The myosin head can bind two actin monomers

Measuring Rotations of a Few Cross-Bridges in Skeletal Muscle

The ability to measure properties of a single cross-bridge in working muscle is important because it avoids averaging the signal from a large number of molecules and because it probes cross-bridges in their native crowded environment. Because the concentration of myosin in muscle is large, observing the kinetics of a single myosin molecule requires that the signal be collected from small volumes. The introduction of small observational volumes defined by diffraction-limited laser beams and confocal detection has made it possible to limit the observational volume to a femtoliter (10 15 liter). By restraining labeling to 1 fluorophore per 100 myosin molecules, we were able to follow the kinetics of approximately 400 cross-bridges. To reduce this number further, we used two-photon (2P) microscopy. The focal plane in which the laser power density was high enough to produce 2P absorption was thinner than in confocal microscopy. Using 2P microscopy, we were able to observe approximately 200 cross-bridges during contraction. The novel method of confocal total internal reflection (CTIR) provides a method to reduce the observational volume even further, to approximately 1 attoliter (10 18 liter), and to measure fluorescence with a high signal-to-noise (S/N) ratio. In this method, the observational volume is made shallow by illuminating the sample with an evanescent field produced by total internal reflection (TIR) of the incident laser beam. To guarantee the small lateral dimensions of the observational volume, a confocal aperture is inserted in the conjugate-image plane of the objective. With a 3.5-μm confocal aperture, we achieved a volume of 1.5 attoliter. Association-dissociation of the myosin head was probed with rhodamine attached at cys707 of the heavy chain of myosin. Signal was contributed by one to five fluorescent myosin molecules. Fluorescence decayed in a series of discrete steps, corresponding to bleaching of individual molecules of rhodamine. The S/N ratio was sufficiently large to make statistically significant comparisons from rigor and contracting myofibrils.

Parking problem and negative cooperativity of binding of myosin subfragment 1 to F-actin

Previously we provided evidence that myosin subfragment 1 (S1) can bind either one (state 1) or two actin monomers (state 2) in solution and in muscle fiber. Here we present results of the kinetics study of binding of S1 to F-actin labeled with fluorescent dye pyrene. A transition from state 1 to state 2 depends on probability that the second actin is free, which is high when molar ratio of S1/actin (R) is less than 0.5, and it decreases dramatically when R>2.0 due to the parking problem. The kinetics data obtained at different molar ratios were well fitted by two binding states model. The sequential binding of myosin head initially with one actin monomer and then with the second actin monomer in F-actin can play a key role in force generation by actin-myosin and their directed movement.

Mechanism of formation of actomyosin interface

Force generation in muscle results from binding of myosin to F-actin. ATP binding to myosin provides energy to dissociate actomyosin complex while the hydrolysis of ATP is needed for re-binding of myosin to F-actin. At the end of each cycle myosin and actin form a tight complex with a substantial interface area. We investigated the dynamics of formation of actomyosin interface in presence and absence of nucleotides by quenched flow cross-linking technique. We showed previously that myosin head (subfragment 1, S1) directly interacts with at least two monomers in the actin filament. The quenched flow cross-linking experiments revealed that the initial contact (in presence or absence of nucleotides) occurs between loop 635-647 of S1 and 1-12 N-terminal residues of one actin and, then, the second contact forms between loop 567-574 of S1 and the N terminus of the second actin. The distance between these two loops in S1 corresponds to the distance between N termini of two actins in the same strand (53 A) but is smaller than that between two actins from the different strands (102 A). The formation of the actomyosin complex proceeds in ordered sequence: S1 initially binds to one actin then binds with the second actin located in the same strand but probably closer to the barbed end of F-actin. The presence of nucleotides slows down the interaction of S1 with the second actin, which correlates with recently proposed cleft movement in a 50 kDa domain of S1. The sequential mechanism of formation of actomyosin interface starting from one end and developing towards the barbed end might be involved in force generation and directional movement in actin-myosin system.

