Studies of adenosine triphosphatase activity and turbidity in myofibril and actomyosin suspensions

Role of sequence 18-29 on actin in actomyosin interactions

Affinity-purified polyclonal antibodies prepared against a synthetic peptide corresponding to sequence 18-29 from the N-terminus of rabbit alpha-skeletal actin reacted with G- and F-actin. Epitope mapping experiments with thrombin and hydroxylamine cleaved actin, and immunochemical assays verified the specificity of antibodies for the 18-29 sequence on actin. The binding of up to 0.5 mol of IgG per mole of actin did not affect the rigor binding of myosin subfragment 1 (S-1) to actin. Similarly, the binding of IgG to actin was not changed by a complete saturation of actin by S-1. In contrast to this, the weak acto-S-1 interactions in the presence of ATP were strongly inhibited by the 18-29 antibodies. At 25 degrees C, the acto-S-1 ATPase activity was inhibited by IgG stronger than the binding of S-1.ATP gamma S to actin. Thus, at this temperature, a catalytic inhibition of the acto-S-1 system appears to account at least in part for the antibody effect. Acto-S-1 ATPase activities at 25 degrees C were inhibited also by F(ab)(18-29). At 5 degrees C, the acto-S-1 ATPase activity and the binding of S-1.ATP to actin were inhibited approximately to the same extent by IgG(18-29). These results are discussed in terms of S-1 binding sites on actin and the possible role of sequence 18-29 in actomyosin interactions.

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.

Interaction of myosin filaments and minifilaments with actin: a comparative study

Various aspects of actin-myosin interaction were investigated using myosin in the form of filaments and minifilaments obtained by dialysis against citrate-Tris buffer or by adding this buffer to performed myosin filaments. Considerable similarities in the behaviour of the two systems were found. (1) Although the minifilaments are soluble structures, they form insoluble complexes with actin, which superprecipitate upon addition of MgATP. Observations in the electron microscope and from centrifugation experiments have shown that the two actomyosin systems undergo essentially similar structural changes during superprecipitation. (2) At low substrate concentrations the rate of ATP hydrolysis in both systems declines with time, which is typical of insoluble superprecipitating actomyosin. (3) In contrast to soluble myosin subfragments, both filamentous and minifilamentous myosin give biphasic actin-activation curves. (4) The Mg2+-ATPase activities of myosin minifilaments and standard myosin preparations at low KCl extrapolate to similar Vmax at infinite actin concentration. Since our values of Vmax for myosin filaments and minifilaments are in the range of those reported for myosin subfragments, the results of this investigation confirm the view that the catalytic properties of myosin subfragments and intact myosin are equivalent. Moreover, the data show that the extent of myosin aggregation in the initial preparations has no appreciable effect on the characteristic features of the interaction between intact myosin and actin at pH 8.

Studies on the special properties of actomyosin in the gel form. II. An analysis of the turbidity changes seen in gel suspensions

The turbidity changes induced by MgATP in suspensions of actomyosin gel particles have been studied systematically over a wide range of MgATP concentrations at different temperatures with and without the regulatory proteins. An analysis of these changes distinguishes three separate protein-protein interactions in the gel: (1) The transient cyclic interactions between actin and myosin involved in the hydrolysis of MgATP and contraction. (2) Cross-links that cause turbidity in the original suspension. (3) Cross-links that cause the high turbidity usually associated with superprecipitation. In general, there was a good correlation between the half-time for reaching maximum turbidity during superprecipitation and the rate of hydrolysis. On the other hand, the actual magnitude of the turbidity increase was progressively diminished as the concentration of substrate was raised in the millimolar range. It appears that some essential phase of the superprecipitation process is limited by the same enzymatic step that limits the rate of hydrolysis. However, whether or not this phase leads to an increase in turbidity depends on the concentration of MgATP. Apparently, high physiological levels of MgATP (1-5 mM) inhibit the formation of the specific cross-links that cause high turbidity in the superprecipitate. It is proposed that MgATP interferes with these links when it can bind to a low-affinity site on myosin that is separate from the high-affinity active sites for hydrolysis. Thus, in contracting muscle, interfilament interactions analogous to those that increase turbidity and cause isodimensional shrinkage in the gel would be prevented by the high level of MgATP in the sarcoplasm. Observations relating the shortening of isolated myofibrils to their turbidity in suspension lend support to this interpretation.

Biphasic steady-state kinetics of myosin adenosine triphosphatase. Evidence for a substrate effector site

The steady-state kinetics of the K+, Ca2+, and Mg2+-activated adenosine triphosphatase (ATPase) activities of rabbit skeletal myosin were investigated in the substrate concentration range from 0.05 microM to 5 mM and found not to follow Michaelis-Menten kinetics but rather to display biphasic behavior. The Ca2+-ATPase activity of myosin chymotryptic subfragment-1 (S-1), which has only one active site, also exhibits biphasic kinetics, thus excluding the possibility that the biphasic behavior is caused by negative cooperativity between the two active sites of myosin. Myosin K+ and Mg2+-ATPase are both activated by 5'-adenyl methylenediphosphonate (AdoPP[CH2]P) in a competitive manner at high substrate concentrations; i.e. the maximal velocity observed at high substrate concentrations is independent of the AdoPP[CH2]P concentration. This result provides evidence for substrate activation via binding to a regulatory site. Pyrophosphate inhibits myosin ATPase in a competitive manner at low substrate concentrations and in an uncompetitive manner at high substrate concentrations, with the uncompetitive Ki being smaller than the competitive Ki; i.e. pyrophosphate binds more tightly to the effector site than to the active site.

