Abstract
Myosin transduces ATP free energy into mechanical work in muscle. Cardiac muscle has dynamically wide-ranging power demands on the motor as the muscle changes modes in a heartbeat from relaxation, via auxotonic shortening, to isometric contraction. The cardiac power output modulation mechanism is explored in vitro by assessing single cardiac myosin step-size selection versus load. Transgenic mice express human ventricular essential light chain (ELC) in wild- type (WT), or hypertrophic cardiomyopathy-linked mutant forms, A57G or E143K, in a background of mouse α-cardiac myosin heavy chain. Ensemble motility and single myosin mechanical characteristics are consistent with an A57G that impairs ELC N-terminus actin binding and an E143K that impairs lever-arm stability, while both species down-shift average step-size with increasing load. Cardiac myosin in vivo down-shifts velocity/force ratio with increasing load by changed unitary step-size selections. Here, the loaded in vitro single myosin assay indicates quantitative complementarity with the in vivo mechanism. Both have two embedded regulatory transitions, one inhibiting ADP release and a second novel mechanism inhibiting actin detachment via strain on the actin-bound ELC N-terminus. Competing regulators filter unitary step-size selection to control force-velocity modulation without myosin integration into muscle. Cardiac myosin is muscle in a molecule.
Highlights
The myosin motor protein powers muscle contraction with chemomechanical transduction of ATP free energy into the mechanical work of actin translation against resisting force [1]
We showed in earlier work using the Qdot in vitro motility assay that porcine cardiac ventricular myosin has three distinct unitary step-sizes of approximately 3, 5 and 8 nm that move actin in the absence of load with approximately 15, 50 and 35% relative stepsize frequencies, respectively [8]
We investigated the effect of load in vitro on three transgenic mouse cardiac myosins wild- type (WT), A57G and E143K
Summary
The myosin motor protein powers muscle contraction with chemomechanical transduction of ATP free energy into the mechanical work of actin translation against resisting force [1]. Skeletal and cardiac muscle have actomyosin interacting cyclically and mostly stochastically generating mechanical energy from ATP hydrolysis under conditions demanding dynamically wide-ranging power [2]. Muscle myosin has a motor domain transducer containing ATP and actin-binding sites, and a mechanical coupler linking impulses from the actin-bound motor to the myosin thick filament. The mechanical coupler is a lever arm stabilized by bound essential and regulatory light chains (ELC and RLC) that rotates cyclically to impel bound actin [3,4]. The cardiac-specific N-terminal extension of myosin ELC mediates the step-size frequency modulation [9] supporting a substantial velocity dynamic range [10]. In addition to cardiac myosins, the three-step-size mechanism appears in other muscle myosins including zebrafish skeletal [11,12]
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