The dynamics of actin and myosin association and the crossbridge model of muscle contraction

Multi-breed host rumen epithelium transcriptome and microbiome associations and their relationship with beef cattle feed efficiency

Understanding host-microbial interactions in the rumen and its influence on desirable production traits may lead to potential microbiota manipulation or genetic selection for improved cattle feed efficiency. This study investigated the host transcriptome and its correlation with the rumen archaea and bacteria differential abundance of two pure beef cattle breeds (Angus and Charolais) and one composite beef hybrid (Kinsella) divergent for residual feed intake (RFI; low-RFI vs. high-RFI). Using RNA-Sequencing of rumen tissue and 16S rRNA gene amplicon sequencing, differentially expressed genes (FDR ≤ 0.05, |log2(Fold-change) >|2) and differentially abundant (p-value < 0.05) archaea and bacteria amplicon sequence variants (ASV) were determined. Significant correlations between gene expression and ASVs (p-value < 0.05) were determine using Spearman correlation. Interesting associations with muscle contraction and the modulation of the immune system were observed for the genes correlated with bacterial ASVs. Potential functional candidate genes for feed efficiency status were identified for Angus (CCL17, CCR3, and CXCL10), Charolais (KCNK9, GGT1 and IL6), and Kinsella breed (ESR2). The results obtained here provide more insights regarding the applicability of target host and rumen microbial traits for the selection and breeding of more feed efficient beef cattle.

Molecular mechanisms of muscle contraction: A historical perspective

Studies of muscle structure and function can be traced to at least 2,000 years ago. However, the modern era of muscle contraction mechanisms started in the 1950s with the classic works by AF Huxley and HE Huxley, both born in the United Kingdom, but not related and working independently. HE Huxley was the first to suggest that muscle contraction occurred through the sliding of two sets of filamentous structures (actin or thin filaments and myosin or thick filaments). AF Huxley then developed a biologically inspired mathematical model suggesting a possible molecular mechanism of how this sliding of actin and myosin might take place. This model then evolved from a two-state to a multi-state model of myosin-actin interactions, and from one that suggested a linear motor causing the sliding to a rotating motor. This model, the cross-bridge model of muscle contraction, is still widely used in biomechanics, and even the more sophisticated cross-bridge models of today still contain many of the features originally proposed by AF Huxley. In 2002, we discovered a hitherto unknown property of muscle contraction that suggested the involvement of passive structures in active force production, the so-called passive force enhancement. It was quickly revealed that this passive force enhancement was caused by the filamentous protein titin, and the three-filament (actin, myosin, and titin) sarcomere model of muscle contraction evolved. There are many suggestions of how these three proteins interact to cause contraction and produce active force, and one such suggestion is described here, but the molecular details of this proposed mechanism still need careful evaluation.

Muscle miR-16 deletion results in impaired insulin sensitivity and contractile function in a sex-dependent manner

microRNAs (miRs) are linked to various human diseases including type 2 diabetes mellitus (T2DM) and emerging evidence suggests that miRs may serve as potential therapeutic targets. Lower miR-16 content is consistent across different models of T2DM; however, the role of miR-16 in muscle metabolic health is still elusive. Therefore, the purpose of this study was to investigate how deletion of miR-16 in mice affects skeletal muscle metabolic health and contractile function in both sexes. This study was conducted using both 1) in vitro and 2) in vivo experiments. In in vitro experiments, we used C2C12 myoblasts to test if inhibition or overexpression of miR-16 affected insulin-mediated glucose handling. In in vivo experiments, we generated muscle-specific miR-16 knockout (KO) mice fed a high-fat diet (HFD) to assess how miR-16 content impacts metabolic and contractile properties including glucose tolerance, insulin sensitivity, muscle contractile function, protein anabolism, and mitochondrial network health. In in vitro experiments, although inhibition of miR-16 induced impaired insulin signaling (P = 0.002) and glucose uptake (P = 0.014), overexpression of miR-16 did not attenuate lipid overload-induced insulin resistance using the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol. In in vivo experiments, miR-16 deletion induced both impaired muscle contractility (P = 0.031-0.033), and mitochondrial network health (P = 0.008-0.018) in both sexes. However, although males specifically exhibited impaired insulin sensitivity following miR-16 deletion (P = 0.030), female KO mice showed pronounced glucose intolerance (P = 0.046), corresponding with lower muscle weights (P = 0.015), and protein hyperanabolism (P = 0.023). Our findings suggest distinct sex differences in muscle adaptation in response to miR-16 deletion and miR-16 may serve as a key regulator for metabolic dysregulation in T2DM.NEW & NOTEWORTHY We set to investigate the role of miR-16 in skeletal muscle during diet-induced insulin resistance. Our data provide novel evidence that the lack of miR-16 induced multiple aberrations in insulin sensitivity, muscle contractility, mitochondrial network health, and protein turnover in a sex-dependent manner. Interestingly, miR-16 deletion leads to insulin resistance in males and exacerbated glucose intolerance in females, suggesting different mechanisms of metabolic dysregulation with a lack of miR-16 between sexes.

Direct visualization of human myosin II force generation using DNA origami-based thick filaments

The sarcomere, the minimal mechanical unit of muscle, is composed of myosins, which self-assemble into thick filaments that interact with actin-based thin filaments in a highly-structured lattice. This complex imposes a geometric restriction on myosin in force generation. However, how single myosins generate force within the restriction remains elusive and conventional synthetic filaments do not recapitulate the symmetric bipolar filaments in sarcomeres. Here we engineered thick filaments using DNA origami that incorporate human muscle myosin to directly visualize the motion of the heads during force generation in a restricted space. We found that when the head diffuses, it weakly interacts with actin filaments and then strongly binds preferentially to the forward region as a Brownian ratchet. Upon strong binding, the two-step lever-arm swing dominantly halts at the first step and occasionally reverses direction. Our results illustrate the usefulness of our DNA origami-based assay system to dissect the mechanistic details of motor proteins.

