Tryptophan-130 is the most reactive tryptophan residue in rabbit skeletal myosin subfragment-1

From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle

Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.

Mass Spectrometry-Based Protein Footprinting for Higher-Order Structure Analysis: Fundamentals and Applications

Proteins adopt different higher-order structures (HOS) to enable their unique biological functions. Understanding the complexities of protein higher-order structures and dynamics requires integrated approaches, where mass spectrometry (MS) is now positioned to play a key role. One of those approaches is protein footprinting. Although the initial demonstration of footprinting was for the HOS determination of protein/nucleic acid binding, the concept was later adapted to MS-based protein HOS analysis, through which different covalent labeling approaches "mark" the solvent accessible surface area (SASA) of proteins to reflect protein HOS. Hydrogen-deuterium exchange (HDX), where deuterium in D2O replaces hydrogen of the backbone amides, is the most common example of footprinting. Its advantage is that the footprint reflects SASA and hydrogen bonding, whereas one drawback is the labeling is reversible. Another example of footprinting is slow irreversible labeling of functional groups on amino acid side chains by targeted reagents with high specificity, probing structural changes at selected sites. A third footprinting approach is by reactions with fast, irreversible labeling species that are highly reactive and footprint broadly several amino acid residue side chains on the time scale of submilliseconds. All of these covalent labeling approaches combine to constitute a problem-solving toolbox that enables mass spectrometry as a valuable tool for HOS elucidation. As there has been a growing need for MS-based protein footprinting in both academia and industry owing to its high throughput capability, prompt availability, and high spatial resolution, we present a summary of the history, descriptions, principles, mechanisms, and applications of these covalent labeling approaches. Moreover, their applications are highlighted according to the biological questions they can answer. This review is intended as a tutorial for MS-based protein HOS elucidation and as a reference for investigators seeking a MS-based tool to address structural questions in protein science.

The identification of tryptophan residues responsible for ATP-induced increase in intrinsic fluorescence of myosin subfragment 1

ATP binding to myosin subfragment 1 (S1) induces an increase in tryptophan fluorescence. Chymotryptic rabbit skeletal S1 has 5 tryptophan residues (Trp113, 131, 440, 510 and 595), and therefore the identification of tryptophan residues perturbed by ATP is quite complex. To solve this problem we resolved the complex fluorescence spectra into log-normal and decay-associated components, and carried out the structural analysis of the microenvironment of each tryptophan in S1. The decomposition of fluorescence spectra of S1 and S1-ATP complex revealed 3 components with maxima at ca. 318, 331 and 339-342 nm. The comparison of structural parameters of microenvironment of 5 tryptophan residues with the same parameters of single-tryptophan-containing proteins with well identified fluorescence properties applying statistical method of cluster analysis, enabled us to assign Trp595 to 318 nm, Trp440 to 331 nm, and Trp 13, 131 and 510 to 342 nm spectral components. ATP induced an almost equal increase in the intensities of the intermediate (331 nm) and long-wavelength (342 nm) components, and a small decrease in the short component (318 nm). The increase in the intermediate component fluorescence most likely results from an immobilization of some quenching groups (Met437, Met441 and/or Arg444) in the environment of Trp440. The increase in the intensity and a blue shift of the long component might be associated with conformational changes in the vicinity of Trp510. However, these conclusions can not be extended directly to the other types of myosins due to the diversity in the tryptophan content and their microenvironments.

Kinetics of the initial steps of rabbit psoas myofibrillar ATPases studied by tryptophan and pyrene fluorescence stopped-flow and rapid flow-quench. Evidence that cross-bridge detachment is slower than ATP binding

The kinetics of the tryptophan fluorescence enhancement that occurs when myofibrils (rabbit psoas) are mixed with Mg-ATP were studied by stopped-flow in different solvents (water, 40% ethylene glycol, 20% methanol) at 4 degrees C. Under relaxing conditions (low Ca(2+)) in water (mu = 0.16 M, pH 7.4) and at high ATP concentrations, the transient was biphasic, giving a k(fast)(max) of 230 s(-)(1) and a k(slow)(max) of 15 s(-)(1). The kinetics of the two phases were compared with those obtained by chemical sampling using [gamma-(32)P]ATP and quenching in acid (P(i) burst experiments: these give unambiguously the ATP cleavage kinetics), or cold Mg-ATP (cold ATP chase: ATP binding kinetics). k(slow) is due to ATP cleavage, as with S1. Interestingly, k(fast) is slower than the ATP binding kinetics. Instead, this constant appears to report ATP-induced cross-bridge detachment from actin because (1) it was identical to the fluorescence transient obtained on addition of ATP to pyrene-labeled myofibrils; (2) when the initial filament overlap in the myofibrils was decreased, the amplitude of the fast phase decreased; (3) there was no fluorescent enhancement upon the addition of ADP to myofibrils. This is different from the situation with S1 or actoS1 where there was also a fast fluorescent ATP-induced transient but whose kinetics were identical to those of the tight ATP binding. To increase the time resolution and to confirm our results, we also carried out transient kinetics in ethylene glycol and methanol. We interpret our results by a scheme in which a rapid equilibrium between attached (AM.ATP) and detached (M.ATP) states is modulated by the fraction of myosin heads in rigor (AM) during the time of experiment.

