Crystal structure of a vertebrate smooth muscle myosin motor domain and its complex with the essential light chain: visualization of the pre-power stroke state

Mutation spectrum of MYO7A and evaluation of a novel nonsyndromic deafness DFNB2 allele with residual function

Recessive mutations of MYO7A, encoding unconventional myosin VIIA, can cause either a deaf-blindness syndrome (type 1 Usher syndrome; USH1B) or nonsyndromic deafness (DFNB2). In our study, deafness segregating as a recessive trait in 24 consanguineous families showed linkage to markers for the DFNB2/USH1B locus on chromosome 11q13.5. A total of 23 of these families segregate USH1 due to 17 homozygous mutant MYO7A alleles, of which 14 are novel. One family segregated nonsyndromic hearing loss DFNB2 due to a novel three-nucleotide deletion in an exon of MYO7A (p.E1716del) encoding a region of the tail domain. We hypothesized that DFNB2 alleles of MYO7A have residual myosin VIIA. To address this question we investigated the effects of several mutant alleles by making green fluorescent protein (GFP) tagged cDNA expression constructs containing engineered mutations of mouse Myo7a at codons equivalent to pathogenic USH1B and DFNB2 alleles of human MYO7A. We show that in transfected mouse hair cells an USH1B mutant GFP-myosin VIIa does not localize properly to inner ear hair cell stereocilia. However, a GFP-myosin VIIa protein engineered to have an equivalent DFNB2 mutation to p.E1716del localizes correctly in transfected mouse hair cells. This finding is consistent with the hypothesis that p.E1716del causes a less severe phenotype (DFNB2) than the USH1B-associated alleles because the resulting protein retains some degree of normal function.

Conservation of the regulated structure of folded myosin 2 in species separated by at least 600 million years of independent evolution

The myosin 2 family of molecular motors includes isoforms regulated in different ways. Vertebrate smooth-muscle myosin is activated by phosphorylation of the regulatory light chain, whereas scallop striated adductor-muscle myosin is activated by direct calcium binding to its essential light chain. The paired heads of inhibited molecules from myosins regulated by phosphorylation have an asymmetric arrangement with motor-motor interactions. It was unknown whether such interactions were a common motif for inactivation used in other forms of myosin-linked regulation. Using electron microscopy and single-particle image processing, we show that indistinguishable structures are indeed found in myosins and heavy meromyosins isolated from scallop striated adductor muscle and turkey gizzard smooth muscle. The similarities extend beyond the shapes of the heads and interactions between them: In both myosins, the tail folds into three segments, apparently at identical sites; all three segments are in close association outside the head region; and two segments are associated in the same way with one head in the asymmetric arrangement. Thus, these organisms, which have different regulatory mechanisms and diverged from a common ancestor >600 Myr ago, have the same quaternary structure. Conservation across such a large evolutionary distance suggests that this conformation is of fundamental functional importance.

Alternative relay domains of Drosophila melanogaster myosin differentially affect ATPase activity, in vitro motility, myofibril structure and muscle function

The relay domain of myosin is hypothesized to function as a communication pathway between the nucleotide-binding site, actin-binding site and the converter domain. In Drosophila melanogaster, a single myosin heavy chain gene encodes three alternative relay domains. Exon 9a encodes the indirect flight muscle isoform (IFI) relay domain, whereas exon 9b encodes one of the embryonic body wall isoform (EMB) relay domains. To gain a better understanding of the function of the relay domain and the differences imparted by the IFI and the EMB versions, we constructed two transgenic Drosophila lines expressing chimeric myosin heavy chains in indirect flight muscles lacking endogenous myosin. One expresses the IFI relay domain in the EMB backbone (EMB-9a), while the second expresses the EMB relay domain in the IFI backbone (IFI-9b). Our studies reveal that the EMB relay domain is functionally equivalent to the IFI relay domain when it is substituted into IFI. Essentially no differences in ATPase activity, actin-sliding velocity, flight ability at room temperature or muscle structure are observed in IFI-9b compared to native IFI. However, when the EMB relay domain is replaced with the IFI relay domain, we find a 50% reduction in actin-activated ATPase activity, a significant increase in actin affinity, abolition of actin sliding, defects in myofibril assembly and rapid degeneration of muscle structure compared to EMB. We hypothesize that altered relay domain conformational changes in EMB-9a impair intramolecular communication with the EMB-specific converter domain. This decreases transition rates involving strongly bound actomyosin states, leading to a reduced ATPase rate and loss of actin motility.

Skeletal muscle performance determined by modulation of number of myosin motors rather than motor force or stroke size

Skeletal muscle can bear a high load at constant length, or shorten rapidly when the load is low. This force-velocity relationship is the primary determinant of muscle performance in vivo. Here we exploited the quasi-crystalline order of myosin II motors in muscle filaments to determine the molecular basis of this relationship by X-ray interference and mechanical measurements on intact single cells. We found that, during muscle shortening at a wide range of velocities, individual myosin motors maintain a force of about 6 pN while pulling an actin filament through a 6 nm stroke, then quickly detach when the motor reaches a critical conformation. Thus we show that the force-velocity relationship is primarily a result of a reduction in the number of motors attached to actin in each filament in proportion to the filament load. These results explain muscle performance and efficiency in terms of the molecular mechanism of the myosin motor.

Three-dimensional structure of vertebrate cardiac muscle myosin filaments

Contraction of the heart results from interaction of the myosin and actin filaments. Cardiac myosin filaments consist of the molecular motor myosin II, the sarcomeric template protein, titin, and the cardiac modulatory protein, myosin binding protein C (MyBP-C). Inherited hypertrophic cardiomyopathy (HCM) is a disease caused mainly by mutations in these proteins. The structure of cardiac myosin filaments and the alterations caused by HCM mutations are unknown. We have used electron microscopy and image analysis to determine the three-dimensional structure of myosin filaments from wild-type mouse cardiac muscle and from a MyBP-C knockout model for HCM. Three-dimensional reconstruction of the wild-type filament reveals the conformation of the myosin heads and the organization of titin and MyBP-C at 4 nm resolution. Myosin heads appear to interact with each other intramolecularly, as in off-state smooth muscle myosin [Wendt T, Taylor D, Trybus KM, Taylor K (2001) Proc Natl Acad Sci USA 98:4361-4366], suggesting that all relaxed muscle myosin IIs may adopt this conformation. Titin domains run in an elongated strand along the filament surface, where they appear to interact with part of MyBP-C and with the myosin backbone. In the knockout filament, some of the myosin head interactions are disrupted, suggesting that MyBP-C is important for normal relaxation of the filament. These observations provide key insights into the role of the myosin filament in cardiac contraction, assembly, and disease. The techniques we have developed should be useful in studying the structural basis of other myosin-related HCM diseases.

Myosin VI walks "wiggly" on actin with large and variable tilting

Myosin VI is an unconventional motor protein with unusual motility properties such as its direction of motion and path on actin and a large stride relative to its short lever arms. To understand these features, the rotational dynamics of the lever arm were studied by single-molecule polarized total internal reflection fluorescence (polTIRF) microscopy during processive motility of myosin VI along actin. The axial angle is distributed in two peaks, consistent with the hand-over-hand model. The changes in lever arm angles during discrete steps suggest that it exhibits large and variable tilting in the plane of actin and to the sides. These motions imply that, in addition to the previously suggested flexible tail domain, there is a compliant region between the motor domain and lever arm that allows myosin VI to accommodate the helical position of binding sites while taking variable step sizes along the actin filament.

