Effects of SH1 and SH2 modifications on myosin: similarities and differences

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.

Electrostatic interactions in the force-generating region of the human cardiac myosin modulate ADP dissociation from actomyosin

Human cardiac myosin has two isoforms, alpha and beta, sharing significant sequence similarity, but different in kinetics: ADP release from actomyosin is an order of magnitude faster in the alpha myosin isoform. Apparently, small differences in the sequence are responsible for distinct local inter-residue interactions within alpha and beta isoforms, leading to such a dramatic difference in the rate of ADP release. Our analysis of structural kinetics of alpha and beta isoforms using molecular dynamics simulations revealed distinct dynamics of SH1:SH2 helix within the force-generation region of myosin head. The simulations showed that the residue R694 of the helix forms two permanent salt bridges in the beta isoform, which are not present in the alpha isoform. We hypothesized that the isoform-specific electrostatic interactions play a role in the difference of kinetic properties of myosin isoforms. We prepared R694N mutant in the beta isoform background to destabilize electrostatic interactions in the force-generating region of the myosin head. Our experimental data confirm faster ADP release from R694N actomyosin mutant, but is not as dramatic as the difference of kinetics of ADP release in the alpha and beta isoforms.

A Drosophila model of dominant inclusion body myopathy type 3 shows diminished myosin kinetics that reduce muscle power and yield myofibrillar defects

Individuals with inclusion body myopathy type 3 (IBM3) display congenital joint contractures with early-onset muscle weakness that becomes more severe in adulthood. The disease arises from an autosomal dominant point mutation causing an E706K substitution in myosin heavy chain type IIa. We have previously expressed the corresponding myosin mutation (E701K) in homozygous Drosophila indirect flight muscles and recapitulated the myofibrillar degeneration and inclusion bodies observed in the human disease. We have also found that purified E701K myosin has dramatically reduced actin-sliding velocity and ATPase levels. Since IBM3 is a dominant condition, we now examine the disease state in heterozygote Drosophila in order to gain a mechanistic understanding of E701K pathogenicity. Myosin ATPase activities in heterozygotes suggest that approximately equimolar levels of myosin accumulate from each allele. In vitro actin sliding velocity rates for myosin isolated from the heterozygotes were lower than the control, but higher than for the pure mutant isoform. Although sarcomeric ultrastructure was nearly wild type in young adults, mechanical analysis of skinned indirect flight muscle fibers revealed a 59% decrease in maximum oscillatory power generation and an approximately 20% reduction in the frequency at which maximum power was produced. Rate constant analyses suggest a decrease in the rate of myosin attachment to actin, with myosin spending decreased time in the strongly bound state. These mechanical alterations result in a one-third decrease in wing beat frequency and marginal flight ability. With aging, muscle ultrastructure and function progressively declined. Aged myofibrils showed Z-line streaming, consistent with the human heterozygote phenotype. Based upon the mechanical studies, we hypothesize that the mutation decreases the probability of the power stroke occurring and/or alters the degree of movement of the myosin lever arm, resulting in decreased in vitro motility, reduced muscle power output and focal myofibrillar disorganization similar to that seen in individuals with IBM3.

Summary: Reduced muscle power output and progressive myofibrillar defects in a Drosophila model of inclusion body myopathy 3 arise from the decreased rate of weak to strong actin-binding transition of myosin.

Calponin-Like Protein from Mussel Smooth Muscle Is a Competitive Inhibitor of Actomyosin ATPase

The goal of this work was to elucidate the mechanism of inhibition of the actin-activated ATPase of myosin subfragment-1 (S1) by the calponin-like protein from mussel bivalve muscle. The calponin-like protein (Cap) is a 40-kDa actin-binding protein from the bivalve muscle of the mussel Crenomytilus grayanus. Kinetic parameters Vmax and KATPase of actomyosin ATPase in the absence and the presence of Cap were determined to investigate the mechanism of inhibition. It was found that Cap mainly causes increase in KATPase value and to a lesser extent the decrease in Vmax, which indicates that it is most likely a competitive inhibitor of actomyosin ATPase. Analysis of Vmax and KATPase parameters in the presence of tropomyosin revealed that the latter is a noncompetitive inhibitor of the actomyosin ATPase.

Expression of the inclusion body myopathy 3 mutation in Drosophila depresses myosin function and stability and recapitulates muscle inclusions and weakness

A Drosophila model of myosin-based inclusion body myopathy type 3 is presented. Muscle function, ATPase activity, and actin sliding velocity were dramatically reduced. The mutant myosin is prone to aggregate, likely accounting for the observed cytoplasmic inclusions and disorganized muscle filaments reminiscent of the human disease.