Interaction of myosin with F-actin: time-dependent changes at the interface are not slow

The kinetics of formation of the actin-myosin complex have been reinvestigated on the minute and second time scales in sedimentation and chemical cross-linking experiments. With the sedimentation method, we found that the binding of the skeletal muscle myosin motor domain (S1) to actin filament always saturates at one S1 bound to one actin monomer (or two S1 per actin dimer), whether S1 was added slowly (17 min between additions) or rapidly (10 s between additions) to an excess of F-actin. The carbodiimide (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, EDC)-induced cross-linking of the actin-S1 complex was performed on the subsecond time scale by a new approach that combines a two-step cross-linking protocol with the rapid flow-quench technique. The results showed that the time courses of S1 cross-linking to either of the two actin monomers are identical: they are not dependent on the actin/S1 ratio in the 0.3-20-s time range. The overall data rule out a mechanism by which myosin rolls from one to the other actin monomer on the second or minute time scales. Rather, they suggest that more subtle changes occur at the actomyosin interface during the ATP cycle.

Actin-related proteins in Actinobacillus pleuropneumoniae and their interactions with actin-binding proteins

A group of prokaryotic actin-related proteins (PARP) with an Mr of 43000 was detected in Actinobacillus pleuropneumoniae. These proteins were enriched by a depolymerization/polymerization cycle, under similar conditions to those used to polymerize muscle actin, and purified by affinity chromatography on a DNase I-Sepharose column. Three isoforms of A. pleuropneumoniae PARP (Ap-PARP) with pI values of 5.8, 6.15 and 6.2 were detected. Ap-PARP were recognized by four different anti-actin antibodies (one anti-muscle and three anti-cytoplasmic isoforms). Ap-PARP were also recognized by antibodies against Anabaena variabilis PARP (Av-PARP) and against actin-binding proteins such as alpha-actinin and spectrin, and also by a monoclonal antibody against heat-shock cognate protein 70 (Hsc70). Specific binding of phalloidin to Ap-PARP was detected both in permeabilized cells and in vitro. Purified Ap-PARP can polymerize under similar conditions to those required for skeletal muscle actin polymerization and the filaments formed appear to be decorated with myosin subfragment-1(S1) as observed by transmission electron microscopy. The amino acid composition of Ap-PARP revealed more similarities to muscle gamma-actin and the cytoplasmic beta-actin isoform than to eukaryotic actin-related proteins.

Orientation of cross-bridges in skeletal muscle measured with a hydrophobic probe

Cis-parinaric acid (PA) binds to a hydrophobic pocket formed between the heavy chain of myosin subfragment-1 (S1) and the 41-residue N-terminal of essential light chain 1 (A1). The binding is strong (Ka = 5.6 x 10(7) M-1) and rigid (polarization = 0.334). PA does not bind to myofibrils in which A1 has been extracted or replaced with alkali light chain 2 (A2). As in the case of S1 labeled with other probes, polarization of fluorescence of S1-PA added to myofibrils depended on fractional saturation of actin filament with S1, i.e., on whether the filaments were fully or partially saturated with myosin heads. Because fluorescence quantum yield of PA is enhanced manyfold upon binding, and because PA binds weakly to myofibrillar structures other then A1, the dye is a convenient probe of cross-bridge orientation in native muscle fibers. The polarization of a fiber irrigated with PA was equal to the polarization of S1-PA added to fibers at nonsaturating concentration. Cross-linking of S1 added to fibers at nonsaturating concentration showed that each S1 bound to two actin monomers of a thin filament. These results suggest that in rigor rabbit psoas muscle fiber each myosin cross-bridge binds to two actins.

Small segmental rearrangements in the myosin head can explain force generation in muscle

Poisson-Boltzmann calculations of the distribution of electrostatic potentials around an actin filament in physiological-strength solutions show that negative isopotential surfaces protrude into the solvent. Each protrusion follows the actin two-start helix and is located on the sites implicated in the formation of the actomyosin complex. Molecular dynamic calculations on the S1 portion of the myosin molecule indicate that in the presence of ATP the crystallographically invisible loops (comprising residues 624-649 and 564-579) remain on the surface, whereas in the absence of ATP they can move toward the actin-binding sites and experience electrostatic forces that range from 1 to 10 pN. The molecular dynamics calculations also suggest that during the ATP cycle there exist at least three states of electrostatic interactions between the loops and actin. Every time a new interaction is formed, the strain in the myosin head increases and the energy of the complex decreases by 2kT to 5kT. This can explain muscular contraction in terms of a Huxley-Simmons-type mechanism, while requiring only rearrangements of small mobile S1 segments rather than the large shape changes in the myosin molecule postulated by the conventional tilting head model.