The primary cause of muscle disfunction associated with substitutions E240K and R244G in tropomyosin is aberrant behavior of tropomyosin and response of actin and myosin during ATPase cycle

Using the polarized photometry technique we have studied the effects of two amino acid replacements, E240K and R244G, in tropomyosin (Tpm1.1) on the position of Tpm1.1 on troponin-free actin filaments and the spatial arrangement of actin monomers and myosin heads at various mimicked stages of the ATPase cycle in the ghost muscle fibres. E240 and R244 are located in the C-terminal, seventh actin-binding period, in f and b positions of the coiled-coil heptapeptide repeat. Actin, Tpm1.1, and myosin subfragment-1 (S1) were fluorescently labeled: 1.5-IAEDANS was attached to actin and S1, 5-IAF was bound to Tpm1.1. The labeled proteins were incorporated in the ghost muscle fibres and changes in polarized fluorescence during the ATPase cycle have been measured. It was found that during the ATPase cycle both mutant tropomyosins occupied a position close to the inner domain of actin. The relative amount of the myosin heads in the strongly-bound conformations and of the switched on actin monomers increased at mimicking different stages of the ATPase cycle. This might be one of the reasons for muscle dysfunction in congenital fibre type disproportion caused by the substitutions E240K and R244G in tropomyosin.

Modeling the Actin.myosin ATPase Cross-Bridge Cycle for Skeletal and Cardiac Muscle Myosin Isoforms

Modeling the complete actin.myosin ATPase cycle has always been limited by the lack of experimental data concerning key steps of the cycle, because these steps can only be defined at very low ionic strength. Here, using human β-cardiac myosin-S1, we combine published data from transient and steady-state kinetics to model a minimal eight-state ATPase cycle. The model illustrates the occupancy of each intermediate around the cycle and how the occupancy is altered by changes in actin concentration for [actin] = 1-20K_m. The cycle can be used to predict the maximal velocity of contraction (by motility assay or sarcomeric shortening) at different actin concentrations (which is consistent with experimental velocity data) and predict the effect of a 5 pN load on a single motor. The same exercise was repeated for human α-cardiac myosin S1 and rabbit fast skeletal muscle S1. The data illustrates how the motor domain properties can alter the ATPase cycle and hence the occupancy of the key states in the cycle. These in turn alter the predicted mechanical response of the myosin independent of other factors present in a sarcomere, such as filament stiffness and regulatory proteins. We also explore the potential of this modeling approach for the study of mutations in human β-cardiac myosin using the hypertrophic myopathy mutation R453C. Our modeling, using the transient kinetic data, predicts mechanical properties of the motor that are compatible with the single-molecule study. The modeling approach may therefore be of wide use for predicting the properties of myosin mutations.

Review: The ATPase mechanism of myosin and actomyosin

Myosins are a large family of molecular motors that use the common P-loop, Switch 1 and Switch 2 nucleotide binding motifs to recognize ATP, to create a catalytic site than can efficiently hydrolyze ATP and to communicate the state of the nucleotide pocket to other allosteric binding sites on myosin. The energy of ATP hydrolysis is used to do work against an external load. In this short review I will outline current thinking on the mechanism of ATP hydrolysis and how the energy of ATP hydrolysis is coupled to a series of protein conformational changes that allow a myosin, with the cytoskeleton track actin, to operate as a molecular motor of distinct types; fast movers, processive motors or strain sensors. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 483-491, 2016.

Kinetic Adaptations of Myosins for Their Diverse Cellular Functions

Members of the myosin superfamily are involved in all aspects of eukaryotic life. Their function ranges from the transport of organelles and cargos to the generation of membrane tension, and the contraction of muscle. The diversity of physiological functions is remarkable, given that all enzymatically active myosins follow a conserved mechanoenzymatic cycle in which the hydrolysis of ATP to ADP and inorganic phosphate is coupled to either actin-based transport or tethering of actin to defined cellular compartments. Kinetic capacities and limitations of a myosin are determined by the extent to which actin can accelerate the hydrolysis of ATP and the release of the hydrolysis products and are indispensably linked to its physiological tasks. This review focuses on kinetic competencies that - together with structural adaptations - result in myosins with unique mechanoenzymatic properties targeted to their diverse cellular functions.

Poorly understood aspects of striated muscle contraction

Muscle contraction results from cyclic interactions between the contractile proteins myosin and actin, driven by the turnover of adenosine triphosphate (ATP). Despite intense studies, several molecular events in the contraction process are poorly understood, including the relationship between force-generation and phosphate-release in the ATP-turnover. Different aspects of the force-generating transition are reflected in the changes in tension development by muscle cells, myofibrils and single molecules upon changes in temperature, altered phosphate concentration, or length perturbations. It has been notoriously difficult to explain all these events within a given theoretical framework and to unequivocally correlate observed events with the atomic structures of the myosin motor. Other incompletely understood issues include the role of the two heads of myosin II and structural changes in the actin filaments as well as the importance of the three-dimensional order. We here review these issues in relation to controversies regarding basic physiological properties of striated muscle. We also briefly consider actomyosin mutation effects in cardiac and skeletal muscle function and the possibility to treat these defects by drugs.

Temperature manifold for a stopped-flow machine to allow measurements from -10 to +40°C

Conducting enzymatic stopped-flow experiments at temperatures far removed from ambient can be very problematic because extremes in temperature (<10°C or >30°C) can damage the machine or the enzyme. We have devised a simple manifold that can be attached to most commercial stopped-flow systems that is independently heated or cooled separate from the main stopped-flow system. Careful calibration of the flow circuit allows the sample to be heated or cooled to the measurement temperature (-8 to +40°C) 1 to 2s before mixing in the reaction chamber. This approach allows measurements at temperatures where the stopped flow or the protein is normally unstable. To validate the manifold, we investigated the well-defined ATP-induced dissociation of rabbit muscle myosin subfragment 1 (S1) from its complex with pyrene-labeled actin. This process has both temperature-dependent and -independent components. Use of ethylene glycol allowed us to measure the reaction below 0°C and up to 42°C, and as expected the second-order rate constant (K1k+2) and the maximum rate of dissociation (k+2) both increased with temperature, whereas 1/K1 is unaffected by the change in temperature.