Hereditary myosin myopathies are characterized by variable clinical features. Inclusion body myopathy 3 (IBM-3) is an autosomal dominant disease associated with a missense mutation (E706K) in the myosin heavy chain IIa gene. Adult patients experience progressive muscle weakness. Biopsies reveal dystrophic changes, rimmed vacuoles with cytoplasmic inclusions, and focal disorganization of myofilaments. We constructed a transgene encoding E706K myosin and expressed it in Drosophila (E701K) indirect flight and jump muscles to establish a novel homozygous organism with homogeneous populations of fast IBM-3 myosin and muscle fibers. Flight and jump abilities were severely reduced in homozygotes. ATPase and actin sliding velocity of the mutant myosin were depressed >80% compared with wild-type myosin. Light scattering experiments and electron microscopy revealed that mutant myosin heads bear a dramatic propensity to collapse and aggregate. Thus E706K (E701K) myosin appears far more labile than wild-type myosin. Furthermore, mutant fly fibers exhibit ultrastructural hallmarks seen in patients, including cytoplasmic inclusions containing aberrant proteinaceous structures and disorganized muscle filaments. Our Drosophila model reveals the unambiguous consequences of the IBM-3 lesion on fast muscle myosin and fibers. The abnormalities observed in myosin function and muscle ultrastructure likely contribute to muscle weakness observed in our flies and patients.

Functional, structural, and chemical changes in myosin associated with hydrogen peroxide treatment of skeletal muscle fibers

To understand the molecular mechanism of oxidation-induced inhibition of muscle contractility, we have studied the effects of hydrogen peroxide on permeabilized rabbit psoas muscle fibers, focusing on changes in myosin purified from these fibers. Oxidation by 5 mM peroxide decreased fiber contractility (isometric force and shortening velocity) without significant changes in the enzymatic activity of myofibrils and isolated myosin. The inhibitory effects were reversed by treating fibers with dithiothreitol. Oxidation by 50 mM peroxide had a more pronounced and irreversible inhibitory effect on fiber contractility and also affected enzymatic activity of myofibrils, myosin, and actomyosin. Peroxide treatment also affected regulation of contractility, resulting in fiber activation in the absence of calcium. Electron paramagnetic resonance of spin-labeled myosin in muscle fibers showed that oxidation increased the fraction of myosin heads in the strong-binding structural state under relaxing conditions (low calcium) but had no effect under activating conditions (high calcium). This change in the distribution of structural states of myosin provides a plausible explanation for the observed changes in both contractile and regulatory functions. Mass spectroscopy analysis showed that 50 mM but not 5 mM peroxide induced oxidative modifications in both isoforms of the essential light chains and in the heavy chain of myosin subfragment 1 by targeting multiple methionine residues. We conclude that 1) inhibition of muscle fiber contractility via oxidation of myosin occurs at high but not low concentrations of peroxide and 2) the inhibitory effects of oxidation suggest a critical and previously unknown role of methionines in myosin function.

Age-related decline in actomyosin structure and function

This review focuses on the role of changes in the contractile proteins actin and myosin in age-related deterioration of skeletal muscle function. Functional and structural changes in contractile proteins have been determined indirectly from specific force and unloaded shortening velocity of permeabilized muscle fibers, and were detected directly from site-directed spectroscopy in muscle fibers and from biochemical analysis of purified actin and myosin. Contractile proteins from aged and young muscle differ in (a) myosin and actomyosin ATPase activities, (b) structural states of myosin in contracting muscle, (c) the state of oxidative modifications. The extent of age-related physiological and molecular changes is dependent on the studied animal, the animal's age, and the type of muscle. Therefore, understanding the aging process requires systematic, multidisciplinary studies on physiological, biochemical, structural, and chemical changes in specific muscles.

