformations.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Adenine nucleoside diphosphates block adaptation of mechanoelectrical transduction in hair cells

By adapting to sustained stimuli, hair cells in the internal ear retain their sensitivity to minute transient displacements. Because one model for adaptation asserts that this process is mediated by a myosin isozyme, we reasoned that we should be able to arrest adaptation by interfering with myosin's ATPase cycle though introduction of ADP into hair cells. During tight-seal, whole-cell recordings of transduction currents in cells isolated from bullfrog (Rana catesbeiana) sacculus, dialysis with 5-25 mM ADP gave variable results. In half of the cells examined, the rate of adaptation remained unchanged or even increased; adaptation was blocked in the remaining cells. Because we suspected that the variable effect of ADP resulted from the conversion of ADP to ATP by adenylate kinase, we employed the ADP analog adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), which is not a substrate for adenylate kinase. Adaptation consistently disappeared in the presence of 1-10 mM ADP[beta S]; in addition, the transduction channels' open probability at rest grew from approximately 0.1 to 0.8 or more. Both effects could be reversed by 2 mM ATP. When used in conjunction with the adenylate kinase inhibitor P1,P5-bis(5'-adenosyl) pentaphosphate (Ap5A), ADP had effects similar to those of ADP[beta S]. These results suggest that adaptation by hair cells involves adenine nucleotides, and they lend support to the hypothesis that the adaptation process is powered by a myosin motor.

Chemomechanical cycle of kinesin differs from that of myosin

Motor proteins move unidirectionally along cytoskeletal polymers by coupling translocation to cycles of ATP hydrolysis. The energy from ATP is required both to generate force and to dissociate the motor-filament complex in order to begin a new chemomechanical cycle. For myosin, force production is associated with phosphate release following ATP hydrolysis, whereas dissociation of actomyosin is tightly coupled to the binding of ATP. Dynein, a microtubule motor, uses a similar cycle, suggesting that all cytoskeletal motors might operate by a common mechanism. Here we investigate kinesin's chemomechanical cycle by assaying microtubule movement by single kinesin molecules when intermediate states in the hydrolysis cycle are prolonged with ATP analogues or inhibitors. In contrast to myosin and dynein, kinesin with bound ADP dissociates from microtubules during translocation, whereas kinesin with unhydrolysed nucleotide remains tightly associated with the polymer. These findings imply that kinesin converts ATP energy into mechanical work by a pathway distinct from that of myosin or dynein.

Coupling between ATPase and force-generating attachment-detachment cycles of actomyosin in vitro

We have developed a high resolution force measurement system in vitro by manipulating a single actin filament attached to a microneedle. The system could resolve forces less than a piconewton, and has time resolution in the submillisecond range. We have used this system to detect force fluctuations produced by individual molecular interactions. We observed large force fluctuations during isometric force generations. Noise analysis of the force fluctuations showed that the force was produced by stochastic and independent attachment-detachment cycles between actin and myosin heads, and one force-generating attachment-detachment cycle corresponded to each ATPase cycle. But, the force fluctuations almost completely disappeared during sliding at the velocities of 20 to 70% of the maximum one at zero load. The analysis indicated that myosin heads produced an almost constant force for most (probably > 70%) of the ATPase cycle time, i.e., the duty ratio > 0.7. Since the myosin step size was given as (velocity) x (the duty ratio) x (the ATPase cycle time, 30 ms), it was calculated to be 40 to 110 nm, corresponding to velocities of 20 to 70% of the maximum one (9 microns/s), respectively. These values are much greater than the displacement by a single attachment-detachment cycle of actomyosin (10-20 nm), indicating that multiple force-generating attachment-detachment cycles correspond to each ATPase cycle during sliding at velocities of > 20% of the maximum one. In conclusion, the coupling between the ATPase and the force-generating attachment-detachment cycles of actomyosin is not rigidly determined in a one-to-one fashion but is variable depending on the load.

Structure of the myosin filaments of relaxed and rigor vertebrate striated muscle studied by rapid freezing electron microscopy

Rapid freezing followed by freeze-substitution has been used to study the ultrastructure of the myosin filaments of live and demembranated frog sartorius muscle in the states of relaxation and rigor. Electron microscopy of longitudinal sections of relaxed specimens showed greatly improved preservation of thick filament ultrastructure compared with conventional fixation. This was revealed by the appearance of a clear helical arrangement of myosin crossbridges along the filament surface and by a series of layer line reflections in computed Fourier transforms of sections, corresponding to the layer lines indexing on a 43 nm repeat in X-ray diffraction patterns of whole, living muscles. Filtered images of single myosin filaments were similar to those of negatively stained, isolated vertebrate filaments and consistent with a three-start helix. M-line and other non-myosin proteins were also very well preserved. Rigor specimens showed, in the region of overlapping myosin and actin filaments, periodicities corresponding to the 36, 24, 14.4 and 5.9 nm repeats detected in X-ray patterns of whole muscle in rigor; in the H-zone they showed a disordered array of crossbridges. Transverse sections, whose Fourier transforms extend to the (3, 0) reflection, supported the view, based on X-ray diffraction and conventional electron microscopy, that in the overlap zone of relaxed muscle most of the crossbridges are detached from the thin filaments while in rigor they are attached. We conclude that the rapid freezing technique preserves the molecular structure of the myofilaments closer to the in vivo state (as monitored by X-ray diffraction) than does normal fixation.

Selective placement of electron spin resonance spin labels: new structural methods for peptides and proteins

Electron spin resonance (ESR) is more powerful than ever as a technique for solving biochemical and biophysical problems. Part of the great utility of ESR arises from the use of modern biochemical methods to place spin labels at important positions along the primary sequence of a peptide or protein.

