Atomistic Models from Orientation and Distance Constraints Using EPR of a Bifunctional Spin Label.

We have used high-resolution orientation and distance measurements derived from electron paramagnetic resonance of a bifunctional spin label (BSL) to build and refine atomistic models of protein structure. We demonstrate this approach by investigating the effects of nucleotide binding on the structure of myosin's catalytic domain while myosin is in complex with actin. Constraints for orientation of individual helices were obtained in a previous study from continuous-wave electron paramagnetic resonance of myosin labeled at specific sites with BSLs in oriented muscle fibers. In this study, new distance constraints were derived from double electron-electron resonance on myosin constructs labeled with a BSL specifically at two sites. Using these complementary constraints together, we thoroughly characterize the BSL's rigid, highly stereoselective attachment to protein α-helices, which permits accurate measurements of orientation and distance. We also leverage these measurements to derive a novel, to our knowledge, structural model for myosin-II in complex with actin and MgADP and compare our model to other recent actomyosin structures. The described approach is applicable to any orientable complex (e.g., membranes or filaments) in which site-specific di-Cys mutation is feasible.

Myosin lever arm orientation in muscle determined with high angular resolution using bifunctional spin labels.

Despite advances in x-ray crystallography, cryo-electron microscopy (cryo-EM), and fluorescence polarization, none of these techniques provide high-resolution structural information about the myosin light chain domain (LCD; lever arm) under ambient conditions in vertebrate muscle. Here, we measure the orientation of LCD elements in demembranated muscle fibers by electron paramagnetic resonance (EPR) using a bifunctional spin label (BSL) with an angular resolution of 4°. To achieve stereoselective site-directed labeling with BSL, we engineered a pair of cysteines in the myosin regulatory light chain (RLC), either on helix E or helix B, which are roughly parallel or perpendicular to the myosin lever arm, respectively. By exchanging BSL-labeled RLC onto oriented muscle fibers, we obtain EPR spectra from which the angular distributions of BSL, and thus the lever arm, can be determined with high resolution relative to the muscle fiber axis. In the absence of ATP (rigor), each of the two labeled helices exhibits both ordered (σ ∼9-11°) and disordered (σ > 38°) populations. Using these angles to determine the orientation of the lever arm (LCD combined with converter subdomain), we observe that the oriented population corresponds to a lever arm that is perpendicular to the muscle fiber axis and that the addition of ATP in the absence of Ca2+ (inducing relaxation) shifts the orientation to a much more disordered orientational distribution. Although the detected orientation of the myosin light chain lever arm is ∼33° different than predicted from a standard "lever arm down" model based on cryo-EM of actin decorated with isolated myosin heads, it is compatible with, and thus augments and clarifies, fluorescence polarization, x-ray interference, and EM data obtained from muscle fibers. These results establish feasibility for high-resolution detection of myosin LCD rotation during muscle contraction.

A bifunctional spin label reports the structural topology of phospholamban in magnetically-aligned bicelles.

We have applied a bifunctional spin label and EPR spectroscopy to determine membrane protein structural topology in magnetically-aligned bicelles, using monomeric phospholamban (PLB) as a model system. Bicelles are a powerful tool for studying membrane proteins by NMR and EPR spectroscopies, where magnetic alignment yields topological constraints by resolving the anisotropic spectral properties of nuclear and electron spins. However, EPR bicelle studies are often hindered by the rotational mobility of monofunctional Cys-linked spin labels, which obscures their orientation relative to the protein backbone. The rigid and stereospecific TOAC label provides high orientational sensitivity but must be introduced via solid-phase peptide synthesis, precluding its use in large proteins. Here we show that a bifunctional methanethiosulfonate spin label attaches rigidly and stereospecifically to Cys residues at i and i+4 positions along PLB's transmembrane helix, thus providing orientational resolution similar to that of TOAC, while being applicable to larger membrane proteins for which synthesis is impractical. Computational modeling and comparison with NMR data shows that these EPR experiments provide accurate information about helix tilt relative to the membrane normal, thus establishing a robust method for determining structural topology in large membrane proteins with a substantial advantage in sensitivity over NMR.

Bifunctional Spin Labeling of Muscle Proteins: Accurate Rotational Dynamics, Orientation, and Distance by EPR.

While EPR allows for the characterization of protein structure and function due to its exquisite sensitivity to spin label dynamics, orientation, and distance, these measurements are often limited in sensitivity due to the use of labels that are attached via flexible monofunctional bonds, incurring additional disorder and nanosecond dynamics. In this chapter, we present methods for using a bifunctional spin label (BSL) to measure muscle protein structure and dynamics. We demonstrate that bifunctional attachment eliminates nanosecond internal rotation of the spin label, thereby allowing the accurate measurement of protein backbone rotational dynamics, including microsecond-to-millisecond motions by saturation transfer EPR. BSL also allows for accurate determination of helix orientation and disorder in mechanically and magnetically aligned systems, due to the label's stereospecific attachment. Similarly, labeling with a pair of BSL greatly enhances the resolution and accuracy of distance measurements measured by double electron-electron resonance (DEER). Finally, when BSL is applied to a protein with high helical content in an assembly with high orientational order (e.g., muscle fiber or membrane), two-probe DEER experiments can be combined with single-probe EPR experiments on an oriented sample in a process we call BEER, which has the potential for ab initio high-resolution structure determination.

High-resolution helix orientation in actin-bound myosin determined with a bifunctional spin label.

Using electron paramagnetic resonance (EPR) of a bifunctional spin label (BSL) bound stereospecifically to Dictyostelium myosin II, we determined with high resolution the orientation of individual structural elements in the catalytic domain while myosin is in complex with actin. BSL was attached to a pair of engineered cysteine side chains four residues apart on known α-helical segments, within a construct of the myosin catalytic domain that lacks other reactive cysteines. EPR spectra of BSL-myosin bound to actin in oriented muscle fibers showed sharp three-line spectra, indicating a well-defined orientation relative to the actin filament axis. Spectral analysis indicated that orientation of the spin label can be determined within <2.1° accuracy, and comparison with existing structural data in the absence of nucleotide indicates that helix orientation can also be determined with <4.2° accuracy. We used this approach to examine the crucial ADP release step in myosin's catalytic cycle and detected reversible rotations of two helices in actin-bound myosin in response to ADP binding and dissociation. One of these rotations has not been observed in myosin-only crystal structures.

Redox-sensitive residue in the actin-binding interface of myosin.

We have examined the chemical and functional reversibility of oxidative modification in myosin. Redox regulation has emerged as a crucial modulator of protein function, with particular relevance to aging. We previously identified a single methionine residue in Dictyostelium discoideum (Dicty) myosin II (M394, near the myosin cardiomyopathy loop in the actin-binding interface) that is functionally sensitive to oxidation. We now show that oxidation of M394 is reversible by methionine sulfoxide reductase (Msr), restoring actin-activated ATPase activity. Sequence alignment reveals that M394 of Dicty myosin II is a cysteine residue in all human isoforms of skeletal and cardiac myosin. Using Dicty myosin II as a model for site-specific redox sensitivity of this Cys residue, the M394C mutant can be glutathionylated in vitro, resulting in reversible inhibition of actin-activated ATPase activity, with effects similar to those of methionine oxidation at this site. This work illustrates the potential for myosin to function as a redox sensor in both non-muscle and muscle cells, modulating motility/contractility in response to oxidative stress.