Nucleotide-induced and actin-induced structural changes in SH1-SH2-modified myosin subfragment 1

We compared the structural properties of myosin subfragment 1 (S1) modified at both reactive SH-groups, SH1 (Cys707) and SH2 (Cys697), with the properties of unmodified S1 and SH1-modified S1. It is shown using differential scanning calorimetry (DSC) that SH1 modification has no noticeable influence on the changes in S1 thermal unfolding induced by the formation of S1 ternary complexes with ADP and P(i) analogs (V(i), AlF(4)(-), and BeF(x)). These changes, however, normally expressed in a significant increase of S1 thermal stability, are almost fully prevented by modification of both SH1 and SH2. In contrast, SH2 modification had no effect on the changes induced by the formation of the ternary complexes S1-ADP-V(i), S1-ADP-AlF(4)(-), and S1-ADP-BeF(x) in EPR spectra of S1 spin-labeled at SH1 group. Interaction of S1 with F-actin substantially increased the thermal stability of S1; a similar effect was observed by DSC with both SH1- and SH1-SH2-modified S1. Overall, our results demonstrate that modification of both reactive SH-groups on S1 has no influence on the actin-induced changes of S1 and on the local nucleotide-induced conformational changes in the SH1 group region, but strongly prevents the global nucleotide-induced structural changes in the entire S1 molecule. The results suggest that modification of SH1 and SH2 impairs the spread of nucleotide-induced conformational changes from the ATPase site throughout the structure of the entire S1 molecule, thus disturbing a coupling between the motor and regulatory domains in the myosin head.

Green fluorescent protein impairs actin-myosin interactions by binding to the actin-binding site of myosin

Green fluorescent proteins (GFP) are widely used in biology for tracking purposes. Although expression of GFP is considered to be innocuous for the cells, deleterious effects have been reported. We recently demonstrated that expression of eGFP in muscle impairs its contractile properties (Agbulut, O., Coirault, C., Niederlander, N., Huet, A., Vicart, P., Hagege, A., Puceat, M., and Menasche, P. (2006) Nat. Meth. 3, 331). This prompted us to identify the molecular mechanisms linking eGFP expression to contractile dysfunction and, particularly, to test the hypothesis that eGFP could inhibit actin-myosin interactions. Therefore, we assessed the cellular, mechanical, enzymatic, biochemical, and structural properties of myosin in the presence of eGFP and F-actin. In vitro motility assays, the maximum actin-activated ATPase rate (V(max)) and the associated constant of myosin for actin (K(m)) were determined at 1:0.5, 1:1, and 1:3 myosin:eGFP molar ratios. At a myosin:eGFP ratio of 1:0.5, there was a nearly 10-fold elevation of K(m). As eGFP concentration increased relative to myosin, the percentage of moving filaments, the myosin-based velocity, and V(max) significantly decreased compared with controls. Moreover, myosin co-precipitated with eGFP. Crystal structures of myosin, actin, and GFP indicated that GFP and actin exhibited similar electrostatic surface patterns and the ClusPro docking model showed that GFP bound preferentially to the myosin head and especially to the actin-binding site. In conclusion, our data demonstrate that expression of eGFP in muscle resulted in the binding of eGFP to myosin, thereby disturbing the actin-myosin interaction and in turn the contractile function of the transduced cells. This potential adverse effect of eGFP should be kept in mind when using this marker to track cells following transplantation.

Fluorescence labeling and computational analysis of the strut of myosin’s 50kDa cleft

A new fluorescent labeling procedure specific for the strut sequence of myosin subfragment-1's 50kDa cleft was developed using CY3 N-hydroxy succinimidyl ester as a hydrophobic tag and hydrophobic interaction chromatography to purify the major labeled species which retained actin-activated ATPase activity. Stern-Volmer analysis suggests that the CY3 is in close proximity to basic residues, consistent with inspection of the mapped labeling site in the atomic model. Fluorescence polarization indicates that the CY3 becomes more mobile upon actin binding, supporting a location near the actomyosin interface. In contrast, nucleotide binding to myosin had little impact on the CY3. Molecular mechanics and stochastic dynamics simulations suggest that this labeling site is sensitive to forced cleft opening and closure, but the upper 50kDa cleft does not move easily. In addition, there appear to be some long-range effects of forced cleft opening and closing that could impact the lever arm position.

Age-Related Decline in Actomyosin Function

To understand the molecular basis of the functional decline in aging muscle, we examined the functional (actomyosin ATPase) and chemical (cysteine content) changes in actin and myosin purified from the muscles of young (4- to 12-month-old) and old (27- to 35-month-old) Fisher 344 rats. Using the soluble, catalytically active myosin fragment, heavy meromyosin (HMM), we determined the maximum rate (V(max)) and actin concentration at half V(max) (K(m)) of the actomyosin ATPase, using four combinations of actin and HMM from old and young rats. V(max) and K(m) were significantly lower when both actin and HMM were obtained from old rats than when both proteins were obtained from young rats. The number of reactive cysteines in HMM significantly decreased with age, but no change was detected in the number of reactive cysteines in actin. We conclude that aging results in chemical changes in myosin (probably oxidation of cysteines) that have inhibitory effects on the actin-activated myosin ATPase.