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

We have measured the orientation of a region of the myosin head, close to the junction with the rod, during active force generation. Paramagnetic probes were attached specifically to a reactive cysteine (Cys 125) of purified myosin light chain 2 (LC2) and exchanged into myosin heads in glycerinated rabbit psoas muscle. Electron paramagnetic resonance spectroscopy was used to monitor the orientation of the probes. Previous work has shown that the LC2 bound spin probes are significantly ordered in rigor and muscle in the presence of adenosine diphosphate (ADP). In contrast, there is a nearly random angular distribution in relaxed muscle. We show here that during the generation of isometric tension, all of the LC2 bound spin probes (98 +/- 1.6%) show an angular distribution similar to that of relaxed muscle. These findings contrast with results obtained from probes attached to Cys 707 on the cross-bridge, located close to the actin binding site, where, during active force generation, a proportion of the spin probes were ordered as in rigor, whereas the remaining probes were disordered as in relaxation. To test the hypothesis that this ordered component is due to modification of Cys 707, we measured the spectra obtained from probes attached to LC2 in fibers modified at Cys 707. The modification of Cys 707 did not produce an ordered component in these spectra. The absence of an ordered component at the LC2 site limits the populations of some states in active fibers. An actin/myosin/ADP state is thought to be the major force-producing state. Our present results show that the populations of states with ordered probes on LC2 are < 2% in active fibers; thus, the major force-producing state is different from the one obtained by addition of ADP to rigor fibers.

Insect crossbridges, relaxed by spin-labeled nucleotide, show well-ordered 90 degrees state by X-ray diffraction and electron microscopy, but spectra of electron paramagnetic resonance probes report disorder

The structure of glycerinated Lethocerus insect flight muscle fibers, relaxed by spin-labeled ATP and vanadate (Vi), was examined using X-ray diffraction, electron microscopy and electron paramagnetic resonance (e.p.r.) spectra. We obtained excellent relaxation of MgATP quality as determined by mechanical criteria, using vanadate trapping of 2' spin-labeled 3' deoxyATP at 3 degree C. In rigor fibers, when the diphosphate analog is bound in the absence of Vi, the probes on myosin heads are well-ordered, in agreement with electron microscopic and X-ray patterns showing that myosin heads are ordered when attached strongly to actin. In relaxed muscle, however, e.p.r. spectra report orientational disorder of bound (Vi-trapped) spin-labeled nucleotide, while electron microscopic and X-ray patterns both show well-ordered bridges at a uniform 90 degrees angle to the filament axis. The spin-labeled nucleotide orientation is highly disordered, but not completely isotropic; the slight anisotropy observed in probe spectra is consistent with a shift of approximately 10% of probes from angles close to 0 degrees to angles close to 90 degrees. Measurements of probe mobility suggest that the interaction between probe and protein remains as tight in relaxed fibers as in rigor, and thus that the disorder in relaxed fibers arises from disorders of (or within) the protein and not from disorder of the probe relative to the protein. Fixation of the relaxed fibers with glutaraldehyde did not alter any aspect of the spectrum of the Vi-trapped analog, including the slight order observed, showing that the extensive inter- and intra-molecular cross-linking of the first step of sample preparation for electron microscopy had not altered relaxed crossbridge orientations. Two models that may reconcile the apparently disparate results obtained on relaxed fibers are presented: (1) a rigid myosin head could possess considerable disorder in the regular array about the thick filament; or (2) the nucleotide site could be on a disordered, probably distal, domain of myosin, while a more proximal region is well ordered on the thick filament backbone. Our findings suggest that when e.p.r. probes signal disorder of a local site or domain, this is complementary, not contradictory, to signals of general order. The e.p.r. spectra show that a portion of the myosin molecule can be disordered at the same time as the X-ray diffraction and electron microscopy show the bulk of myosin head mass to be uniformly oriented and regularly arrayed.

Two different acto-S1 complexes

Based on change in anisotropy of fluorescently labelled S1 and on increase in turbidity of acto-S1 complex when S1 bound to F-actin, we reported previously that depending on the molar ratio of S1 to actin two different complexes of actin monomer (A) and myosin subfragment 1 (S1) could be formed: A1*S1 (one actin with one S1) and A2*S1 (two actins with one S1). Here we extend these findings to F-actin labelled with pyrene and cross-linked to S1 with 1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide (EDC). The fluorescence of pyrene F-actin decreased with increase in S1 concentration and reached saturation at a molar ratio of S1 to actin of either 0.5 or 1.0, depending on whether S1 was added slowly (5 min) or quickly (10-20 s between additions). Incubation of A2*S1 complex in excess of S1 for > 1 h caused a shift in equilibrium towards the A1*S1 complex. The A2*S1 complexes were not formed at high S1 to actin ratios (> 1.0) owing to competition between heads. Crosslinking experiments showed that the formation of EDC crosslinked products, 175-185 kDa doublet and 265 kDa band, depended on the ratio S1 to actin. To assess the relative ratio of S1 and actin in crosslinked products, we labelled S1 and F-actin with different fluorescent probes (5-IAF and IATR). The S1 to actin ratio was proportional to the ratio of intensities of fluorescence of labelled S1 and actin. The S1 to actin ratio in 265 kDa product was two times smaller than in 175-185 kDa doublet (which is believed to be A1*S1 complex) and therefore 265 kDa band corresponded to A2*S1. Transition between two types of binding may be important to understanding how muscle contracts.

Early steps of the Mg(2+)-ATPase of relaxed myofibrils. A comparison with Ca(2+)-activated myofibrils and myosin subfragment 1

The early steps of the Mg(2+)-ATPase activity of relaxed rabbit psoas myofibrils were studied in a buffer of near-physiological ionic strength at 4 degrees C by the rapid flow quench technique. The initial ATP binding steps were studied by the ATP chase, and the cleavage and release of product steps by the Pi burst method. The data obtained were interpreted by [formula: see text] where M represents the myosin heads with or without actin interaction. This work is a continuation of our study on Ca(2+)-activated myofibrils [Houadjeto, M., Travers, F., & Barman, T. (1992) Biochemistry 31, 1564-1569]. Here the constants obtained with relaxed myofibrils were compared with those with activated myofibrils and myosin subfragment 1 (S1). We find that whereas Ca2+ increases 80X the release of products (k4), it has little effect upon the kinetics of the initial binding and cleavage steps. As with activated myofibrils and S1, the second-order binding constant for ATP (k2/K1) was about 1 microM-1 s-1 and the ATP was bound very tightly. With activated myofibrils, it was difficult to obtain an estimate for the koff for ATP(k-2) but it is much less than kcat. Here with relaxed myofibrils we estimate k-2 less than 8 x 10(-4) s-1, which is considerably smaller than kcat (0.019 s-1) and also previous estimates for this constant. The overall Kd for ATP to relaxed myofibrils is less than 8 x 10(-10) M. With S1 this Kd is about 10(-11) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Cooperativity of thiol-modified myosin filaments. ATPase and motility assays of myosin function

