Rigidity of myosin and myosin rod by electric birefringence

Predicting allosteric switches in myosins

The sequences of several members of the myosin family of molecular motors are evaluated using ASP (Ambivalent Structure Predictor), a new computational method. ASP predicts structurally ambivalent sequence elements by analyzing the output from a secondary structure prediction algorithm. These ambivalent sequence elements form secondary structures that are hypothesized to function as switches by undergoing conformational rearrangement. For chicken skeletal muscle myosin, 13 discrete structurally ambivalent sequence elements are identified. All 13 are located in the heavy chain motor domain. When these sequence elements are mapped into the myosin tertiary structure, they form two compact regions that connect the actin binding site to the adenosine 5'-triphosphate (ATP) site, and the ATP site to the fulcrum site for the force-producing bending of the motor domain. These regions, predicted by the new algorithm to undergo conformational rearrangements, include the published known and putative switches of the myosin motor domain, and they form plausible allosteric connections between the three main functional sites of myosin. The sequences of several other members of the myosin I and II families are also analyzed.

Hydrodynamics of segmentally flexible macromolecules

Segmentally flexible macromolecules are composed of a few rigid subunits linked by joints which are more or less flexible. The dynamics in solution of this type of macromolecule present special aspects that are reviewed here. Three alternative approaches are described. One is the rigid-body treatment, which is shown to be valid for overall dynamic properties such as translational diffusion and intrinsic viscosity. Another approach is the Harvey-Wegener treatment, which is particularly suited for rotational diffusion. The simplest version of this treatment, which ignores hydrodynamic interaction (HI) effects, is found to be quite accurate when compared to a more rigorous version including HI. A third approach is the Brownian dynamics simulation that, albeit at some computational cost, might describe rigorously cases of arbitrary complexity. This technique has been used to test the approximations in the rigid-body and Harvey-Wegener treatments, thus allowing a better understanding of their validity. Brownian trajectories of simplified models such as the trumbbell and the broken rod have been simulated. The comparison of the decay rates of some correlation functions with the predictions of the two treatments leads to a general conclusion: the Harvey-Wegener treatment determines the initial rate, while the long-time behavior is dominated by the rigid-body relaxation time. As an example of application to a specific biological macromolecule, we present a simulation of an immunoglobulin molecule, showing how Brownian Dynamics can be used to predict rotational and internal dynamics. Another typical example is myosin. Literature data of hydrodynamic properties of whole myosin and the myosin rod are compared with predictions from the Harvey-Wegener and rigid-body treatments. The present situation of the problem on myosin flexibility is analyzed, and some indications are given for future experimental and simulation work.

On the flexibility of myosin in solution

Rabbit skeletal muscle myosin from the same rabbit was prepared by two different methods, and then purified by either Sephadex or hydroxylapatite chromatography. The resulting myosin samples were analyzed in 2-10 mM sodium pyrophosphate solutions at pH 9 using transient electric birefringence. The birefringence decay signals were fitted using a Fortran program called DISCRETE and two relaxation times, 49.7 +/- 5.6 and 11.2 +/- 2.5 microseconds, were determined. These relaxation times were independent of the method of myosin preparation, the method of myosin purification, the concentration of sodium pyrophosphate between 2 and 10 mM, the concentration of myosin between 0.08 and 1.59 mg/mL, and the temperature between 4.0 and 20.0 degrees C, after correction to 20.0 degrees C. The longer relaxation time is consistent with a rigid, linear myosin molecule. The shorter relaxation time is consistent with myosin that has a completely flexible hinge region in the myosin tail. Both relaxation times are inconsistent with the previously reported single relaxation time of myosin obtained by fitting the birefringence decay data to only 90% of the decay signal. By forcing some of the birefringence decay data in the presence work to fit 90% of the decay signal with a single relaxation time, approximately the same relaxation time as previously reported was obtained.

