Josh E. Baker

The Light Chain Binding Domain of Expressed Smooth Muscle Heavy Meromyosin Acts as a Mechanical Lever

Structural data led to the proposal that the molecular motor myosin moves actin by a swinging of the light chain binding domain, or “neck.” To test the hypothesis that the neck functions as a mechanical lever, smooth muscle heavy meromyosin (HMM) mutants were expressed with shorter or longer necks by either deleting or adding light chain binding sites. The mutant HMMs were characterized kinetically and mechanically, with emphasis on measurements of unitary displacements and forces in the laser trap assay. Two shorter necked constructs had smaller unitary step sizes and moved actin more slowly than WT HMM in the motility assay. A longer necked construct that contained an additional essential light chain binding site exhibited a 1.4-fold increase in the unitary step size compared with its control. Kinetic changes were also observed with several of the constructs. The mutant lacking a neck produced force at a somewhat reduced level, while the force exerted by the giraffe construct was higher than control. The single molecule displacement and force data support the hypothesis that the neck functions as a rigid lever, with the fulcrum for movement and force located at a point within the motor domain.

The Biochemical Kinetics Underlying Actin Movement Generated by One and Many Skeletal Muscle Myosin Molecules

To better understand how skeletal muscle myosin molecules move actin filaments, we determine the motion-generating biochemistry of a single myosin molecule and study how it scales with the motion-generating biochemistry of an ensemble of myosin molecules. First, by measuring the effects of various ligands (ATP, ADP, and P(i)) on event lifetimes, tau(on), in a laser trap, we determine the biochemical kinetics underlying the stepwise movement of an actin filament generated by a single myosin molecule. Next, by measuring the effects of these same ligands on actin velocities, V, in an in vitro motility assay, we determine the biochemistry underlying the continuous movement of an actin filament generated by an ensemble of myosin molecules. The observed effects of P(i) on single molecule mechanochemistry indicate that motion generation by a single myosin molecule is closely associated with actin-induced P(i) dissociation. We obtain additional evidence for this relationship by measuring changes in single molecule mechanochemistry caused by a smooth muscle HMM mutation that results in a reduced P(i)-release rate. In contrast, we observe that motion generation by an ensemble of myosin molecules is limited by ATP-induced actin dissociation (i.e., V varies as 1/tau(on)) at low [ATP], but deviates from this relationship at high [ATP]. The single-molecule data uniquely provide a direct measure of the fundamental mechanochemistry of the actomyosin ATPase reaction under a minimal load and serve as a clear basis for a model of ensemble motility in which actin-attached myosin molecules impose a load.

Mechanochemical Coupling in Spin-Labeled, Active, Isometric Muscle

Observed effects of inorganic phosphate (P(i)) on active isometric muscle may provide the answer to one of the fundamental questions in muscle biophysics: how are the free energies of the chemical species in the myosin-catalyzed ATP hydrolysis (ATPase) reaction coupled to muscle force? Pate and Cooke (1989. Pflugers Arch. 414:73-81) showed that active, isometric muscle force varies logarithmically with [P(i)]. Here, by simultaneously measuring electron paramagnetic resonance and the force of spin-labeled muscle fibers, we show that, in active, isometric muscle, the fraction of myosin heads in any given biochemical state is independent of both [P(i)] and force. These direct observations of mechanochemical coupling in muscle are immediately described by a muscle equation of state containing muscle force as a state variable. These results challenge the conventional assumption mechanochemical coupling is localized to individual myosin heads in muscle.

A thermodynamic muscle model and a chemical basis for A.V. Hill's muscle equation.

Direct measurements of a relationship between force and actin–myosin biochemistry in muscle suggest that molecular forces in active muscle rapidly equilibrate among, not within, individual myosin crossbridges [Baker et al. (1999) Biophys J77: 2657–2664]. This observation suggests a thermodynamic model of muscle contraction in which muscle, not an individual myosin crossbridge, is treated as a near-equilibrium system. The general approach can be applied to any ensemble of molecular motors that undergo a physicochemical step against a constant external potential. In this paper we apply the model to a simple two-state crossbridge scheme like that proposed by A.F. Huxley (1957) [Prog Biophys7: 255–317], and we immediately obtain A.V. Hills muscle equation. We show that this equation accurately describes steady-state muscle mechanics, biochemistry and energetics. This thermodynamic model provides a novel description of force-dependent actin–myosin kinetics in muscle and provides precise chemical expressions for myosin cooperativity, myosin duty ratios, the number of working strokes per ATP hydrolyzed, muscle efficiency, and energy transfer.

