Following the rotational trajectory of the principal hydrodynamic frame of a protein using multiple probes

Measuring Rotations of a Few Cross-Bridges in Skeletal Muscle

The ability to measure properties of a single cross-bridge in working muscle is important because it avoids averaging the signal from a large number of molecules and because it probes cross-bridges in their native crowded environment. Because the concentration of myosin in muscle is large, observing the kinetics of a single myosin molecule requires that the signal be collected from small volumes. The introduction of small observational volumes defined by diffraction-limited laser beams and confocal detection has made it possible to limit the observational volume to a femtoliter (10 15 liter). By restraining labeling to 1 fluorophore per 100 myosin molecules, we were able to follow the kinetics of approximately 400 cross-bridges. To reduce this number further, we used two-photon (2P) microscopy. The focal plane in which the laser power density was high enough to produce 2P absorption was thinner than in confocal microscopy. Using 2P microscopy, we were able to observe approximately 200 cross-bridges during contraction. The novel method of confocal total internal reflection (CTIR) provides a method to reduce the observational volume even further, to approximately 1 attoliter (10 18 liter), and to measure fluorescence with a high signal-to-noise (S/N) ratio. In this method, the observational volume is made shallow by illuminating the sample with an evanescent field produced by total internal reflection (TIR) of the incident laser beam. To guarantee the small lateral dimensions of the observational volume, a confocal aperture is inserted in the conjugate-image plane of the objective. With a 3.5-μm confocal aperture, we achieved a volume of 1.5 attoliter. Association-dissociation of the myosin head was probed with rhodamine attached at cys707 of the heavy chain of myosin. Signal was contributed by one to five fluorescent myosin molecules. Fluorescence decayed in a series of discrete steps, corresponding to bleaching of individual molecules of rhodamine. The S/N ratio was sufficiently large to make statistically significant comparisons from rigor and contracting myofibrils.

Mesoscopic analysis of motion and conformation of cross-bridges

The orientation of a cross-bridge is widely used as a parameter in determining the state of muscle. The conventional measurements of orientation, such as that made by wide-field fluorescence microscopy, electron paramagnetic resonance (EPR) or X-ray diffraction or scattering, report the average orientation of 10¹²-10⁹ myosin cross-bridges. Under conditions where all the cross-bridges are immobile and assume the same orientation, for example in normal skeletal muscle in rigor, it is possible to determine the average orientation from such global measurements. But in actively contracting muscle, where a parameter indicating orientation fluctuates in time, the measurements of the average value provide no information about cross-bridge kinetics. To avoid problems associated with averaging information from trillions of cross-bridges, it is necessary to decrease the number of observed cross-bridges to a mesoscopic value (i.e. the value affected by fluctuations around the average). In such mesoscopic regimes, the averaging of the signal is minimal and dynamic behavior can be examined in great detail. Examples of mesoscopic analysis on skeletal and cardiac muscle are provided.

Myosin cross-bridges do not form precise rigor bonds in hypertrophic heart muscle carrying troponin T mutations

Distribution of orientations of myosin was examined in ex-vivo myofibrils from hearts of transgenic (Tg) mice expressing Familial Hypertrophic Cardiomyopathy (FHC) troponin T (TnT) mutations I79N, F110I and R278C. Humans are heterozygous for sarcomeric FHC mutations and so hypertrophic myocardium contains a mixture of the wild-type (WT) and mutated (MUT) TnT. If mutations are expressed at a low level there may not be a significant change in the global properties of heart muscle. In contrast, measurements from a few molecules avoid averaging inherent in the global measurements. It is thus important to examine the properties of only a few molecules of muscle. To this end, the lever arm of one out of every 60,000 myosin molecules was labeled with a fluorescent dye and a small volume within the A-band (~1 fL) was observed by confocal microscopy. This volume contained on average 5 fluorescent myosin molecules. The lever arm assumes different orientations reflecting different stages of acto-myosin enzymatic cycle. We measured the distribution of these orientations by recording polarization of fluorescent light emitted by myosin-bound fluorophore during rigor and contraction. The distribution of orientations of rigor WT and MUT myofibrils was significantly different. There was a large difference in the width and of skewness and kurtosis of rigor distributions. These findings suggest that the hypertrophic phenotype associated with the TnT mutations can be characterized by a significant increase in disorder of rigor cross-bridges.