The effects of chemical modifications of myosin's reactive cysteines on actomyosin adenosine triphosphatase (ATPase) activities and sliding velocities in the in vitro motility assays were examined in this work. The three types of modifications studied were 4-[N-[(iodoacetoxy)ethyl]-N-methylamino]-7-nitrobenz-2-oxa-1,3- diazole labeling of SH2 (based on Ajtai and Burghart. 1989. Biochemistry. 28:2204-2210.), phenylmaleimide labeling of SH1, and phenylmaleimide labeling of myosin in myofibrils under rigor conditions. Each type of modified myosin inhibited the sliding of actin in motility assays. The sliding velocities of actin over copolymers of modified and unmodified myosins in the motility assay were slowest with rigor-modified myosin and most rapid with SH2-labeled myosin. The actin-activated ATPase activities of similarly copolymerized myosins were lowest with SH2-labeled myosin and highest with rigor-modified myosin. The actin-activated ATPase activities of myosin subfragment-1 obtained from these modified myosins decreased in the same linear manner with the fraction of modified heads. These results are interpreted using a model in which the sliding of actin filaments over myosin filaments decreases the probability of myosin activation by actin. The sliding velocity of actin over monomeric rigor-modified myosin exceeded that over the filamentous form, which suggests for this myosin that filament structure is important for the inhibition of actin sliding in motility assays. The fact that all cysteine modifications examined inhibited the actomyosin ATPase activities and sliding velocities of actin over myosin poses questions concerning the information about the activated crossbridge obtained from probes attached to SH1 or SH2 on myosin.

Myosin head movements are synchronous with the elementary force-generating process in muscle

Motor proteins such as myosin, dynein and kinesin use the free energy of ATP hydrolysis to produce force or motion, but despite recent progress their molecular mechanism is unknown. The best characterized system is the myosin motor which moves actin filaments in muscle. When an active muscle fibre is rapidly shortened the force first decreases, then partially recovers over the next few milliseconds. This elementary force-generating process is thought to be due to a structural 'working stroke' in the myosin head domain, although structural studies have not provided definitive support for this. X-ray diffraction has shown that shortening steps produce a large decrease in the intensity of the 14.5 nm reflection arising from the axial repeat of the myosin heads along the filaments. This was interpreted as a structural change at the end of the working stroke, but the techniques then available did not allow temporal resolution of the elementary force-generating process itself. Using improved measurement techniques, we show here that myosin heads move by about 10 nm with the same time course as the elementary force-generating process.

Spontaneous oscillation of tension and sarcomere length in skeletal myofibrils. Microscopic measurement and analysis

We have devised a simple method for measuring tension development of single myofibrils by micromanipulation with a pair of glass micro-needles. The tension was estimated from the deflection of a flexible needle under an inverted phase-contrast microscope equipped with an image processor, so that the tension development is always accompanied by the shortening of the myofibril (auxotonic condition) in the present setup. The advantage of this method is that the measurement of tension (1/30 s for time resolution and about 0.05 micrograms for accuracy of tension measurement; 0.05 microns as a spatial resolution for displacement of the micro-needle) and the observation of sarcomere structure are possible at the same time, and the technique to hold myofibrils, even single myofibrils, is very simple. This method has been applied to study the tension development of glycerinated skeletal myofibrils under the condition where spontaneous oscillation of sarcomeres is induced, i.e., the coexistence of MgATP, MgADP and inorganic phosphate without free Ca2+. Under this condition, we found that the tension of myofibrils spontaneously oscillates accompanied by the oscillation of sarcomere length with a main period of a few seconds; the period was lengthened and shortened with stretch and release of myofibrils. A possible mechanism of the oscillation is discussed.

Contraction of glycerinated rabbit slow-twitch muscle fibers as a function of MgATP concentration

We have measured the isometric tension and force-velocity relationships of glycerinated rabbit slow-twitch semimembranosus muscle as a function of MgATP concentration ([MgATP]) and have compared the results with those obtained previously from fast-twitch psoas muscle. We find that isometric tension decreases as [MgATP] increases. The magnitude of the decrease is not as great as observed in psoas. Maximum shortening velocity (Vmax) exhibits classical Michaelian saturation behavior with respect to [MgATP] with a Michaelis constant (Km) for half-maximal velocity of 18 microM and a value at saturating [MgATP] of 0.6 muscle lengths/s. Similar values were observed in fibers from soleus, another slow-twitch muscle. The corresponding values in rabbit psoas muscle are 150 microM and 1.6 lengths/s. Compared with psoas, in semimembranosus muscle Km decreases by a factor of approximately 10, whereas Vmax decreases by about a factor of 3. Thus, although in a nonphysiological regime, at low [MgATP], a "fast" muscle actually has a lower shortening velocity than a "slow" muscle.

Ca(2+)-dependence of structural changes in troponin-C in demembranated fibers of rabbit psoas muscle

The Ca(2+)-dependence of structural changes in troponin-C (TnC) has been detected by monitoring the fluorescence from TnC labeled at Methionine-25, in the NH2-terminal domain, with danzylaziridine (TnC-DANZ) and then exchanged for endogenous TnC in glycerinated single fibers. The fluorescence-pCa relation obtained from fibers stretched to a sarcomere length greater than 4.0 microns evidenced two transitions: a small one, attributable to the binding of Ca2+ to the high affinity, Ca(2+)-Mg(2+)-binding sites of TnC; and a large one, attributable to the binding of Ca2+ to the low affinity, Ca(2+)-specific binding sites of TnC. In the fluorescence-pCa relation determined with fibers set to a sarcomere length of 2.4 microns, hence obtained in the presence of cycling cross-bridges, the large transition had the same Ca(2+)-dependence as did the development of tension. These results indicate that the NH2-terminal globular domain of TnC is modified by the binding of Ca2+ to sites located in both globular domains and that the structural changes in TnC resulting from the binding of Ca2+ to the low-affinity sites, but not to the high-affinity sites, are directly associated with the triggering of contraction.

