Analytical Comparison of Natural and Pharmaceutical Ventricular Myosin Activators

Ventricular myosin (beta Mys) is the motor protein in cardiac muscle generating force using ATP hydrolysis free energy to translate actin. In the cardiac muscle sarcomere, myosin and actin filaments interact cyclically and undergo rapid relative translation facilitated by the low duty cycle motor. It contrasts with high duty cycle processive myosins for which persistent actin association is the priority. The only pharmaceutical beta Mys activator, omecamtive mecarbil (OM), upregulates cardiac contractility in vivo and is undergoing testing for heart failure therapy. In vitro beta Mys step-size, motility velocity, and actin-activated myosin ATPase were measured to determine duty cycle in the absence and presence of OM. A new parameter, the relative step-frequency, was introduced and measured to characterize beta Mys motility due to the involvement of its three unitary step-sizes. Step-size and relative step-frequency were measured using the Qdot assay. OM decreases motility velocity 10-fold without affecting step-size, indicating a large increase in duty cycle converting beta Mys to a near processive myosin. The OM conversion dramatically increases force and modestly increases power over the native beta Mys. Contrasting motility modification due to OM with that from the natural myosin activator, specific beta Mys phosphorylation, provides insight into their respective activation mechanisms and indicates the boilerplate screening characteristics desired for pharmaceutical beta Mys activators. New analytics introduced here for the fast and efficient Qdot motility assay create a promising method for high-throughput screening of motor proteins and their modulators.