The covalent modification of myosin's proteolytic fragments by a purine disulfide analog of adenosine triphosphate. Reaction at a binding site other than the active site

A purine disulfide analog of ATP, 6,6'-dithiobis(inosinyl imidodiphosphate), forms mixed disulfide bonds between the 6 thiol group on the purine ring and certain key cysteines on myosin, heavy meromyosin, and subfragment one. The EDTA ATPase activities of myosin and heavy meromyosin were completely inactivated when 4 mol of thiopurine nucleotide was bound. When similarly inactivated, subfragment one, depending on its method of preparation, incorporated either 1 or 2 mol of thiopurine nucleotide. Modification of a single cysteine on subfragment one resulted in an inhibition of both the Ca2+ and the EDTA ATPase activities, but the latter always to a greater extent. Modification of two cysteines per head of heavy meromyosin had the same effect suggesting that the active sites were not blocked by the thiopurine nucleotides. Direct evidence for this suggestion was provided by equilibrium dialysis experiments. Heavy meromyosin and subfragment one bound 1.9 and 0.8 mol of [8-3H]adenylyl imidodiphosphate per mol of enzyme, respectively, with an average dissociation constant of 5 X 10(-7) M. Heavy meromyosin with four thiopurine nucleotides bound or subfragment one with two thiopurine nucleotides bound retained 65-80% of these tight adenylyl imidodiphosphate binding sites confirming the above suggestion. Thus previous work assuming reaction of thiopurine nucleotide analogs at the active site of myosin must be reevaluated. Ultracentrifugation studies showed that heavy meromyosin which had incorporated four thiopurine nucleotides did not bind to F-actin while subfragment one with one thiopurine nucleotide bound interacted only very weakly with F-actin. Thus reaction of 6,6'-dithiobis(inosinyl imidodiphosphate) at nucleotide binding sites other than the active sites reduces the rate of ATP hydrolysis and inhibits actin binding. It is suggested that these second sites may function as regulatory sites on myosin.

Tension in skinned frog muscle fibers in solutions of varying ionic strength and neutral salt composition

The maximal calcium-activated isometric tension produced by a skinned frog single muscle fiber falls off as the ionic strength of the solution bathing this fiber is elevated declining to zero near 0.5 M as the ionic strength is varied using KCl. When other neutral salts are used, the tension always declines at high ionic strength, but there is some difference between the various neutral salts used. The anions and cations can be ordered in terms of their ability to inhibit the maximal calcium-activated tension. The order of increasing inhibition of tension (decreasing tension) at high ionic strength for anions is propionate(-) approximately SO(4) (--) < Cl(-) < Br(-). The order of increasing inhibition of calcium-activated tension for cations is K(+) approximately Na(+) approximately TMA(+) < TEA(+) < TPrA(+) < TBuA(+). The decline of maximal calcium-activated isometric tension with elevated salt concentration (ionic strength) can quantitatively explain the decline of isometric tetanic tension of a frog muscle fiber bathed in a hypertonic solution if one assumes that the internal ionic strength of a muscle fiber in normal Ringer's solution is 0.14-0.17 M. There is an increase in the base-line tension of a skinned muscle fiber bathed in a relaxing solution (no added calcium and 3 mM EGTA) of low ionic strength. This tension, which has no correlate in the intact fiber in hypotonic solutions, appears to be a noncalcium-activated tension and correlates more with a declining ionic strength than with small changes in [MgATP], [Mg], pH buffer, or [EGTA]. It is dependent upon the specific neutral salts used with cations being ordered in increasing inhibition of this noncalcium-activated tension (decreasing tension) as TPrA(+) < TMA(+) < K(+) approximately Na(+). Measurements of potentials inside these skinned muscle fibers bathed in relaxing solutions produced occasional small positive values (<6 mV) which were not significantly different from zero.

Correlations of length and volume measurements in myofibril suspensions

Instrumentation has been developed for the rapid electronic sizing of large numbers of myofibrils. The response of myofibrils in the presence of ATP to changes in Ca(++) concentration was examined. Shortening of myofibrils upon addition of Ca(++) was accompanied by an increased protein effective volume of approximately 10-40%. Whereas ATPase activation and increased turbidity of myofibrils upon addition of Ca(++) were reversible upon subsequent addition of EGTA, the shortening and swelling were irreversible. It is proposed that the swelling may result from the breaking of hydrophobic bonds within myosin. The ATPase activity and turbidity are measures of the input, while the shortening and swelling are measures of the output of a coupled nonequilibrium process; failure of reversal of the output indicates an uncoupling under the experimental conditions.