Electron microscopy of the actin-myosin head complex in the presence of ATP

The structure of the actin-myosin head complex during the ATPase cycle has been studied by electron microscopy of negatively stained acto-heavy-meromyosin. In the absence of ATP, heavy meromyosin molecules generally showed a regular, angled appearance, with both heads attached to the actin filament. In the presence of ATP, attached molecules showed a less ordered structure, often with only one head attached. We conclude that configurations other than the rigor structure occur during the actomyosin cross-bridge cycle.

Transients in orientation of a fluorescent cross-bridge probe following photolysis of caged nucleotides in skeletal muscle fibres

In muscle fibres labelled with iodoacetamidotetramethylrhodamine at Cys707 of the myosin heavy chain, the probes have been reported to change orientation when the fibre is activated, relaxed or put into rigor. In order to test whether these motions are indications of the cross-bridge power stroke, we monitored tension and linear dichroism of the probes in single glycerol-extracted fibres of rabbit psoas muscle during mechanical transients initiated by laser pulse photolysis of caged ATP and caged ADP. In rigor dichroism is negative, indicating average probe absorption dipole moments oriented more than 54.7 degrees away from the fibre axis. During activation from rigor induced by photoliberation of ATP from caged ATP in the presence of calcium, the dichroism reversed sign promptly (half-time 12.5 ms for 500 microM-ATP) upon release of ATP, but then changed only slightly during tension development 20 to 100 milliseconds later. During the onset of rigor following transfer of the fibre from an ATP-containing relaxing solution to a rigor medium lacking ATP, force generation preceded the change in dichroism. The dichroism change occurred slowly (half-time 47 s), because binding of ADP to sites within the muscle fibre limited its rate of diffusion out of the fibre. When ADP was introduced or removed, the dichroism transient was similar in time course and magnitude to that obtained after the introduction or removal of ATP. Neither adding nor removing ADP produced substantial changes in force. These results demonstrate that orientation of the rhodamine probes on the myosin head reflects mainly structural changes linked to nucleotide binding and release, rather than rotation of the cross-bridge during force generation.

Reversal of the cross-bridge force-generating transition by photogeneration of phosphate in rabbit psoas muscle fibres

1. Orthophosphate (P(i), 0.1-2.0 mM) was photogenerated within the filament lattice of isometrically contracting glycerinated fibres of rabbit psoas muscle at 10 and 20 degrees C. The P(i) was produced by laser flash photolysis of the photolabile compound 1-(2-nitrophenyl)ethylphosphate (caged P(i)). Caged P(i) caused a depression of tension that was much smaller than that caused by P(i). 2. Photolysis of caged P(i) produced a decline in isometric force composed of four phases: phase I, a lag phase (e.g. 1-4 ms at 10 degrees C) during which force did not change; phase II, an exponential decline by as much as 20% of the pre-pulse force; phase III, a partial force recovery (0-3% of the pre-pulse force); and phase IV, a further slow (0.5-3 s) decline to the steady value. Phases I, III and IV were largely independent of [P(i)] and are likely to be indirect effects caused by the caged P(i) photolysis. 3. Both the rate and amplitude of phase II depended markedly on [P(i)]. The amplitude of phase II was similar to the reduction of steady-state force by P(i). The rate of phase II increased with increasing temperature and [P(i)]. At high [P(i)] the rate began to saturate, and approached limits of 123 s-1 at 10 degrees C and 194 s-1 at 20 degrees C. 4. The rate of phase II was independent of sarcomere overlap, while the amplitude was proportional to tension at partial filament overlap. A control experiment using caged ATP showed that phase II was not produced by the photolytic by-products or the light pulse. The results suggest that phase II is associated with the force-generating transition of the cross-bridge cycle. 5. Sinusoidal length oscillations at 0.5 and 2 kHz were used to measure muscle stiffness during phase II. Stiffness declined in a single exponential phase, with the same time course as phase II of the tension transient. The change in stiffness was 83 +/- 6% (mean +/- S.E.M., n = 10, 0.5 kHz) of the change in tension when both signals were normalized to their pre-flash values. 6. Analysis of the data shows that two steps are involved in force generation and P(i) release. The non-force exerting AM-ADP-P(i) cross-bridge state first isomerizes to form a force-exerting cross-bridge state (AM'-ADP-P(i)). P(i) is then released to form a second force-generating state, AM'-ADP.(ABSTRACT TRUNCATED AT 400 WORDS)

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Regulatory light chain influences alterations of myosin head induced by actin

The effect of magnesium-for-calcium exchange and phosphorylation of regulatory light chain (LC2) on structural organization of rabbit skeletal myosin head was studied by limited tryptic digestion. In the presence of actin, exchange of magnesium bound to LC2 by calcium in dephosphorylated myosin accelerates the digestion of myosin and heavy meromyosin heavy chain and increases the accumulation of a 50 kDa fragment. This effect is significantly diminished in the case of phosphorylated myosin. Thus, both phosphorylation and cation exchange influences the effect of actin binding on the structural organization of myosin head.

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.

Quantitative model for Schädler's isometric oscillations in insect flight and cardiac muscle

Schädler and colleagues (1969, 1971) and Steiger (1977a) have found that tetanized insect fibrillar and cardiac muscles exhibit damped isometric oscillations in tension following a quick stretch. This behaviour cannot be explained by the conventional sliding filament model at full activation, or by including stretch activation in the obvious way. However, it is predicted by a sliding filament model which allows these muscles to be further activated by an increase in thin-filament tension even at high calcium levels (above 10(-5) M), providing the strength gamma of strain-activation coupling exceeds a critical value. Calculations from a comprehensive model of the actin-myosin contraction cycle suggest that this can be achieved if the phosphate release and head rotation steps are both regulated by calcium and thin-filament tension. The model also predicts a delayed tension rise following a quick release for subcritical values of gamma. Current knowledge of sarcomere structure and regulation of contractility in striated muscle indicates that this strain-activation mechanism alone cannot account for all stretch-activation phenomena, although many can be predicted if the regulatory filament is allowed to carry passive tension.

