[Kinetic regularities and mechanisms of action of calix[4]arene C-99 on ATPase activity of myosin subfragment-1 of myometrium]

It has been shown that calix[4]arene C-99 inhibited myosin subfragment-1 ATPase of myometrium. This inhibition is noncompetitive as to ATP and Mg2+. At the same time, this compound reduces the seeming enzymatic hydrolysis maximum rate of nucleoside triphosphate with respect to ATP and Mg2+. With the help of computer design the interaction of mentioned calix[4]arene with myosin subfragment-1 of myometrium has been investigated. Several mechanisms involved in the calix[4]arene C-99 inhibition of myosin head ATPase were supposed and participation of hydrogen, hydrophobic and electrostatic interactions in these mechanisms was discussed.

Effect of nucleotides on the orientation and mobility of myosin subfragment-1 in ghost muscle fiber

Using polarization fluorimetry, the orientation and mobility of 1,5-IAEDANS specifically bound to Cys707 of myosin subfragment-1 (S1) were studied in ghost muscle tropomyosin-containing fibers in the absence and in the presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes in the structural state of the myosin head during the ATPase cycle. Maximal differences in the probe orientation by 4 degrees and its mobility by 30% were found between actomyosin states in the presence of MgADP and MgATP. It is suggested that interaction of S1 with F-actin induces nucleotide-dependent rotation of the whole motor domain of the myosin head or only the dye-binding site and also change in the head mobility.

Inhibitory effect of ATP analogs and actin on the modification of myosin subfragment 1 with 9-anthroylnitrile

The fluorescent probe, 9-anthroylnitrile (ANN), can selectively attach to Ser-180 at the ATP-binding site of subfragment 1 (S1) of skeletal muscle myosin [J. Biol. Chem. 278 (2003) 31891]. We have found that MgATP, MgATPgammaS, MgADP.AlF(4) or MgPP(i), but not MgADP, inhibit the incorporation of ANN into S1. The inhibitory effect of the nucleotide gamma-phosphate group (or its analog) on the modification of S1 with ANN can be explained by the contribution of Ser-180 to the binding of the nucleotide gamma-phosphate at the active site of S1. We have also observed that the incorporation of ANN into S1.MgADP complex is inhibited by actin. These experimental data strongly support the existence of nucleotide-promoted conformational changes revealed by crystal structures of S1 complexes with various nucleotide analogs. They also convincingly show an effect of actin on the environment of Ser-180 at the nucleotide binding site of S1.

Effect of high hydrostatic pressure on chicken myosin subfragment-1

Hydrostatic pressure-induced structural changes in subfragment-1 (S1) of myosin molecule were studied. ATP-induced emission spectra of S1 were used to detect global structural change of S1 by pressure treatment. The fluorescence intensity of unpressurized S1 increased by addition of ATP. The increment of fluorescence of pressurized S1 up to 150 MPa was almost the same as control, whereas it became smaller above 200 MPa. ATP binding ability of S1 examined using 1, N(6)-ethenoadenosine 5'-diphosphate (epsilon -ADP) indicated that the binding of epsilon -ADP to S1 decreased in the range of 250-300 MPa. S1 pressurized below 250 MPa and unpressurized S1 similarly bound to F-actin, although binding of S1 pressurized above 250 MPa decreased. Electron microscopic observation revealed arrowhead structure in control acto-S1, while disordered arrowhead structure was observed in acto-S1 prepared from pressurized S1 at 300 MPa. S1 pressurized below 250 MPa retained the same actin activated ATPase activity as the control, whereas the activity decreased to 60% at 300 MPa. Pressure treated S1 was easily cleaved by tryptic digestion into three domains, i.e. 27 kDa (N-terminal), 50 and 20 kDa (C-terminal) fragments, which were the same as those in unpressurized one. It is concluded that pressure-induced global structural changes of S1 begin to occur about 150 MPa, and the local structural changes in ATPase and actin binding sites followed with elevating pressure to 250-300 MPa.