Differential scanning calorimetric study of the thermal unfolding of myosin rod, light meromyosin, and subfragment 2

The thermal unfolding of myosin rod, light meromyosin (LMM), and myosin subfragment 2 (S-2) was studied by differential scanning calorimetry (DSC) over the pH range of 6.5-9.0 in 0.5M KCl and either 0.20 M sodium phosphate or 0.15M sodium pyrophosphate. Two rod samples were examined: one was purified by Sephadex G-200 without prior denaturation (native rod), and the other was purified by a cycle of denaturation-renaturation followed by Sephacryl S-200 chromatography (renatured rod). There were clearly distinguishable differences in the calorimetric behavior of these two samples. At pH 7.0 in phosphate the DSC curves of native rod were deconvoluted into six endothermic two-state transitions with melting temperatures in the range of 46-67 degrees C and a total enthalpy of 4346 kJ/mol. Under identical conditions the melting profile of LMM was resolved into five endothermic peaks with transition temperatures in the range of 45-66 degrees C, and the thermal profile of long S-2 was resolved into two endotherms, 46 and 57 degrees C. Transition 4 observed with native rod was present in the deconvoluted DSC curve for long S-2, but absent in the DSC curve for LMM. This transition was identified with the high-temperature transition detected with long S-2 and attributed to the melting of the coiled-coil alpha-helical segment of subfragment 2 (short S-2). The low-temperature transition of long S-2 was attributed to the unfolding of the hinge region. The smallest transition temperatures observed for all three fragments were 45-46 degrees C. It is suggested that the most unstable domain in rod (domain 1) responsible for the 46 degrees C transition includes both the hinge region, which is the C-terminal segment of long S-2, and a short N-terminal segment of LMM. This domain, accounting for 21% of the rod structure, contains the S-2/LMM junction, and upon proteolytic cleavage yields the C-terminal and N-terminal ends of long S-2 and LMM, respectively. Over the pH range of 6.5-7.5, the observed specific heat of denaturation of rod was approximately equal to the sum of the specific heats of LMM and S-2. This finding provides an additional argument for the existence of independent domains in myosin rod.

Transport properties of rigid bent-rod macromolecules and of semiflexible broken rods in the rigid-body treatment. Analysis of the flexibility of myosin rod

The translational diffusion coefficients, rotational relaxation times and intrinsic viscosities of rigid bent rods, composed by two rodlike arms joined rigidly at an angle alpha, have been evaluated for varying conformation using the latest advances in hydrodynamic theory. We have considered semiflexible rods in which the joint is an elastic hinge or swivel, with a potential V(alpha) = 1/2Q alpha 2 with constant Q. Accepting the rigid-body treatment, we calculate properties of broken rods by averaging alpha-dependent values for rigid rods. The results are finally used to interpret literature values of the properties of myosin rod. Q is regarded as an adjustable parameter, and the value fitted is such that the average bending angle of myosin rod is approximately 60 degrees.

Electric linear dichroism and birefringence of biological polyelectrolytes

The phenomenon of electro-optic orientation was discovered by John Kerr in 1875 and has been used extensively for determining the optical polarizability anisotropy of small molecules and for high-speed transmission of optical signals. Measurements on biopolymers have been made at least since 1950, but only in the last decade have these yielded definitive structural and physical information. In the course of this review, it should become obvious that among the reasons for this late development is the inherent difficulty of analysing optical data that depend simultaneously on intrinsic optical-structural properties of the molecules, and on their degree of orientation under the conditions of the experiment. The problem has been particularly difficult far biopolymers such as the nucleic acids, whose polarization in an electric field is dependent on their special polyelectrolyte properties. These unique electrostatic properties are an important feature in the interpretation of the experimental observations.

Possible role of helix-coil transitions in the microscopic mechanism of muscle contraction

