Alternative versions of the myosin relay domain differentially respond to load to influence Drosophila muscle kinetics

Alternative relay regulates the adenosine triphosphatase activity of Locusta migratoria striated muscle myosin

Locust (Locusta migratoria) has a single striated muscle myosin heavy chain (Mhc) gene, which contains 5 clusters of alternative exclusive exons and 1 differently included penultimate exon. The alternative exons of Mhc gene encode 4 distinct regions in the myosin motor domain, that is, the N-terminal SH3-like domain, one lip of the nucleotide-binding pocket, the relay, and the converter. Here, we investigated the role of the alternative regions on the motor function of locust muscle myosin. Using Sf9-baculovirus protein expression system, we expressed and purified 5 isoforms of the locust muscle myosin heavy meromyosin (HMM), including the major isoform in the thorax dorsal longitudinal flight muscle (FL1) and 4 isoforms expressed in the abdominal intersegmental muscle (AB1 to AB4). Among these 5 HMMs, FL1-HMM displayed the highest level of actin-activated adenosine triphosphatase (ATPase) activity (hereafter referred as ATPase activity). To identify the alternative region(s) responsible for the elevated ATPase activity of FL1-HMM, we produced a number of chimeras of FL1-HMM and AB4-HMM. Substitution with the relay of AB4-HMM (encoded by exon-14c) substantially decreased the ATPase activity of FL1-HMM, and conversely, the relay of FL1-HMM (encoded by exon-14a) enhanced the ATPase activity of AB4-HMM. Mutagenesis showed that the exon-14a-encoded residues Gly474 and Asn509 are responsible for the elevated ATPase activity of FL1-HMM. Those results indicate that the alternative relay encoded by exon-14a/c play a key role in regulating the ATPase activity of FL1-HMM and AB4-HMM.

Contributions of alternative splicing to muscle type development and function

Animals possess a wide variety of muscle types that support different kinds of movements. Different muscles have distinct locations, morphologies and contractile properties, raising the question of how muscle diversity is generated during development. Normal aging processes and muscle disorders differentially affect particular muscle types, thus understanding how muscles normally develop and are maintained provides insight into alterations in disease and senescence. As muscle structure and basic developmental mechanisms are highly conserved, many important insights into disease mechanisms in humans as well as into basic principles of muscle development have come from model organisms such as Drosophila, zebrafish and mouse. While transcriptional regulation has been characterized to play an important role in myogenesis, there is a growing recognition of the contributions of alternative splicing to myogenesis and the refinement of muscle function. Here we review our current understanding of muscle type specific alternative splicing, using examples of isoforms with distinct functions from both vertebrates and Drosophila. Future exploration of the vast potential of alternative splicing to fine-tune muscle development and function will likely uncover novel mechanisms of isoform-specific regulation and a more holistic understanding of muscle development, disease and aging.

X-ray Crystallographic and Molecular Dynamic Analyses of Drosophila melanogaster Embryonic Muscle Myosin Define Domains Responsible for Isoform-Specific Properties

Drosophila melanogaster is a powerful system for characterizing alternative myosin isoforms and modeling muscle diseases, but high-resolution structures of fruit fly contractile proteins have not been determined. Here we report the first x-ray crystal structure of an insect myosin: the D melanogaster skeletal muscle myosin II embryonic isoform (EMB). Using our system for recombinant expression of myosin heavy chain (MHC) proteins in whole transgenic flies, we prepared and crystallized stable proteolytic S1-like fragments containing the entire EMB motor domain bound to an essential light chain. We solved the x-ray crystal structure by molecular replacement and refined the resulting model against diffraction data to 2.2 Å resolution. The protein is captured in two slightly different renditions of the rigor-like conformation with a citrate of crystallization at the nucleotide binding site and exhibits structural features common to myosins of diverse classes from all kingdoms of life. All atom molecular dynamics simulations on EMB in its nucleotide-free state and a derivative homology model containing 61 amino acid substitutions unique to the indirect flight muscle isoform (IFI) suggest that differences in the identity of residues within the relay and the converter that are encoded for by MHC alternative exons 9 and 11, respectively, directly contribute to increased mobility of these regions in IFI relative to EMB. This suggests the possibility that alternative folding or conformational stability within these regions contribute to the observed functional differences in Drosophila EMB and IFI myosins.