E22K mutation of RLC that causes familial hypertrophic cardiomyopathy in heterozygous mouse myocardium: effect on cross-bridge kinetics

Familial hypertrophic cardiomyopathy is a disease characterized by left ventricular and/or septal hypertrophy and myofibrillar disarray. It is caused by mutations in sarcomeric proteins, including the ventricular isoform of myosin regulatory light chain (RLC). The E22K mutation is located in the RLC Ca(2+)-binding site. We have studied transgenic (Tg) mouse cardiac myofibrils during single-turnover contraction to examine the influence of E22K mutation on 1) dissociation time (tau(1)) of myosin heads from thin filaments, 2) rebinding time (tau(2)) of the cross bridges to actin, and 3) dissociation time (tau(3)) of ADP from the active site of myosin. tau(1) was determined from the increase in the rate of rotation of actin monomer to which a cross bridge was bound. tau(2) was determined from the rate of anisotropy change of the recombinant essential light chain of myosin labeled with rhodamine exchanged for native light chain (LC1) in the cardiac myofibrils. tau(3) was determined from anisotropy of muscle preloaded with a stoichiometric amount of fluorescent ADP. Cross bridges were induced to undergo a single detachment-attachment cycle by a precise delivery of stoichiometric ATP from a caged precursor. The times were measured in Tg-mutated (Tg-m) heart myofibrils overexpressing the E22K mutation of human cardiac RLC. Tg wild-type (Tg-wt) and non-Tg muscles acted as controls. tau(1) was statistically greater in Tg-m than in controls. tau(2) was shorter in Tg-m than in non-Tg, but the same as in Tg-wt. tau(3) was the same in Tg-m and controls. To determine whether the difference in tau(1) was due to intrinsic difference in myosin, we estimated binding of Tg-m and Tg-wt myosin to fluorescently labeled actin by measuring fluorescent lifetime and time-resolved anisotropy. No difference in binding was observed. These results suggest that the E22K mutation has no effect on mechanical properties of cross bridges. The slight increase in tau(1) was probably caused by myofibrillar disarray. The decrease in tau(2) of Tg hearts was probably caused by replacement of the mouse RLC for the human isoform in the Tg mice.

Rotation of the lever arm of Myosin in contracting skeletal muscle fiber measured by two-photon anisotropy

The rotation of the lever arm of myosin cross-bridges is believed to be responsible for muscle contraction. To resolve details of this rotation, it is necessary to observe a single cross-bridge. It is still impossible to do so in muscle fiber, but it is possible to investigate a small population of cross-bridges by simultaneously activating myosin in a femtoliter volume by rapid release of caged ATP. In earlier work, in which the number of observed cross-bridges was limited to approximately 600 by confocal microscopy, we were able to measure the rates of cross-bridge detachment and rebinding. However, we were unable to resolve the power stroke. We speculated that the reason for this was that the number of observed cross-bridges was too large. In an attempt to decrease this number, we used two-photon microscopy which permitted observation of approximately 1/2 as many cross-bridges as before with the same signal/noise ratio. With the two-photon excitation, the number of cross-bridges was small enough to resolve the beginning of the power stroke. The results indicated that the power stroke begins approximately 170 ms after the rigor cross-bridge first binds ATP.

A maximum entropy analysis of protein orientations using fluorescence polarization data from multiple probes

Techniques have recently become available to label protein subunits with fluorescent probes at predetermined orientation relative to the protein coordinates. The known local orientation enables quantitative interpretation of fluorescence polarization experiments in terms of orientation and motions of the protein within a larger macromolecular assembly. Combining data obtained from probes placed at several distinct orientations relative to the protein structure reveals functionally relevant information about the axial and azimuthal orientation of the labeled protein segment relative to its surroundings. Here we present an analytical method to determine the protein orientational distribution from such data. The method produces the broadest distribution compatible with the data by maximizing its informational entropy. The key advantages of this approach are that no a priori assumptions are required about the shape of the distribution and that a unique, exact fit to the data is obtained. The relative orientations of the probes used for the experiments have great influence on information content of the maximum entropy distribution. Therefore, the choice of probe orientations is crucial. In particular, the probes must access independent aspects of the protein orientation, and two-fold rotational symmetries must be avoided. For a set of probes, a "figure of merit" is proposed, based on the independence among the probe orientations. With simulated fluorescence polarization data, we tested the capacity of maximum entropy analysis to recover specific protein orientational distributions and found that it is capable of recovering orientational distributions with one and two peaks. The similarity between the maximum entropy distribution and the test distribution improves gradually as the number of independent probe orientations increases. As a practical example, ME distributions were determined with experimental data from muscle fibers labeled with bifunctional rhodamine at known orientations with respect to the myosin regulatory light chain (RLC). These distributions show a complex relationship between the axial orientation of the RLC relative to the fiber axis and the azimuthal orientation of the RLC about its own axis. Maximum entropy analysis reveals limitations in available experimental data and supports the design of further probe angles to resolve details of the orientational distribution.

Conformational changes of contractile proteins and their role in muscle contraction

The review summarizes the results of studies on conformational changes in contractile proteins that occur during muscle contraction. Polarized fluorescence of tryptophan residues in actin and of fluorescent probes bound specifically to different sites on actin, myosin, or tropomyosin in muscle fibers was measured. The results show that the transition of actomyosin complex from the weak to the strong-binding state is accompanied by a change in the orientation of F-actin subunits with the C and N termini moving opposite to a large part of the subunit. Myosin light chains and some areas in the 20-kDa domain of myosin head move in the same direction as the C- and N-terminal regions of actin. It is established that troponin, caldesmon, calponin, and myosin systems of regulation of muscle contraction modify intramolecular actomyosin rearrangements in a Ca(2+)-dependent manner. The role of intramolecular movements of contractile proteins in muscle contraction is discussed.