Orientational disorder and motion of weakly attached cross-bridges

In a relaxed muscle fiber at low ionic strength, the cross-bridges may well be in states comparable to the one that precedes the cross-bridge power stroke (Schoenberg, M. 1988. Adv. Exp. Med. Biol. 226:189-202). Using electron paramagnetic resonance (EPR) and (saturation transfer) electron paramagnetic resonance (ST-EPR) techniques on fibers labeled with maleimide spin label, under low ionic strength conditions designed to produce a majority of weakly-attached heads, we have established that (a) relaxed labeled fibers show a speed dependence of chord stiffness identical to that of unlabeled, relaxed fibers, suggesting similar rapid dissociation and reassociation of cross-bridges; (b) the attached relaxed heads at low ionic strength are nearly as disordered as in relaxation at physiological ionic strength where most of the heads are detached from actin; and (c) the microsecond rotational mobility of the relaxed heads was only slightly restricted compared to normal ionic strength, implying great motional freedom despite attachment. The differences in head mobility between low and normal ionic strength scale with filament overlap and are thus due to acto-myosin interactions. The spectra can be modeled in terms of two populations: one identical to relaxed heads at normal ionic strength (83%), the other representing a more oriented population of heads (17%). The spectrum of the latter is centered at approximately the same angle as the spectrum in rigor but exhibits larger (40 degrees) axial probe disorder with respect to the fiber axis. Alternatively, assuming that the chord stiffness is proportional to the fraction of attached crossbridges, the attached fraction must be even more disordered than 400, with rotational mobility nearly as great as for detached cross-bridges.

Regulation of skeletal muscle stiffness and elasticity by titin isoforms: a test of the segmental extension model of resting tension

To explore the role of titin filaments in muscle elasticity, we measured the resting tension-sarcomere length curves of six rabbit skeletal muscles that express three size classes of titin isoform. The stress-strain curves of the split fibers of these muscles displayed a similar multiphasic shape, with an exponential increase in tension at low sarcomere strain followed by a leveling of tension and a decrease in stiffness at and beyond an elastic limit (yield point) at higher sarcomere strain. Significantly, positive correlations exist between the size of the expressed titin isoform, the sarcomere length at the onset of exponential resting tension, and the yield point of each muscle. Immunoelectron microscopic studies of an epitope in the extensible segment of titin revealed a transition in the elastic behavior of the titin filaments near the yield point sarcomere length of these muscles, providing direct evidence of titin's involvement in the genesis of resting tension. Our data led to the formulation of a segmental extension model of resting tension that recognizes the interplay of three major factors in shaping the stress-strain curves: the net contour length of an extensible segment of titin filaments (between the Z line and the ends of the thick filaments), the intrinsic molecular elasticity of titin, and the strength of titin thick filament anchorage. Our data further suggest that skeletal muscle cells may control and modulate stiffness and elastic limit coordinately by selective expression of specific titin isoforms.

Simulation of stochastic processes in motile crossbridge systems

The underlying stochastic nature of many models of the actomyosin interaction should result in fluctuations in both force and shortening velocity. In classical experimental approaches involving intact or glycerinated muscle preparations these fluctuations are too small to resolve owing to the large numbers of crossbridges involved. However, new experimental techniques allow mechanical measurements to be made in systems in which small numbers of myosin heads act on a single actin filament, or small numbers of kinesin molecules act on a single tubulin filament. In these systems, stochastic effects should be evident. To understand better the nature of the expected stochastic effects, we have used computer simulation to investigate the fluctuations predicted by the original model for muscle crossbridge mechanics proposed by A.F. Huxley. We consider three situations: (1) the translation of actin or tubulin filaments by myosin or kinesin motors immobilized on a fixed substrate, (2) the production of tension by ensembles of immobilized myosin which involve the displacement of an elastic load, and (3) the fluctuations in axial displacement of a single, bipolar myosin thick filament interacting with actin filaments as in a sarcomere. In all three cases, fluctuations are clearly evident in simulations involving small numbers of motors. For case (1), we show that translation velocities can vary with crossbridge density. Whether one motor translates a filament faster, slower or at the same speed as many motors depends on the relative magnitudes of the attachment and detachment rate functions. Analytical expressions are provided to quantitate this relationship. For case (2), we show that fluctuations predicted assuming perfectly isometric conditions differ form those observed when the 'isometric state' is achieved against an elastic load. 'Elastic damping' of the fluctuations in the system results from the presence of many attached motors. In case (3) we show that in spite of the presence of stochastic fluctuations which can destabilize the uniformity of filament overlap in a sarcomere, the magnitude of thick filament displacement is less than might be anticipated over time periods of in vivo contraction. Taken together, these simulations allow one to better interpret experimental data in terms of current models of motor function.

Sub-piconewton force fluctuations of actomyosin in vitro

A new system has been developed for measuring the forces produced by a small number (less than 5-150) of myosin molecules interacting with a single actin filament in vitro. The technique can resolve forces of less than a piconewton and has a time resolution in the submillisecond range. It can thus detect fluctuations of force caused by individual molecular interactions. From analysis of these force fluctuations, the coupling between the enzymatic ATPase activity of actomyosin and the resulting mechanical impulses can be elucidated.