Tertiary structural changes in the cleft containing the ATP sensitive tryptophan and reactive thiol are consistent with pivoting of the myosin heavy chain at Gly699

The conformation of myosin subfragment 1 (S1) in the vicinity of the ATP sensitive tryptophan (Trp510) and the highly reactive thiol (SH1), both residing in the "probe-binding" cleft at the junction of the catalytic and lever arm domains, was studied to ascertain its role in the mechanism of energy transduction and force generation. In glycerinated muscle fibers in rigor, a fluorescent probe linked to SH1 detects a strained probe-binding cleft conformation following a length transient by altering emission intensity without detectably rotating. In myosin S1 in solution, the optical activity of Trp510 senses conformation change in the probe-binding cleft caused by substrate analog trapping of S1 in various structures attainable transiently during normal energy transduction. Also in S1 in solution, the induced optical activity of a fluorescein probe linked to SH1 shows sensitivity to changing probe-binding cleft conformation caused by nucleotide binding to the S1 active site. The changes in the optical activity of Trp510 and SH1 bound fluorescein in response to nucleotide or nucleotide analog binding are interpreted structurally using the S1 crystallographic coordinates and aided by a model of energy transduction that pivots at Gly699 to change probe-binding cleft conformation and to displace the S1 lever arm as during force generation. The crystallographic structure of the probe-binding cleft in S1 resembles most the nucleotide bound conformation in the native protein. A different structure, generated by pivoting at Gly699, better resembles the native rigor conformation of the probe-binding cleft. Pivoting at Gly699 rotates probes at SH1 suggesting that length transients on fibers in rigor do not cause pivoting at Gly699 or reverse the power stroke.

Optical activity of a nucleotide-sensitive tryptophan in myosin subfragment 1 during ATP hydrolysis

The xanthene probes 5'-iodoacetamido-fluorescein and -tetramethylrhodamine specifically modify skeletal muscle myosin subfragment 1 (S1) at the reactive thiol residue (SH1) and fully quench the fluorescence emission from tryptophan residue 510 (Trp510) in S1 (T.P. Burghardt and K. Ajtai, Biophys. Chem., 60 (1996) 119; K. Ajtai and T.P. Burghardt, Biochemistry, 34 (1995) 15943). The difference between the fluorescence intensity obtained from S1 and probe-modified S1 comes solely from Trp510 in chymotryptic S1, a protein fragment that contains five tryptophan residues. The rotary strength and quantum efficiency of Trp510 were measured using difference signals from fluorescence detected circular dichroism (FDCD) and fluorescence emission spectroscopy. These structure-sensitive signals indicate that the binding of nucleotide or nucleotide analogs to the active site of S1 causes structural changes in S1 at Trp510 and that a one-to-one correspondence exists between Trp510 conformation and transient states of myosin during contraction. The Trp510 rotary strength and quantum efficiency were interpreted structurally in terms of the indole side-chain conformation using model structures and established computational methods.

Cleft containing reactive thiol of myosin closes during ATP hydrolysis

The probe binding cleft of myosin subfragment 1 (S1) contains the reactive thiol, SH1, and tryptophan 510 (Trp-510). Solvent accessibility to Trp-510, measured using the acrylamide quenching of its fluorescence, is highest in rigor and decreases during the ATPase cycle prior to force generation. These data suggest the probe binding cleft closes during ATP hydrolysis and opens during force generation. The closing of the probe binding cleft may be the origin of the shape change of S1 during ATP hydrolysis.

Fluorescence resonance energy transfer within the regulatory light chain of myosin