Velocities of unloaded muscle filaments are not limited by drag forces imposed by myosin cross-bridges

Significance In vitro experiments that measure relative sliding velocities between actin and myosin filaments give insight into how the muscle motor protein myosin works in muscle. We show that when the physiological filamentous myosin moves along actin filaments, the velocities cannot be explained by the prevailing theory. Instead, we propose a paradigm in which myosin heads undergo unloaded “powerstrokes” rather than work in mechanical opposition to each other during contraction. This model is consistent with unloaded shortening velocities measured in muscle. It is not known which kinetic step in the acto-myosin ATPase cycle limits contraction speed in unloaded muscles (V0). Huxley’s 1957 model [Huxley AF (1957) Prog Biophys Biophys Chem 7:255–318] predicts that V0 is limited by the rate that myosin detaches from actin. However, this does not explain why, as observed by Bárány [Bárány M (1967) J Gen Physiol 50(6, Suppl):197–218], V0 is linearly correlated with the maximal actin-activated ATPase rate (vmax), which is limited by the rate that myosin attaches strongly to actin. We have observed smooth muscle myosin filaments of different length and head number (N) moving over surface-attached F-actin in vitro. Fitting filament velocities (V) vs. N to a detachment-limited model using the myosin step size d = 8 nm gave an ADP release rate 8.5-fold faster and ton (myosin’s attached time) and r (duty ratio) ∼10-fold lower than previously reported. In contrast, these data were accurately fit to an attachment-limited model, V = N·v·d, over the range of N found in all muscle types. At nonphysiologically high N, V = L/ton rather than d/ton, where L is related to the length of myosin’s subfragment 2. The attachment-limited model also fit well to the [ATP] dependence of V for myosin-rod cofilaments at three fixed N. Previously published V0 vs. vmax values for 24 different muscles were accurately fit to the attachment-limited model using widely accepted values for r and N, giving d = 11.1 nm. Therefore, in contrast with Huxley’s model, we conclude that V0 is limited by the actin–myosin attachment rate.

Effects of actin-myosin kinetics on the calcium sensitivity of regulated thin filaments

Activation of thin filaments in striated muscle occurs when tropomyosin exposes myosin binding sites on actin either through calcium-troponin (Ca-Tn) binding or by actin-myosin (A-M) strong binding. However, the extent to which these binding events contributes to thin filament activation remains unclear. Here we propose a simple analytical model in which strong A-M binding and Ca-Tn binding independently activates the rate of A-M weak-to-strong binding. The model predicts how the level of activation varies with pCa as well as A-M attachment, N·katt, and detachment, kdet, kinetics. To test the model, we use an in vitro motility assay to measure the myosin-based sliding velocities of thin filaments at different pCa, N·katt, and kdet values. We observe that the combined effects of varying pCa, N·katt, and kdet are accurately fit by the analytical model. The model and supporting data imply that changes in attachment and detachment kinetics predictably affect the calcium sensitivity of striated muscle mechanics, providing a novel A-M kinetic-based interpretation for perturbations (e.g. disease-related mutations) that alter calcium sensitivity.

Thermodynamics and Kinetics of a Molecular Motor Ensemble

If, contrary to conventional models of muscle, it is assumed that molecular forces equilibrate among rather than within molecular motors, an equation of state and an expression for energy output can be obtained for a near-equilibrium, coworking ensemble of molecular motors. These equations predict clear, testable relationships between motor structure, motor biochemistry, and ensemble motor function, and we discuss these relationships in the context of various experimental studies. In this model, net work by molecular motors is performed with the relaxation of a near-equilibrium intermediate step in a motor-catalyzed reaction. The free energy available for work is localized to this step, and the rate at which this free energy is transferred to work is accelerated by the free energy of a motor-catalyzed reaction. This thermodynamic model implicitly deals with a motile cell system as a dynamic network (not a rigid lattice) of molecular motors within which the mechanochemistry of one motor influences and is influenced by the mechanochemistry of other motors in the ensemble.

HSP20 phosphorylation and airway smooth muscle relaxation.