Regulation of the actin-myosin interaction by titin

Titin is known to interact with actin thin filaments within the I-band region of striated muscle sarcomeres. In this study, we have used a titin fragment of 800 kDa (T800) purified from striated skeletal muscle to measure the effect of this interaction on the functional properties of the actin-myosin complex. MALDI-TOF MS revealed that T800 contains the entire titin PEVK (Pro, Glu, Val, Lys-rich) domain. In the presence of tropomyosin-troponin, T800 increased the sliding velocity (both average and maximum values) of actin filaments on heavy-meromyosin (HMM)-coated surfaces and dramatically decreased the number of stationary filaments. These results were correlated with a 30% reduction in actin-activated HMM ATPase activity and with an inhibition of HMM binding to actin N-terminal residues as shown by chemical cross-linking. At the same time, T800 did not affect the efficiency of the Ca(2+)-controlled on/off switch, nor did it alter the overall binding energetics of HMM to actin, as revealed by cosedimentation experiments. These data are consistent with a competitive effect of PEVK domain-containing T800 on the electrostatic contacts at the actin-HMM interface. They also suggest that titin may participate in the regulation of the active tension generated by the actin-myosin complex.

Engineering lysine reactivity as a conformational sensor in the Dictyostelium myosin II motor domain

Lys84 of skeletal muscle myosin located at the interface between the motor and neck domains has long been utilized as a useful chemical probe sensing motor domain conformational changes and tilting of the lever arm. Here we report the first site-directed mutagenesis study on this side chain and its immediate chemical environment. We made Dictyostelium myosin II motor domain constructs in which Lys84 was replaced by either a methionine or a glutamic acid residue and another mutant containing an Arg704Glu substitution. By following trinitrophenylation of the mutant constructs, we first unambiguously identify Lys84 as the reactive lysine in Dictyostelium myosin. Analysis of the reaction profiles also reveals that the Lys84-Arg704 interaction at the interface of two subdomains of the myosin head has a significant effect on Lys84 reactivity, but it is not the only determinant of this property. Our findings imply that the nucleotide sensitivity of the trinitrophenylation reaction is a general feature of conventional myosins that reflects similar changes in the conformational dynamics of the different orthologs during the ATPase cycle.

Conformation and dynamics of the SH1-SH2 helix in scallop myosin

Atomic structures of scallop myosin subfragment 1(S1) with the bound MgADP, MgAMPPNP, and MgADP.BeF(x) provide crystallographic evidence for a destabilization of the helix containing reactive thiols SH1 (Cys703) and SH2 (Cys693). A destabilization of this helix was not observed in previous structures of S1 (from chicken skeletal, Dictyostelium discoideum, and smooth muscle myosins), including complexes for which solution experiments indicated such a destabilization. In this study, the factors that influence the SH1-SH2 helix in scallop S1 were examined using monofunctional and bifunctional thiol reagents. The rate of monofunctional labeling of scallop S1 was increased in the presence of MgADP and MgATPgammaS but was inhibited by MgADP.V(i) and actin. The resulting changes in ATPase activities of S1 were symptomatic of SH2 and not SH1 modification, which was confirmed by mass spectrometry analysis. With bifunctional reagents of various lengths, cross-linking did not occur on a short time scale in the absence of nucleotides. In the presence of MgADP, cross-linking was greatly enhanced for all of the reagents. These reactions, as well as the formation of a disulfide bond between SH1 and SH2, were much faster in scallop S1.ADP than in rabbit skeletal S1.ADP and were rate-limited by the initial attachment of the reagent to scallop S1. The cross-linking sites were mapped by mass spectrometry to SH1 and SH2. These results reveal isoform-specific differences in the conformation and dynamics of the SH1-SH2 helix, providing a possible explanation for destabilization of this helix in some scallop S1 but not in other S1 isoform structures.

