ATP hydrolysis and sliding movement of actomyosin complex in the presence of ethanol

The excluded volume effect induced by poly(ethylene glycol) modulates the motility of actin filaments interacting with myosin

To examine the motility of actomyosin complexes in the presence of high concentrations of polymers, we investigated the effect of poly(ethylene glycol) on the sliding velocities of actin filaments and regulated thin filaments on myosin molecules in the presence of ATP. Increased concentrations and relative molecular masses of poly(ethylene glycol) decreased the sliding velocities of actin and regulated thin filaments. The decreased ratio of velocity in regulated thin filaments at - log[Ca(2+) ] of 4 was higher than that of actin filaments. Furthermore, in the absence of Ca(2+) , regulated thin filaments were moderately motile in the presence of poly(ethylene glycol). The excluded volume change (∆V), defined as the change in water volume surrounding actomyosin during the interactions, was estimated by determining the relationship between osmotic pressure exerted by poly(ethylene glycol) and the decreased ratio of the velocities in the presence and absence of poly(ethylene glycol). The ∆V increased up to 3.7 × 10(5) Å(3) as the Mr range of poly(ethylene glycol) was increased up to 20,000. Moreover, the ∆V for regulated thin filaments was approximately two-fold higher than that of actin filaments. This finding suggests that differences in the conformation of filaments according to whether troponin-tropomyosin complexes lie on actin filaments alter the ∆V during interactions of actomyosin complexes and influence motility.

1,3-Diethylurea-enhanced Mg-ATPase activity of skeletal muscle myosin with a converse effect on the sliding motility

We investigate the effects of urea and its derivatives on the ATPase activity and on the in vitro motility of chicken skeletal muscle actomyosin. Mg-ATPase rate of myosin subfragment-1 (S1) is increased by 4-fold by 0.3M 1,3-diethylurea (DEU), but it is unaffected by urea, thiourea, and 1,3-dimethylurea at ≤1M concentration. Thus, we further examine the effects of DEU in comparison to those of urea as reference. In in vitro motility assay, we find that in the presence of 0.3M DEU, the sliding speeds of actin filaments driven by myosin and heavy meromyosin (HMM) are significantly decreased to 1/16 and 1/6.6, respectively, compared with the controls. However, the measurement of the actin-activated ATPase activity of HMM shows that the maximal rate, Vmax, is almost unchanged with DEU. Thus, the myosin-driven sliding motility of actin filaments is significantly impeded in the presence of 0.3M DEU, whereas the cyclic interaction of myosin with F-actin occurs during the ATP turnover, the rate of which is close to that without DEU. In contrast to DEU, 0.3M urea exhibits only modest effects on both actin-activated ATPase and sliding motility of actomyosin. Thus, DEU has the effect of uncoupling the sliding motility of actomyosin from its ATP turnover.

Effects of urea and guanidine hydrochloride on the sliding movement of actin filaments with ATP hydrolysis by myosin molecules

To evaluate the role of the hydration layer on the protein surface of actomyosin, we compared the effects of urea and guanidine-HCl on the sliding velocities and ATPase activities of the actin-heavy meromyosin (HMM) system. Both chemicals denature proteins, but only urea perturbs the hydration layer. Both the sliding velocity of actin filaments and actin-activated ATPase activity decreased with increasing urea concentrations. The sliding movement was completely inhibited at 1.0 M urea, while actin filaments were bound to HMM molecules fixed on the glass surface. Guanidine-HCl (0-0.05 M) drastically decreased both the sliding velocity and ATPase activation of acto-HMM complexes. Under this condition, actin filaments almost detached from HMM molecules. In contrast, the ATPase activity of HMM without actin filaments was almost independent of urea concentrations <1.0 M and guanidine-HCl concentrations <0.05 M. An increase in urea concentrations up to 2.0 M partly induced changes in the ternary structure of HMM molecules, while the actin filaments were stable in this concentration range. Hydration changes around such actomyosin complexes may alter both the stability of part of the myosin molecules, and the affinity for force transmission between actin filaments and myosin heads.