Rabbit skeletal muscle myosin regulatory light chain-2 (LC2) contains two reactive cysteine residues, Cys125 and Cys154, and one tryptophan at position 137. Using wild-type rabbit LC2 or its genetically engineered mutant with Cys125-->Arg (C125R), these residues can be selectively modified with fluorescent or chromophoric probes for spectroscopic studies. We have bound suitable donor/acceptor probe pairs to the two cysteine residues and Trp137 in LC2 or C125R, and measured the distance in solution between the probes by fluorescence resonance energy transfer spectroscopy. C125R was made to facilitate specific labelling of the less reactive Cys154, thus allowing the distance between Cys154 and Trp137 to be measured. Our measurements show that these residues are in close proximity to each other, the distance between them ranging from 1.7 nm (between Cys125 and Trp137) to 2.7 nm (Cys125 and Cys154). These results suggest that Cys125, Trp137 and Cys154, spanning up to 29 residues in the sequence of LC2, are spatially close, consistent with these residues residing within a C-terminal globular domain. The distances we obtained are in agreement with previous crosslinking studies [Huber, P. A., Brunner, U.T. & Schaub, M. C. (1989) Biochemistry 28, 9116-9123; Saraswat, L. & Lowey, S. (1991) J. Biol. Chem. 266, 19777-19785] and structure predictions of LC2. LC2 is located at the head-rod junction of the myosin crossbridge, and provides the primary regulatory mechanism in molluscan and smooth muscle. In skeletal muscle, its functional role is unclear, although it has been implicated in modulating actomyosin interaction [Metzger, J. M. & Moss, R. L. (1992) Biophys. J. 63, 460-468]. The incorporation of spectroscopic probes onto the light chains of myosin in solution or in fibres has become a valuable tool for evaluating the dynamic properties of the crossbridge during force generation.

The ATP-induced myosin subfragment-1 fluorescence intensity increase is due to one tryptophan

The irradiation of skeletal muscle myosin subfragment 1 (S1) in the presence of 2,2,2-trichloroethanol (TCE) reduces S1 fluorescence intensity in two phases. In the first phase, there is an increase in MgATPase activity, and no significant change in the fluorescence intensity increase upon ATP binding. In the second phase, the activity remains elevated, but there is a complete loss of the ATP-induced intensity increase. Measurements on denatured S1 indicate that fluorescence intensity reductions of one fifth of the total occur during each of the two phases, consistent with the fluorescence intensity increase upon forming S1.MgADP.P(i) being due to one of the five heavy-chain tryptophans.

[2-3H]ATP synthesis and 3H NMR spectroscopy of enzyme-nucleotide complexes: ADP and ADP.Vi bound to myosin subfragment 1

The synthesis of [2-3H]ATP with specific activity high enough to use for 3H NMR spectroscopy at micromolar concentrations was accomplished by tritiodehalogenation of 2-Br-ATP. ATP with greater than 80% substitution at the 2-position and negligible tritium levels at other positions had a single 3H NMR peak at 8.20 ppm in 1D spectra obtained at 533 MHz. This result enables the application of tritium NMR spectroscopy to ATP utilizing enzymes. The proteolytic fragment of skeletal muscle myosin, called S1, consists of a heavy chain (95 kDa) and one alkali light chain (16 or 21 kDa) complex that retains myosin ATPase activity. In the presence of Mg2+, S1 converts [2-3H]ATP to [2-3H]ADP and the complex S1.Mg[2-3H]ADP has ADP bound in the active site. At 0 degrees C, 1D 3H NMR spectra of S1.Mg[2-3H]ADP have two broadened peaks shifted 0.55 and 0.90 ppm upfield from the peak due to free [2-3H]ADP. Spectra with good signal-to-noise for 0.10 mM S1.Mg[2-3H]ADP were obtained in 180 min. The magnitude of the chemical shift caused by binding is consistent with the presence of an aromatic side chain being in the active site. Spectra were the same for S1 with either of the alkali light chains present, suggesting that the alkali light chains do not interact differently with the active site. The two broad peaks appear to be due to the two conformations of S1 that have been observed previously by other techniques. Raising the temperature to 20 degrees C causes small changes in the chemical shifts, narrows the peak widths from 150 to 80 Hz, and increases the relative area under the more upfield peak. Addition of orthovanadate (Vi) to produce S1.Mg[2-2H]ADP.Vi shifts both peaks slightly more upfield without changing their widths or relative areas.

A search for protein structural changes accompanying the contractile interaction

It appears that small movements (detected hitherto only by fluorescence resonance energy transfer measurements and crosslinking studies) in a region of the myosin S-1 particle may mediate chemomechanical energy transduction in the contractile system. Here we find under conditions of high precision at 10 degrees C and 20 degrees C that ATP binding to S-1 causes small (0.4%) changes in CD signal, delta epsilon 222, as do temperature changes in the regime below 16 degrees C. ATP binding perturbs tryptophan residues that we now think are in the mobile region, and we find here that temperature affects tryptophan fluorescence in much the same way that it affects the CD signal, so we believe that the CD signal reports transduction-related movements in S-1. If S-1 is exposed to the range 16-30 degrees C, CD signal falls with temperature; ATP counteracts this fall. Analysis of vacuum-UV CD spectra yields 42% alpha-helix, 9% antiparallel beta-sheet, 7% parallel beta-sheet, 14% beta-turns, and 29% other structures.