The globular tail domain puts on the brake to stop the ATPase cycle of myosin Va

Myosin Va is a well known processive motor involved in transport of organelles. A tail-inhibition model is generally accepted for the regulation of myosin Va: inhibited myosin Va is in a folded conformation such that the tail domain interacts with and inhibits myosin Va motor activity. Recent studies indicate that it is the C-terminal globular tail domain (GTD) that directly inhibits the motor activity of myosin Va. In the present study, we identified a conserved acidic residue in the motor domain (Asp-136) and two conserved basic residues in the GTD (Lys-1706 and Lys-1779) as critical residues for this regulation. Alanine mutations of these conserved charged residues not only abolished the inhibition of motor activity by the GTD but also prevented myosin Va from forming a folded conformation. We propose that Asp-136 forms ionic interactions with Lys-1706 and Lys-1779. This assignment locates the GTD-binding site in a pocket of the motor domain, formed by the N-terminal domain, converter, and the calmodulin in the first IQ motif. We propose that binding of the GTD to the motor domain prevents the movement of the converter/lever arm during ATP hydrolysis cycle, thus inhibiting the chemical cycle of the motor domain.

Thiol reactivity as a sensor of rotation of the converter in myosin

Smooth muscle myosin has two reactive thiols located near the C-terminal region of its motor domain, the "converter", which rotates by approximately 70 degrees upon the transition from the "nucleotide-free" state to the "pre-power stroke" state. The incorporation rates of a thiol reagent, 5-(((2-iodoacetyl)amino)ethyl)aminonaphthalene-1-sulfonic acid (IAEDANS), into these thiols were greatly altered by adding ATP or changing the myosin conformation. Comparisons of the myosin structures in the pre-power stroke state and the nucleotide-free state explained why the reactivity of both thiols is especially sensitive to a conformational change around the converter, and thus can be used as a sensor of the rotation of the converter. Modeling of the myosin structure in the pre-power stroke state, in which the most reactive thiol, "SH1", was selectively modified with IAEDANS, revealed that this label becomes an obstacle when the converter completely rotates toward its position in the pre-power stroke state, thus resulting in incomplete rotation of the converter. Therefore, we suggest that the limitation of the converter rotation by modification causes the as-yet unexplained phenomena of SH1-modified myosin, including the inhibition of 10S myosin formation and the losses in phosphorylation-dependent regulation of the basic and actin-activated Mg-ATPase activities of myosin.

Myosin dynamics on the millisecond time scale

Myosin is a motor protein associating with actin and ATP. It translates along actin filaments against a force by transduction of free energy liberated with ATP hydrolysis. Various myosin crystal structures define time points during ATPase showing the protein undergoes large conformation change during transduction over a cycle with approximately 10 ms periodicity. The protein conformation trajectory between two intermediates in the cycle is surmised by non-equilibrium Monte Carlo simulation utilizing free-energy minimization. The trajectory shows myosin transduction of free energy to mechanical work giving evidence for: (i) a causal relationship between product release and work production in the native isoform that is correctly disrupted in a chemically modified protein, (ii) the molecular basis of ATP-sensitive tryptophan fluorescence enhancement and acrylamide quenching, (iii) an actin-binding site peptide containing the free-energy barrier to ATPase product release defining the rate limiting step and, (iv) a scenario for actin-activation of myosin ATPase.

The R403Q myosin mutation implicated in familial hypertrophic cardiomyopathy causes disorder at the actomyosin interface

Background

Mutations in virtually all of the proteins comprising the cardiac muscle sarcomere have been implicated in causing Familial Hypertrophic Cardiomyopathy (FHC). Mutations in the β-myosin heavy chain (MHC) remain among the most common causes of FHC, with the widely studied R403Q mutation resulting in an especially severe clinical prognosis. In vitro functional studies of cardiac myosin containing the R403Q mutation have revealed significant changes in enzymatic and mechanical properties compared to wild-type myosin. It has been proposed that these molecular changes must trigger events that ultimately lead to the clinical phenotype.

Principal Findings

Here we examine the structural consequences of the R403Q mutation in a recombinant smooth muscle myosin subfragment (S1), whose kinetic features have much in common with slow β-MHC. We obtained three-dimensional reconstructions of wild-type and R403Q smooth muscle S1 bound to actin filaments in the presence (ADP) and absence (apo) of nucleotide by electron cryomicroscopy and image analysis. We observed that the mutant S1 was attached to actin at highly variable angles compared to wild-type reconstructions, suggesting a severe disruption of the actin-myosin interaction at the interface.

Significance

These results provide structural evidence that disarray at the molecular level may be linked to the histopathological myocyte disarray characteristic of the diseased state.

Ternary borate-nucleoside complex stabilization by ribonuclease A demonstrates phosphate mimicry

Phosphate esters play a central role in cellular energetics, biochemical activation, signal transduction and conformational switching. The structural homology of the borate anion with phosphate, combined with its ability to spontaneously esterify hydroxyl groups, suggested that phosphate ester recognition sites on proteins might exhibit significant affinity for nonenzymatically formed borate esters. (11)B NMR studies and activity measurements on ribonuclease A (RNase A) in the presence of borate and several cytidine analogs demonstrate the formation of a stable ternary RNase A.3'-deoxycytidine-2'-borate ternary complex that mimics the complex formed between RNase A and a 2'-cytidine monophosphate (2'-CMP) inhibitor. Alternatively, no slowly exchanging borate resonance is observed for a ternary RNase A, borate, 2'-deoxycytidine mixture, demonstrating the critical importance of the 2'-hydroxyl group for complex formation. Titration of the ternary complex with 2'-CMP shows that it can displace the bound borate ester with a binding constant that is close to the reported inhibition constant of RNase A by 2'-CMP. RNase A binding of a cyclic cytidine-2',3'-borate ester, which is a structural homolog of the cytidine-2',3'-cyclic phosphate substrate, could also be demonstrated. The apparent dissociation constant for the cytidine-2',3'-borate.RNase A complex is 0.8 mM, which compares with a Michaelis constant of 11 mM for cytidine-2',3'-cyclic phosphate at pH 7, indicating considerably stronger binding. However, the value is 1,000-fold larger than the reported dissociation constant of the RNase A complex with uridine-vanadate. These results are consistent with recent reports suggesting that in situ formation of borate esters that mimic the corresponding phosphate esters supports enzyme catalysis.

