Effect of myosin on conformational changes of F-actin in thin filament in vivo induced by calcium ions

Myosin-Induced Gliding Patterns at Varied [MgATP] Unveil a Dynamic Actin Filament

Actin filaments have key roles in cell motility but are generally claimed to be passive interaction partners in actin-myosin-based motion generation. Here, we present evidence against this static view based on an altered myosin-induced actin filament gliding pattern in an in vitro motility assay at varied [MgATP]. The statistics that characterize the degree of meandering of the actin filament paths suggest that for [MgATP] ≥ 0.25 mM, the flexural rigidity of heavy meromyosin (HMM)-propelled actin filaments is similar (without phalloidin) or slightly lower (with phalloidin) than that of HMM-free filaments observed in solution without surface tethering. When [MgATP] was reduced to ≤0.1 mM, the actin filament paths in the in vitro motility assay became appreciably more winding in both the presence and absence of phalloidin. This effect of lowered [MgATP] was qualitatively different from that seen when HMM was mixed with ATP-insensitive, N-ethylmaleimide-treated HMM (NEM-HMM; 25-30%). In particular, the addition of NEM-HMM increased a non-Gaussian tail in the path curvature distribution as well as the number of events in which different parts of an actin filament followed different paths. These effects were the opposite of those observed with reduced [MgATP]. Theoretical modeling suggests a 30-40% lowered flexural rigidity of the actin filaments at [MgATP] ≤ 0.1 mM and local bending of the filament front upon each myosin head attachment. Overall, the results fit with appreciable structural changes in the actin filament during actomyosin-based motion generation, and modulation of the actin filament mechanical properties by the dominating chemomechanical actomyosin state.

Elementary steps of the cross-bridge cycle in bovine myocardium with and without regulatory proteins

The role of regulatory proteins in the elementary steps of the cross-bridge cycle in bovine myocardium was investigated. The thin filament was selectively removed by gelsolin and the actin filament was reconstituted without tropomyosin or troponin. Further reconstitution was achieved by adding tropomyosin and troponin. The effects of MgATP and phosphate (Pi) on the rate constants of exponential processes were studied in control, actin filament-reconstituted, and thin filament-reconstituted myocardium at pCa < or = 4.66, pH 7.00, 25 degrees C. In control myocardium, the MgATP association constant was 9.1 +/- 1.3 mM(-1), and the Pi association constant 0.14 +/- 0.04 mM(-1). The equilibrium constant of the cross-bridge detachment step was 2.6 +/- 0.4, and the equilibrium constant of the force generation step was 0.59 +/- 0.04. In actin filament-reconstituted myocardium without regulatory proteins, the MgATP association constant was approximately the same, and the Pi association constant increased to 2.8x. The equilibrium constant of cross-bridge detachment decreased to 0.2x, but the equilibrium constant of the force generation step increased to 4x. These kinetic constants regained control values after reconstitution of the thin filament. These results indicate that tension/cross-bridge in the presence of regulatory proteins is approximately 1.5-1.7x of that in the absence of regulatory proteins. These results further indicate that regulatory proteins promote detachment of cross-bridges.

An EPR study of the rotational dynamics of actins from striated and smooth muscle and their complexes with heavy meromyosin

The rotational motions of the actin from rabbit skeletal muscle and from chicken gizzard smooth muscle were measured by conventional and saturation transfer electron paramagnetic resonance (EPR) spectroscopy using maleimide spin-label rigidly bound at Cys-374. The conventional EPR spectra indicate a slight difference in the polarity of the environment of the label and in the rotational mobility of the monomeric gizzard actin compared to its skeletal muscle counterpart. These differences disappear upon polymerization. The EPR spectra of the two actins in their F form and in their complexes with heavy meromyosin (HMM) did not reveal any difference in the rotational dynamic properties that might be correlated with the known differences in the activation of myosin ATPase activity by smooth and skeletal muscle actin. Our results agree with earlier EPR studies on skeletal muscle actin in showing that polymerization stops the nanosecond rotational motion of actin monomers and that F-actin undergoes rotational motion having an effective correlation time of the order of 0.1 ms. However, our measurements show that complete elimination of the nanosecond motions requires prolonged incubation of F-actin, suggesting that the slow formation of interfilamental cross-links in concentrated F-actin solutions contributes to this process. We have also used the EPR spectroscopy to study the interaction between HMM and actin in the F and G form. Our results show that in the absence of salt one HMM molecule can cooperatively interact with eight monomers to produce a polymer which closely resembles F-actin in its rotational mobility but differs from the complex of F-actin with HMM. The results indicate that salt is necessary for further slowing down, in a cooperative manner, the sub-millisecond internal motion in actin polymer and for a non-cooperative change in the intramonomer conformation around Cys-374 on the binding of HMM.