Interaction of the N-terminal part of the A1 essential light chain with the myosin heavy chain

The kinetics of actin-dependent MgATPase activity of skeletal muscle myosin subfragment 1 (S1) isoform containing the A1 essential light chain differ from those of the S1 isoform containing the A2 essential light chain. The differences are due to the presence of the extra N-terminal peptide comprising 42 amino acid residues in the A1 light chain. This peptide can interact with actin; heretofore, there have no been reports of the direct interaction between this peptide and the heavy chain of S1. Here, using the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and S. aureus V8 protease, we show for the first time that the N-terminal part of the A1-light chain can interact with the 22-kDa fragment of the S1 heavy chain. No such interaction has been observed for the S1(A2) isoenzyme. Localization of residues which can possibly react with the cross-linker suggests that the interaction might involve the N-terminal residues of the A1 light chain and the converter region of the heavy chain.

The identification of tryptophan residues responsible for ATP-induced increase in intrinsic fluorescence of myosin subfragment 1

ATP binding to myosin subfragment 1 (S1) induces an increase in tryptophan fluorescence. Chymotryptic rabbit skeletal S1 has 5 tryptophan residues (Trp113, 131, 440, 510 and 595), and therefore the identification of tryptophan residues perturbed by ATP is quite complex. To solve this problem we resolved the complex fluorescence spectra into log-normal and decay-associated components, and carried out the structural analysis of the microenvironment of each tryptophan in S1. The decomposition of fluorescence spectra of S1 and S1-ATP complex revealed 3 components with maxima at ca. 318, 331 and 339-342 nm. The comparison of structural parameters of microenvironment of 5 tryptophan residues with the same parameters of single-tryptophan-containing proteins with well identified fluorescence properties applying statistical method of cluster analysis, enabled us to assign Trp595 to 318 nm, Trp440 to 331 nm, and Trp 13, 131 and 510 to 342 nm spectral components. ATP induced an almost equal increase in the intensities of the intermediate (331 nm) and long-wavelength (342 nm) components, and a small decrease in the short component (318 nm). The increase in the intermediate component fluorescence most likely results from an immobilization of some quenching groups (Met437, Met441 and/or Arg444) in the environment of Trp440. The increase in the intensity and a blue shift of the long component might be associated with conformational changes in the vicinity of Trp510. However, these conclusions can not be extended directly to the other types of myosins due to the diversity in the tryptophan content and their microenvironments.

Influence of ADP on cross-bridge-dependent activation of myofibrillar thin filaments

Contraction of skeletal muscle is regulated by calcium at the level of the thin filament via troponin and tropomyosin. Studies have indicated that strong cross-bridge binding is also involved in activation of the thin filament. To further test this, myofibrils were incubated with a wide range of fluorescent myosin subfragment 1(fS1) at pCa 9 or pCa 4 with or without ADP. Sarcomere fluorescence intensity and the fluorescence intensity ratio (non-overlap region/overlap region) were measured to determine the amount and location of bound fS1 in the myofibril. There was lower sarcomere fluorescence intensity with ADP compared to without ADP for both calcium levels. Similar data were obtained from biochemical measures of bound fS1, validating the fluorescence microscopy measurements. The intensity ratio, which is related to activation of the thin filament, increased with increasing [fS1] with or without ADP. At pCa 9, the fluorescence intensity ratio was constant until 80-160 nM fS1 without ADP conditions, then it went up dramatically and finally attained saturation. The dramatic shift of the ratio demonstrated the cooperative character of strong cross-bridge binding, and this was not observed at high calcium. A similar pattern was observed with ADP in that the ratio was right-shifted with respect to total [fS1]. Saturation was obtained with both the fluorescence intensity and ratio data. Plots of intensity ratio as a function of normalized sarcomere intensity (bound fS1) showed little difference between with and without ADP. This suggests that the amount of strongly bound fS1, not fS1 state (with or without ADP) is related to activation of the thin filament.

