Domain motion between the regulatory light chain and the nucleotide site in skeletal myosin

Asymmetric myosin binding to the thin filament as revealed by a fluorescent nanocircuit

The interplay between myosin, actin, and striated muscle regulatory proteins involves complex cooperative interactions that propagate along the thin filament. A repeating unit of the tropomyosin dimer, troponin heterotrimer, and the actin protofilament heptamer is sometimes assumed to be able to bind myosin at any of its seven actins when activated even though the regulatory proteins are asymmetrically positioned along this repeating unit. Analysis of the impact of this asymmetry on actin and myosin interactions by sensitized emission luminescence resonance energy transfer spectroscopy and a unique fluorescent nanocircuit design reveals that the troponin affects the structure and function of myosin heads bound nearby in a different manner than myosin heads bound further away from the troponin. To test this hypothesis, a fluorescent nanocircuit reported the position of the myosin lever arm only when the myosin was bound adjacent to the troponin, or in controls, only when the myosin was bound distant from the troponin. Confirming the hypothesis, the myosin lever arm is predominantly in the pre powerstroke orientation when bound near troponin, but is predominantly in the post powerstroke orientation when bound distant from troponin. These data are consistent with the hypothesis that troponin is responsible for the formation of myosin binding target zones along the thin filament.

Site-directed spectroscopic probes of actomyosin structural dynamics

Spectroscopy of myosin and actin has entered a golden age. High-resolution crystal structures of isolated actin and myosin have been used to construct detailed models for the dynamic actomyosin interactions that move muscle. Improved protein mutagenesis and expression technologies have facilitated site-directed labeling with fluorescent and spin probes. Spectroscopic instrumentation has achieved impressive advances in sensitivity and resolution. Here we highlight the contributions of site-directed spectroscopic probes to understanding the structural dynamics of myosin II and its actin complexes in solution and muscle fibers. We emphasize studies that probe directly the movements of structural elements within the myosin catalytic and light-chain domains, and changes in the dynamics of both actin and myosin due to their alternating strong and weak interactions in the ATPase cycle. A moving picture emerges in which single biochemical states produce multiple structural states, and transitions between states of order and dynamic disorder power the actomyosin engine.

Close proximity of myosin loop 3 to troponin determined by triangulation of resonance energy transfer distance measurements

Cooperative activation of the thin filament is known to be influenced by the tight binding of myosin to actin, but the molecular mechanism underlying this contribution of myosin is not well understood. To better understand the structural relationship of myosin with the regulatory troponin complex, resonance energy transfer measurements were used to map the location of troponin relative to a neighboring myosin bound to actin using atomic models. Using a chicken troponin T isoform that contains a single cysteine near the binding interface between troponins T, I, and C, this uniquely labeled cysteine on troponin was found to be remarkably near loop 3 of myosin. This loop has previously been localized near the actin and myosin interface by chemical cross-linking methods, but its functional contributions have not been established. The implications of this close proximity are examined by molecular modeling, which suggests that only restricted conformations of actomyosin can accommodate the presence of troponin at this location near the cross-bridge. This potential for interaction between troponin and myosin heads that bind near it along the thin filament raises the possibility of models in which direct myosin and troponin interactions may play a role in the regulatory mechanism.

Myosin regulatory domain orientation in skeletal muscle fibers: application of novel electron paramagnetic resonance spectral decomposition and molecular modeling methods

Reorientation of the regulatory domain of the myosin head is a feature of all current models of force generation in muscle. We have determined the orientation of the myosin regulatory light chain (RLC) using a spin-label bound rigidly and stereospecifically to the single Cys-154 of a mutant skeletal isoform. Labeled RLC was reconstituted into skeletal muscle fibers using a modified method that results in near-stoichiometric levels of RLC and fully functional muscle. Complex electron paramagnetic resonance spectra obtained in rigor necessitated the development of a novel decomposition technique. The strength of this method is that no specific model for a complex orientational distribution was presumed. The global analysis of a series of spectra, from fibers tilted with respect to the magnetic field, revealed two populations: one well-ordered (+/-15 degrees ) with the spin-label z axis parallel to actin, and a second population with a large distribution (+/-60 degrees ). A lack of order in relaxed or nonoverlap fibers demonstrated that regulatory domain ordering was defined by interaction with actin rather than the thick filament surface. No order was observed in the regulatory domain during isometric contraction, consistent with the substantial reorientation that occurs during force generation. For the first time, spin-label orientation has been interpreted in terms of the orientation of a labeled domain. A Monte Carlo conformational search technique was used to determine the orientation of the spin-label with respect to the protein. This in turn allows determination of the absolute orientation of the regulatory domain with respect to the actin axis. The comparison with the electron microscopy reconstructions verified the accuracy of the method; the electron paramagnetic resonance determined that axial orientation was within 10 degrees of the electron microscopy model.