Force Generation by Myosin Motors: A Structural Perspective

Generating force and movement is essential for the functions of cells and organisms. A variety of molecular motors that can move on tracks within cells have evolved to serve this role. How these motors interact with their tracks and how that, in turn, leads to the generation of force and movement is key to understanding the cellular roles that these motor-track systems serve. This review is focused on the best understood of these systems, which is the molecular motor myosin that moves on tracks of filamentous (F-) actin. The review highlights both the progress and the limits of our current understanding of how force generation can be controlled by F-actin-myosin interactions. What has emerged are insights they may serve as a framework for understanding the design principles of a number of types of molecular motors and their interactions with their tracks.

The load dependence of muscle's force-velocity curve is modulated by alternative myosin converter domains

The hyperbolic shape of the muscle force-velocity relationship (FVR) is characteristic of all muscle fiber types. The degree of curvature of the hyperbola varies between muscle fiber types and is thought to be set by force-dependent properties of different myosin isoforms. However, the structural elements in myosin and the mechanism that determines force dependence are unresolved. We tested our hypothesis that the myosin converter domain plays a critical role in the force-velocity relationship (FVR) mechanism. Drosophila contains a single myosin heavy chain gene with five converters encoded by alternative exons. We measured FVR properties of Drosophila jump muscle fibers from five transgenic lines each expressing a single converter. Consistent with our hypothesis, we observed up to 2.4-fold alterations in FVR curvature. Maximum shortening velocity (v0) and optimal velocity for maximum power generation were also altered, but isometric tension and maximum power generation were unaltered. Converter 11a, normally found in the indirect flight muscle (IFM), imparted the highest FVR curvature and v0, whereas converter 11d, found in larval body wall muscle, imparted the most linear FVR and slowest v0. Jump distance strongly correlated with increasing FVR curvature and v0, meaning flies expressing the converter from the IFM jumped farther than flies expressing the native jump muscle converter. Fitting our data with Huxley's two-state model and a biophysically based four-state model suggest a testable hypothesis that the converter sets muscle type FVR curvature by influencing the detachment rate of negatively strained myosin via changes in the force dependence of product release.

Conserved functions of RNA-binding proteins in muscle

Animals require different types of muscle for survival, for example for circulation, motility, reproduction and digestion. Much emphasis in the muscle field has been placed on understanding how transcriptional regulation generates diverse types of muscle during development. Recent work indicates that alternative splicing and RNA regulation are as critical to muscle development, and altered function of RNA-binding proteins causes muscle disease. Although hundreds of genes predicted to bind RNA are expressed in muscles, many fewer have been functionally characterized. We present a cross-species view summarizing what is known about RNA-binding protein function in muscle, from worms and flies to zebrafish, mice and humans. In particular, we focus on alternative splicing regulated by the CELF, MBNL and RBFOX families of proteins. We discuss the systemic nature of diseases associated with loss of RNA-binding proteins in muscle, focusing on mis-regulation of CELF and MBNL in myotonic dystrophy. These examples illustrate the conservation of RNA-binding protein function and the marked utility of genetic model systems in understanding mechanisms of RNA regulation.

