The performance of atrial and ventricular myosin isoforms purified from bovine heart and measured on a half-sarcomere like synthetic nanomachine

The functional differences of the α- and β-cardiac myosins that make up the atrial and ventricular chambers of the human heart have been described in detail with solution kinetics analysis, showing that α-cardiac myosin has faster ATP hydrolysis and faster ADP release so that the ATPase cycle time is ten times shorter, but the duty ratio (r) is similar to that of β-cardiac myosin. Instead, the isoform-dependence of mechanical performances remains elusive as the force of the β-cardiac myosin was found either twice higher (attributed to a larger r) with in vitro loaded motility assay or similar in Ca2+-activated skinned myocytes and myofibrils. Here, we use our synthetic nanomachine to determine the performance of an array of ∼15 heavy-meromyosin fragments (HMM) purified from bovine atrium and ventricle pulling on the actin filament in either isometric or isotonic condition. We find that the α-HMM array has an isometric force (6.6 ± 0.6 pN) ∼3-fold lower than the β-HMM (23.7 ± 3.9 pN), and an unloaded shortening-velocity ∼3-fold higher (1.4 ± 0.2 and 0.5 ± 0.1 μms−1). The force-velocity relation underpins a maximum power, an essential parameter for cardiac function, twofold lower for α- (0.7 ± 0.1 aW) compared to β-HMM (1.4 ± 0.1 aW). This quantitative description of the performances of cardiac myosin isoforms demonstrates the potential of the nanomachine for characterizing cardiomyopathy-causing myosin mutations and drugs currently developed for different therapeutic indications. Supported by Myokardia and the Italian Society for Pure and Applied Biophysics (SIBPA).