GFP-tagged regulatory light chain monitors single myosin lever-arm orientation in a muscle fiber

Myosin is the molecular motor in muscle-binding actin and executing a power stroke by rotating its lever arm through an angle of approximately 70 degrees to translate actin against resistive force. A green fluorescent protein (GFP)-tagged human cardiac myosin regulatory light chain (HCRLC) was constructed to study in situ lever arm orientation one molecule at a time by polarized fluorescence emitted from the GFP probe. The recombinant protein physically and functionally replaced the native RLC on myosin lever arms in the thick filaments of permeabilized skeletal muscle fibers. Detecting single molecules in fibers where myosin concentration reaches 300 microM is accomplished using total internal reflection fluorescence microscopy. With total internal reflection fluorescence, evanescent field excitation, supercritical angle fluorescence detection, and CCD detector pixel size limits detection volume to just a few attoliters. Data analysis manages both the perturbing effect of the TIR interface on probe emission and the effect of high numerical aperture collection of light. The natural myosin concentration gradient in a muscle fiber allows observation of fluorescence polarization from C-term GFP-tagged HCRLC exchanged myosin from regions in the thick filament containing low and high myosin concentrations. In rigor, cross-bridges at low concentration at the end of the thick filament maintain GFP dipole moments at two distinct polar angles relative to the fiber symmetry axis. The lower angle, where the dipole is nearly parallel to fiber axis, is more highly populated than the alternative, larger angle. Cross-bridges at higher concentration in the center of the thick filament are oriented in a homogeneous band at approximately 45 degrees to the fiber axis. The data suggests molecular crowding impacts myosin conformation, implying mutual interactions between cross-bridges alter how the muscle generates force. The GFP-tagged RLC is a novel probe to assess single-lever-arm orientation characteristics in situ.

Predicting allosteric communication in myosin via a pathway of conserved residues

We present a computational method that predicts a pathway of residues that mediate protein allosteric communication. The pathway is predicted using only a combination of distance constraints between contiguous residues and evolutionary data. We applied this analysis to find pathways of conserved residues connecting the myosin ATP binding site to the lever arm. These pathway residues may mediate the allosteric communication that couples ATP hydrolysis to the lever arm recovery stroke. Having examined pre-stroke conformations of Dictyostelium, scallop, and chicken myosin II as well as Dictyostelium myosin I, we observed a conserved pathway traversing switch II and the relay helix, which is consistent with the understood need for allosteric communication in this conformation. We also examined post-rigor and rigor conformations across several myosin species. Although initial residues of these paths are more heterogeneous, all but one of these paths traverse a consistent set of relay helix residues to reach the beginning of the lever arm. We discuss our results in the context of structural elements and reported mutational experiments, which substantiate the significance of the pre-stroke pathways. Our method provides a simple, computationally efficient means of predicting a set of residues that mediate allosteric communication. We provide a refined, downloadable application and source code (on https://simtk.org) to share this tool with the wider community (https://simtk.org/home/allopathfinder).

An unusual transduction pathway in human tonic smooth muscle myosin

The motor protein myosin binds actin and ATP, producing work by causing relative translation of the proteins while transducing ATP free energy. Smooth muscle myosin has one of four heavy chains encoded by the MYH11 gene that differ at the C-terminus and in the active site for ATPase due to alternate splicing. A seven-amino-acid active site insert in phasic muscle myosin is absent from the tonic isoform. Fluorescence increase in the nucleotide sensitive tryptophan (NST) accompanies nucleotide binding and hydrolysis in several myosin isoforms implying it results from a common origin within the motor. A wild-type tonic myosin (smA) construct of the enzymatic head domain (subfragment 1 or S1) has seven tryptophan residues and nucleotide-induced fluorescence enhancement like other myosins. Three smA mutants probe the molecular basis for the fluorescence enhancement. W506+ contains one tryptophan at position 506 homologous to the NST in other myosins. W506F has the native tryptophans except phenylalanine replaces W506, and W506+(Y499F) is W506+ with phenylalanine replacing Y499. W506+ lacks nucleotide-induced fluorescence enhancement probably eliminating W506 as the NST. W506F has impaired ATPase activity but retains nucleotide-induced fluorescence enhancement. Y499F replacement in W506+ partially rescues nucleotide sensitivity demonstrating the role of Y499 as an NST facilitator. The exceptional response of W506 to active site conformation opens the possibility that phasic and tonic isoforms differ in how influences from active site ATPase propagate through the protein network.

A closer look at energy transduction in muscle

Muscular force is the sum of unitary force interactions generated as filaments of myosins move forcibly along parallel filaments of actins, understanding that the free energy required comes from myosin-catalyzed ATP hydrolysis. Using results from conventional biochemistry, our own mutational studies, and diffraction images from others, we attempt, in molecular detail, an account of a unitary interaction, i.e., what happens after a traveling myosin head, bearing an ADP-P(i), reaches the next station of an actin filament in its path. We first construct a reasonable model of the myosin head and actin regions that meet to form the "weakly bound state". Separately, we consider Holmes' model of the rigor state [Holmes, K. C., Angert, I., Kull, F. J., Jahn, W. & Schröder, R. R. (2003) Nature 425, 423-427], supplemented with several heretofore missing residues, thus realizing the "strongly bound state." Comparing states suggests how influences initiated at the interface travel elsewhere in myosin to discharge various functions, including striking the actins. Overall, state change seems to occur by attachment of a hydrophobic triplet (Trp-546, Phe-547, and Pro-548) of myosin to an actin conduit with a hydrophobic guiding rail (Ile-341, Ile-345, Leu-349, and Phe-352) and the subsequent linear movement of the triplet along the rail.

Structures of smooth muscle myosin and heavy meromyosin in the folded, shutdown state

Remodelling the contractile apparatus within smooth muscle cells allows effective contractile activity over a wide range of cell lengths. Thick filaments may be redistributed via depolymerisation into inactive myosin monomers that have been detected in vitro, in which the long tail has a folded conformation. Using negative stain electron microscopy of individual folded myosin molecules from turkey gizzard smooth muscle, we show that they are more compact than previously described, with heads and the three segments of the folded tail closely packed. Heavy meromyosin (HMM), which lacks two-thirds of the tail, closely resembles the equivalent parts of whole myosin. Image processing reveals a characteristic head region morphology for both HMM and myosin, with features identifiable by comparison with less compact molecules. The two heads associate asymmetrically: the tip of one motor domain touches the base of the other, resembling the blocked and free heads of this HMM when it forms 2D crystals on lipid monolayers. The tail of HMM lies between the heads, contacting the blocked motor domain, unlike in the 2D crystal. The tail of whole myosin is bent sharply and consistently close to residues 1175 and 1535. The first bend position correlates with a skip in the coiled coil sequence, the second does not. Tail segments 2 and 3 associate only with the blocked head, such that the second bend is near the C-lobe of the blocked head regulatory light chain. Quantitative analysis of tail flexibility shows that the single coiled coil of HMM has an apparent Young's modulus of about 0.5 GPa. The folded tail of the whole myosin is less flexible, indicating interactions between the segments. The folded tail does not modify the compact head arrangement but stabilises it, indicating a structural mechanism for the very low ATPase activity of the folded molecule.

Differential scanning calorimetry study of glycerinated rabbit psoas muscle fibres in intermediate state of ATP hydrolysis

Background

Thermal denaturation experiments were extended to study the thermal behaviour of the main motor proteins (actin and myosin) in their native environment in striated muscle fibres. The interaction of actin with myosin in the highly organized muscle structure is affected by internal forces; therefore their altered conformation and interaction may differ from those obtained in solution. The energetics of long functioning intermediate states of ATP hydrolysis cycle was studied in muscle fibres by differential scanning calorimetry (DSC).