Five Alternative Myosin Converter Domains Influence Muscle Power, Stretch Activation, and Kinetics

Muscles have evolved to power a wide variety of movements. A protein component critical to varying power generation is the myosin isoform present in the muscle. However, how functional variation in muscle arises from myosin structure is not well understood. We studied the influence of the converter, a myosin structural region at the junction of the lever arm and catalytic domain, using Drosophila because its single myosin heavy chain gene expresses five alternative converter versions (11a-e). We created five transgenic fly lines, each forced to express one of the converter versions in their indirect flight muscle (IFM) fibers. Electron microscopy showed that the converter exchanges did not alter muscle ultrastructure. The four lines expressing converter versions (11b-e) other than the native IFM 11a converter displayed decreased flight ability. IFM fibers expressing converters normally found in the adult stage muscles generated up to 2.8-fold more power and displayed up to 2.2-fold faster muscle kinetics than fibers with converters found in the embryonic and larval stage muscles. Small changes to stretch-activated force generation only played a minor role in altering power output of IFM. Muscle apparent rate constants, derived from sinusoidal analysis of the chimeric converter fibers, showed a strong positive correlation between optimal muscle oscillation frequency and myosin attachment kinetics to actin, and an inverse correlation with detachment related cross-bridge kinetics. This suggests the myosin converter alters at least two rate constants of the cross-bridge cycle with changes to attachment and power stroke related kinetics having the most influence on setting muscle oscillatory power kinetics.

The Relay/Converter Interface Influences Hydrolysis of ATP by Skeletal Muscle Myosin II

The interface between relay and converter domain of muscle myosin is critical for optimal myosin performance. Using Drosophila melanogaster indirect flight muscle S1, we performed a kinetic analysis of the effect of mutations in the converter and relay domain. Introduction of a mutation (R759E) in the converter domain inhibits the steady-state ATPase of myosin S1, whereas an additional mutation in the relay domain (N509K) is able to restore the ATPase toward wild-type values. The R759E S1 construct showed little effect on most steps of the actomyosin ATPase cycle. The exception was a 25-30% reduction in the rate constant of the hydrolysis step, the step coupled to the cross-bridge recovery stroke that involves a change in conformation at the relay/converter domain interface. Significantly, the double mutant restored the hydrolysis step to values similar to the wild-type myosin. Modeling the relay/converter interface suggests a possible interaction between converter residue 759 and relay residue 509 in the actin-detached conformation, which is lost in R759E but is restored in N509K/R759E. This detailed kinetic analysis of Drosophila myosin carrying the R759E mutation shows that the interface between the relay loop and converter domain is important for fine-tuning myosin kinetics, in particular ATP binding and hydrolysis.

Myofilament proteins in the synchronous flight muscles of Manduca sexta show both similarities and differences to Drosophila melanogaster

Insect flight muscles have been classified as either synchronous or asynchronous based on the coupling between excitation and contraction. In the moth Manduca sexta, the flight muscles are synchronous and do not display stretch activation, which is a property of asynchronous muscles. We annotated the M. sexta genes encoding the major myofibrillar proteins and analyzed their isoform pattern and expression. Comparison with the homologous genes in Drosophila melanogaster indicates both difference and similarities. For proteins such as myosin heavy chain, tropomyosin, and troponin I the availability and number of potential variants generated by alternative spicing is mostly conserved between the two insects. The exon usage associated with flight muscles indicates that some exon sets are similarly used in the two insects, whereas others diverge. For actin the number of individual genes is different and there is no evidence for a flight muscle specific isoform. In contrast for troponin C, the number of genes is similar, as well as the isoform composition in flight muscles despite the different calcium regulation. Both troponin I and tropomyosin can include COOH-terminal hydrophobic extensions similar to tropomyosinH and troponinH found in D. melanogaster and the honeybee respectively.