Conformational selection during weak binding at the actin and myosin interface

The molecular mechanism of the powerstroke in muscle is examined by resonance energy transfer techniques. Recent models suggesting a pre-cocking of the myosin head involving an enormous rotation between the lever arm and the catalytic domain were tested by measuring separation distances among myosin subfragment-2, the nucleotide site, and the regulatory light chain in the presence of nucleotide transition state analogs. Only small changes (<0.5 nm) were detected that are consistent with internal conformational changes of the myosin molecule, but not with extreme differences in the average lever arm position suggested by some atomic models. These results were confirmed by stopped-flow resonance energy transfer measurements during single ATP turnovers on myosin. To examine the participation of actin in the powerstroke process, resonance energy transfer between the regulatory light chain on myosin subfragment-1 and the C-terminus of actin was measured in the presence of nucleotide transition state analogs. The efficiency of energy transfer was much greater in the presence of ADP-AlF(4), ADP-BeF(x), and ADP-vanadate than in the presence of ADP or no nucleotide. These data detect profound differences in the conformations of the weakly and strongly attached cross-bridges that appear to result from a conformational selection that occurs during the weak binding of the myosin head to actin.

Native purification of biomolecules with temperature-mediated hydrophobic modulation liquid chromatography

The high-resolution purification of native enzymes is impeded by the limitations in the mobile-phase choices required for conventional hydrophobic separations such as in reverse-phase chromatography. To avoid problems associated with varying the composition of the mobile phase, we developed a stationary phase with a hydrophobicity that can be modulated by slight variations in temperature to bind and elute biomolecules. This chromatographic matrix was tested on nucleotide analogs, amino acids, and protein samples. Visualization of the temperature-dependent hydrophobic interaction with the chromatographic matrix was performed with fluorescence microscopy of CY3-ATP. Amino acids adsorbed to the column according to their known hydrophobicities, confirming the hydrophobic nature of their interaction with the matrix. Biomolecules were separated by modulating the hydrophobicity of the column matrix with slight adjustments to the running temperature between 22 and 37 degrees C without changing the mobile phase. Freedom in the choice of a mobile phase for both the loading and the elution of samples provides great practical advantages by eliminating the need for buffer-exchange steps and allowing more native conditions for purifying delicate enzymes, such as myosin.

Determination of fluorescent probe orientations on biomolecules by conformational searching: algorithm testing and applications to the atomic model of myosin

The ability of a localized conformational searching method to predict probe orientation was tested on model nucleic acid and protein structures and applied to the prediction of skeletal myosin integrity upon chemical modification of its reactive thiols. Double-stranded oligonucleotides were chemically labeled with donor and acceptor resonance energy transfer probes at each end for distance determinations. These measurements were made independently using a terbium chelate as a donor to each of four chemically and spectroscopically distinct acceptor probes from the xanthene and cyanine dye groups. The choice of acceptor significantly affected the separation distance measured. Conformational searching algorithms on the atomic model corrected for the differences to within 0.2 nm on average. Verifying its usefulness on proteins, the localized conformational searching method determined the orientation of a fluorescent probe on RNase A that corresponds closely to available crystallographic models of the labeled protein (RMS deviation = 0.1 nm). Also, analysis of the symmetry of the fluorophores' structures suggests why FRET orientation factors are often closer to their dynamic average value than might normally be expected. Furthermore, the computational method provides insights about FRET data that are important for assessing the stability of the alpha-helix separating the SH1 and SH2 reactive thiols in skeletal myosin.