A new mechanokinetic model for muscle contraction, where force and movement are triggered by phosphate release

The atomic structure of myosin-S1 suggests that its working stroke, which generates tension and shortening in muscle, is triggered by the release of inorganic phosphate from the active site. This mechanism is the basis of a new mechanokinetic model for contractility, using the biochemical actomyosin ATPase cycle, strain-dependent kinetics and dimeric myosins on buckling rods. In this model, phosphate-dependent aspects of contractility arise from a rapid reversible release of phosphate from the initial bound state (A.M.ADP.Pi), which triggers the stroke. Added phosphate drives bound myosin towards this initial state, and the transient tension response to a phosphate jump reflects the rate at which it detaches from actin. Predictions for the tensile and energetic properties of striated muscle as a function of phosphate level, including the tension responses to length steps and Pi-jumps, are compared with experimental data from rabbit psoas fibres at 10 °C. The phosphate sensitivity of isometric tension is maximal when the actin affinity of M.ADP.Pi is near unity. Hence variations in actin affinity modulate the phosphate dependence of isometric tension, and may explain why phosphate sensitivity is temperature-dependent or absent in different muscles.

Ultrafast force-clamp spectroscopy of single molecules reveals load dependence of myosin working stroke

We describe a dual-trap force-clamp configuration that applies constant loads between a binding protein and an intermittently interacting biological polymer. The method has a measurement delay of only ∼10 μs, allows detection of interactions as brief as ∼100 μs and probes sub-nanometer conformational changes with a time resolution of tens of microseconds. We tested our method on molecular motors and DNA-binding proteins. We could apply constant loads to a single motor domain of myosin before its working stroke was initiated (0.2-1 ms), thus directly measuring its load dependence. We found that, depending on the applied load, myosin weakly interacted (<1 ms) with actin without production of movement, fully developed its working stroke or prematurely detached (<5 ms), thus reducing the working stroke size with load. Our technique extends single-molecule force-clamp spectroscopy and opens new avenues for investigating the effects of forces on biological processes.

Modification of interface between regulatory and essential light chains hampers phosphorylation-dependent activation of smooth muscle myosin

We examined the regulatory importance of interactions between regulatory light chain (RLC), essential light chain (ELC), and adjacent heavy chain (HC) in the regulatory domain of smooth muscle heavy meromyosin. After mutating the HC, RLC, and/or ELC to disrupt their predicted interactions (using scallop myosin coordinates), we measured basal ATPase, V(max), and K(ATPase) of actin-activated ATPase, actin-sliding velocities, rigor binding to actin, and kinetics of ATP binding and ADP release. If unphosphorylated, all mutants were similar to wild type showing turned-off behaviors. In contrast, if phosphorylated, mutation of RLC residues smM129Q and smG130C in the F-G helix linker, which interact with the ELC (Ca(2+) binding in scallop), was sufficient to abolish motility and diminish ATPase activity, without altering other parameters. ELC mutations within this interacting ELC loop (smR20M and smK25A) were normal, but smM129Q/G130C-R20M or -K25A showed a partially recovered phenotype suggesting that interaction between the RLC and ELC is important. A molecular dynamics study suggested that breaking the RLC/ELC interface leads to increased flexibility at the interface and ELC-binding site of the HC. We hypothesize that this leads to hampered activation by allowing a pre-existing equilibrium between activated and inhibited structural distributions (Vileno, B., Chamoun, J., Liang, H., Brewer, P., Haldeman, B. D., Facemyer, K. C., Salzameda, B., Song, L., Li, H. C., Cremo, C. R., and Fajer, P. G. (2011) Broad disorder and the allosteric mechanism of myosin II regulation by phosphorylation. Proc. Natl. Acad. Sci. U.S.A. 108, 8218-8223) to be biased strongly toward the inhibited distribution even when the RLC is phosphorylated. We propose that an important structural function of RLC phosphorylation is to promote or assist in the maintenance of an intact RLC/ELC interface. If the RLC/ELC interface is broken, the off-state structures are no longer destabilized by phosphorylation.

Multiscale modeling of structural dynamics underlying force generation and product release in actomyosin complex

To decrypt the mechanistic basis of myosin motor function, it is essential to probe the conformational changes in actomyosin with high spatial and temporal resolutions. In a computational effort to meet this challenge, we have performed a multiscale modeling of the allosteric couplings and transition pathway of actomyosin complex by combining coarse-grained modeling of the entire complex with all-atom molecular dynamics simulations of the active site. Our modeling of allosteric couplings at the pre-powerstroke state has pinpointed key actin-activated couplings to distant myosin parts which are critical to force generation and the sequential release of phosphate and ADP. At the post-powerstroke state, we have identified isoform-dependent couplings which underlie the reciprocal coupling between actin binding and nucleotide binding in fast Myosin II, and load-dependent ADP release in Myosin V. Our modeling of transition pathway during powerstroke has outlined a clear sequence of structural events triggered by actin binding, which lead to subsequent force generation, twisting of central beta-sheet, and the sequential release of phosphate and ADP. Finally we have performed atomistic simulations of active-site dynamics based on an on-path "transition-state" myosin conformation, which has revealed significantly weakened coordination of phosphate by Switch II, and a disrupted key salt bridge between Switch I and II. Meanwhile, the coordination of MgADP by Switch I and P loop is less perturbed. As a result, the phosphate can be released prior to MgADP. This study has shed new lights on the controversy over the structural mechanism of actin-activated phosphate release and force generation in myosin motor.

Caldesmon inhibits the rotation of smooth actin subdomain-1 and alters its mobility during the ATP hydrolysis cycle

Smooth muscle thin filaments have been reconstituted in muscle ghost fibers by incorporation of smooth muscle actin, tropomyosin and caldesmon. For the first time, rotation of subdomain-1 and changes of its mobility in IAEDANS-labeled actin during the ATP hydrolysis cycle simulated using nucleotides and non-hydrolysable ATP analogs have been demonstrated directly. Binding of caldesmon altered the mobility and inhibited the rotation of actin subdomain-1 during the transition from AM * *.ADP.Pi to AM state, resulting in inhibition of both strong and weak-binding intermediate states. These new results imply that regulation of actomyosin interaction by caldesmon during the ATPase cycle is fulfilled via the inhibition of actin subdomain-1 rotation toward the periphery of the thin filament, which decreases the area of the specific binding between actin and myosin molecules and is likely to underlie at least in part the mechanism of caldesmon-induced contractility suppression.

