Kinetics of Mouse Alpha-Myosin S1 a Fast Myosin

Abstract

The fast skeletal muscle myosins and the slow/β-cardiac myosins have been well described but few kinetic studies of the α cardiac myosin have been presented. Here we show that the mouse alpha cardiac myosin has more in common with fast skeletal type myosin than the slower B-type myosin. The data for individual rate constant for mouse α myosin S1 are compared to our published values for the rabbit fast muscle and slow/β myosin S1. The ADP affinity for acto.S1 is much weaker (250 µM) than beta-S1 (10 µM) and similar to that of fast rabbit muscle S1 (120 μM). The rate constant for ADP release is too fast to measure for mouse α -S1 and rabbit fast muscle S1 but 63 s−1 for the rabbit β-S1. For all myosins affinity of actin form myosin is reduced by ADP and the affinity of ADP for S1 is reduced by actin. This thermodynamic coupling ratio is 10 for β-S1 but ∼50-100 for fast muscle S1 and α-S1. The ATP hydrolysis step rate constant estimated from the protein fluorescence changes is 150 s−1 similar tor fast muscle S1 (120 s−1) and faster that β-S1 (21 s−1). These values suggest that the ADP release from AMD and the hyrdolysis step are changed in parallel and this is consistent with the hydrolysis step having a role in defining the lifetime of the detached state (M.ATP) while ADP release controls the lifetime of the attached state. Thus the duty ratio can remain similar while the cycle time is altered. All other values assayed show no major difference between the three isoforms of myosin. Overall despite the greater sequence similarity between α and β cardic S1 the α-S1 shares kinetic properties with the fast muscle S1.