The roles of troponin C isoforms in the mechanical function of Drosophila indirect flight muscle

Stretch activation (SA) is a fundamental property of all muscle types that increases power output and efficiency, yet its mechanism is unknown. Recently, studies have implicated troponin isoforms as important in the SA mechanism. The highly stretch-activated Drosophila IFMs express two isoforms of the Ca(2+)-binding subunit of troponin (TnC). TnC1 (TnC-F2 in Lethocerus IFM) has two calcium binding sites, while an unusual isoform, TnC4 (TnC-F1 in Lethocerus IFM), has only one binding site. We investigated the roles of these two TnC isoforms in Drosophila IFM by targeting RNAi to each isoform. IFMs with TnC4 expression (normally ~90% of total TnC) replaced by TnC1 did not generate isometric tension, power or display SA. However, TnC4 knockdown resulted in sarcomere ultrastructure disarray, which could explain the lack of mechanical function and thus make interpretation of the influence of TnC4 on SA difficult. Elimination of TnC1 expression (normally ~10% of total TnC) by RNAi resulted in normal muscle structure. In these IFMs, fiber power generation, isometric tension, stretch-activated force and calcium sensitivity were statistically identical to wild type. When TnC1 RNAi was driven by an IFM specific driver, there was no decrease in flight ability or wing beat frequency, which supports our mechanical findings suggesting that TnC1 is not essential for the mechanical function of Drosophila IFM. This finding contrasts with previous work in Lethocerus IFM showing TnC1 is essential for maximum isometric force generation. We propose that differences in TnC1 function in Lethocerus and Drosophila contribute to the ~40-fold difference in IFM isometric tension generated between these species.

A new experimental model to study force depression: the Drosophila jump muscle

Force depression (FD) is a decrease in isometric force following active muscle shortening. Despite being well characterized experimentally, its underlying mechanism remains unknown. To develop a new, genetically manipulatable experimental model that would greatly improve our ability to study the underlying mechanism(s) of FD, we tested the Drosophila jump muscle for classical FD behavior. Steady-state force generation following active shortening decreased by 2, 8, and 11% of maximum isometric force with increasing shortening amplitudes of 5, 10, and 20% of optimal fiber length, and decreased by 11, 8, and 5% with increasing shortening velocities of 4, 20, and 200% of optimal fiber length per second. These steady-state FD (FDSS) characteristics of Drosophila jump muscle mimic those observed in mammalian skeletal muscle. A double exponential fit of transient force recovery following shortening identified two separate phases of force recovery: a rapid initial force redevelopment, and a slower recovery toward steady state. This analysis showed the slower rate of force redevelopment to be inversely proportional to the amount of FDSS, while the faster rate did not correlate with FDSS. This suggests that the mechanism behind the slower, most likely cross-bridge cycling rate, influences the amount of FDSS. Thus the jump muscle, when coupled with the genetic mutability of its sarcomere proteins, offers a unique and powerful experimental model to explore the underlying mechanism behind FD.

Mapping interactions between myosin relay and converter domains that power muscle function

Intramolecular communication within myosin is essential for its function as motor, but the specific amino acid residue interactions required are unexplored within muscle cells. Using Drosophila melanogaster skeletal muscle myosin, we performed a novel in vivo molecular suppression analysis to define the importance of three relay loop amino acid residues (Ile(508), Asn(509), and Asp(511)) in communicating with converter domain residue Arg(759). We found that the N509K relay mutation suppressed defects in myosin ATPase, in vitro motility, myofibril stability, and muscle function associated with the R759E converter mutation. Through molecular modeling, we define a mechanism for this interaction and suggest why the I508K and D511K relay mutations fail to suppress R759E. Interestingly, I508K disabled motor function and myofibril assembly, suggesting that productive relay-converter interaction is essential for both processes. We conclude that the putative relay-converter interaction mediated by myosin residues 509 and 759 is critical for the biochemical and biophysical function of skeletal muscle myosin and the normal ultrastructural and mechanical properties of muscle.