Time course and strain dependence of ADP release during contraction of permeabilized skeletal muscle fibers

A phosphorylated, single cysteine mutant of nucleoside diphosphate kinase, labeled with N-[2-(iodoacetamido)ethyl]-7-diethylaminocoumarin-3-carboxamide (P approximately NDPK-IDCC), was used as a fluorescence probe for time-resolved measurement of changes in [MgADP] during contraction of single permeabilized rabbit psoas fibers. The dephosphorylation of the phosphorylated protein by MgADP occurs within the lattice environment of permeabilized fibers with a second-order rate constant at 12 degrees C of 10(5) M(-1) s(-1). This dephosphorylation is accompanied by a change in coumarin fluorescence. We report the time course of P approximately NDPK-IDCC dephosphorylation during the period of active isometric force redevelopment after quick release of fiber strain at pCa(2+) of 4.5. After a rapid length decrease of 0.5% was applied to the fiber, the extra NDPK-IDCC produced during force recovery, above the value during the approximately steady state of isometric contraction, was 2.7 +/- 0.6 microM and 4.7 +/- 1.5 microM at 12 and 20 degrees C, respectively. The rates of P approximately NDPK-IDCC dephosphorylation during force recovery were 28 and 50 s(-1) at 12 and 20 degrees C, respectively. The time courses of isometric force and P approximately NDPK-IDCC dephosphorylation were simulated using a seven-state reaction scheme. Relative isometric force was modeled by changes in the occupancy of strongly bound A.M.ADP.P(i) and A.M.ADP states. A strain-sensitive A.M.ADP isomerization step was rate-limiting (3-6 s(-1)) in the cross-bridge turnover during isometric contraction. At 12 degrees C, the A.M.ADP.P(i) and the pre- and postisomerization A.M.ADP states comprised 56%, 38%, and 7% of the isometric force-bearing AM states, respectively. At 20 degrees C, the force-bearing A.M.ADP.P(i) state was a lower proportion of the total force-bearing states (37%), whereas the proportion of postisomerization A.M.ADP states was higher (19%). The simulations suggested that release of cross-bridge strain caused rapid depopulation of the preisomerization A.M.ADP state and transient accumulation of MgADP in the postisomerization A.M.ADP state. Hence, the strain-sensitive isomerization of A.M.ADP seems to explain the rate of change of P approximately NDPK-IDCC dephosphorylation during force recovery. The temperature-dependent isometric distribution of myosin states is consistent with the previous observation of a small decrease in amplitude of the P(i) transient during force recovery at 20 degrees C and the current observation of an increase in amplitude of the ADP-sensitive NDPK-IDCC transient.

Modulation of the effects of tropomyosin on actin and myosin conformational changes by troponin and Ca2+

The molecular mechanisms by which troponin (TN)-tropomyosin (TM) regulates the myosin ATPase cycle were investigated using fluorescent probes specifically bound to Cys36 of TM, Cys707 of myosin subfragment-1, and Cys374 of actin incorporated into ghost muscle fibers. Intermediate states of actomyosin were simulated by using nucleotides and non-hydrolysable ATP analogs. Multistep changes in mobility and spatial arrangement of SH1 helix of myosin motor domain and actin subdomain-1 during the ATPase cycle were observed. Each intermediate state of actomyosin induced a definite conformational state and specific position of TM strands on the surface of thin filament. TM increased the amplitude of myosin SH1 helix and actin subdomain-1 movements at transition from weak- to strong-binding states shifting to the center of thin filament at strong-binding and to the periphery of thin filament at weak-binding states. TN modulated those movements in a capital ES, Cyrillicsmall a, Cyrillic(2+)-dependent manner. At high-Ca(2+), TN enhanced the effect of TM on SH1 helix and subdomain-1 movements by transferring TM further to the center of thin filament at strong-binding states. In contrast, at low-Ca(2+), TN inhibited the effect of TM movements, "freezing" actin structure in "OFF" state and TM in the position typical for weak-binding states, resulting in disturbing the interplay of actin and myosin.

Towards a unified theory of muscle contraction. I: foundations

Molecular models of contractility in striated muscle require an integrated description of the action of myosin motors, firstly in the filament lattice of the half-sarcomere. Existing models do not adequately reflect the biochemistry of the myosin motor and its sarcomeric environment. The biochemical actin-myosin-ATP cycle is reviewed, and we propose a model cycle with two 4- to 5-nm working strokes, where phosphate is released slowly after the first stroke. A smaller third stroke is associated with ATP-induced detachment from actin. A comprehensive model is defined by applying such a cycle to all myosin-S1 heads in the half-sarcomere, subject to generic constraints as follows: (a) all strain-dependent kinetics required for actin-myosin interactions are derived from reaction-energy landscapes and applied to dimeric myosin, (b) actin-myosin interactions in the half-sarcomere are controlled by matching rules derived from the structure of the filaments, so that each dimer may be associated with a target zone of three actin sites, and (c) the myosin and actin filaments are treated as elastically extensible. Numerical predictions for such a model are presented in the following paper.

The elastic properties of the structurally characterized myosin II S2 subdomain: a molecular dynamics and normal mode analysis

The elastic properties (stretching and bending moduli) of myosin are expected to play an important role in its function. Of particular interest is the extended alpha-helical coiled-coil portion of the molecule. Since there is no high resolution structure for the entire coiled-coil, a study is made of the scallop myosin II S2 subdomain for which an x-ray structure is available (Protein Data Bank 1nkn). We estimate the stretching and bending moduli of the S2 subdomain with an atomic level model by use of molecular simulations. Results were obtained from nonequilibrium molecular dynamics simulations in the presence of an external force, from the fluctuations in equilibrium molecular dynamics simulations and from normal modes. In addition, a poly-Ala (78 amino acid residues) alpha-helix model was examined to test the methodology and because of its interest as part of the lever arm. As expected, both the alpha-helix and coiled-coil S2 subdomain are very stiff for stretching along the main axis, with the stretching stiffness constant in the range 60-80 pN/nm (scaled to the 60 nm long S2). Both molecules are much more flexible for bending with a lateral stiffness of approximately 0.010 pN/nm for the S2 and 0.0055 pN/nm for the alpha-helix (scaled to 60 nm). These results are expected to be useful in estimating cross-bridge elasticity, which is required for understanding the strain-dependent transitions in the actomyosin cycle and for the development of three-dimensional models of muscle contraction.