Effect of nucleotides and actin on the orientation of the light chain-binding domain in myosin subfragment 1

The X-ray structure of myosin head (S1) reveals the presence of a long alpha-helical structure that supports both the essential and the regulatory light chains. It has been proposed that small structural changes in the catalytic domain of S1 are amplified by swinging the long alpha-helix (the "lever arm") to produce approximately 11 nm steps. To probe the spatial position of the putative lever in various S1 states, we have measured, by fluorescence resonance energy transfer (FRET), the effect of nucleotides and actin on the distances between Cys-177 of the essential light chain A1 (which is attached to the alpha-helix) and three loci in the catalytic domain. Cys-177 (donor) was labeled with 1,5-IAEDANS. The trinitrophenylated ADP analog (TNP-ADP, acceptor) was used to measure the distance to the active site. Lys-553 at the actin-binding site, labeled with a fluorescein derivative, and Lys-83 modified with trinitrobenzenesulfonic acid served as two other acceptors. FRET measurements were performed for S1 alone, for its complexes with MgADP and MgATP, for the analogs of the transition state of the ATPase reaction, S1.ADP.BeFx, S1.ADP.AlF4, and S1.ADP.VO4, and for acto-S1 in the absence and in the presence of ADP. When the transition state and acto-S1 complexes were formed, the change in the Cys-177 --> Lys-83 distance was <1.1 A, for the distance Cys-177 --> Lys-553, the change was +/-2.5 A. These distance changes correspond to rotations by <10 degrees and approximately 25 degrees, respectively. For the Cys-177 --> TNP-ADP the interprobe separation decreased by approximately 6 A in the presence of BeFx and AlF4- but only 1.9 A in the presence of vanadate; we do not interpret the 6 A change as resulting from the lever rotation. Using the coordinates of the acto-S1 complex, we have computed the expected changes in these distances resulting from rotation of the lever. These changes were much greater than the ones observed. The above results are inconsistent with models of force generation by S1 in which the head assumes two distinct conformations characterized by large differences in the angle between the motor and the light chain-binding domain. Several alternative mechanisms are proposed.

Effect of complexes of ADP and phosphate analogs on the conformation of the Cys707-Cys697 region of myosin subfragment 1

Recent crystallographic studies have suggested structural differences between the complexes of S1.Mg.ADP with the phosphate analogs aluminium fluoride (AlF4-), vanadate (VO(4)3-) and beryllium fluoride (BeFx) [Fisher, A. J., Smith, C. A., Thoden, J. B., Smith, R., Sutoh, K., Holden, H. M. & Rayment, I. (1995) Biochemistry 34, 8960-8972; Smith, R. & Rayment, I. (1996) Biochemistry 35, 5404-5417]. In this work, chemical modifications, namely labeling of Cys707 (the reactive SH1 thiol) and Cys707-Cys697 (SH1-SH2) cross-linking, were used to compare the S1.ADP.BeFx, S1.ADP. AlF4- and S1.ADP-VO(4)3- complexes with specific states of the myosin-ATPase pathway. Modification of Cys707 with the fluorescent monofunctional reagents 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin and N-iodoacetyl-N'-(5-sulfo-1-naphtyl)ethylenediamine has shown that the reactivity of the SH1 group depends on the nucleotide bound to S1. The observed rates of Cys707 modification at 20 degrees C lead to the conclusion that S1.ADP.BeFx is similar to S1*.ATP, while S1.ADP.AlF4- and S1.ADP.VO(4)3- are more similar to S1**.ADP.Pi. The conformations of the analog states were also compared by monitoring the dissociation of the fluorescent nucleotide analog 1-N6-ethenoadenosine diphosphate (ADP[C2H2]) from the active site of Cys707-modified (by N-ethylmaleimide) and Cys707-Cys697-cross-linked (by N,N'-p-phenylene dimaleimide) S1.ADP[C2H2].AlF4- and S1.ADP[C2H2]. BeFx. Our results suggest that the conformations of the S1.ADP.AlF4-, S1.ADP.VO(4)3- and S1.ADP.BeFx complexes in the Cys707-Cys697 region are distinct from each other, with the former two at least partially resembling the S1**.ADP.Pi state, while the latter is similar to the prehydrolyzed S1*.ATP state.