An embryonic myosin isoform enables stretch activation and cyclical power in Drosophila jump muscle

The mechanism behind stretch activation (SA), a mechanical property that increases muscle force and oscillatory power generation, is not known. We used Drosophila transgenic techniques and our new muscle preparation, the jump muscle, to determine if myosin heavy chain isoforms influence the magnitude and rate of SA force generation. We found that Drosophila jump muscles show very low SA force and cannot produce positive power under oscillatory conditions at pCa 5.0. However, we transformed the jump muscle to be moderately stretch-activatable by replacing its myosin isoform with an embryonic isoform (EMB). Expressing EMB, jump muscle SA force increased by 163% and it generated net positive power. The rate of SA force development decreased by 58% with EMB expression. Power generation is Pi dependent as >4 mM Pi was required for positive power from EMB. Pi increased EMB SA force, but not wild-type SA force. Our data suggest that when muscle expressing EMB is stretched, EMB is more easily driven backward to a weakly bound state than wild-type jump muscle. This increases the number of myosin heads available to rapidly bind to actin and contribute to SA force generation. We conclude that myosin heavy chain isoforms influence both SA kinetics and SA force, which can determine if a muscle is capable of generating oscillatory power at a fixed calcium concentration.

An embryonic myosin converter domain influences Drosophila indirect flight muscle stretch activation, power generation and flight

Stretch activation (SA) is critical to the flight ability of insects powered by asynchronous, indirect flight muscles (IFMs). An essential muscle protein component for SA and power generation is myosin. Which structural domains of myosin are significant for setting SA properties and power generation levels is poorly understood. We made use of the transgenic techniques and unique single muscle myosin heavy chain gene of Drosophila to test the influence of the myosin converter domain on IFM SA and power generation. Replacing the endogenous converter with an embryonic version decreased SA tension and the rate of SA tension generation. The alterations in SA properties and myosin kinetics from the converter exchange caused power generation to drop to 10% of control fiber power when the optimal conditions for control fibers - 1% muscle length (ML) amplitude and 150 Hz oscillation frequency - were applied to fibers expressing the embryonic converter (IFI-EC). Optimizing conditions for IFI-EC fiber power production, by doubling ML amplitude and decreasing oscillation frequency by 60%, improved power output to 60% of optimized control fiber power. IFI-EC flies altered their aerodynamic flight characteristics to better match optimal fiber power generation conditions as wing beat frequency decreased and wing stroke amplitude increased. This enabled flight in spite of the drastic changes to fiber mechanical performance.

Calcium signalling indicates bilateral power balancing in the Drosophila flight muscle during manoeuvring flight

Manoeuvring flight in animals requires precise adjustments of mechanical power output produced by the flight musculature. In many insects such as fruit flies, power generation is most likely varied by altering stretch-activated tension, that is set by sarcoplasmic calcium levels. The muscles reside in a thoracic shell that simultaneously drives both wings during wing flapping. Using a genetically expressed muscle calcium indicator, we here demonstrate in vivo the ability of this animal to bilaterally adjust its calcium activation to the mechanical power output required to sustain aerodynamic costs during flight. Motoneuron-specific comparisons of calcium activation during lift modulation and yaw turning behaviour suggest slightly higher calcium activation for dorso-longitudinal than for dorsoventral muscle fibres, which corroborates the elevated need for muscle mechanical power during the wings' downstroke. During turning flight, calcium activation explains only up to 54 per cent of the required changes in mechanical power, suggesting substantial power transmission between both sides of the thoracic shell. The bilateral control of muscle calcium runs counter to the hypothesis that the thorax of flies acts as a single, equally proportional source for mechanical power production for both flapping wings. Collectively, power balancing highlights the precision with which insects adjust their flight motor to changing energetic requirements during aerial steering. This potentially enhances flight efficiency and is thus of interest for the development of technical vehicles that employ bioinspired strategies of power delivery to flapping wings.

Mechanical analysis of Drosophila indirect flight and jump muscles

The genetic advantages of Drosophila make it a very appealing choice for investigating muscle development, muscle physiology and muscle protein structure and function. To take full advantage of this model organism, it has been vital to develop isolated Drosophila muscle preparations that can be mechanically evaluated. We describe techniques to isolate, prepare and mechanically analyze skinned muscle fibers from two Drosophila muscle types, the indirect flight muscle and the jump muscle. The function of the indirect flight muscle is similar to vertebrate cardiac muscle, to generate power in an oscillatory manner. The indirect flight muscle is ideal for evaluating the influence of protein mutations on muscle and cross-bridge stiffness, oscillatory power, and deriving cross-bridge rate constants. Jump muscle physiology and structure are more similar to skeletal vertebrate muscle than indirect flight muscle, and it is ideal for measuring maximum shortening velocity, force-velocity characteristics and steady-state power generation.