Quantification of myosin heavy chain RNA in human laryngeal muscles: differential expression in the vertical and horizontal posterior cricoarytenoid and thyroarytenoid

BACKGROUND

Human laryngeal muscles are composed of fibers that express type I, IIA, and IIX myosin heavy chains (MyHC), but the presence and quantity of atypical myosins such as perinatal, extraocular, IIB, and alpha (cardiac) remain in question. These characteristics have been determined by biochemical or immunohistologic tissue sampling but with no complementary evidence of gene expression at the molecular level. The distribution of myosin, the main motor protein, in relation to structure-function relationships in this specialized muscle group will be important for understanding laryngeal function in both health and disease.

OBJECTIVES

We determined the quantity of MyHC genes expressed in human posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscle using real-time quantitative reverse-transcriptase polymerase chain reaction in a large number of samples taken from laryngectomy subjects. The PCA muscle was divided into vertical (V) and horizontal (H) portions for analysis.

RESULTS AND CONCLUSIONS

No extraocular or IIB myosin gene message is present in PCA or TA, but IIB is expressed in human extraocular muscle. Low but detectable amounts of perinatal and alpha gene message are present in both of the intrinsic laryngeal muscles. In H- and V-PCA, MyHC gene amounts were beta greater than IIA greater than IIX, but amounts of fast myosin RNA were greater in V-PCA. In TA, the order was beta greater than IIX greater than IIA. The profiles of RNA determined here indicate that, in humans, neither PCA nor TA intrinsic laryngeal muscles express unique very fast-contracting MyHCs but instead may rely on differential synthesis and use of beta, IIA, and IIX isoforms to perform their specialized contractile functions.

Modulation of the actomyosin interaction during fatigue of skeletal muscle

Fatigue of skeletal muscle involves many systems beginning with the central nervous system and ending with the contractile machinery. This review concentrates on those factors that directly affect the actomyosin interaction: the build-up of metabolites; myosin phosphorylation; and oxidation of the myofibrillar proteins by free radicals. The decrease in [ATP] and increase in [ADP] appear to play little role in modulating function. The increase in phosphate inhibits tension. The decrease in pH, long thought to be a major factor, is now known to play a more minor role. Myosin phosphorylation potentiates the force achieved in a twitch, and a further role in inhibiting velocity is proposed. Protein oxidation can both potentiate and inhibit the actomyosin interaction. It is concluded that these factors, taken together, do not fully explain the inhibition of the actomyosin interaction observed in living fibers, and thus additional modulators of this interaction remain to be discovered.

Human myosin Vc is a low duty ratio, nonprocessive molecular motor

Myosin Vc is the product of one of the three genes of the class V myosin found in vertebrates. It is widely found in secretory and glandular tissues, with a possible involvement in transferrin trafficking. Transient and steady-state kinetic studies of human myosin Vc were performed using a truncated, single-headed construct. Steady-state actin-activated ATPase measurements revealed a V(max) of 1.8 +/- 0.3 s(-1) and a K(ATPase) of 43 +/- 11 microm. Unlike previously studied vertebrate myosin Vs, the rate-limiting step in the actomyosin Vc ATPase pathway is the release of inorganic phosphate (~1.5 s(-1)), rather than the ADP release step (~12.0-16.0 s(-1)). Nevertheless, the ADP affinity of actomyosin Vc (K(d) = 0.25 +/- 0.02 microm) reflects a higher ADP affinity than seen in other myosin V isoforms. Using the measured kinetic rates, the calculated duty ratio of myosin Vc was approximately 10%, indicating that myosin Vc spends the majority of the actomyosin ATPase cycle in weak actin-binding states, unlike the other vertebrate myosin V isoforms. Consistent with this, a fluorescently labeled double-headed heavy meromyosin form showed no processive movements along actin filaments in a single molecule assay, but it did move actin filaments at a velocity of approximately 24 nm/s in ensemble assays. Kinetic simulations reveal that the high ADP affinity of actomyosin Vc may lead to elevations of the duty ratio of myosin Vc to as high as 64% under possible physiological ADP concentrations. This, in turn, may possibly imply a regulatory mechanism that may be sensitive to moderate changes in ADP concentration.

Plasticity of muscle function in a thermoregulating ectotherm (Crocodylus porosus): biomechanics and metabolism

Thermoregulation and thermal sensitivity of performance are thought to have coevolved so that performance is optimized within the selected body temperature range. However, locomotor performance in thermoregulating crocodiles (Crocodylus porosus) is plastic and maxima shift to different selected body temperatures in different thermal environments. Here we test the hypothesis that muscle metabolic and biomechanical parameters are optimized at the body temperatures selected in different thermal environments. Hence, we related indices of anaerobic (lactate dehydrogenase) and aerobic (cytochrome c oxidase) metabolic capacities and myofibrillar ATPase activity to the biomechanics of isometric and work loop caudofemoralis muscle function. Maximal isometric stress (force per muscle cross-sectional area) did not change with thermal acclimation, but muscle work loop power output increased with cold acclimation as a result of shorter activation and relaxation times. The thermal sensitivity of myofibrillar ATPase activity decreased with cold acclimation in caudofemoralis muscle. Neither aerobic nor anaerobic metabolic capacities were directly linked to changes in muscle performance during thermal acclimation, although there was a negative relationship between anaerobic capacity and isometric twitch stress in cold-acclimated animals. We conclude that by combining thermoregulation with plasticity in biomechanical function, crocodiles maximize performance in environments with highly variable thermal properties.

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.

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.