Probes bound to myosin Cys-707 rotate during length transients in contraction

It is widely conjectured that muscle shortens because portions of myosin molecules (the "cross-bridges") impel the actin filament to which they transiently attach and that the impulses result from rotation of the cross-bridges. Crystallography indicates that a cross-bridge is articulated-consisting of a globular catalytic/actin-binding domain and a long lever arm that may rotate. Conveniently, a rhodamine probe with detectable attitude can be attached between the globular domain and the lever arm, enabling the observer to tell whether the anchoring region rotates. Well-established signature effects observed in shortening are tension changes resulting from the sudden release or quick stretch of active muscle fibers. In this investigation we found that closely correlated with such tension changes are changes in the attitude of the rhodamine probes. This correlation strongly supports the conjecture about how shortening is achieved.

The orientation of eosin-5-maleimide on human erythrocyte band 3 measured by fluorescence polarization microscopy

The dominant motional mode for membrane proteins is uniaxial rotational diffusion about the membrane normal axis, and investigations of their rotational dynamics can yield insight into both the oligomeric state of the protein and its interactions with other proteins such as the cytoskeleton. However, results from the spectroscopic methods used to study these dynamics are dependent on the orientation of the probe relative to the axis of motion. We have employed polarized fluorescence confocal microscopy to measure the orientation of eosin-5-maleimide covalently reacted with Lys-430 of human erythrocyte band 3. Steady-state polarized fluorescence images showed distinct intensity patterns, which were fit to an orientation distribution of the eosin absorption and emission dipoles relative to the membrane normal axis. This orientation was found to be unchanged by trypsin treatment, which cleaves band 3 between the integral membrane domain and the cytoskeleton-attached domain. this result suggests that phosphorescence anisotropy changes observed after trypsin treatment are due to a rotational constraint change rather than a reorientation of eosin. By coupling time-resolved prompt fluorescence anisotropy with confocal microscopy, we calculated the expected amplitudes of the e-Dt and e-4Dt terms from the uniaxial rotational diffusion model and found that the e-4Dt term should dominate the anisotropy decay. Delayed fluorescence and phosphorescence anisotropy decays of control and trypsin-treated band 3 in ghosts, analyzed as multiple uniaxially rotating populations using the amplitudes predicted by confocal microscopy, were consistent with three motional species with uniaxial correlation times ranging from 7 microseconds to 1.4 ms.

The conformation of xanthene dyes in the myosin sulfhydryl one binding site. Part I. Methods and model systems

Derivatives of the fluorescent probes fluorescein and rhodamine specifically and covalently modify the highly reactive thiol (SH1) of myosin subfragment 1 (S1). Both probes develop circular dichroism (CD) upon modification of SH1 at the visible absorption band of the chromophore. A model system of chiral complexing agents (aromatic chiral amines) interacting with fluorescein in solvent develops a CD signal that mimics that produced by S1. The model system suggests that a specific interaction of the probe with an aromatic chiral residue in the SH1 binding pocket induces the CD signal. Several other spectroscopic signals, including absorption and fluorescence intensity and anisotropy, characterize the fluorescein or rhodamine binding to SH1. A coupled dipole method is adapted to interpret these spectroscopic signals in terms of the probe-S1 complex conformation. The computation of the orientation of the principal hydrodynamic frame (PHF) of S1 from its crystallographic alpha-carbon backbone structure permits the known orientation of the probe in the PHF of S1 to further constrain the conformation of the probe-S1 complex. The coupled dipole interpretation of spectroscopic data combined with constraints relating the probe dipole orientation to the PHF of S1 determines the conformation of the probe-S1 complex. The methods developed here are applied to the spectroscopic signals from fluorescein or rhodamine in the SH1 binding site of S1 to obtain an atomic resolution model of the probe-S1 conformation [Ajtai and Burghardt, Biochemistry, 34 (1995) 15943-15952].

Fluorescence emission and anisotropy from rhodamine dimers

The absorption, fluorescence emission, and excitation fluorescence anisotropy spectra of a rhodamine dye in a water-glycerol solution at high concentration were investigated to determine spectroscopic properties of the ground state dimer. The combination of data from these spectra measured at several dye concentrations contained sufficient constraints on the model for dimer association to permit an estimate of: the association constant, the extinction coefficients, the relative fluorescence quantum yield, and the emission spectra of the monomeric and dimeric species. The rhodamine dimer is an efficient fluorescence emitter with fluorescence anisotropy equivalent to that of the pure monomeric species over the range of excitation wavelengths covering its two lowest energy transitions.