ABSTRACT Striated muscle myosin is highly organized into thick filaments that bear the molecular forces generated by the myosin heads. While thick filament structure and stability are essential for contractility, the mechanisms that allow fully developed muscles to replace myosin molecules while maintaining contractile fidelity are unclear. Critical questions include; what are the temporal dynamics of myosin synthesis and degradation (i.e. turnover) and how are molecules selected for degradation? Do striated myosin molecules exist in a dynamic equilibrium with thick filaments to allow for their exchange out of and into thick filaments? If thick filament structure is dynamic, what are the molecular mechanisms governing this equilibrium? Most importantly, is this mechanism tunable to modify striated muscle structure and/or function? We will address these questions in an adult mouse model in three aims. Our overall hypothesis is that myosin turnover is a stochastic process which involves the exchange of individual myosin molecules between a cytosolic pool of monomers and thick filaments, by a mechanism governed by the folding of the monomers within the cytosol. Aim 1 will define the turnover rate of cardiac myosin in our model and determine whether myosin degradation occurs via a stochastic (i.e. random) mechanism by using a combination of isotope labeling strategies and mass spectrometry. Aim 2 will test the hypothesis that the organization of striated muscle myosin is highly dynamic to allow for the rapid exchange of individual molecules between thick filaments and a cytosolic pool of monomers by virally labeling myosin with a fluorescent tag in vivo and examining the mobility of the myosin within hearts using multiphoton fluorescence recovery after photobleaching. Aim 3 will test the hypotheses that the structural conformation (i.e. folded vs. extended) of individual myosin molecules in the cytosol regulates the exchange of myosin molecules between pools. Aim 3 will take advantage of a drug that folds myosin and reduces cardiac mass. We will test our overall hypothesis that tuning myosin folding, affects the effective concentration of myosin with the cytosol, and regulates its availability for degradation. The proposed studies will be the first to examine myosin turnover and macromolecular exchange in a striated muscle system in any intact animal model. The results will provide conceptual innovation that fully developed muscle is designed in such a way to allow for structural rearrangement of myosin on a minute-to-minute timescale. The mechanistic findings have the potential to add to the current paradigm regarding thick filament structure and explain how striated muscle is maintained from the single molecule to whole organ level. The new knowledge gained may allow us to take advantage of this mechanism for tuning striated muscle structure and/or function in whole animals.