Results

SETARAM Micro DSC-II was used to monitor the thermal denaturation of the fibre system in rigor and in the presence of nucleotide and nucleotide analogues. The AM.ADP.P_i state of the ATP hydrolysis cycle has a very short lifetime therefore, we mimicked the different intermediate states with AMP.PNP and/or inorganic phosphate analogues V_i and AlF₄ or BeF_x. Studying glycerol-extracted muscle fibres from the rabbit psoas muscle by DSC, three characteristic thermal transitions were detected in rigor. The thermal transitions can be assigned to myosin heads, myosin rods and actin with transition temperatures (T_m) of 52.9 ± 0.7°C, 57.9 ± 0.7°C, 63.7 ± 1.0°C. In different intermediate states of the ATP hydrolysis mimicked by nucleotide analogues a fourth thermal transition was also detected which is very likely connected with nucleotide binding domain of myosin and/or actin filaments. This transition temperature T_m4 depended on the mimicked intermediate states, and varied in the range of 66°C – 77°C.

Conclusion

According to DSC measurements, strongly and weakly binding states of myosin to actin were significantly different. In the presence of ADP only a moderate change of the DSC pattern was detected in comparison with rigor, whereas in ADP.P_i state trapped by V_i, AlF₄ or BeF_x a remarkable stabilization was detected on the myosin head and actin filament which is reflected in a 3.0 – 10.0°C shift in T_m to higher temperature. A similar effect was observed in the case of the nonhydrolyzable AMP.PNP analogue. Differential DSC measurements suggest that stabilization actin structure in the intermediate states of ATP hydrolysis may play an additional role in actin-myosin interaction.

Distribution and structure-function relationship of myosin heavy chain isoforms in the adult mouse heart

The two cardiac myosin heavy chain isoforms, alpha and beta, exhibit distinct functional characteristics and therefore may be distributed regionally within the heart to match the functional demands of a specific region. In adult mouse hearts, which predominantly express alpha-myosin heavy chain, we observed high concentrations of beta-myosin in distinct areas such as at the tip of papillary muscles and at the base close to the valvular annulus. In light of these distinct distribution patterns of the myosin isoforms, we subsequently explored the isoform-specific structure-function relationships of the myosins. The alpha- and beta-isoforms are 93% identical in amino acid sequence, but it remains unclear which of the nonidentical residues determines isoform functionality. We hypothesized that residues situated within or close to the actin-binding interface of the myosin head influence actin binding and thereby modulate actin-activated ATPase activity. A chimeric myosin was created containing beta-sequence from amino acid 417 to 682 within the alpha-backbone. In mice, approximately 70% of the endogenous cardiac protein was replaced with the chimeric myosin. Myofibrils containing chimeric myosin exhibited ATPase activities that were depressed to the levels observed in hearts expressing approximately 70% beta-myosin. In vitro motility assays showed that the actin filament sliding velocity generated by chimeric myosin was similar to that of alpha-myosin, almost twice the velocities observed with beta-myosin. These data indicate that this large domain sequence switch conferred beta-like actin-activated ATPase activities to the chimeric myosin, suggesting that this region is responsible for the distinct hydrolytic properties of these myosin isoforms.

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.

Diversity of structural behavior in vertebrate conventional myosins complexed with actin

Low-resolution three-dimensional structures of acto-myosin subfragment-1 (S1) complexes were retrieved from X-ray fiber diffraction patterns, recorded either in the presence or absence of ADP. The S1 was obtained from various myosin-II isoforms from vertebrates, including rabbit fast-skeletal and cardiac, chicken smooth and human non-muscle IIA and IIB species, and was diffused into an array of overstretched, skinned skeletal muscle fibers. The S1 attached to the exposed actin filaments according to their helical symmetry. Upon addition of ADP, the diffraction patterns from acto-S1 showed an increasing magnitude of response in the order as listed above, with features of a lateral compression of the whole diffraction pattern (indicative of increased radius of the acto-S1 complex) and an enhancement of the fifth layer-line reflection. The structure retrieval indicates that these changes are mainly due to the swing of the light chain (LC) domain in the direction consistent with the cryo-electron microscopic results. In the non-muscle isoforms, the swing is large enough to affect the manner of quasi-crystal packing of the S1-decorated actin filaments and their lattice dimension, with a small change in the twist of actin filaments. Variations also exist in the behavior of the 50K-cleft, which apparently opens upon addition of ADP to the non-muscle isoforms but not to other isoforms. The fast-skeletal S1 remains as the only isoform that does not clearly exhibit either of the structural changes. The results indicate that the "conventional" myosin-II isoforms exhibit a wide variety of structural behavior, possibly depending on their functions and/or the history of molecular evolution.

Myosin II isoforms in smooth muscle: heterogeneity and function

Both smooth muscle (SM) and nonmuscle class II myosin molecules are expressed in SM tissues comprising hollow organ systems. Individual SM cells may express one or more of multiple myosin II isoforms that differ in myosin heavy chain (MHC) and myosin light chain (MLC) subunits. Although much has been learned, the expression profiles, organization within contractile filaments, localization within cells, and precise roles in various contractile functions of these different myosin molecules are still not well understood. However, data supporting unique physiological roles for certain isoforms continues to build. Isoform differences located in the S1 head region of the MHC can alter actin binding and rates of ATP hydrolysis. Differences located in the MHC tail can alter the formation, stability, and size of the myosin thick filament. In these distinct ways, both head and tail isoform differences can alter force generation and muscle shortening velocities. The MLCs that are associated with the lever arm of the S1 head can affect the flexibility and range of motion of this domain and possibly the motion of the S2 and motor domains. Phosphorylation of MLC(20) has been associated with conformational changes in the S1 and/or S2 fragments regulating enzymatic activity of the entire myosin molecule. A challenge for the future will be delineation of the physiological significance of the heterogeneous expression of these isoforms in developmental, tissue-specific, and species-specific patterns and or the intra- and intercellular heterogeneity of myosin isoform expression in SM cells of a given organ.

Rigor-like Structures from Muscle Myosins Reveal Key Mechanical Elements in the Transduction Pathways of This Allosteric Motor

Unlike processive cellular motors such as myosin V, whose structure has recently been determined in a "rigor-like" conformation, myosin II from contracting muscle filaments necessarily spends most of its time detached from actin. By using squid and sea scallop sources, however, we have now obtained similar rigor-like atomic structures for muscle myosin heads (S1). The significance of the hallmark closed actin-binding cleft in these crystal structures is supported here by actin/S1-binding studies. These structures reveal how different duty ratios, and hence cellular functions, of the myosin isoforms may be accounted for, in part, on the basis of detailed differences in interdomain contacts. Moreover, the rigor-like position of switch II turns out to be unique for myosin V. The overall arrangements of subdomains in the motor are relatively conserved in each of the known contractile states, and we explore qualitatively the energetics of these states.