Cryoenzymic studies on actomyosin ATPase: kinetic evidence for communication between the actin and ATP sites on myosin

The post-ATP binding steps of myosin subfragment 1 (S1) and actomyosin subfragment 1 (actoS1) ATPases were studied at -15 degrees C with 40% ethylene glycol as antifreeze. The cleavage and release of Pi steps were studied by the rapid-flow quench method and the interaction of actin with S1 plus ATP by light scattering in a stopped-flow apparatus. At -15 degrees C, the interaction of actin with S1 remains tight, and the Km for the activation of S1 ATPase is very small (0.3 microM). The chemical data were interpreted by E + ATP----E*.ATP----E**.ADP.Pi----E*.ADP----products, where E is S1 or actoS1. In Pi burst experiments with S1, there was a large Pi burst of free Pi, but E**.ADP.Pi could not be detected. Here the predominant complex in the seconds time range is E*.ATP and in the steady-state E*.ADP. With actoS1, there was a small Pi burst of E**.ADP.Pi, evidence that the cleavage steps for S1 and actoS1 are different. From the stopped-flow experiments, the dissociation of actoS1 by ATP was complete, even at actin concentrations 60X its Km. Further, no interaction of actin with the key intermediate M*.ATP could be detected. Therefore, at -15 degrees C, actoS1 ATPase occurs by a dissociative pathway; in particular, the cleavage step appears to occur in the absence of actin.(ABSTRACT TRUNCATED AT 250 WORDS)

Assembly of avian skeletal muscle myosins: evidence that homodimers of the heavy chain subunit are the thermodynamically stable form

Using a double antibody sandwich ELISA we examined the heavy chain isoform composition of myosin molecules isolated from chicken pectoralis major muscle during different stages of development. At 2- and 40-d posthatch, when multiple myosin heavy chain isoforms are being synthesized, we detected no heterodimeric myosins, suggesting that myosins are homodimers of the heavy chain subunit. Chymotryptic rod fragments of embryonic, neonatal, and adult myosins were prepared and equimolar mixtures of embryonic and neonatal rods and neonatal and adult rods were denatured in 8 M guanidine. The guanidine denatured myosin heavy chain fragments were either dialyzed or diluted into renaturation buffer and reformed dimers which were electrophoretically indistinguishable from native rods. Analysis of these renatured rods using double antibody sandwich ELISA showed them to be predominantly homodimers of each of the isoforms. Although hybrids between the different heavy chain fragments were not detected, exchange was possible under these conditions since mixture of biotinylated neonatal rods and fluoresceinated neonatal rods formed a heterodimeric biotinylated-fluoresceinated species upon renaturation. Therefore, we propose that homodimers are the thermodynamically stable form of skeletal muscle myosin isoforms and that there is no need to invoke compartmentalization or other cellular regulatory processes to explain the lack of heavy chain heterodimers in vivo.

Mechanics of glycerinated muscle fibers using nonnucleoside triphosphate substrates

We have investigated the ability of the photoaffinity, nonnucleotide ATP analogues, 2-[(4-azido-2-nitrophenyl) amino] ethyl triphosphate (NANTP) and 2-[(4-azido-2-nitrophenyl) amino] propyl triphosphate (PrNANTP), to support active contraction in glycerinated rabbit psoas fibers. At millimolar concentrations, in the absence of calcium, both analogues relaxed fibers. In the presence of calcium, MgNANTP produced isometric tension and stiffness that were one-half to two-thirds the values obtained in MgATP. Maximum shortening velocity and the calcium-activated, myofibrillar catalyzed rate of hydrolysis were approximately the same for MgNANTP as for MgATP. With MgNANTP as the substrate, increasing concentrations of the diphosphate analogue, MgNANDP, inhibited shortening velocity but did not change isometric tension. The addition of increased concentrations of orthophosphate (P) decreased tension while shortening velocity increased. Thus, the effects of the hydrolysis products of NANTP were quite similar to those observed previously for ADP and P in the presence of MgATP. Taken together, these observations show that MgNANTP binds to, and functions in the active site of myosin in a manner quite analogous to MgATP. Thus, the aryl azido group should serve as a valid photoaffinity label for the purine portion of the active site. In contrast, MgPrNANTP, which differs from MgNANTP only in an extra CH2 spacer between the nitrophenyl ring and the triphosphate moiety did not support isometric tension or active shortening in the presence of calcium. Fiber stiffness increased in the presence of calcium and MgPrNANTP, with a calcium-activated, myofibrillar MgPrNANTPase which was about half that obtained with MgATP. Thus, in the presence of MgPrNANTP, cross-bridges appeared to be cycling through states that were attached to actin, but not producing force.

Cross-bridge kinetics in the presence of MgADP investigated by photolysis of caged ATP in rabbit psoas muscle fibres

1. The interaction between MgADP and rigor cross-bridges in glycerol-extracted single fibres from rabbit psoas muscle has been investigated using laser pulse photolysis of caged ATP (P3-1(2-nitrophenyl)ethyladenosine 5'-triphosphate) in the presence of MgADP and following small length changes applied to the rigor fibre. 2. Addition of 465 microM-MgADP to a rigor fibre caused rigor tension to decrease by 15.3 +/- 0.7% (S.E.M., n = 24 trials in thirteen fibres). The half-saturation value for this tension reduction was 18 +/- 4 microM (n = 23, thirteen fibres). 3. Relaxation from rigor by photolysis of caged ATP in the absence of Ca2+ was markedly slowed by inclusion of 20 microM-2 mM-MgADP in the photolysis medium. 4. Four phases of tension relaxation occurred with MgADP in the medium: at, a quick partial relaxation (in pre-stretch fibres); bt, a slowing of relaxation or a rise in tension for 50-100 ms; ct, a sudden acceleration of relaxation; and dt, a final, nearly exponential relaxation. 5. Experiments at varied MgATP and MgADP concentrations suggested that phase at is due to MgATP binding to nucleotide-free cross-bridges. 6. Phase bt was abbreviated by including 1-20 mM-orthophosphate (Pi) in the photolysis medium, or by applying quick stretches before photolysis or during phase bt. These results suggest that phases bt and ct are complex processes involving ADP dissociation, cross-bridge reattachment and co-operative detachment involving filament sliding and the Ca(2+)-regulatory system. 7. Stretching relaxed muscle fibres to 3.2-3.4 microns striation spacing followed by ATP removal and release of the rigor fibre until tension fell below the relaxed level allowed investigation of the strain dependence of relaxation in the regions of negative cross-bridge strain. In the presence of 50 microM-2 mM-MgADP and either 10 mM-Pi or 20 mM-2,3-butanedione monoxime, relaxation following photolysis of caged ATP was 6- to 8-fold faster for negatively strained cross-bridges than for positively strained ones. This marked strain dependence of cross-bridge detachment is predicted from the model of A. F. Huxley (1957). 8. In the presence of Ca2+, activation of contraction following photolysis of caged ATP was slowed by inclusion of 20-500 microM-MgADP in the medium. An initial decrease in tension related to cross-bridge detachment by MgATP was markedly suppressed in the presence of MgADP. 9. Ten millimolar Pi partly suppressed active tension generation in the presence of MgADP.(ABSTRACT TRUNCATED AT 400 WORDS)