Transgenic expression and purification of myosin isoforms using the Drosophila melanogaster indirect flight muscle system

Biophysical and structural studies on muscle myosin rely upon milligram quantities of extremely pure material. However, many biologically interesting myosin isoforms are expressed at levels that are too low for direct purification from primary tissues. Efforts aimed at recombinant expression of functional striated muscle myosin isoforms in bacterial or insect cell culture have largely met with failure, although high level expression in muscle cell culture has recently been achieved at significant expense. We report a novel method for the use of strains of the fruit fly Drosophila melanogaster genetically engineered to produce histidine-tagged recombinant muscle myosin isoforms. This method takes advantage of the single muscle myosin heavy chain gene within the Drosophila genome, the high level of expression of accessible myosin in the thoracic indirect flight muscles, the ability to knock out endogenous expression of myosin in this tissue and the relatively low cost of fruit fly colony production and maintenance. We illustrate this method by expressing and purifying a recombinant histidine-tagged variant of embryonic body wall skeletal muscle myosin II from an engineered fly strain. The recombinant protein shows the expected ATPase activity and is of sufficient purity and homogeneity for crystallization. This system may prove useful for the expression and isolation of mutant myosins associated with skeletal muscle diseases and cardiomyopathies for their biochemical and structural characterization.

Alternative relay and converter domains tune native muscle myosin isoform function in Drosophila

Myosin isoforms help define muscle-specific contractile and structural properties. Alternative splicing of myosin heavy chain gene transcripts in Drosophila melanogaster yields muscle-specific isoforms and highlights alternative domains that fine-tune myosin function. To gain insight into how native myosin is tuned, we expressed three embryonic myosin isoforms in indirect flight muscles lacking endogenous myosin. These isoforms differ in their relay and/or converter domains. We analyzed isoform-specific ATPase activities, in vitro actin motility and myofibril structure/stability. We find that dorsal acute body wall muscle myosin (EMB-9c11d) shows a significant increase in MgATPase V(max) and actin sliding velocity, as well as abnormal myofibril assembly compared to cardioblast myosin (EMB-11d). These properties differ as a result of alternative exon-9-encoded relay domains that are hypothesized to communicate signals among the ATP-binding pocket, actin-binding site and the converter domain. Further, EMB-11d shows significantly reduced levels of basal Ca- and MgATPase as well as MgATPase V(max) compared to embryonic body wall muscle isoform (EMB) (expressed in a multitude of body wall muscles). EMB-11d also induces increased actin sliding velocity and stabilizes myofibril structure compared to EMB. These differences arise from exon-11-encoded alternative converter domains that are proposed to reposition the lever arm during the power and recovery strokes. We conclude that relay and converter domains of native myosin isoforms fine-tune ATPase activity, actin motility and muscle ultrastructure. This verifies and extends previous studies with chimeric molecules and indicates that interactions of the relay and converter during the contractile cycle are key to myosin-isoform-specific kinetic and mechanical functions.