Binding modes of decavanadate to myosin and inhibition of the actomyosin ATPase activity

Decavanadate, a vanadate oligomer, is known to interact with myosin and to inhibit the ATPase activity, but the putative binding sites and the mechanism of inhibition are still to be clarified. We have previously proposed that the decavanadate (V(10)O(28)(6-)) inhibition of the actin-stimulated myosin ATPase activity is non-competitive towards both actin and ATP. A likely explanation for these results is that V(10) binds to the so-called back-door at the end of the Pi-tube opposite to the nucleotide-binding site. In order to further investigate this possibility, we have carried out molecular docking simulations of the V(10) oligomer on three different structures of the myosin motor domain of Dictyostelium discoideum, representing distinct states of the ATPase cycle. The results indicate a clear preference of V(10) to bind at the back-door, but only on the "open" structures where there is access to the phosphate binding-loop. It is suggested that V(10) acts as a "back-door stop" blocking the closure of the 50-kDa cleft necessary to carry out ATP-gamma-phosphate hydrolysis. This provides a simple explanation to the non-competitive behavior of V(10) and spurs the use of the oligomer as a tool to elucidate myosin back-door conformational changes in the process of muscle contraction.

Haematological characteristics of MYH9 disorders due to MYH9 R702 mutations

OBJECTIVE

MYH9 disorders are characterised by giant platelets, thrombocytopenia, and Döhle body-like cytoplasmic granulocyte inclusion bodies that result from mutations in MYH9, the gene for non-muscle myosin heavy chain-IIA (NMMHC-IIA). MYH9 R702 mutations are highly associated with Alport manifestations and result in Epstein syndrome. The aim of our study was to determine the haematological characteristics of MYH9 disorders as a result of R702 mutations to aid in making a proper diagnosis.

PATIENTS AND METHODS

Platelet size of patients with MYH9 disorders was determined as platelet diameter by microscopic observation of 200 platelets on stained peripheral blood smears. Double in situ hybridisation using a biotinylated oligo(dT) probe and immunofluorescence analysis of neutrophil NMMHC-IIA was performed on peripheral blood smears.

RESULTS

Patients carrying R702 mutations had significantly larger platelets than those with other MYH9 mutations. Although granulocyte inclusion bodies were mostly invisible on stained blood smears, immunofluorescence analysis for NMMHC-IIA showed an abnormal type II localisation in all neutrophils. We first showed that poly(A)+ RNA coincided with accumulated NMMHC-IIA at inclusion bodies in patients with MYH9 disorders. However, no condensation of poly(A)+ RNA at inclusion bodies was observed in patients with R702 mutations.

CONCLUSION

Our study shows that R702 mutations result in especially large platelets and inclusion bodies being faint and mostly invisible on conventionally stained blood smears. We further demonstrated that poly(A)+ RNA content but not NMMHC-IIA accumulation is responsible for the morphological appearance/stainability of inclusion bodies on stained blood smears and the amount of poly(A)+ RNA is decreased in those with R702 mutations.

Quantum Chemical Studies of the Myosin ATPase Mechanism

Muscle contraction is one of the molecular motions in the living body. The elemental process of muscle contraction is a mutual sliding of two kinds of filaments of proteins, myosin and actin, using energy liberated by ATP hydrolysis. Myosin enzymatically hydrolyzes ATP and interacts with actin. This article reviews previous quantum chemical studies to help determine how the chemical energy of ATP hydrolysis is converted to mechanical energy during sliding.

Evidence about the structural behaviour of myosin crossbridges during muscle contraction

It has been a great honor and a particular pleasure to participate in this meeting to celebrate the fortieth anniversary of the discovery of troponin by Professor Ebashi, whom I have been privileged to know for many years of my scientific life. I thought therefore it would be appropriate to described briefly what was happening in studies of another aspect of muscle contraction over somewhat the same time period.

Myosin light chain phosphorylation inhibits muscle fiber shortening velocity in the presence of vanadate

We have shown that myosin light chain phosphorylation inhibits fiber shortening velocity at high temperatures, 30 degrees C, in the presence of the phosphate analog vanadate. Vanadate inhibits tension by reversing the transition to force-generating states, thus mimicking a prepower stroke state. We have previously shown that at low temperatures vanadate also inhibits velocity, but at high temperatures it does not, with an abrupt transition in inhibition occurring near 25 degrees C (E. Pate, G. Wilson, M. Bhimani, and R. Cooke. Biophys J 66: 1554-1562, 1994). Here we show that for fibers activated in the presence of 0.5 mM vanadate, at 30 degrees C, shortening velocity is not inhibited in dephosphorylated fibers but is inhibited by 37 +/- 10% in fibers with phosphorylated myosin light chains. There is no effect of phosphorylation on fiber velocity in the presence of vanadate at 10 degrees C. The K(m) for ATP, defined by the maximum velocity of fibers partially inhibited by vanadate at 30 degrees C, is 20 +/- 4 microM for phosphorylated fibers and 192 +/- 40 microM for dephosphorylated fibers, showing that phosphorylation also affects the binding of ATP. Fiber stiffness is not affected by phosphorylation. Inhibition of velocity by phosphorylation at 30 degrees C depends on the phosphate analog, with approximately 12% inhibition in fibers activated in the presence of 5 mM BeF(3) and no inhibition in the presence of 0.25 mM AlF(4). Our results show that myosin phosphorylation can inhibit shortening velocity in fibers with large populations of myosin heads trapped in prepower stroke states, such as occurs during muscle fatigue.

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.

Myosin essential light chain in health and disease

The essential light chain of myosin (ELC) is known to be important for structural stability of the alpha-helical lever arm domain of the myosin head, but its function in striated muscle contraction is poorly understood. Two ELC isoforms are expressed in fast skeletal muscle, a long isoform and its NH(2)-terminal approximately 40 amino acid shorter counterpart, whereas only the long ELC is observed in the heart. Biochemical and structural studies revealed that the NH(2)-terminus of the long ELC can make direct contacts with actin, but the effects of the ELC on the affinity of myosin for actin, ATPase, force, and the kinetics of force generating myosin cross-bridges are inconclusive. Myosin containing the long ELC has been shown to have slower cross-bridge kinetics than myosin with the short isoform. A difference was also reported among myosins with long isoforms. Increased shortening velocity was observed in atrial compared with ventricular muscle fibers. The common findings suggest that ELC provides the fine tuning of the myosin motor function, which is regulated in an isoform and tissue-dependent manner. The functional importance of the ELC is further implicated by the discovery of ELC mutations associated with Familial Hypertrophic Cardiomyopathy. The pathological phenotypes vary in severity, but more notably, almost all ELC mutations result in sudden cardiac death at a young age. This review summarizes the functional roles of striated muscle ELC in normal healthy muscle and in disease. Transgenic animal models and phenotypic characterization of ELC-mediated remodeling of the heart are also discussed.

Complex movements of motor protein relay helices during the power stroke

We use the Toda soliton formalism to propose a possible complex movement of alpha helices with a very important role in energy transduction during the power stroke of motor proteins. We find that this approach has advantages in comparison with the Davydov soliton model and its variants. We estimated the model's parameters and calculated corresponding properties of the predicted solitary waves including propagation velocities and energies. The energies are found to be within the expected range.