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

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

Structure of the 265-kilodalton complex formed upon EDC cross-linking of subfragment 1 to F-actin

The conventional model of force generation in muscle requires the presence of at least two different contact areas between the myosin head (S1) and the actin filament. It has been found that S1 has two sites available for carbodiimide cross-linking, but it is generally believed that the myosin head can be cross-linked to only one actin through either site. We provide here, for the first time, evidence that one S1 can be cross-linked to two separate actin molecules. The covalent complex of one S1 with two actins was found to have an apparent molecular mass of 265 kDa. The formation of the 265-kDa acto-S1 complex was strongly dependent on the ratio of S1 to actin. Limited tryptic digestion converted the 265-kDa product into the 240-kDa complex by releasing a 27-kDa N-terminal S1 fragment. Limited subtilisin digestion of the 265-kDa covalent acto-S1 complex yielded 29-, 93-, and 66-kDa peptides which corresponded to the 29-kDa N-terminal domain of S1, actin-44-kDa (central domain of S1) and actin-22-kDa (C-terminal domain of S1) complexes, respectively. These peptides could be generated only if a single S1 has been cross-linked to two separate actins. The 265-kDa acto-S1 complex (S1:actin ratio = 0.5) had 60% of the ATPase activity of the 175-185-kDa acto-S1 complex (S1:actin ratio = 1).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Cooperativity in F-actin: chemical modifications of actin monomers affect the functional interactions of myosin with unmodified monomers in the same actin filament

We have chemically modified a fraction of the monomers in actin filaments, and then measured the effects on the functional interaction of myosin with unmodified monomers within the same filament. Two modifications were used: (a) covalent attachment of various amounts of myosin subfragment-1 (S1) with the bifunctional reagent disuccinimidyl suberate and (b) copolymerization of unmodified actin monomers with monomers cross-linked internally with 1-ethyl-3-(dimethylaminopropyl)-carbodiimide. Each of these modifications abolished the interaction of the modified monomers with myosin, so the remaining interactions were exclusively with unmodified monomers. The two modifications had similar effects on the interaction of actin with myosin in solution: decreased affinity of myosin heads for unmodified actin monomers, without a change in the Vmax of actin-activated myosin ATPase activity. However, modification (b) produced much greater inhibition of actin sliding on a myosin-coated surface, as measured by an in vitro motility assay. These results provide insight into the functional consequences of cooperative interactions within the actin filament.

Binding of myosin subfragment-1 to F-actin

During a part of the hydrolytic cycle, myosin head (S1) carries no nucleotide and binds strongly to an actin filament forming a rigor bond. At saturating concentration of S1 in rigor, S1 is well known to form 1:1 complex with actin. However, we have provided evidence that under certain conditions S1 could also form a complex with 2 actin monomers in a filament (Andreev, O.A. & Borejdo, J. (1991) Biochem. Biophys. Res. Comm. 177, 350-356). This view was recently challenged by Carlier & Didry (Carlier, M-F. & Didry, D. (1992) Biochem. Biophys. Res. Comm. 183, 970-974) who interpreted our data by suggesting that F-actin underwent a simple depolymerization and implied that, when only actin in the F-form was scored, the real stoichiometry in our experiments was 1:1. We show here that under conditions of our experiments less than 8% of actin was depolymerized. Moreover, we have repeated the experiments in the presence of phalloidin and show that under these conditions too, when S1 was added slowly to a fixed concentration of F-actin, it formed a different complex with F-actin than when it was added quickly. This confirms our original conclusion that S1 can bind actin in two different ways and shows that depolymerization of F-actin is not responsible for this finding.

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.

Interaction of myosin subfragment-1 with F- and G-actin in equilibrium

The structural changes of the F-actin-myosin head (S1) complex during the cross-bridge cycle are essential in muscle contraction. Although a large body of evidence has accumulated showing that the actin: S1 stoichiometry in the decorated F-actin-S1 filament is 1:1 at saturation by S1, a recent report by Andreev and Borejdo (1991, Biochem. Biophys. Res. Comm. 177, 350-356) indicated that under some conditions, the actin: S1 stoichiometry could be 2:1 at saturation by S1. Because of the important implications of this result in the mechanism of acto-myosin motility, we have re-investigated this issue. It is shown here that evidence for the 2:1 stoichiometry was circumstantial and was only observed under conditions where 50% of the actin was F-actin, i.e. at a total actin concentration twice as large as the critical concentration. The interaction of S1 with both F- and G-actin in dynamic equilibrium is studied in detail. The present data fully support the 1:1 actin: S1 stoichiometry in the decorated filament at saturation by S1.