Disrupting the myosin converter-relay interface impairs Drosophila indirect flight muscle performance

Structural interactions between the myosin converter and relay domains have been proposed to be critical for the myosin power stroke and muscle power generation. We tested this hypothesis by mutating converter residue 759, which interacts with relay residues I508, N509, and D511, to glutamate (R759E) and determined the effect on Drosophila indirect flight muscle mechanical performance. Work loop analysis of mutant R759E indirect flight muscle fibers revealed a 58% and 31% reduction in maximum power generation (P(WL)) and the frequency at which maximum power (f(WL)) is generated, respectively, compared to control fibers at 15 °C. Small amplitude sinusoidal analysis revealed a 30%, 36%, and 32% reduction in mutant elastic modulus, viscous modulus, and mechanical rate constant 2πb, respectively. From these results, we infer that the mutation reduces rates of transitions through work-producing cross-bridge states and/or force generation during strongly bound states. The reductions in muscle power output, stiffness, and kinetics were physiologically relevant, as mutant wing beat frequency and flight index decreased about 10% and 45% compared to control flies at both 15 °C and 25 °C. Thus, interactions between the relay loop and converter domain are critical for lever-arm and catalytic domain coordination, high muscle power generation, and optimal Drosophila flight performance.

Projectin PEVK domain, splicing variants and domain structure in basal and derived insects

The third elastic filament of striated muscles consists of giant proteins: titin (in vertebrates) and kettin/projectin (in insects). In all three proteins, elasticity is at least partly associated with the so-called PEVK domain. The projectin PEVK domains of diverse insects are highly divergent compared with an otherwise conserved protein organization. We present the characterization of the PEVK domain in two dragonflies and in human lice. A conserved segment at the end of the PEVK, the NH(2)-terminal conserved segment-1 (NTCS-1), may serve as an anchor point for projectin to either myosin or actin, providing a mechanical link. The analysis of alternative splicing variants identifies the shortest PEVK isoform as the predominant form in the flight muscles of several insects, possibly contributing to myofibrillar stiffness.

Nucleotide pocket thermodynamics measured by EPR reveal how energy partitioning relates myosin speed to efficiency

We have used spin-labeled ADP to investigate the dynamics of the nucleotide-binding pocket in a series of myosins, which have a range of velocities. Electron paramagnetic resonance spectroscopy reveals that the pocket is in equilibrium between open and closed conformations. In the absence of actin, the closed conformation is favored. When myosin binds actin, the open conformation becomes more favored, facilitating nucleotide release. We found that faster myosins favor a more closed pocket in the actomyosin•ADP state, with smaller values of ΔH(0) and ΔS(0), even though these myosins release ADP at a faster rate. A model involving a partitioning of free energy between work-generating steps prior to rate-limiting ADP release explains both the unexpected correlation between velocity and opening of the pocket and the observation that fast myosins are less efficient than slow myosins.

The mechanical properties of Drosophila jump muscle expressing wild-type and embryonic Myosin isoforms

Transgenic Drosophila are highly useful for structure-function studies of muscle proteins. However, our ability to mechanically analyze transgenically expressed mutant proteins in Drosophila muscles has been limited to the skinned indirect flight muscle preparation. We have developed a new muscle preparation using the Drosophila tergal depressor of the trochanter (TDT or jump) muscle that increases our experimental repertoire to include maximum shortening velocity (V(slack)), force-velocity curves and steady-state power generation; experiments not possible using indirect flight muscle fibers. When transgenically expressing its wild-type myosin isoform (Tr-WT) the TDT is equivalent to a very fast vertebrate muscle. TDT has a V(slack) equal to 6.1 +/- 0.3 ML/s at 15 degrees C, a steep tension-pCa curve, isometric tension of 37 +/- 3 mN/mm(2), and maximum power production at 26% of isometric tension. Transgenically expressing an embryonic myosin isoform in the TDT muscle increased isometric tension 1.4-fold, but decreased V(slack) 50% resulting in no significant difference in maximum power production compared to Tr-WT. Drosophila expressing embryonic myosin jumped <50% as far as Tr-WT that, along with comparisons to frog jump muscle studies, suggests fast muscle shortening velocity is relatively more important than high tension generation for Drosophila jumping.