X-ray Interference Studies of Crossbridge Action in Muscle Contraction: Evidence from Quick Releases

We have used a high-resolution small angle X-ray scattering system, together with a high-performance CCD camera, on the BioCAT beamline at the APS synchrotron radiation facility at the Argonne National Laboratory, to study X-ray interference effects in the meridional reflections generated by the arrays of myosin crossbridges in contracting muscle. These give information about axial movements of the myosin heads during contraction with sub-nanometer resolution. Using whole intact muscle preparations (frog sartorius) we have been able to record the detailed behavior of M3 (the first order meridional reflection from the myosin crossbridges, at 14.56 nm) at each of a number of quick releases of increasing magnitude, on the same specimen, and at the same time make similar measurements on higher order myosin meridional reflections, particularly M6. The latter provides information about the dispersion of lever arm angles of the actin-attached myosin heads. The observations show that in isometric contraction the lever arm angles are dispersed through +/- 20-25 degrees on either side of a mean orientation that is about 60 degrees away from their orientation at the end of the working stroke: and that they move towards that orientation in synchronized fashion, with constant dispersion, during quick releases. The relationship between the shift in the interference fringes (which measures the shift of the myosin heads scattering mass towards the center of the sarcomere, and the changes in the total intensity of the reflections, which measures the changes in the axial profile of the heads, is consistent with the tilting lever arm mechanism of muscle contraction. Significant fixed contributions to the meridional reflections come from unattached myosin heads and from backbone components of the myosin filaments, and the interaction of these with the contributions from actin-attached myosin heads determines the behavior of these reflections.

Kinetic mechanism of human myosin IIIA

Myosin IIIA is specifically expressed in photoreceptors and cochlea and is important for the phototransduction and hearing processes. In addition, myosin IIIA contains a unique N-terminal kinase domain and C-terminal tail actin-binding motif. We examined the kinetic properties of baculovirus expressed human myosin IIIA containing the kinase, motor, and two IQ domains. The maximum actin-activated ATPase rate is relatively slow (k(cat) = 0.77 +/- 0.08 s(-1)), and high actin concentrations are required to fully activate the ATPase rate (K(ATPase) = 34 +/- 11 microm). However, actin co-sedimentation assays suggest that myosin III has a relatively high steady-state affinity for actin in the presence of ATP (K(actin) approximately 7 microm). The rate of ATP binding to the motor domain is quite slow both in the presence and absence of actin (K(1)k(+2) = 0.020 and 0.001 microm(-1).s(-1), respectively). The rate of actin-activated phosphate release is more than 100-fold faster (85 s(-1)) than the k(cat), whereas ADP release in the presence of actin follows a two-step mechanism (7.0 and 0.6 s(-1)). Thus, our data suggest a transition between two actomyosin-ADP states is the rate-limiting step in the actomyosin III ATPase cycle. Our data also suggest the myosin III motor spends a large fraction of its cycle in an actomyosin ADP state that has an intermediate affinity for actin (K(d) approximately 5 microm). The long lived actomyosin-ADP state may be important for the ability of myosin III to function as a cellular transporter and actin cross-linker in the actin bundles of sensory cells.

Native nonmuscle myosin II stability and light chain binding inDrosophila melanogaster

Native nonmuscle myosin IIs play essential roles in cellular and developmental processes throughout phylogeny. Individual motor molecules consist of a heterohexameric complex of three polypeptides which, when properly assembled, are capable of force generation. Here, we more completely characterize the properties, relationships and associations that each subunit has with one another in Drosophila melanogaster. All three native nonmuscle myosin II polypeptide subunits are expressed in close to constant stoichiometry to each other throughout development. We find that the stability of two subunits, the heavy chain and the regulatory light chain, depend on one another whereas the stability of the third subunit, the essential light chain, does not depend on either the heavy chain or regulatory light chain. We demonstrate that heavy chain aggregates, which form when regulatory light chain is lacking, associate with the essential light chain in vivo-thus showing that regulatory light chain association is required for heavy chain solubility. By immunodepletion we find that the majority of both light chains are associated with the nonmuscle myosin II heavy chain but pools of free light chain and/or light chain bound to other proteins are present. We identify four myosins (myosin II, myosin V, myosin VI and myosin VIIA) and a microtubule-associated protein (asp/Abnormal spindle) as binding partners for the essential light chain (but not the regulatory light chain) through mass spectrometry and co-precipitation. Using an in silico approach we identify six previously uncharacterized genes that contain IQ-motifs and may be essential light chain binding partners.

RECENT IMPROVEMENTS IN SMALL ANGLE X-RAY DIFFRACTION FOR THE STUDY OF MUSCLE PHYSIOLOGY

The molecular mechanism of muscle contraction is one of the most important unresolved problems in Biology and Biophysics. Notwithstanding the great advances of recent years, it is not yet known in detail how the molecular motor in muscle, the class II myosin, converts the free energy of ATP hydrolysis into work by interacting with its track, the actin filament, neither it is understood how the high efficiency in energy conversion depends on the cooperative action of myosin motors working in parallel along the actin filament. Researches in muscle contraction imply the combination of mechanical, biochemical and structural methods in studies that span from tissue to single molecule. Therefore, more than for any other research field, progresses in the comprehension of muscle contraction at molecular level are related to, and in turn contribute to, the advancement of methods in Biophysics.This review will focus on the progresses achieved by time resolved small angle X-ray scattering (SAXS) from muscle, an approach made possible by the highly ordered arrangement of both the contractile proteins myosin and actin in the ca 2 mum long structural unit the sarcomere that repeats along the whole length of the muscle cell. Among the time resolved structural techniques, SAXS has proved to be the most powerful method of investigation, as it allows the molecular motor to be studied in situ, in intact single muscle cells, where it is possible to combine the structural study with fast mechanical methods that synchronize the action of the molecular motors. The latest development of this technique allows Angstrom-scale measurements of the axial movement of the motors that pull the actin filament toward the centre of the sarcomere, by exploiting the X-ray interference between the two arrays of myosin motors in the two halves of the sarcomere.

Functional characterization of the N-terminal region of myosin-2

All class 2 myosins contain an N-terminal extension of approximately 80 residues that includes an Src homology 3 (SH3)-like subdomain. To explore the functional importance of this region, which is also present in most other myosin classes, we generated truncated constructs of Dictyostelium discoideum myosin-2. Truncation at position 80 resulted in the complete loss of myosin-2 function in vivo. Actin affinity was more than 80-fold, and the rate of ADP release approximately 40-fold decreased in this mutant. In contrast, a myosin construct that lacks only the SH3-like subdomain, corresponding to residues 33-79, displayed much smaller functional defects. In complementation experiments with myosin-2 null cells, this construct rescued myosin-2-dependent processes such as cytokinesis, fruiting body formation, and sporogenesis. An 8-fold reduction in motile activity and changes of similar extent in the affinity for ADP and filamentous actin indicate the importance of the SH3-like subdomain for correct communication between the functional regions within the myosin motor domain and suggest that local perturbations in this region can play a role in modulating myosin-2 motor activity.

Protein-protein interactions in actin-myosin binding and structural effects of R405Q mutation: a molecular dynamics study

Detailed residue-wise interactions involved in the binding of myosin to actin in the rigor conformation without nucleotides have been examined using molecular dynamics simulations of the chicken skeletal myosin head complexed with two actin monomers, based on the cryo-microscopic model of Holmes et al. (Nature 2003;425:423-427). The overall interaction is largely electrostatic in nature, because of the charged residues in the four loops surrounding the central primary binding site. The 50k/20k loop, disordered in crystal structures and in simulations of free myosin in solution, was found to be in a conformation stabilized with 1 - 2 internal salt bridges. The cardiomyopathy loop forms 2 - 3 interprotein salt bridges with actin monomers upon binding, whereas its Arg405 residue, the mutation site associated with the hypertrophic cardiomyopathy, forms a strong salt bridge with Glu605 in the neighboring helix away from actin in the actin-bound myosin. The myopathy loop of the R405Q mutant maintains a high degree of two-strand beta-sheet character when bound to actin with the corresponding salt bridges broken.