Derivation and Left Ventricular Pressure Phase Plane Based Validation of a Time Dependent Isometric Crossbridge Attachment Model

Huxley's crossbridge attachment model predicts tension (contractile force) development in isometric (fixed length) cells using constant attachment and detachment rates. Alternative models incorporating time-varying calcium concentrations are complex (coupled linear differential equations) and use time-dependent inputs (calcium, elastance, etc.) to model multiple states. We hypothesize that by incorporating the known significant rise and fall in intracellular calcium, via either an asymmetric damped function or a symmetric Gaussian function, into a time-varying, rather than constant, attachment rate function, the Huxley model prediction for tension (i.e., chamber pressure) in isovolumic (isometric) non-ejecting beats will improve. To test the hypothesis that the time-dependent model-predicted (TDM) pressure fits the in vivo isometric (isovolumic) LV pressure phase-plane (PPP) contour better than the constant attachment rate predicted pressure, we used the TDM to fit non-ejecting, premature ventricular contraction (PVC) PPP contours in 6 subjects. Conventional model fit was poor (relative error 74.0%+/-12.5%), while the asymmetric damped TDM rate function provided slight improvement relative to the conventional time-independent model (relative error 55.4%+/-9.8%). The symmetric Gaussian rate function TDM provided the best PPP fit to all non-ejecting beats tested (relative error 19.8%+/-4.8%). We conclude that approximating the lumped attachment rate via a time-varying, rather than constant, rate function generates a physiologically viable model of crossbridge behavior. The PPP provides the optimal arena for alternate mathematical formulation assessment of LVP contour prediction by time-dependent attachment rate functions and facilitates modeling of cardiac contraction and relaxation.

Dynamics of the nucleotide pocket of myosin measured by spin-labeled nucleotides

We have used electron paramagnetic probes attached to the ribose of ATP (SL-ATP) to monitor conformational changes in the nucleotide pocket of myosin. Spectra for analogs bound to myosin in the absence of actin showed a high degree of immobilization, indicating a closed nucleotide pocket. In the Actin.Myosin.SL-AMPPNP, Actin.Myosin.SL-ADP.BeF(3), and Actin.Myosin.SL-ADP.AlF(4) complexes, which mimic weakly binding states near the beginning of the power stroke, the nucleotide pocket remained closed. The spectra of the strongly bound Actin.Myosin.SL-ADP complex consisted of two components, one similar to the closed pocket and one with increased probe mobility, indicating a more open pocket, The temperature dependence of the spectra showed that the two conformations of the nucleotide pocket were in equilibrium, with the open conformation more favorable at higher temperatures. These results, which show that opening of the pocket occurs only in the strongly bound states, appear reasonable, as this would tend to keep ADP bound until the end of the power stroke. This conclusion also suggests that force is initially generated by a myosin with a closed nucleotide pocket.

Caldesmon freezes the structure of actin filaments during the actomyosin ATPase cycle

Hybrid contractile apparatus was reconstituted in skeletal muscle ghost fibers by incorporation of skeletal muscle myosin subfragment 1 (S1), smooth muscle tropomyosin and caldesmon. The spatial orientation of FITC-phalloidin-labeled actin and IAEDANS-labeled S1 during sequential steps of the acto-S1 ATPase cycle was studied by measurement of polarized fluorescence in the absence or presence of nucleotides conditioning the binding affinity of both proteins. In the fibers devoid of caldesmon addition of nucleotides evoked unidirectional synchronous changes in the orientation of the fluorescent probes attached to F-actin or S1. The results support the suggestion on the multistep rotation of the cross-bridge (myosin head and actin monomers) during the ATPase cycle. The maximal cross-bridge rotation by 7 degrees relative to the fiber axis and the increase in its rigidity by 30% were observed at transition between A**.M**.ADP.Pi (weak binding) and A--.M--.ADP (strong binding) states. When caldesmon was present in the fibers (OFF-state of the thin filament) the unidirectional changes in the orientation of actin monomers and S1 were uncoupled. The tilting of the myosin head and of the actin monomer decreased by 29% and 90%, respectively. It is suggested that in the "closed" position caldesmon "freezes" the actin filament structure and induces the transition of the intermediate state of actomyosin towards the weak-binding states, thereby inhibiting the ATPase activity of the actomyosin.

Orientation and mobility of actin in different intermediate states of the ATP hydrolysis cycle

Using polarization fluorimetry, we have investigated conformational changes of FITC-phalloidin-labeled F-actin in ghost muscle fibers. These changes were induced by myosin subfragment-1 (S1) in the absence and presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes of orientation and mobility of actin monomers during the ATPase cycle. The most pronounced differences in orientation (~4 degrees ) and in mobility (~43%) of actin were found between the actomyosin states induced by MgADP and MgATP.

Effect of nucleotides on the orientation and mobility of myosin subfragment-1 in ghost muscle fiber

Using polarization fluorimetry, the orientation and mobility of 1,5-IAEDANS specifically bound to Cys707 of myosin subfragment-1 (S1) were studied in ghost muscle tropomyosin-containing fibers in the absence and in the presence of MgADP, MgAMP-PNP, MgATPgammaS, or MgATP. Modeling of various intermediate states was accompanied by discrete changes in actomyosin orientation and mobility of fluorescent dye dipoles. This suggests multistep changes in the structural state of the myosin head during the ATPase cycle. Maximal differences in the probe orientation by 4 degrees and its mobility by 30% were found between actomyosin states in the presence of MgADP and MgATP. It is suggested that interaction of S1 with F-actin induces nucleotide-dependent rotation of the whole motor domain of the myosin head or only the dye-binding site and also change in the head mobility.

Does phosphate release limit the ATPases of soleus myofibrils? Evidence that (A)M. ADP.Pi states predominate on the cross-bridge cycle

The ATPases (+/-Ca2+) of myofibrils from rabbit soleus (a slow muscle) and psoas (a fast muscle) have different Ea: -Ca2+, 78 and 60 kJ/mol and +Ca2+, 155 and 71 kJ/mol, respectively. At physiological temperatures, the two types of myofibrillar ATPase are very similar and yet the mechanical properties of the muscles are different (Candau et al. (2003) Biophys J 85: 3132-3141). Muscle contraction relies on specific interactions of the different chemical states on the myosin head ATPase pathway with the thin filament. An explanation for the Ea data is that different states populate the pathways of the two types of myofibril because the rate limiting steps are different. Here, we put this to the test by a comparison of the transient kinetics of the initial steps of the ATPases of the two types of myofibril at 4 degrees C. We used two methods: rapid flow quench ('cold ATP chase': titration of active sites, ATP binding kinetics, k(cat); 'Pi burst': ATP cleavage kinetics) and fluorescence stopped-flow (MDCC-phosphate binding protein for free Pi; myofibrillar tryptophan fluorescence for myosin head-thin filament detachment and ATP cleavage kinetics). We find that, as with psoas myofibrils, the most populated state on the cross-bridge cycle of soleus myofibrils, whether relaxed or activated, is (A)M.ADP.Pi. We propose a reaction pathway that includes several (A)M.ADP.Pi sub-states that are either 'weak' or 'strong', depending on the mechanical condition.

