Cardiac and Skeletal Actin Substrates Uniquely Tune Cardiac Myosin Strain-Dependent Mechanics

Abstract

ABSTRACTNative cardiac ventricular myosin (βmys) translates actin under load by transducing ATP free energy into mechanical work on actin during muscle contraction. Unitary βmys translation of actin is the myosin step-size. In vitro and in vivo βmys regulates contractile force and velocity by remixing 3 different step-sizes with stepping frequencies autonomously adapted to workload. Cardiac and skeletal actin isoforms have a specific 1:4 stoichiometry in normal adult human ventriculum. Human adults with inheritable hypertrophic cardiomyopathy (HCM) up-regulate skeletal actin in ventriculum suggesting that increasing skeletal/cardiac actin stoichiometry also adapts myosin force-velocity to respond to the muscle’s inability to meet demand.Nanometer scale displacement of quantum dot (Qdot) labeled actin under resistive load when impelled by βmys measures single myosin force-velocity in vitro in the Qdot assay. Unitary displacement classification constraints introduced here better separates myosin based signal from background upgrading step-size spatial resolution to the sub-nanometer range. Single βmys force-velocity for skeletal vs cardiac actin substrates was compared using the Qdot assay.Two competing myosin strain-sensitive mechanisms regulate step-size choices dividing mechanical characteristics into low- and high-force regimes. The actin isoforms alter myosin strain-sensitive regulation such that onset of the high-force regime, where a short step-size is a large or major contributor, is offset to higher loads by a unique cardiac ELC N-terminus/cardiac-actin contact at Glu6/Ser358. It modifies βmys force-velocity by stabilizing the ELC N-terminus/cardiac-actin association. Uneven onset of the high-force regime for skeletal vs cardiac actin dynamically changes force-velocity characteristics as skeletal/cardiac actin fractional content increases in diseased muscle.