Varying the Kinetics of Actin and Myosin of the Bovine Cardiac Muscle ATPase Cycle
Biochemistry & Molecular Biology
Biochem and Molecular Biology
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Understanding the kinetics and mechanics of cardiac muscle myosin (CMM) is important for understanding how cardiac muscle contraction occurs in both a normal and a diseased states and can also provide insights into how drugs may be affecting cardiac muscle. In this study, various aspects of the stability and kinetic parameters of CMM filaments were investigated to determine how various models of muscle contraction describe CMM filament motility. The stability of CMM filaments was determined using a sedimentation assay. It was found that CMM filaments are not as stable as skeletal myosin filaments at very low concentration s that are required for direct imaging of myosin movement. Therefore, to observe motility it was necessary to stabilize the filaments by crosslinking. Inverted motility assays of fluorescently-labeled myosin filaments and co-filaments (made with rod-myosin mixtures) of varying length were performed to determine the dependence of unloaded velocity (V) on numbers of heads (N) in a filament. However, to fit to the data to our current model, the maximal turnover rate of CMM under these conditions was required. To obtain this value, the steady-state ATPase was measured using CMM S1 at increasing actin concentrations to determine vmax. With the kinetics of CMM S1 known, it was shown that an attachment-detachment limited model provides a good explanation for the relationship between V and N. This suggests that CMM motility is not simply detachment-limited as previously thought. Lastly, an experiment was done using fascin to determine if it is capable of bundling actin because use of actin bundles could further improve the standard motility assay.