Spare the rod, spoil the regulation: necessity for a myosin rod

Regulation of a variety of cellular contractile events requires that vertebrate smooth and non-muscle myosin II can achieve an "off" state. To examine the role of the myosin rod in this process, we determined the minimal size at which a myosin molecule is capable of regulation via light chain phosphorylation. Expressed smooth muscle myosin subfragments with as many as 100 amino acids of the coiled-coil rod sequence did not dimerize and were active independently of phosphorylation. To test whether dimerization per se restores regulation of ATPase activity, mutants were expressed with varying lengths of rod sequence, followed by C-terminal leucine zippers to stabilize the coiled-coil. Dimerization restored partial regulation, but the presence of a length of rod approximately equal to the myosin head was necessary to achieve a completely off state. Partially regulated short dimers could be converted into fully regulated molecules by addition of native rod sequence after the zipper. These results suggest that the myosin rod mediates specific interactions with the head that are required to obtain the completely inactive state of vertebrate smooth and non-muscle myosins. If these interactions are prohibited under cellular conditions, unphosphorylated crossbridges can slowly cycle.

Chemomechanical transduction in the actomyosin molecular motor by 2',3'-dideoxydidehydro-ATP and characterization of its interaction with myosin subfragment 1 in the presence and absence of actin

The effect of torsional freedom about the N-glycoside bond of ATP in the ability of the nucleoside triphosphate to support chemomechanical transduction (Takenaka et al., 1978) has been investigated by examining the ability of the nucleotide analogue 2',3'-dideoxy-2',3'-didehydro-ATP (1b, enf-ATP) to act as a substrate for myosin subfragment 1 in the presence and absence of actin and to support actin sliding in the standard in vitro motility assay. By converting the ribosyl ring of the natural substrate to the rigid and almost planar enofuranosyl ring, effects on torsional freedom about the N-glycoside bond due to changes in ribosyl ring pucker and/or by steric interferences of the protons attached to the 2' and 3' carbons are eliminated allowing for increased torsional freedom about the N-glycoside bond. The data indicate that this enofuranosyl analogue is an excellent substrate for subfragment 1 and actosubfragment 1 and produces actin sliding velocities which are twice as fast as those observed with ATP in the standard in vitro motility assay. The analogue diphosphate is trapped in S1 by the common P(i) analogues, but the rate of formation of the ternary complex formed with Vi is very slow compared to that observed with MgADP. Similar conformations of S1 are formed with Mg.enf-ATP and MgATP under steady-state conditions, but S1 with bound Mg.enf-ADP differs significantly from that observed with MgADP.

Light-directed generation of the actin-activated ATPase activity of caged heavy meromyosin

An understanding of the molecular mechanism of muscle contraction will require a complete description of the kinetics of the myosin motor in vitro and in vivo. To this end chemical relaxation studies employing light-directed generation of ATP from caged ATP have provided detailed kinetic information in muscle fibers. A more direct approach would be to trigger the actin-activated ATPase activity from a caged myosin, i.e., myosin whose activity is blocked upon derivatization with a photolabile protection group. Herein we report that a new type of caged reagent can be used to prepare a caged heavy meromyosin by modification of critical thiol groups, i.e., a chemically modified motor without activity that can be reactivated at will using a pulse of near-ultraviolet light. Heavy meromyosin modified at Cys-707 with the thiol reactive reagent 1-(bromomethyl)-2-nitro-4,5-dimethoxybenzene does not exhibit an actin-activated ATPase activity and may be viewed as a caged protein. Absorption spectroscopy showed that the thioether bond linking the cage group to Cys-707 is cleaved following irradiation (340-400 nm) via a transient aci-nitro intermediate which has an absorption maximum at 440 nm and decays with a rate constant of 45.6 s(-1). The in vitro motility assay showed that caged heavy meromyosin cannot generate the force necessary to move actin filaments although following irradiation of the image field with a 30 ms pulse of 340-400 nm light the caged group was removed with the concomitant movement of most filaments at a velocity of 0.5-2 micron/s compared to 3-4 micron/s for unmodified HMM. The specificity and simplicity of labeling myosin with the caged reagent should prove useful in studies of muscle contraction in vivo.