Local helix-coil transitions in the coiled coil portion of myosin have long been implicated as a possible origin of tension generation in muscle. From a statistical mechanical theory of conformational transitions in coiled coils, the free energy required to form a randomly coiled bubble in the hinge region of myosin of the type conjectured by Harrington (Harrington, W. F., 1979, Proc. Natl. Acad. Sci. USA, 76:5066-5070) is estimated to be approximately 25 kcal/mol. Unfortunately this is far more than the free energy available from ATP hydrolysis if the crossbridges operate independently. Thus, in solution such bubbles are predicted to be absent, and the theory requires that the rod portion of myosin be a hingeless, continuously deforming rod. While such bubble formation in vivo cannot be entirely ruled out, it appears to be unlikely. We further conjecture that in solution the swivel located between myosin subfragments 1 and 2 (S-2 and S-1) is due to a locally random conformation of the chains caused by the presence of a proline residue at the point that physically separates the coiled coil from the globular portion of myosin. On attachment of S-1 to actin in the strong binding state, the configurational entropy of the random coil in the swivel region is greatly reduced relative to the case where the ends are free. This produces a spontaneous coil-to-helix transition in the swivel region that causes rotation of S-1 and the translation of actin. Thus, the model predicts that the actin filaments are pushed rather than pulled past the thick filaments by the crossbridges. The specific mechanism of force generation is examined in detail, and a simple statistical mechanical realization of the model is proposed. We find that the model gives a substantial number of qualitative and at times quantitative predictions in accord with experiment, and is particularly appealing in that it provides a simple means of free energy transduction--the well known fact that topological constraints shift the equilibrium between helical and random coil states.

Transient electrical birefringence characterization of heavy meromyosin

Heavy meromyosin (HMM) and myosin subfragment 1 (S1) were prepared from myosin by using low concentrations of alpha-chymotrypsin. The light chain distribution in HMM was identical with that of myosin, within experimental error, when analyzed on 12% polyacrylamide gels after electrophoresis. Specific birefringences and birefringence decay times were measured by transient electrical birefringence in 5 mM KCl, 5 mM tris(hydroxymethyl)aminomethane (pH 7), and 1 mM MgCl2 at 4 degrees C under gentle conditions that reduced the CaATPase activity by less than 10%. For solutions of HMM, by use of electric field pulses shorter than 0.5 microseconds, the birefringence decay signal from the S1 portions of HMM could be resolved and the rotational motions of the S1 moieties observed directly. The rotation relaxation time, adjusted to 20 degrees C, was 0.34 microseconds; this is in quantitative agreement with previous hydrodynamic results obtained by using covalently attached probes. The assignment of the fast decay time obtained with HMM to the S1 portions was confirmed by birefringence decay measurements on free S1, for which the relaxation time was 0.13 microseconds, corrected to 20 degrees C. The specific birefringences for S1 and HMM, respectively, were 0.37 X 10(-6) and 12.8 X 10(-6) (cm/statvolt)2. Thus, for much longer electric field pulses, the signal from HMM is due almost entirely to its subfragment 2 (S2) portion, and its rotational dynamics can also be monitored directly by using electrical birefringence. The decay of the signal from the S2 portion could be adequately fit without evoking bending of the S2 portion of HMM other than at its junction with S1.

Reactivities of thiols in myosin rod: effect of magnesium and ionic strength

There are six cysteines in each chain of myosin rod of rabbit skeletal muscle: three are in the S-2 portion, at residues 66, 108 and 410 (Lu, R.C. and Lehrer, S. (1984) Biochemistry 23, 5975-5981). The other three are in the light meromyosin portion, assigned at residues 572, 600 and 770 on the basis of homology between the amino acid sequence in the vicinity of these thiols and that of the rod of nematode myosin (McLachlan, A.D. and Karn, J. (1982) Nature 299, 226-231). Since the thiols are distributed in different regions of the rod, measuring their reactivities under various conditions may provide information on the conformations of these regions. Myosin rod was carboxymethylated with radioactive iodoacetic acid under various conditions. The cysteine-containing peptides were isolated using HPLC from the tryptic digests, and the radioactivity incorporated into each thiol was measured. In the denatured state all six thiols were equally reactive. In the native state, all thiols have low reactivity, the reactivity of Cys-108 or -410 is only 0.1% of that in the denatured state, Cys-600 exhibited the highest reactivity, about 20-times that of Cys-410; Cys-66, -572 and -770 had 2-4-times that of Cys-410. When the rods formed filaments, the reactivities of all cysteines further decreased: Cys-66, -108 and -770 were reduced to 50%, while Cys-410, -572 and -600, located in the middle of the rod, were reduced to 20-30% of their reactivities in the monomeric form. In the presence of Mg2+ the reactivity of Cys-108 increased by 20%, whereas Cys-572 decreased by 50%. The results are consistent with the view that metal ions affect the conformation of the rod. This may play a role in the mechanism of filament formation and the movement of crossbridges.