Fluorescence depolarization of actin filaments in reconstructed myofibers: the effect of S1 or pPDM-S1 on movements of distinct areas of actin

Fluorescence polarization measurements were used to study changes in the orientation and order of different sites on actin monomers within muscle thin filaments during weak or strong binding states with myosin subfragment-1. Ghost muscle fibers were supplemented with actin monomers specifically labeled with different fluorescent probes at Cys-10, Gln-41, Lys-61, Lys-373, Cys-374, and the nucleotide binding site. We also used fluorescent phalloidin as a probe near the filament axis. Changes in the orientation of the fluorophores depend not only on the state of acto-myosin binding but also on the location of the fluorescent probes. We observed changes in polarization (i.e., orientation) for those fluorophores attached at the sites directly involved in myosin binding (and located at high radii from the filament axis) that were contrary to the fluorophores located at the sites close to the axis of thin filament. These altered probe orientations suggest that myosin binding alters the conformation of F-actin. Strong binding by myosin heads produces changes in probe orientation that are opposite to those observed during weak binding.

The force exerted by a muscle cross-bridge depends directly on the strength of the actomyosin bond

Myosin produces force in a cyclic interaction, which involves alternate tight binding to actin and to ATP. We have investigated the energetics associated with force production by measuring the force generated by skinned muscle fibers as the strength of the actomyosin bond is changed. We varied the strength of the actomyosin bond by addition of a polymer that promotes protein-protein association or by changing temperature or ionic strength. We estimated the free energy available to generate force by measuring isometric tension, as the free energy of the states that precede the working stroke are lowered with increasing phosphate. We found that the free energy available to generate force and the force per attached cross-bridge at low [Pi] were both proportional to the free energy available from the formation of the actomyosin bond. We conclude that the formation of the actomyosin bond is involved in providing the free energy driving the production of isometric tension and mechanical work. Because the binding of myosin to actin is an endothermic, entropically driven reaction, work must be performed by a "thermal ratchet" in which a thermal fluctuation in Brownian motion is captured by formation of the actomyosin bond.

Alternative N-terminal regions of Drosophila myosin heavy chain tune muscle kinetics for optimal power output

We assessed the influence of alternative versions of a region near the N-terminus of Drosophila myosin heavy chain on muscle mechanical properties. Previously, we exchanged N-terminal regions (encoded by alternative exon 3s) between an embryonic (EMB) isoform and the indirect flight muscle isoform (IFI) of myosin, and demonstrated that it influences solution ATPase rates and in vitro actin sliding velocity. Because each myosin is expressed in Drosophila indirect flight muscle, in the absence of other myosin isoforms, this allows for muscle mechanical and whole organism locomotion assays. We found that exchanging the flight muscle specific exon 3 region into the embryonic isoform (EMB-3b) increased maximum power generation (P(max)) and optimal frequency of power generation (f(max)) threefold and twofold compared to fibers expressing EMB, whereas exchanging the embryonic exon 3 region into the flight muscle isoform (IFI-3a) decreased P(max) and f(max) to approximately 80% of IFI fiber values. Drosophila expressing IFI-3a exhibited a reduced wing beat frequency compared to flies expressing IFI, which optimized power generation from their kinetically slowed flight muscle. However, the slower wing beat frequency resulted in a substantial loss of aerodynamic power as manifest in decreased flight performance of IFI-3a compared to IFI. Thus the N-terminal region is important in tuning myosin kinetics to match muscle speed for optimal locomotory performance.

KIF1A alternately uses two loops to bind microtubules

The motor protein kinesin moves along microtubules, driven by adenosine triphosphate (ATP) hydrolysis. However, it remains unclear how kinesin converts the chemical energy into mechanical movement. We report crystal structures of monomeric kinesin KIF1A with three transition-state analogs: adenylyl imidodiphosphate (AMP-PNP), adenosine diphosphate (ADP)-vanadate, and ADP-AlFx (aluminofluoride complexes). These structures, together with known structures of the ADP-bound state and the adenylyl-(beta,gamma-methylene) diphosphate (AMP-PCP)-bound state, show that kinesin uses two microtubule-binding loops in an alternating manner to change its interaction with microtubules during the ATP hydrolysis cycle; loop L11 is extended in the AMP-PNP structure, whereas loop L12 is extended in the ADP structure. ADP-vanadate displays an intermediate structure in which a conformational change in two switch regions causes both loops to be raised from the microtubule, thus actively detaching kinesin.

One-dimensional diffusion on microtubules of particles coated with cytoplasmic dynein and immunoglobulins

We characterized and compared the diffusion of beads coated with proteins such as cytoplasmic dynein, alpha-casein, and some immunoglobulins on microtubules. Such weak binding interactions could be common and convenient for concentrating proteins at the surface of cytoplasmic structures such as microtubules. In studying the motile behavior of anionic latex beads coated with limiting dilutions of cytoplasmic dynein, we observed that in addition to active movement, 20-50% of the beads moved back and forth in a random manner. The random movement was inhibited by depletion of ATP or addition of ADP or AMP-PNP. Mean-square-displacement analysis showed that the movement is a one-dimensional diffusion along the microtubule axis with a diffusion coefficient of 2.16 x 10(-10) cm2/sec. Histogram analysis of off-axis movements suggested that approximately 60% of the diffusing beads followed the path of a single microtubule protofilament. Beads coated with proteins such as alpha-casein or a monoclonal immunoglobulin were also observed to diffuse on microtubules with a similar diffusion coefficient to cytoplasmic dynein. However, alpha-casein or immunoglobulin-bead diffusion was not ATP dependent and did not follow the paths of single protofilaments. Thus, although the environment of the microtubule surface can trap a variety of different protein-coated beads, cytoplasmic dynein's interaction is unusual in its ATP dependence and tracking on a single protofilament, which is consistent with its specific interaction with microtubules. Diffusive interactions could concentrate associating proteins and still allow for freedom of movement.

Why choose myofibrils to study muscle myosin ATPase?

