Comparison of unitary displacements and forces between 2 cardiac myosin isoforms by the optical trap technique - Molecular basis for cardiac adaptation

To provide information on the mechanism of cardiac adaptation at the molecular level, we compared the unitary displacements and forces between the 2 rat cardiac myosin isoforms, V(1) and V(3). A fluorescently labeled actin filament, with a polystyrene bead attached, was caught by an optical trap and brought close to a glass surface sparsely coated with either of the 2 isoforms, so that the actin-myosin interaction took place in the presence of a low concentration of ATP (0.5 mu mol/L). Discrete displacement events were recorded with a low trap stiffness (0.03 to 0.06 pN/nm). Frequency distribution of the amplitude of the displacements consisted of 2 gaussian curves with peaks at 9 to 10 and 18 to 20 nm for both V(1) and V(3), suggesting that 9 to 10 nm is the unitary displacement for both isoforms, The duration of the displacement events was longer for V(3) than for V(1). On the other hand, discrete force transients were recorded with a high trap stiffness (2.1 pN/nm), and their amplitude showed a broad distribution with mean values between 1 and 2 pN for V(1) and V(3). The durations of the force transients were also longer for V(3) than for V(1). These results indicate that both the unitary displacements and forces are similar in amplitude but different in duration between the 2 cardiac myosin isoforms, being consistent with the reports that the tension cost is higher in muscles consisting mainly of V(1) than those consisting mainly of V(3).