Extensively methylated myosin subfragment-1: examination of local structure, interactions with nucleotides and actin, and ligand-induced conformational changes

The atomic structure of myosin subfragment-1 (S1) has been recently solved for crystals of extensively methylated S1 [Rayment et al. (1993) Science 261, 50-58]. In this study, the effect of such a modification on S1 structure and function was examined. According to the far- and near-ultraviolet CD spectra, the methylation does not affect the secondary structure of S1 but causes limited changes in its tertiary structure. The methylation significantly decreases the affinity of S1 for actin in rigor and, to a lesser degree, that of S1 to actin in the presence of MgATP gamma S. This modification, like the trinitrophenylation of Lys-83, accelerates the dissociation of a nucleotide trapped on S1 either by phosphate analogs or by cross-linking of the SH1 and SH2 thiols. Methylation strongly impairs the coupling between the actin- and nucleotide-binding sites as revealed by the reduced effect of actin on the release of epsilon ADP from the active site. It also causes a complete loss of in vitro motility of actin filaments over methylated HMM. In addition to this, methylation also impairs the communication between other sites on S1 including that between the nucleotide-binding site and SH1, and the actin-binding site and the 27/50 kDa junction and a site at 74 kDa from the N-terminus of S1. These changes are revealed in SH1 modification, thermolysin digestion, and vanadate-dependent photocleavage experiments, respectively. The increased rate of thermal denaturation of S1 and the loss of S1 protection by ADP and actin from this process also indicate flawed communications in methylated S1. It is concluded that these relatively mild but numerous and important changes impair the function of methylated S1.

Vanadate-induced changes in myosin subfragment-1 from cardiac muscle

The interaction between myosin subfragment-1 from bovine cardiac muscle (CS1) and 1,N6-ethenoadenosine diphosphate (epsilon ADP) was studied using steady-state and time-resolved fluorescence methods. The binding constant was found to be 1.52 x 10(6) M-1 at pH 7.5 and 5 degrees C. The intensity decay of epsilon ADP bound to CS1 was resolved into two components over a narrow range of temperatures. The long component was about 22 ns and the short component was between 5 and 7 ns, with fractional amplitudes of about 0.6-0.7 for the long component and 0.3-0.4 for the short component. These data suggest a two-state temperature-sensitive transition of the CS1.epsilon ADP complex. In the presence of orthovanadate (Vi) at 5 degrees C, the decay time of the long component was little affected, whereas the short decay time increased by over 3 ns and the fractional amplitude of the long component decreasing by a factor of 2 to about 0.3 and that of the short component increasing to 0.7. The anisotropy decay of bound epsilon ADP was monoexponential regardless of whether vanadate was present. The recovered single rotational correlation time was 110 ns in the absence of vanadate and 79 ns in the presence of vanadate. The decrease in correlation time suggests an increase in molecular symmetry of the CS1.epsilon ADP.Vi complex. The results are in agreement with previous results obtained from skeletal S1 and indicate that S1 from both isoforms of myosin experiences similar vanadate-induced changes in its hydrodynamic shape. Since the ternary vanadate complex is a stable analogue of the S1.ADP.Pi state, the ligand-induced change in hydrodynamic shape of S1 may be related to the conformational change which myosin head experiences during the ATPase cycle and this change in myosin could be a structural basis for force generation in striated muscle.

Effect of protein kinase inhibitor H-7 on the contractility, integrity, and membrane anchorage of the microfilament system