Studies on conformation of F-actin in muscle fibers in the relaxed state, rigor, and during contraction using fluorescent phalloidin

F-actin in a glycerinated muscle fiber was specifically labeled with fluorescent phalloidin-(fluorescein isothiocyanate) FITC complex at 1:1 molar ratio. Binding of phalloidin-FITC to F-actin affected neither contraction of the fiber nor its regulation by Ca2+. Comparison of polarized fluorescence from phalloidin-FITC bound to F-actin in the relaxed state, rigor, and during isometric contraction of the fiber revealed that the changes in polarization accompanying activation are quantitatively as well as qualitatively different from those accompanying transition of the fiber from the relaxed state to rigor. The extent of the changes of polarized fluorescence during isometric contraction increased with decreasing ionic strength, in parallel with increase in isometric tension. On the other hand, polarized fluorescence was not affected by addition of ADP or by stretching of the fiber in rigor solution. It is concluded from these observations that conformational changes in F-actin are involved in the process of active tension development.

Studies on calcium ion-induced conformation changes in the actin-tropomyosin-troponin system by fluorimetry. IV. Conformational changes in the region containing fluorescence-labeled sulfhydryl group(s) of troponin

Conformational changes associated with the functional states of the molecule of troponin were studied using SH-direct fluorogenic reagents, N-(p-(2-benzimidazolyl)phenyl) maleimide (BIPM) and N-(1-anilinonaphthyl-4) maleimide (ANM). 1. The fluorescence parameters of ANM-troponin, intensity, and polarization, did not change on combining it with tropomyosin alone, but markedly changed when F-actin was further added to the system. 2. The conformation around the dye-labeled sulfhydryl group(s) was shown to be susceptible to Ca2+ in terms of fluorescence intensity of the label, thermal transition of the conformation, and the microenvironment near the label. 3. On addition of Ca2+, the fluorescence characteristics of the two systems, ANM-troponin . tropomyosin and ANM-troponin . tropomyosin . F-actin complexes, were altered in opposite directions. When BIPM was used in place of ANM, similar changes were observed: a simple decrease in the intensity when pCa was decreased from 7.4 to 5.5 in the system without F-actin and a sigmoidal increase in the range from pCa 7 to 6 in the system with F-actin. Heavy meromyosin, when added to the latter complex (the reconstituted thin filaments), made the profile of its Ca2+ concentration dependence of fluorescence similar to that of the former complex. When tropomyosin was labeled in place of troponin, similar results were obtained. The data obtained imply that the Ca2+-induced conformational changes of troponin are markedly modified when detached from actin, and that heavy meromyosin weakens the interaction of the troponin . tropomyosin complex with F-actin.

The use of [125I]C1q subcomponent for the measurement of complement binding antibodies on cell surfaces

An assay using 125I-labelled human C1q has been developed for the measurement of complement fixing antibodies bound to cell monolayers or cell suspensions. The method has been adapted for use either during or after sensitisation of the cells with antiserum, is simple to perform and does not require require prelabelling of the target cells.

Electronmicroscopic investigation of the flexibility of F-actin

The contour lenghts and the end-to-end distances of a large number of F-actin filaments were measured in electronmicrographs. Preparation of F-actin for electron-microscopy was made at three different temperatures. The flexibility parameter or the elastic modulus for bending of F-actin was determined from the relation between the contour length and the end-to-end distance after statistical treatment. The value of the flexibility parameter obtained here showed good agreement with that obtained by the quasielastic light scattering at all temperatures examined. The flexibility of F-actin increased with raising temperature and with the binding of heavy meromyosin.

Birefringence of glycerinated crab muscle fiber under various conditions

By using glycerinated single fibers of crab muscle (Sesarma haematocheir) which has long sarcomeres, the birefringence of the I band, H band and the overlapping region between thin and thick filaments was measured separately, under various environmental conditions. At the resting length, the birefringence of the fiber was decreased by the addition of Ca2+ in the absence of ATP, by about 0.35%. This birefringence decrease was found to take place in the overlapping region. The decrease corresponded to about 2% of the birefringence of thin filaments in this region. The birefringence of the fiber was increased by the addition of ATP in the absence of Ca2+, by about 6%. This birefringence increase also took place mostly in the overlapping region. The increase of birefringence by pyrophosphate was about half of that by ATP. The birefringence of the fiber was decreased by the increase of the ionic strength from 0.12 to 0.20. The origin of the observed changes of birefringence is discussed.