Orientation of spin-labeled light chain-2 exchanged onto myosin cross-bridges in glycerinated muscle fibers

Electron paramagnetic resonance (EPR) spectroscopy has been used to study the angular distribution of a spin label attached to rabbit skeletal muscle myosin light chain 2. A cysteine reactive spin label, 3-(5-fluoro-2,4-dinitroanilino)-2,2,5,5- tetramethyl-1-pyrrolidinyloxy (FDNA-SL) was bound to purified LC2. The labeled LC2 was exchanged into glycerinated muscle fibers and into myosin and its subfragments. Analysis of the spectra of labeled fibers in rigor showed that the probe was oriented with respect to the fiber axis, but that it was also undergoing restricted rotations. The motion of the probe could be modeled assuming rapid rotational diffusion (rotational correlation time faster than 5 ns) within a "cone" whose full width was 70 degrees. Very different spectra of rigor fibers were obtained with the fiber oriented parallel and perpendicular to the magnetic field, showing that the centroid of each cone had the same orientation for all myosin heads, making an angle of approximately 74 degrees to the fiber axis. Binding of light chains or labeled myosin subfragment-1 to ion exchange heads immobilized the probes, showing that most of the motion of the probe arose from protein mobility and not from mobility of the probe relative to the protein. Relaxed labeled fibers produced EPR spectra with a highly disordered angular distribution, consistent with myosin heads being detached from the thin filament and undergoing large angular motions. Addition of pyrophosphate, ADP, or an ATP analogue (AMPPNP), in low ionic strength buffer where these ligands do not dissociate cross-bridges from actin, failed to perturb the rigor spectrum. Applying static strains as high as 0.16 N/mm2 to the labeled rigor fibers also failed to change the orientation of the spin label. Labeled light chain was exchanged into myosin subfragment-1 (S1) and the labeled S1 was diffused into fibers. EPR spectra of these fibers had a component similar to that seen in the spectra of fibers into which labeled LC2 had been exchanged directly. However, the fraction of disordered probes was greater than seen in fibers. In summary, the above data indicate that the region of the myosin head proximal to the thick filament is ordered in rigor, and disordered in relaxation.

Inhibitory influence of phosphate and arsenate on contraction of skinned skeletal and cardiac muscle

It has been widely observed that Pi decreases maximum calcium-activated force (Fmax) and calcium sensitivity of skinned skeletal and cardiac muscle. However, whether a particular ionic species of Pi (i.e., H2PO4-) is responsible for these effects is controversial. To clarify this issue, we examined the influence of Pi and its structural analogue arsenate (Asi) on contraction of skinned rabbit psoas (fast twitch), soleus (slow twitch), and cardiac papillary muscle. Asi decreased Fmax of all three muscles types to a greater extent than Pi. Both Pi and Asi decreased calcium sensitivity of psoas and cardiac muscles, with Asi having the greater effect. The effect of the protonated form of Pi and Asi on Fmax was evaluated by measuring the response to 30 mM total Pi or Asi at pH 7.4, 7.0, 6.6, and 6.2. In psoas fibers we found that both Pi and Asi were more effective in decreasing Fmax as the pH was lowered (i.e., as the concentration of the diprotonated forms increased). On the contrary, soleus and cardiac fibers did not exhibit this behavior. These differences in the effects of Pi and Asi on Fmax in psoas vs. cardiac and soleus muscles may be related to differences in their myosin heavy chains other than the binding site for the gamma-phosphate of ATP which appears to be conserved for all myosins.

Bead movement by single kinesin molecules studied with optical tweezers

Kinesin, a mechanoenzyme that couples ATP hydrolysis to movement along microtubules, is thought to power vesicle transport and other forms of microtubule-based motility. Here, microscopic silica beads were precoated with carrier protein, exposed to low concentrations of kinesin, and individually manipulated with a single-beam gradient-force optical particle trap ('optical tweezers') directly onto microtubules. Optical tweezers greatly improved the efficiency of the bead assay, particularly at the lowest kinesin concentrations (corresponding to approximately 1 molecule per bead). Beads incubated with excess kinesin moved smoothly along a microtubule for many micrometres, but beads carrying from 0.17-3 kinesin molecules per bead, moved, on average, only about 1.4 microns and then spontaneously released from the microtubule. Application of the optical trap directly behind such moving beads often pulled them off the microtubule and back into the centre of the trap. This did not occur when a bead was bound by an AMP.PNP-induced rigor linkage, or when beads were propelled by several kinesin molecules. Our results are consistent with a model in which kinesin detaches briefly from the microtubule during a part of each mechanochemical cycle, rather than a model in which kinesin remains bound at all times.