Our objective is to propose an overview of the usefulness of skeletal myofibril as an experimental system for studying mechanochemical coupling of skeletal muscles and myosin ATPase activity. The myofibril is a true functional mini-muscle that is able to contract in the presence of ATP. It also contains the machinery necessary for the calcium sensitivity of the contraction. In the absence of calcium, myofibrillar ATPase activity is basal, no shortening occurs and no active force is developed. In the presence of calcium, myofibrillar ATPase is activated and myofibrils either shorten with no external load (native myofibrils) or contract isometrically (cross-linked myofibrils). With this organised system, both chemical and mechanical studies can be carried out. For a decade, our laboratory has been using rabbit psoas myofibrils for exploring myosin ATPase activity. The first challenge was to successfully apply rapid kinetic approaches, such as rapid-flow-quench, to this organised system. Another challenge was to work with myofibrils in cryoenzymic conditions, i.e. in the presence of organic solvents and at sub-zero temperatures. In this overview, we highlight differences between the myosin ATPase in organised systems (myofibrils or fibres) and that of contractile proteins in solution (S1 or actoS1) that we observed using these approaches. We discuss the importance of these differences in terms of mechanochemical coupling. It is concluded that great care should be taken when extrapolating mechanochemical properties of the contractile proteins in solution to the whole muscle.

An electromechanical model of myosin molecular motors

There is a long-running debate on the working mechanism of myosin molecular motors, which, by interacting with actin filaments, convert the chemical energy of ATP into a variety of mechanical work. After the development of technologies for observing and manipulating individual working molecules, experimental results negating the widely accepted 'lever-arm hypothesis' have been reported. In this paper, based on the experimental results so far accumulated, an alternative hypothesis is proposed, in which motor molecules are modelled as electromechanical components that interact with each other through electrostatic force. Electrostatic attractive force between myosin and actin is assumed to cause a conformational change in the myosin head during the attachment process. An elastic energy resulting from the conformational change then produces the power stroke. The energy released at the ATP hydrolysis is mainly used to detach the myosin head from actin filaments. The mechanism presented in this paper is compatible with the experimental results contradictory to the previous theories. It also explains the behavior of myosins V and VI, which are engaged in cellular transport and move processively along actin filaments.

A kinetic mechanism for the fast movement of Chara myosin

Endoplasmic streaming of characean cells of Nitella or Chara is known to be in the range 30-100 microm/second. The Chara myosin extracted from the cells and fixed onto a glass surface was found to move muscle actin filaments at a velocity of 60 microm/second. This is ten times faster than that of skeletal muscle myosin (myosin II). In this study, the displacement caused by single Chara myosin molecules was measured using optical trapping nanometry. The step size of Chara myosin was approximately 19nm. This step size is longer than that of skeletal muscle myosin but shorter than that of myosin V. The dwell time of the steps was relatively long, and this most likely resulted from two rate-limiting steps, the dissociation of ADP and the binding of ATP. The rate of ADP release from Chara myosin after the completion of the force-generation step was similar to that of myosin V, but was considerably slower than that of skeletal muscle myosin. The 19nm step size and the dwell time obtained could not explain the fast movement. The fast movement could be explained by the load-dependent release of ADP. As the load imposed on the myosin decreased, the rate of ADP release increased. We propose that the interaction of Chara myosin with an actin filament resulted in a negative load being imposed on other myosin molecules interacting with the same actin filament. This resulted in an accelerated release of ADP and the fast sliding movement.

Cooperative regulation of myosin-actin interactions by a continuous flexible chain I: actin-tropomyosin systems

We present a model for cooperative myosin binding to the regulated actin filament, where tropomyosins are treated as a weakly-confined continuous flexible chain covering myosin binding sites. Thermal fluctuations in chain orientation are initially required for myosin binding, leaving kinked regions under which subsequent myosins may bind without further distortion of the chain. Statistical mechanics predicts the fraction of sites with bound myosin-S1 as a function of their affinities. Published S1 binding curves to regulated filaments with different tropomyosin isoforms are fitted by varying the binding constant, chain persistence length nu (in actin monomers), and chain kink energy A from a single bound S1. With skeletal tropomyosin, we find an S1 actin-binding constant of 2.2 x 10(7) M(-1), A = 1.6 k(B)T and nu = 2.7. Similar persistence lengths are found with yeast tropomyosin. Larger values are found for tropomyosin-troponin in the presence of calcium (nu = 3.7) and tropomyosins from smooth muscle and fibroblasts (nu = 4.5). The relationship of these results to structural information and the rigid-unit model of McKillop and Geeves is discussed.

Effect of an ADP analog on isometric force and ATPase activity of active muscle fibers

The role played by ADP in modulating cross-bridge function has been difficult to study, because it is hard to buffer ADP concentration in skinned muscle preparations. To solve this, we used an analog of ADP, spin-labeled ADP (SL-ADP). SL-ADP binds tightly to myosin but is a very poor substrate for creatine kinase or pyruvate kinase. Thus ATP can be regenerated, allowing well-defined concentrations of both ATP and SL-ADP. We measured isometric ATPase rate and isometric tension as a function of both [SL-ADP], 0.1-2 mM, and [ATP], 0.05-0.5 mM, in skinned rabbit psoas muscle, simulating fresh or fatigued states. Saturating levels of SL-ADP increased isometric tension (by P'), the absolute value of P' being nearly constant, approximately 0.04 N/mm(2), in variable ATP levels, pH 7. Tension decreased (50-60%) at pH 6, but upon addition of SL-ADP, P' was still approximately 0.04 N/mm(2). The ATPase was inhibited competitively by SL-ADP with an inhibition constant, K(i), of approximately 240 and 280 microM at pH 7 and 6, respectively. Isometric force and ATPase activity could both be fit by a simple model of cross-bridge kinetics.

Considerations on muscle contraction

The independent force generator and the power-stroke cross-bridge model have dominated the thinking on mechanisms of muscular contraction for nearly the past five decades. Here, we review the evolution of the cross-bridge theory from its origins as a two-state model to the current thinking of a multi-state mechanical model that is tightly coupled with the hydrolysis of ATP. Finally, we emphasize the role of skeletal muscle myosin II as a molecular motor whose actions are greatly influenced by Brownian motion. We briefly consider the conceptual idea of myosin II working as a ratchet rather than a power stroke model, an idea that is explored in detail in the companion paper.