Addition of protein kinase inhibitor H-7 leads to major changes in cell structure and dynamics. In previous studies [Citi, 1992: J. Cell Biol. 117:169-178] it was demonstrated that intercellular junctions in H-7-treated epithelial cells become calcium independent. To elucidate the mechanism responsible for this effect we have examined the morphology, dynamics, and cytoskeletal organization of various cultured cells following H-7-treatment. We show here that drug treated cells display an enhanced protrusive activity. Focal contact-attached stress fibers and the associated myosin, vinculin, and talin deteriorated in such cells while actin, vinculin, and N-cadherin associated with cell-cell junctions were retained. Furthermore, we demonstrate that even before these cytoskeletal changes become apparent, H-7 suppresses cellular contractility. Thus, short pretreatment with H-7 leads to strong inhibition of the ATP-induced contraction of saponin permeabilized cells. Comparison of H-7 effects with those of other kinase inhibitors revealed that H-7-induced changes in cell shape, protrusional activity, and actin cytoskeleton structure are very similar to those induced by selective inhibitor of myosin light chain kinase, KT5926. Specific inhibitors of protein kinase C (Ro31-8220 and GF109203X), on the other hand, did not induce similar alterations. These results suggest that the primary effect of H-7 on cell morphology, motility, and junctional interactions may be attributed to the inhibition of actomyosin contraction. This effect may have multiple effects on cell behavior, including general reduction in cellular contractility, destruction of stress fibers, and an increase in lamellipodial activity. It is proposed that this reduction in tension also leads to the apparent stability of cell-cell junctions in low-calcium medium.

Formation of the stable myosin-ADP-aluminum fluoride and myosin-ADP-beryllium fluoride complexes and their analysis using 19F NMR

The effects of aluminum fluoride and beryllium fluoride on smooth muscle myosin and its subfragments were studied. Mg(2+)-ATPase activity was inhibited in the presence of aluminum fluoride (beryllium fluoride). [3H]ADP bound to heavy meromyosin (HMM) in the presence of aluminum fluoride (beryllium fluoride) and was not dissociated after 3 days of dialysis demonstrating that [3H]ADP was trapped in HMM. These results suggest the formation of a stable HMM-ADP-fluoroaluminate (fluoroberyllate) complex. The intrinsic tryptophane fluorescence intensity was increased in the presence of ADP and aluminum fluoride (beryllium fluoride). Acto-S1 was dissociated upon the formation of S1-ADP-fluoroberyllate and actin destabilized S1-ADP-fluoroberyllate complex, while S1-ADP-fluoroaluminate failed to bind to actin. Furthermore, when S1 formed the complex with actin, nucleotide trapping did not occur in the presence of fluoraluminate. These results indicated that the myosin-ADP-fluoroberyllate complex resembles a weak binding state while myosin-ADP-fluoroaluminate complex is a distinct conformation although the binding to actin was also weak. The structure of the ternary complex was investigated using 19F NMR. The 19F NMR spectrum of the S1-ADP-fluoroaluminate complex showed a peak at -66.7 ppm which is due to the binding of fluoraluminate to S1. The peak was not observed when 5'-adenylylimidodiphosphate was substituted for ADP suggesting that aluminum fluoride plays a role as a phosphate analogue. The stoichiometry of the bound fluoride was determined to be 3.8 mol/mol S1 suggesting that the bound species is AlF-4.

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.

Inhibition of actin-activated myosin Mg(2+)-ATPase in smooth muscle by ruthenium red

Ruthenium red was found to inhibit actin-activated myosin Mg(2+)-ATPase in smooth muscle and to bind to myosin heavy chain, but not to F-actin. The inhibition by Ruthenium red of actin-activated Mg(2+)-ATPase was of the competitive type with respect to actin (Ki 4.4 microM) and of the non-competitive type with respect to ATP (Ki 6.6 microM). However, Ruthenium red scarcely dissociated the acto-heavy meromyosin complex during the ATPase reaction. These results suggest that Ruthenium red interacts directly with the binding site for F-actin on the myosin heavy chain. This site is considered to be necessary not for maintaining the binding affinity of myosin for F-actin, but for activation of the Mg(2+)-ATPase.

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.

Troponin I and caldesmon restrict alterations in actin structure occurring on binding of myosin subfragment 1

The effect of troponin I and caldesmon on phalloidin-rhodamine- and 1,5-IAEDANS-labelled actin in skeletal muscle ghost fibers was investigated by polarized fluorescence. Both these proteins inhibited the structural alterations in the actin monomer and the increase of flexibility of actin filaments occurring on binding of myosin heads, and their effects were potentiated by tropomyosin. This immobilization of the actin filament through troponin I and caldesmon seems to originate from restriction of the relative motions of the two domains within the monomer.