Conformational transitions within the head and at the head-rod junction in smooth muscle myosin studied with a limited proteolysis method

It was previously shown that tryptic digestion of subfragment 1 (S1) of skeletal muscle myosins at 0 degree C results in cleavage of the heavy chain at a specific site located 5 kDa from the NH2-terminus. This cleavage is enhanced by nucleotides and suppressed by actin and does not occur at 25 degrees C, except in the presence of nucleotide. Here we show a similar temperature sensitivity and protection by actin of an analogous chymotryptic cleavage site in the heavy chain of gizzard S1. The results support the view that the myosin head, in general, can exist in two different conformational states even in the absence of nucleotides and actin, and indicate that the heavy chain region 5 kDa from the NH2-terminus is involved in the communication between the sites of nucleotide and actin binding. We also show here for the first time that the S1-S2 junction in gizzard myosin can be cleaved by chymotrypsin and that this cleavage (observed in papain-produced S1 devoid of the regulatory light chain) is also temperature-dependent but insensitive to nucleotides and actin. It is suggested that the temperature-dependent alteration in the flexibility of the head-rod junction, which is apparent from these and similar observations on skeletal muscle myosin [Miller, L. & Reisler, E. (1985) J. Mol. Biol. 182, 271-279; Redowicz, M.J. & Strzelecka-Gołaszewska, H. (1988) Eur. J. Biochem. 177, 615-624], may contribute to the temperature dependence of some steps in the cross-bridge cycle.

The theory of sliding filament models for muscle contraction. II. Biochemically-based models of the contraction cycle

A seven-state sliding filament model is proposed which differs from the model of Eisenberg & Greene. It is based on a simplified version of the in-vitro contraction cycle of Stein et al., and also has some desirable dynamical features of the empirical three-state model of Nishiyama & Murase. Appropriate x-dependences for all reaction rates are derived from the transition-state theory. The seventh-state is assumed to be a high-tension intermediate of A.M.ATP, from which direct but x-dependent dissociation can occur. If the final A.M.ATP state has a sufficiently lower tension than that of A.M.ADP.Pi, then the dominant escape path from the intermediate state is shown to be direct dissociation of the actin-myosin bond. This leads to an approximate five-state model for active and relaxed muscle in which A.M and the final A.M.ATP state are omitted.

Subtilisin cleavage of actin inhibits in vitro sliding movement of actin filaments over myosin

Subtilisin cleaved actin was shown to retain several properties of intact actin including the binding of heavy meromyosin (HMM), the dissociation from HMM by ATP, and the activation of HMM ATPase activity. Similar Vmax but different Km values were obtained for acto-HMM ATPase with the cleaved and intact actins. The ATPase activity of HMM stimulated by copolymers of intact and cleaved actin showed a linear dependence on the fraction of intact actin in the copolymer. The most important difference between the intact and cleaved actin was observed in an in vitro motility assay for actin sliding movement over an HMM coated surface. Only 30% of the cleaved actin filaments appeared mobile in this assay and moreover, the velocity of the mobile filaments was approximately 30% that of intact actin filaments. These results suggest that the motility of actin filaments can be uncoupled from the activation of myosin ATPase activity and is dependent on the structural integrity of actin and perhaps, dynamic changes in the actin molecule.

Orientation of spin-labeled light chain 2 of myosin heads in muscle fibers

Electron paramagnetic resonance (e.p.r.) spectroscopy has been used to monitor the orientation of spin labels attached rigidly to a reactive SH residue on the light chain 2 (LC2) of myosin heads in muscle fibers. e.p.r. spectra from spin-labeled myosin subfragment-1 (S1), allowed to diffuse into unlabeled rigor (ATP-free) fibers, were roughly approximated by a narrow angular distribution of spin labels centered at 66 degrees relative to the fiber axis, indicating a uniform orientation of S1 bound to actin. On the other hand, spectra from spin-labeled heavy meromyosin (HMM) were roughly approximated by two narrow angular distributions centered at 42 degrees and 66 degrees, suggesting that the LC2 domains of the two HMM heads have different orientations. In contrast to S1 or HMM, the spectra from rigor fibers, in which LC2 of endogenous myosin heads was labeled, showed a random orientation which may be due to distortion imposed by the structure of the filament lattice and the mismatch of the helical periodicities of the thick and thin filaments. However, spectra from the fibers in the presence of ATP analog 5'-adenylyl imidodiphosphate (AMPPNP) were approximated by two narrow angular distributions similar to those obtained with HMM. Thus, AMPPNP may cause the LC2 domain to be less flexible and/or the S2 portion to be more flexible, so as to release the distortion of the LC2 domain and make it return to its natural position. At high ionic strength, AMPPNP disoriented the spin labels as ATP did under relaxing conditions, suggesting that the myosin head is detached from and/or weakly (flexibly) attached to a thin filament.

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP

To study the orientation of spin-labeled myosin heads in the first few seconds after the production of saturating ATP, we have used a laser flash to photolyze caged ATP during EPR data acquisition. Rabbit psoas muscle fibers were labeled with maleimide spin label, modifying 60% of the myosin heads without impairing muscle fiber biochemical and physiological activity (ATPase and force). The muscle bundles were incubated for 30 min with 5 mM caged ATP prior to the UV flash. The flash, from an excimer laser, liberated 2-3 mM ATP, generating maximum force in the presence of Ca2+ and relaxing fully in the absence of Ca2+. Control experiments, using fibers decorated with labeled myosin subfragment, showed that the flash liberates sufficient ATP to saturate myosin active sites in all regions of the muscle bundles. To increase the time resolution, and to minimize the time of the contraction, we followed in time the intensity at a single spectral position (P2), which is associated with the high degree of orientational order in rigor. ATP liberation produced a rapid decrease of P2 with liberation of ATP, indicating a large decrease in orientational order in both relaxation and contraction. This transient was absent when caged AMP was used, ruling out nonspecific effects of the UV flash and subsequent photochemistry. The steady-state level of P2 during contraction was almost as low as that reached in relaxation, although the duration of the steady state was much more brief in contraction. Upon depletion of ATP in contraction, the P2 intensity reverted to the original rigor level, accompanied by development of rigor tension. The steady-state results obtained in the brief contractions induced by caged ATP are quantitatively consistent with those obtained in longer contractions by continuously perfusing fibers with ATP. In isometric contraction, most (88% +/- 4%) of the heads are in a population characterized by a high degree of axial disorder, comparable to that observed for all heads in relaxation. Since the stiffness of these fibers in contraction is 80% of the stiffness in rigor, it is likely that most of the heads in this highly disoriented population are attached to actin in contraction and that most actin-attached heads in contraction are in this disoriented population.