The influence of myosin converter and relay domains on cross-bridge kinetics of Drosophila indirect flight muscle

We are investigating the influence of the converter and relay domains on elementary rate constants of the actomyosin cross-bridge cycle. The converter and relay domains vary between Drosophila myosin heavy chain isoforms due to alternative mRNA splicing. Previously, we found that separate insertions of embryonic myosin isoform (EMB) versions of these domains into the indirect flight muscle (IFM) myosin isoform (IFI) both decreased Drosophila IFM power and slowed muscle kinetics. To determine cross-bridge mechanisms behind the changes, we employed sinusoidal analysis while varying phosphate and MgATP concentrations in skinned Drosophila IFM fibers. Based on a six-state cross-bridge model, the EMB converter decreased myosin rate constants associated with actin attachment and work production, k(4), but increased rates related to cross-bridge detachment and work absorption, k(2). In contrast, the EMB relay domain had little influence on kinetics, because only k(4) decreased. The main alteration was mechanical, in that work production amplitude decreased. That both domains decreased k(4) supports the hypothesis that these domains are critical to lever-arm-mediated force generation. Neither domain significantly influenced MgATP affinity. Our modeling suggests the converter domain is responsible for the difference in rate-limiting cross-bridge steps between EMB and IFI myosin--i.e., a myosin isomerization associated with MgADP release for EMB and Pi release for IFI.

Chronic heart failure decreases cross-bridge kinetics in single skeletal muscle fibres from humans

Skeletal muscle function is impaired in heart failure patients due, in part, to loss of myofibrillar protein content, in particular myosin. In the present study, we utilized small-amplitude sinusoidal analysis for the first time in single human skeletal muscle fibres to measure muscle mechanics, including cross-bridge kinetics, to determine if heart failure further impairs contractile performance by altering myofibrillar protein function. Patients with chronic heart failure (n = 9) and controls (n = 6) were recruited of similar age and physical activity to diminish the potentially confounding effects of ageing and muscle disuse. Patients showed decreased cross-bridge kinetics in myosin heavy chain (MHC) I and IIA fibres, partially due to increased myosin attachment time (t(on)). The increased t(on) compensated for myosin protein loss previously found in heart failure patients by increasing the fraction of the total cycle time myosin is bound to actin, resulting in a similar number of strongly bound cross-bridges in patients and controls. Accordingly, isometric tension did not differ between patients and controls in MHC I or IIA fibres. Patients also had decreased calcium sensitivity in MHC IIA fibres and alterations in the viscoelastic properties of the lattice structure of MHC I and IIA fibres. Collectively, these results show that heart failure alters skeletal muscle contraction at the level of the myosin-actin cross-bridge, leading to changes in muscle mechanics which could contribute to impaired muscle function. Additionally, we uncovered a unique kinetic property of MHC I fibres, a potential indication of two distinct populations of cross-bridges, which may have important physiological consequences.

Alternative exon 9-encoded relay domains affect more than one communication pathway in the Drosophila myosin head

We investigated the biochemical and biophysical properties of one of the four alternative regions within the Drosophila myosin catalytic domain: the relay domain encoded by exon 9. This domain of the myosin head transmits conformational changes in the nucleotide-binding pocket to the converter domain, which is crucial to coupling catalytic activity with mechanical movement of the lever arm. To study the function of this region, we used chimeric myosins (IFI-9b and EMB-9a), which were generated by exchange of the exon 9-encoded domains between the native embryonic body wall (EMB) and indirect flight muscle isoforms (IFI). Kinetic measurements show that exchange of the exon 9-encoded region alters the kinetic properties of the myosin S1 head. This is reflected in reduced values for ATP-induced actomyosin dissociation rate constant (K(1)k(+2)) and ADP affinity (K(AD)), measured for the chimeric constructs IFI-9b and EMB-9a, compared to wild-type IFI and EMB values. Homology models indicate that, in addition to affecting the communication pathway between the nucleotide-binding pocket and the converter domain, exchange of the relay domains between IFI and EMB affects the communication pathway between the nucleotide-binding pocket and the actin-binding site in the lower 50-kDa domain (loop 2). These results suggest an important role of the relay domain in the regulation of actomyosin cross-bridge kinetics.