HSP20 (HSPB6) is a small heat shock protein expressed in smooth muscles that is hypothesized to inhibit contraction when phosphorylated by cAMP-dependent protein kinase. To investigate this hypothesis in airway smooth muscle (ASM) we showed that HSP20 was constitutively expressed as well as being inducible in cultured hASM cells by treatment with 1 μM isoproterenol or 10 μM salmeterol. In contrast, a mixture of proinflammatory mediators (interleukin-1β, tumor necrosis factor α, and interferon γ) inhibited expression of HSP20 by about 50% in 48 hours. To determine whether phosphorylation of HSP20 is sufficient to induce relaxation, canine tracheal smooth muscle was treated with a cell permeant phosphopeptide that mimics the phosphorylation of HSP20. The HSP20 phosphopeptide antagonized carbachol-induced contraction by 60% with no change in myosin light chain phosphorylation. Recombinant full length HSP20 inhibited skeletal actin binding to smooth muscle myosin subfragment 1 (S1), and recombinant cell permeant TAT-HSP20 S16D mutant reduced F-actin filaments in cultured hASM cells. Carbachol stimulation of canine tracheal smooth muscle tissue caused redistribution of HSP20 from large macromolecular complexes (200-500 kDa) to smaller complexes (<60 kDa). The results are consistent with HSP20 expression and macromolecular structure being dynamically regulated in airway smooth muscle. HSP20 is upregulated by beta agonists and downregulated by proinflammatory cytokines. HSP20 is phosphorylated in vivo in a cAMP-dependent manner and the phosphorylated form promotes airway smooth muscle relaxation, possibly through depolymerization of F-actin as well as inhibition of myosin binding to actin.

The Kinetics Underlying the Velocity of Smooth Muscle Myosin Filament Sliding on Actin Filaments in Vitro

Background: Myosin polymerizes into filaments that move on actin. Results: ATPase and moving velocities of filaments may be limited by the weak to strong transition. Conclusion: Filaments moving on top of actin may have fewer drag heads than actin filaments moving on myosin monomers. Significance: Understanding kinetics of intact myosin filaments with actin is important to understand muscle mechanics. Actin-myosin interactions are well studied using soluble myosin fragments, but little is known about effects of myosin filament structure on mechanochemistry. We stabilized unphosphorylated smooth muscle myosin (SMM) and phosphorylated smooth muscle myosin (pSMM) filaments against ATP-induced depolymerization using a cross-linker and attached fluorescent rhodamine (XL-Rh-SMM). Electron micrographs showed that these side polar filaments are very similar to unmodified filaments. They are ∼0.63 μm long and contain ∼176 molecules. Rate constants for ATP-induced dissociation and ADP release from acto-myosin for filaments and S1 heads were similar. Actin-activated ATPases of SMM and XL-Rh-SMM were similarly regulated. XL-Rh-pSMM filaments moved processively on F-actin that was bound to a PEG brush surface. ATP dependence of filament velocities was similar to that for solution ATPases at high [actin], suggesting that both processes are limited by the same kinetic step (weak to strong transition) and therefore are attachment-limited. This differs from actin sliding over myosin monomers, which is primarily detachment-limited. Fitting filament data to an attachment-limited model showed that approximately half of the heads are available to move the filament, consistent with a side polar structure. We suggest the low stiffness subfragment 2 (S2) domain remains unhindered during filament motion in our assay. Actin-bound negatively displaced heads will impart minimal drag force because of S2 buckling. Given the ADP release rate, the velocity, and the length of S2, these heads will detach from actin before slack is taken up into a backwardly displaced high stiffness position. This mechanism explains the lack of detachment-limited kinetics at physiological [ATP]. These findings address how nonlinear elasticity in assemblies of motors leads to efficient collective force generation.

Modification of Interface between Regulatory and Essential Light Chains Hampers Phosphorylation-dependent Activation of Smooth Muscle Myosin

Background: SMM is activated by RLC phosphorylation in the lever arm. Results: Modifying RLC-ELC interaction hampers the ability of phosphorylation to activate motor functions. Conclusion: A major consequence of phosphorylation is to stabilize RLC-ELC interactions and associated conformations of the lever arm elbow. Significance: Learning how this myosin is regulated furthers the understanding of activation and relaxation of smooth muscle contraction. 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, Vmax, and KATPase 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 (Ca2+ 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.