Surface Of Actin Filament Research Articles

Machine learning model predicts the effects of tropomyosin rigidity on cardiac muscle contraction

Tropomyosin (Tm) oscillates periodically between three equilibrium states over the surface of actin filament during the cardiac thin filament activation process. This motion has shown to be essential in regulating the access of Myosin heads to actin binding sites, hence the cross-bridge cycling dynamics. Since the trajectory landscape of Tm motions depends critically on its micromechanical characteristics represented primarily by its stiffness or rigidity. It is important to study the effects of Tm stiffness variations on the sarcomere contraction.

Predicting Effects of Tropomyosin Stiffness on Cardiac Muscle Contraction Using Coarse-Grained Stochastic Modeling

During the cardiac thin filament activation process, Tropomyosin (Tm) oscillates over the surface of actin filament in the azimuthal direction to regulate the access of Myosin heads to actin binding sites. These dynamical modes of oscillation and the resultant cooperative activation effects depend critically on the stiffness characteristics and the mobility of Tm molecules. In this study, we developed a stochastic coarse-grained computational model to describe the Tm motions over the surface of actin filament using Langevin-Brownian dynamics. The model represents the structural arrangement of Tm molecules as a flexible chain with variable torsional stiffness using a crystal elastic network approach. The model accounts for the spatial interactions among nearest-neighbor regulatory units (RUs), which are thought to appear from the structural coupling of adjacent Tropomyosins. This elastic coupling between RUs is accomplished by assigning a multi-well potential energy for each RU. The model is then used to study the effects of Tm torsional stiffness variations on the cooperative activation of the thin filament. The results suggest that small perturbations in Tm torsional stiffness can lead to a significant effect on force-Ca2+ sensitivity, the rate of tension redevelopment, relaxation rate, and other contraction characteristics. The present stochastic computational model draws for the first time a more detailed molecular connection between Tm torsional stiffness, Tm modes of oscillations over actin surface, cooperativity among RUs, and dynamic muscle twitches. Thus, this coarse-graining approach may be useful in explaining many cardiomyopathies induced by structural remodeling and stiffening in Tm molecules as a result of point mutations in the human gene TPM1.

Turning the Azimuthal Motions of Adjacent Tropomyosins into a Coupled N-body Problem in a Brownian Model of Cardiac Thin Filament Activation

During thin filament activation process, tropomyosin (Tm) oscillates over the surface of actin filament in the azimuthal direction to regulate the access of myosin head to actin binding sites. This motion is characterized by the existence of three distinct equilibrium positions, namely; blocked (B), closed (C), and open (M) regulatory states. The exact dynamics and energy landscape that governs Tm motions between these conformational states is not yet fully understood. In this study, we developed a Brownian flashing ratchet model to mathematically describe Tm motions over the surface of actin filament. The model approximates the Tm flexible chain by a system of discrete and coupled Brownian bodies. In other words, the thin filament regulatory units (RUs) are prescribed by a set of N-body stochastic ODEs that govern the azimuthal motion of adjacent tropomyosins. The model explicitly accounts for spatial interactions among nearest neighbor RUs, which were thought to appear from structural coupling of adjacent tropomyosins. This coupling is achieved by assigning each Brownian Tm-body with a local multi-well potential. The energetic barrier, asymmetry, and bias of each individual energy landscape are hypothesized to be influenced by the Tm status of its neighbors. This coupling approach is anticipated to be sufficient to produce observed cooperativity in both force-Ca2+ relationship and twitch dynamic. The present analysis is expected to perform better than many of current phenomenological myofilament models which were mostly derived based on the classical mean-field approximation. Additionally, this analysis draws a more detailed molecular connection between Tm dynamics and sarcomeric twitch production. Therefore, the Brownian ratchet modeling methodology of Tm motions during thin filament activation may offer an enhanced mechanistic approach to describe cardiac muscle contraction.

The effects of stabilizing mutations in the central part of the α-chain of tropomyosin on the structural and functional properties of αβ-tropomyosin heterodimers

The effects of the D137L/G126R double mutation in the central part of the tropomyosin α-chain via the simultaneous replacement of two highly conserved non-canonical residues, viz., Asp137 and Gly126, by canonical residues Leu and Arg, respectively, on the properties of the αβ-tropomyosin heterodimer have been studied. It has been shown using circular dichroism that this mutation substantially increases the thermal stability of αβ-tropomyosin heterodimers, which, nevertheless, remains lower than that of αα-tropomyosin homodimers with these mutations in both α-chains. The stability of tropomyosin complexes with F-actin has also been studied by measuring the temperature dependences of their dissociation, which is detected by a decrease in light scattering. It has been revealed that αβ-tropomyosin heterodimers carrying the D137L/G126R mutation in the α-chain dissociate from the surface of actin filaments at a higher temperature than ββ-homodimers but at a lower temperature than αα-homodimers with these mutations in both α-chains. It has also been shown using the in vitro motility assay that D137L/G126R substitution in the α-chain increases the sliding velocity of regulated actin filaments in the case of αα-homodimers, while it noticeably decreases the velocity in the case of αβ-tropomyosin heterodimers. Thus, we can conclude that mutations in one of the chains of the tropomyosin dimeric molecule may have different effects on the properties of tropomyosin homodimers and heterodimers.