Effect of adenosine 5'-[beta,gamma-imido]triphosphate on myosin head domain movements

Conventional and saturation transfer electron paramagnetic resonance spectroscopy (EPR and ST EPR) was used to study the orientation of probe molecules in muscle fibers in different intermediate states of the ATP hydrolysis cycle. A separate procedure was used to obtain ST EPR spectra with precise phase settings even in the case of samples with low spectral intensity. Fibers prepared from rabbit psoas muscle were labeled with isothiocyanate spin labels at the reactive thiol sites of the catalytic domain of myosin. In comparison with rigor, a significant difference was detected in the orientation-dependence of spin labels in the ADP and adenosine 5'-[beta,gamma-imido]triphosphate (AdoPP[CH2]P) states, indicating changes in the internal dynamics and domain orientation of myosin. In the AdoPP[CH2]P state, approximately half of the myosin heads reflected the motional state of ADP-myosin, and the other half showed a different dynamic state with greater mobility.

Calcium-sensitive regions of GCAP1 as observed by chemical modifications, fluorescence, and EPR spectroscopies

Guanylyl cyclase-activating proteins are EF-hand Ca(2+)-binding proteins that belong to the calmodulin superfamily. They are involved in the regulation of photoreceptor membrane-associated guanylyl cyclases that produce cGMP, a second messenger of vertebrate vision. Here, we investigated changes in GCAP1 structure using mutagenesis, chemical modifications, and spectroscopic methods. Two Cys residues of GCAP1 situated in spatially distinct regions of the N-terminal domain (positions 18 and 29) and two Cys residues located within the C-terminal lobe (positions 106 and 125) were employed to detect conformational changes upon Ca(2+) binding. GCAP1 mutants with only a single Cys residue at each of these positions, modified with N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)ethylenediamine, an environmentally sensitive fluorophore, and with (1-oxy-2,2,5,5-tetramethylpyrroline-3-methyl)methanethiosulfonate, a spin label reagent, were studied using fluorescence and EPR spectroscopy, respectively. Only minor structural changes around Cys(18), Cys(29), Cys(106), and Cys(125) were observed as a function of Ca(2+) concentration. No Ca(2+)-dependent oligomerization of GCAP1 was observed at physiologically relevant Ca(2+) concentrations, in contrast to the observation reported by others for GCAP2. Based on these results and previous studies, we propose a photoreceptor activation model that assumes changes within the flexible central helix upon Ca(2+) dissociation, causing relative reorientation of two structural domains containing a pair of EF-hand motifs and thus switching its partner, guanylyl cyclase, from an inactive (or low activity) to an active conformation.

Nucleotide binding induces global and local structural changes of myosin head in muscle fibres

Thermal stability and internal dynamics of myosin heads in fiber bundles from rabbit psoas muscle has been studied by electron paramagnetic resonance (EPR) spectroscopy and differential scanning calorimetry (DSC). Using ADP, ATP and orthovanadate (V(i)), three intermediate states of the ATP hydrolysis cycle were simulated in glycerinated muscle fibers. DSC transitions contained three overlapping endotherms in each state. Deconvolution showed that the transition temperature of 58.4 degrees C was almost independent of the intermediate state of myosin, while nucleotide binding shifted the melting temperatures of 54.0 and 62.3 degrees C, and changed the enthalpies. These changes suggest global rearrangements of the internal structure in myosin head. In the presence of ADP and ADP plus V(i), the conventional EPR spectra showed changes in the ordering of the probe molecules, suggesting local conformational and motional changes in the internal structure of myosin heads. Saturation transfer EPR measurements reported increased rotational mobility of spin labels in the presence of ATP plus orthovanadate corresponding to a weakly binding state of myosin to actin.

Changes in the thermal unfolding of p-phenylenedimaleimide-modified myosin subfragment 1 induced by its 'weak' binding to F-actin

Differential scanning calorimetry (DSC) was used to analyze the thermal unfolding of myosin subfragment 1 (S1) with the SH1 (Cys-707) and SH2 (Cys-697) groups cross-linked by N,N'-p-phenylenedimaleimide (pPDM-S1). It has been shown that F-actin affects the thermal unfolding of pPDM-S1 only at very low ionic strength, when some part of pPDM-S1 binds weakly to F-actin, but not at higher ionic strength (200 mM KCl). The weak binding of pPDM-S1 to F-actin shifted the thermal transition of pPDM-S1 by about 5 degrees C to a higher temperature. This actin-induced increase in thermal stability of pPDM-S1 was similar to that observed with 'strong' binding of unmodified S1 to F-actin. Our results show that actin-induced structural changes revealed by DSC in the myosin head occur not only upon strong binding but also on weak binding of the head to F-actin, thus suggesting that these changes may occur before the power-stroke and play an important role in the motor function of the head.