Exploiting reaction intermediates of the ATPase reaction to elucidate the mechanism of transport by P-glycoprotein (ABCB1)

The transport cycle of ABC transporters in general and P-glycoprotein in particular has been extensively studied, but the molecular mechanism remains controversial. We identify stable reaction intermediates in the progression of the P-glycoprotein-mediated ATPase reaction equivalent to the enzyme-substrate (E.S, P-glycoprotein.ATP) and enzyme-product (E.P, P-glycoprotein.ADP.P(i)) reaction intermediates. These have been characterized using the photoaffinity analog 8-azido-[alpha-32P]ATP as well as under equilibrium conditions using [alpha-32P]ATP, in which a cross-linking step is not involved. Similar results were obtained when 8-azido-[alpha-32P]ATP or [alpha-32P]ATP was used. The reaction intermediates were characterized based on their kinetic properties and the nature (triphosphate/diphosphate) of the trapped nucleotide. Using this defined framework and the Walker B E556Q/E1201Q mutant that traps nucleotide in the absence of vanadate or beryllium fluoride, the high to low affinity switch in the transport substrate binding site can be attributed to the formation of the E.S reaction intermediate of the ATPase reaction. Importantly, the posthydrolysis E.P state continues to have low affinity for substrate, suggesting that conformational changes that form the E.S complex are coupled to the conformational change at the transport substrate site to do mechanical work. Thus, the formation of E.S reaction intermediate during a single turnover of the catalytic cycle appears to provide the initial power stroke for movement of drug substrate from inner leaflet to outer leaflet of lipid bilayer. This novel approach applies transition state theory to elucidate the mechanism of P-glycoprotein and other ABC transporters and has wider applications in testing cause-effect hypotheses in coupled systems.

Exploration of the conformational space of myosin recovery stroke via molecular dynamics

Muscle contractions are driven by cyclic conformational changes of myosin, whose molecular mechanisms of operation are being elucidated by recent advances in crystallographic studies and single molecule experiments. To complement such structural studies and consider the energetics of the conformational changes of myosin head, umbrella sampling molecular dynamics (MD) simulations were performed with the all-atom model of the scallop myosin sub-fragment 1 (S1) with a bound ATP in solution in explicit water using the crystallographic near-rigor and transition state conformations as two references. The constraints on RMSD reaction coordinates used for the umbrella sampling were found to steer the conformational changes efficiently, and relatively close correlations have been observed between the set of characteristic structural changes including the lever arm rotation and the closing of the nucleotide binding pocket. The lever arm angle and key residue interaction distances in the nucleotide binding pocket and the relay helix show gradual changes along the recovery stroke reaction coordinate, consistent with previous crystallographic and computational minimum energy studies. Thermal fluctuations, however, appear to make the switch-2 coordination of ATP more flexible than suggested by crystal structures. The local solvation environment of the fluorescence probe, Trp 507 (scallop numbering), also appears highly mobile in the presence of thermal fluctuations.

Myosin at work: motor adaptations for a variety of cellular functions

Cells have evolved multiple mechanisms to overcome the effects of entropy and diffusion to create a highly ordered environment. For cells to function properly, some components must be anchored to provide a framework or structure. Others must be rapidly transported over long distances to generate asymmetries in cell morphology and composition. To accomplish long-range transport, cells cannot rely on diffusion alone as many large organelles and macromolecular complexes are essentially immobilized by the dense meshwork of the cytosol. One strategy used by cells to overcome diffusion is to harness the free energy liberated by ATP hydrolysis through molecular motors. Myosins are a family of actin based molecular motors that have evolved a variety of ways to contribute to cellular organization through numerous modifications to the manner they convert that free energy into mechanical work.

Minimization of detection volume by surface-plasmon-coupled emission

We report theoretical predictions and experimental observations of the reduced detection volume with the use of surface-plasmon-coupled emission (SPCE). The effective fluorescence volume (detection volume) in SPCE experiments depends on two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. With direct excitation of the sample (reverse Kretschmann excitation) the detection volume is restricted only by the distance-dependent coupling of the excitation to the surface plasmons. However, with the excitation through the glass prism at surface plasmon resonance angle (Kretschmann configuration), the detection volume is a product of evanescent wave penetration depth and distance-dependent coupling. In addition, the detection volume is further reduced by a metal quenching of excited fluorophores at a close proximity (below 10nm). The height of the detected volume size is 40-70nm, depending on the orientation of the excited dipoles. We show that, by using the Kretschmann configuration in a microscope with a high-numerical-aperture objective (1.45) together with confocal detection, the detection volume can be reduced to 1-2attoL. The strong dependence of the coupling to the surface plasmons on the orientation of excited dipoles can be used to study the small conformational changes of macromolecules.

Temperature change does not affect force between regulated actin filaments and heavy meromyosin in single-molecule experiments

The temperature dependence of sliding velocity, force and the number of cross-bridges was studied on regulated actin filaments (reconstituted thin filaments) when they were placed on heavy meromyosin (HMM) attached to a glass surface. The regulated actin filaments were used because our previous study on muscle fibres demonstrated that the temperature effect was much reduced in the absence of regulatory proteins. A fluorescently labelled thin filament was attached to the gelsolin-coated surface of a polystyrene bead. The bead was trapped by optical tweezers, and HMM-thin filament interaction was performed at 20-35 degrees C to study the temperature dependence of force at the single-molecule level. Our experiments showed that there was a small increase in force with temperature (Q10 = 1.43) and sliding velocity (Q10 = 1.46). The small increase in force was correlated with the small increase in the number of cross-bridges (Q10 = 1.49), and when force was divided by the number of cross-bridges, the result did not depend on the temperature (Q(10) = 1.03). These results demonstrate that the force each cross-bridge generates is fixed and independent of temperature. Our additional experiments demonstrate that tropomyosin (Tm) in the presence of troponin (Tn) and Ca2+ enhances both force and velocity, and a truncated mutant, Delta23Tm, diminishes force and velocity. These results are consistent with the hypothesis that Tm in the presence of Tn and Ca2+ exerts a positive allosteric effect on actin to make actomyosin linkage more secure so that larger forces can be generated.