Dimethyl sulphoxide enhances the effects of P(i) in myofibrils and inhibits the activity of rabbit skeletal muscle contractile proteins

In the catalytic cycle of skeletal muscle, myosin alternates between strongly and weakly bound cross-bridges, with the latter contributing little to sustained tension. Here we describe the action of DMSO, an organic solvent that appears to increase the population of weakly bound cross-bridges that accumulate after the binding of ATP, but before P(i) release. DMSO (5-30%, v/v) reversibly inhibits tension and ATP hydrolysis in vertebrate skeletal muscle myofibrils, and decreases the speed of unregulated F-actin in an in vitro motility assay with heavy meromyosin. In solution, controls for enzyme activity and intrinsic tryptophan fluorescence of myosin subfragment 1 (S1) in the presence of different cations indicate that structural changes attributable to DMSO are small and reversible, and do not involve unfolding. Since DMSO depresses S1 and acto-S1 MgATPase activities in the same proportions, without altering acto-S1 affinity, the principal DMSO target apparently lies within the catalytic cycle rather than with actin-myosin binding. Inhibition by DMSO in myofibrils is the same in the presence or the absence of Ca(2+) and regulatory proteins, in contrast with the effects of ethylene glycol, and the Ca(2+) sensitivity of isometric tension is slightly decreased by DMSO. The apparent affinity for P(i) is enhanced markedly by DMSO (and to a lesser extent by ethylene glycol) in skinned fibres, suggesting that DMSO stabilizes cross-bridges that have ADP.P(i) or ATP bound to them.

Viscosity and solute dependence of F-actin translocation by rabbit skeletal heavy meromyosin

We tested the hypothesis that solvent viscosity affects translocation of rhodamine phalloidin-labeled F-actin by rabbit skeletal heavy meromyosin (HMM). When viscosity was increased using either glycerol, fructose, sucrose, or dextran (1.5, 6.0, or 15-20 kDa mol mass), there was little or no effect on the fraction of moving filaments, whereas sliding speed decreased in inverse proportion to viscosity. The results could be explained neither by an effect of osmotic pressure at high solute concentrations nor by altered solvent drag on the actin filament. Elevated viscosity inhibited HMM ATPase activity in solution, but only at much higher viscosities than were needed to reduce sliding speed. Polyethylene glycols (300, 1,000, or 3,000 mol wt) also inhibited speed via elevated viscosity but secondarily inhibited by enhancing electrostatic interactions. These results demonstrate that a diffusion-controlled process intrinsic to cross-bridge cycling can be limiting to actomyosin function.

Alcohol-induced biphasic inhibition of myosin subfragment 1 K-EDTA-ATPase

Butanol-induced inhibition of K-EDTA-ATPase of myosin subfragment 1 proceeded by biphasic kinetics, consisting of rapid and slow inactivations. The extent of the rapid inactivation, which was estimated by extrapolating the process of slow inactivation to zero time of the incubation period, was saturated with butanol concentration. Recovery of activity by dilution in the rapid phase indicates that the rapid process is reversible. The slow inactivation was concomitant with a partial denaturation of the 50 kDa domain of S1, which was detected by limited tryptic digestion. Other alcohols (methanol, ethanol, propanol and hexanol) also inhibited the K-EDTA-ATPase in the rapid phase. The Ki decreased with an increase in the number of methylene groups of alcohol. When K-EDTA-ATPase activity in the rapid phase was plotted against viscosity, surface tension or dielectric constant, the curves were different for each of the various alcohol solutions. The rapid inactivation appears to be caused by a binding of the alkyl group to S1, rather than by solvent effects. The kinetics of rapid butanol inhibitions indicate that butanol reduces the maximum activity of ATPase but enhances an apparent affinity of S1 with ATP. These indications suggest that alcohol stabilizes S1.KATP intermediate. The rapid K-EDTA-ATPase inhibition was observed at the same alcohol concentration where S1 Mg-ATPase was activated.

Delayed dissociation of in vitro moving actin filaments from heavy meromyosin induced by low concentrations of Triton X-100

The in vitro motility of fluorescent actin filaments over heavy meromyosin (HMM) was studied in the presence of the nonionic detergent Triton X-100. Below 0.004% Triton X-100 concentration, motility was not affected. Above 0.007%, motility was not observed because actin filaments were dissociated from HMM. In the Triton X-100 concentration range of 0.004-0.007%, the sliding actin filaments dissociated from HMM with a delay. The dissociation delay time decreased with increasing Triton X-100 concentration, increasing ATP (adenosine-5'-triphosphate) concentration, and increasing temperature. The delayed acto-HMM dissociation was absent when weak-binding kinetic intermediates of the myosin ATPase cycle (M.ATP and M.ADP-Pi) were used. The presence of sliding movement was necessary to evoke the delayed acto-HMM dissociation. The acto-HMM dissociation delay was independent of actin filament length. For a given Triton X-100 concentration, the dissociation delay time was found to be inversely proportional to sliding velocity, indicating that actin filaments travel a more or less constant distance prior to dissociation from HMM. The actin-activated HMM ATPase activity was not inhibited by Triton X-100; rather, it was slightly enhanced. The results imply the presence of a motility-associated conformational change in acto-HMM.