Visualizing myosin's power stroke in muscle contraction

The long-standing swinging crossbridge or lever arm hypothesis for the motor action of myosin heads finds support in recent results from 3-D tomograms of insect flight muscle (IFM) fast frozen during active contraction and from both fluorescence polarization and X-ray diffraction during rapid stretches or releases of isometrically contracting fibers. The latter provide direct evidence for lever arm movements synchronous with force changes. Rebuilding the atomic model of nucleotide-free subfragment 1 (S1) to fit fast-frozen, active IFM crossbridges suggests a two-stage power stroke in which the catalytic domain rolls on actin from weak to strong binding; this is followed by a 5-nm lever arm swing of the light chain domain, which gives a total interaction distance of approx. 12 nm. Comparison of S1 crystal structures with in situ myosin heads suggests that actin binding may be necessary in order to view the full repertoire of myosin motor action. The differing positions of the catalytic domains of actin-attached myosin heads in contracting IFM suggest that both the actin-myosin binding energy and the hydrolysis of ATP may be used to cock the crossbridge and drive the power stroke.

Stabilization of the actomyosin complex by negative charges on myosin

Sequence comparisons of members of the myosin superfamily show a high degree of charge conservation in a surface exposed helix (Dictyostelium discoideum myosin II heavy chain residues S510 to K546). Most myosins display a triplet of acidic residues at the equivalent positions to D. discoideummyosin II residues D530, E531, and Q532. The high degree of charge conservation suggests strong evolutionary constrain and that this region is important for myosin function. Mutations at position E531 were shown to strongly affect actin binding [Giese, K. C., and Spudich, J. A. (1997) Biochemistry 36, 8465-8473]. Here, we used steady-state and transient kinetics to characterize the enzymatic competence of mutant constructs E531Q and Q532E, and their properties were compared with those of a loop 2 mutant with a 20 amino acid insertion containing 12 positive charges (20/+12) [Furch et al. (1998) Biochemistry 37, 6317-6326], double mutant Q532E(20/+12), and the native motor domain constructs. Our results confirm that charge changes at residues 531 and 532 primarily affect actin binding with little change being communicated to the nucleotide pocket. Mutation D531Q reduces actin affinity (K(A)) 10-fold, while Q532E leads to a 5-fold increase. The observed changes in K(A)() stem almost exclusively from variations in the dissociation rate constant (k(-A)), with the introduction of a single negative charge at position 532 having the same effect on k(-A) as the introduction of 12 positive charges in the loop 2 region.

Is SH1-SH2-cross-linked myosin subfragment 1 a structural analog of the weakly-bound state of myosin?

Myosin subfragment 1 (S1) with SH1 (Cys(707)) and SH2 (Cys(697)) groups cross-linked by p-phenylenedimaleimide (pPDM-S1) is thought to be an analog of the weakly bound states of myosin bound to actin. The structural properties of pPDM-S1 were compared in this study to those of S1.ADP.BeF(x) and S1.ADP.AlF(4)(-), i.e., the established structural analogs of the myosin weakly bound states. To distinguish between the conformational effects of SH1-SH2 cross-linking and those due to their monofunctional modification, we used S1 with the SH1 and SH2 groups labeled with N-phenylmaleimide (NPM-S1) as a control in our experiments. The state of the nucleotide pocket was probed using a hydrophobic fluorescent dye, 3-[4-(3-phenyl-2-pyrazolin-1-yl)benzene-1-sulfonylamido]phen ylboronic acid (PPBA). Differential scanning calorimetry (DSC) was used to study the thermal stability of S1. By both methods the conformational state of pPDM-S1 was different from that of unmodified S1 in the S1.ADP.BeF(x) and S1.ADP.AlF(4)(-) complexes and closer to that of nucleotide-free S1. Moreover, BeF(x) and AlF(4)(-) binding failed to induce conformational changes in pPDM-S1 similar to those observed in unmodified S1. Surprisingly, when pPDM cross-linking was performed on S1.ADP.BeF(x) complex, ADP.BeF(x) protected to some extent the nucleotide pocket of S1 from the effects of pPDM modification. NPM-S1 behaved similarly to pPDM-S1 in our experiments. Overall, this work presents new evidence that the conformational state of pPDM-S1 is different from that of the weakly bound state analogs, S1.ADP.BeF(x) and S1.ADP.AlF(4)(-). The similar structural effects of pPDM cross-linking of SH1 and SH2 groups and their monofunctional labeling with NPM are ascribed to the inhibitory effects of these modifications on the flexibility/mobility of the SH1-SH2 helix.