Regulatory and catalytic domain dynamics of smooth muscle myosin filaments

Domain dynamics of the chicken gizzard smooth muscle myosin catalytic domain (heavy chain Cys-717) and regulatory domain (regulatory light chain Cys-108) were determined in the absence of nucleotides using saturation-transfer electron paramagnetic resonance. In unphosphorylated synthetic filaments, the effective rotational correlation times, tau(r), were 24 +/- 6 micros and 441 +/- 79 micros for the catalytic and regulatory domains, respectively. The corresponding amplitudes of motion were 42 +/- 4 degrees and 24 +/- 9 degrees as determined from steady-state phosphorescence anisotropy. These results suggest that the two domains have independent mobility due to a hinge between the two domains. Although a similar hinge was observed for skeletal myosin (Adhikari and Fajer (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 9643-9647. Brown et al. (2001) Biochemistry 40, 8283-8291), the latter displayed higher regulatory domain mobility, tau(r)= 40 +/- 3 micros, suggesting a smooth muscle specific mechanism of constraining regulatory domain dynamics. In the myosin monomers the correlation times for both domains were the same (approximately 4 micros) for both smooth and skeletal myosin, suggesting that the motional difference between the two isoforms in the filaments was not due to intrinsic variation of hinge stiffness. Heavy chain/regulatory light chain chimeras of smooth and skeletal myosin pinpointed the origin of the restriction to the heavy chain and established correlation between the regulatory domain dynamics with the ability of myosin to switch off but not to switch on the ATPase and the actin sliding velocity. Phosphorylation of smooth muscle myosin filaments caused a small increase in the amplitude of motion of the regulatory domain (from 24 +/- 4 degrees to 36 +/- 7 degrees ) but did not significantly affect the rotational correlation time of the regulatory domain (441 to 408 micros) or the catalytic domain (24 to 17 micros). These data are not consistent with a stable interaction between the two catalytic domains in unphosphorylated smooth muscle myosin filaments in the absence of nucleotides.

Refined structure of bony fish muscle myosin filaments from low-angle X-ray diffraction data

Application of X-ray diffraction methods to the elucidation of the arrangement of the myosin heads on myosin filaments in resting muscles is made simpler when the muscles themselves are well ordered in 3D. Bony fish muscle for the vertebrates and insect flight muscle for the invertebrates are the muscles of choice for this analysis. The rich, well-sampled, low-angle X-ray diffraction pattern from bony fish muscle has previously been modelled with an R-factor of 3.4% between observed and calculated transforms on the assumption that the two heads in one myosin molecule have the same shape. However, recent evidence from other kinds of analysis of other muscles has shown that this assumption may not be valid. There is evidence that the motor domain of one head in each pair may interact with the neck region of the second head. This possibility has been tested directly in the present analysis which extends the X-ray modelling of fish muscle myosin filaments by permitting independent shape changes of the two heads in one molecule. The new model, with a computed R-factor of 1.19% against 56 independent reflections, shows that in fish muscle also there is a marked asymmetry in the organisation of each head pair.

Constriction velocities of renal afferent and efferent arterioles of mice are not related to SMB expression

BACKGROUND

Constriction of renal arterioles contributes significantly to the control of perfusion and glomerular filtration. Afferent but not efferent arterioles express smooth muscle myosin heavy chain B (SMB) (with a 5'-insert of seven amino acids). The aim of the present study was to investigate (1) the constriction characteristics of afferent and efferent arterioles under physiologic load and (2) whether expression of SMB may causally contribute to these constriction characteristics.

METHODS

We compared constriction parameters [constriction amplitude, maximal rate of constriction velocity ("dc/dt(max)"), and time to half-maximal constriction (t(1/2)) of in vitro perfused renal afferent and efferent arterioles of wild-type (smb(+/+)] and homozygous SMB knockout [smb(-/-)] mice upon stimulation with angiotensin II (Ang II) (10(-8) mol/L) and potassium chloride (KCl) (100 mmol/L). SMB expression was investigated by double-labeling immunofluorescence.

RESULTS

Contraction amplitude and dc/dt(max) of mouse afferent arterioles upon Ang II stimulation were significantly greater compared to efferent arterioles. However, constriction amplitudes, dc/dt(max), and t(1/2) of afferent as well as efferent arterioles upon Ang II stimulation were similar in smb(+/+) and smb(-/-) mice. Constriction amplitudes upon KCl stimulation of afferent arterioles were similar in both smb(+/+) and smb(-/-) mice. Furthermore, KCl-induced dc/dt(max) and t(1/2) of afferent arterioles were similar in both smb(+/+) and smb(-/-) mice. SMB expression could be detected in afferent but not efferent arterioles in smb(+/+) mice. No SMB expression in either arteriole could be observed in smb(-/-) mice.

CONCLUSION

Our results suggest that the presence of different alternatively 5'-spliced smooth muscle-myosin heavy chain (SM-MHC) isoforms does not dominate the different contractile features of physiologically loaded renal afferent or efferent arterioles.

Structural Evidence for Non-canonical Binding of Ca2+ to a Canonical EF-hand of a Conventional Myosin

We have previously identified a single inhibitory Ca2+-binding site in the first EF-hand of the essential light chain of Physarum conventional myosin (Farkas, L., Malnasi-Csizmadia, A., Nakamura, A., Kohama, K., and Nyitray, L. (2003) J. Biol. Chem. 278, 27399-27405). As a general rule, conformation of the EF-hand-containing domains in the calmodulin family is "closed" in the absence and "open" in the presence of bound cations; a notable exception is the unusual Ca2+-bound closed domain in the essential light chain of the Ca2+-activated scallop muscle myosin. Here we have reported the 1.8 A resolution structure of the regulatory domain (RD) of Physarum myosin II in which Ca2+ is bound to a canonical EF-hand that is also in a closed state. The 12th position of the EF-hand loop, which normally provides a bidentate ligand for Ca2+ in the open state, is too far in the structure to participate in coordination of the ion. The structure includes a second Ca2+ that only mediates crystal contacts. To reveal the mechanism behind the regulatory effect of Ca2+, we compared conformational flexibilities of the liganded and unliganded RD. Our working hypothesis, i.e. the modulatory effect of Ca2+ on conformational flexibility of RD, is in line with the observed suppression of hydrogen-deuterium exchange rate in the Ca2+-bound form, as well as with results of molecular dynamics calculations. Based on this evidence, we concluded that Ca2+-induced change in structural dynamics of RD is a major factor in Ca2+-mediated regulation of Physarum myosin II activity.

An alternative domain near the nucleotide-binding site of Drosophila muscle myosin affects ATPase kinetics

In Drosophila melanogaster expression of muscle myosin heavy chain isoforms occurs by alternative splicing of transcripts from a single gene. The exon 7 domain is one of four variable regions in the catalytic head and is located near the nucleotide-binding site. To ascribe a functional role to this domain, we created two chimeric myosin isoforms (indirect flight isoform-exon 7a and embryonic-exon 7d) that differ from the native indirect flight muscle and embryonic body-wall muscle isoforms only in the exon 7 region. Germline transformation and subsequent expression of the chimeric myosins in the indirect flight muscle of myosin-null Drosophila allowed us to purify the myosin for in vitro studies and to assess in vivo structure and function of transgenic muscles. Intriguingly, in vitro experiments show the exon 7 domain modulates myosin ATPase activity but has no effect on actin filament velocity, a novel result compared to similar studies with other Drosophila variable exons. Transgenic flies expressing the indirect flight isoform-exon 7a have normal indirect flight muscle structure, and flight and jump ability. However, expression of the embryonic-exon 7d chimeric isoform yields flightless flies that show improvements in both the structural stability of the indirect flight muscle and in locomotor abilities as compared to flies expressing the embryonic isoform. Overall, our results suggest the exon 7 domain participates in the regulation of the attachment of myosin to actin in order to fine-tune the physiological properties of Drosophila myosin isoforms.