Tropomyosin Dynamics during Cardiac Thin Filament Activation as Governed by a Multi-Well Energy Landscape

The thin filament of cardiac muscle is composed of multi-subunits of machinery proteins, namely; actin, myosin, tropomyosin (Tm), and the troponin complex. Contraction depends upon highly nonlinear dynamical processes that rely on interactions between these components. During the thin filament activation process, Tm slides over the surface of actin filament. Its dynamics governs access of myosin head (S1) to binding sites on actin and, hence, it plays an important role in muscle contraction.Currently, there are two debating hypothetical mechanisms (sliding vs. rolling) proposed to describe Tm motions on actin's surface. In both scenarios, Tm undergoes distinct movements to uncover the S1-binding sites. These motions are characterized by three equilibrium positions, namely; the blocked position “B” (binding sites are blocked), closed position “C” (weak binding is permissible), and the open position “M” (strong binding of S1). Although, the three-state-model describes the final Tm's regulatory states, however, still the actual intrinsic mechanism by which Tm oscillates between these states are not yet fully understood.In this study, we propose a novel mathematical model that can explain Tm motions on actin's surface using principles from nonlinear dynamical systems, energy landscape, and chaotic theory. Tm oscillations between the three observed equilibrium states (B-C-M) are modelled using an energy potential with multiple wells. Each well is hypothesized to mimic one of the regulatory positions proposed by the three-state-model. The Jacobian and Lyapunov methods are then used to study the system's local and global stability. A bifurcation analysis using Melinkov function is used to find conditions (parameter values) that signify Tm transitions from simple harmonic oscillation to chaotic behaviors. Results demonstrate that Tm's dynamical motions on the surface of actin filament may exhibit periodic, aperiodic, and chaotic behaviors during muscle contraction.

Tropomyosin Movement on F-actin Analyzed by Energy Landscape Determination

Muscle contraction is regulated by movement of tropomyosin over the surface of actin filaments. At low-Ca2+, tropomyosin blocks myosin-binding on actin, whereas, in a two-step process, Ca2+-saturated troponin and myosin open the myosin-binding site leading to contraction. No obvious steric obstructions or geometrical barriers on actin limit such tropomyosin movement. However, lacking atomic models for these transitions, the pathways taken by tropomyosin during regulatory movements are uncertain. Here, end-points for regulatory transitions were determined by fitting tropomyosin to EM reconstructions. Reconstructions of negatively-stained low-Ca2+ thin filaments yield an atomic model very close to that described for troponin-free actin-tropomyosin by Li et al. (2011), while comparable high-Ca2+ filament maps suggest an azimuthal sliding of tropomyosin parallel to its superhelical path. Additionally, Raunser et al. (2012-Biophys. Soc. abst.) describe an atomic model of tropomyosin based on high-resolution cryo-EM reconstructions of myosin-decorated filaments, showing expected azimuthal movement and pronounced ∼15A axial displacement of tropomyosin toward the pointed-end of the thin filament. To evaluate transitions between these various regulatory positions, we explored the energy landscape between “end-states” over a comprehensive grid of 832 tropomyosin locations relative to F-actin coordinates. The position of tropomyosin was varied azimuthally and axially relative to F-actin, and then the structure energy-minimized. The resulting electrostatic energy landscape shows a wide energy basin with a minimum centered near the blocked-state. The width of this basin indicates that large azimuthal and axial oscillations of tropomyosin are possible. By contrast, in the myosin-induced open-position, tropomyosin is located at an energy peak, representing a region with no obvious complementary electrostatic interactions between tropomyosin and F-actin. Our results therefore suggest that the open-position is reached only because of tropomyosin interaction with myosin, while binding of tropomyosin to the F-actin surface becomes less important.

Tropomyosin: Double helix from the protein world

This review concerns the structure and functions of tropomyosin (TM), an actin-binding protein that plays a key role in the regulation of muscle contraction. The TM molecule is a dimer of α-helices, which form a coiled-coil. Recent views on the TM structure are analyzed, and special attention is concentrated on those structural traits of the TM molecule that distinguish it from the other coiled-coil proteins. Modern data are presented on TM functional properties, such as its interaction with actin and ability to move on the surface of actin filaments, which underlies the regulation of the actin-myosin interaction upon contraction of skeletal and cardiac muscles. Also, part of the review is devoted to analysis of the effects of mutations in TM genes associated with muscle diseases (myopathies) on the structure and functions of TM.

Visualization in the EM of the intermediates in the actin-myosin ATPase cycle prepared by a rapid-freezing/stopped-flow method

Motion and force are produced by myosin and actin in muscle and nonmuscle cells when myosin heads move along the surface of actin filaments. The energy is provided by the hydrolysis of ATP on the head of the myosin molecule. The details of the molecular motion in the myosin crossbridge have resisted characterization, in part because the crossbridge act asynchronously, even in the semicrystalline array of filaments found in striated muscles. Consequently, averaging techniques, including spectroscopic methods, tend to record a blurred signal from a large population of crossbridges.One way to circumvent this problem is :to examine individual myosin heads as they go through their cycle of ATP hydrolysis on the surface of the actin filament. The heads of myosin are large enough to image by EM, but the low affinity of active crossbridges for actin filaments complicates the situation.