Differential Effects of Alpha Vs Beta Myosin Heavy Chain on the Kinetcs and Mechanics of Familial Hypertrophic Cardiomyopathy Mutations in the Myosin Regulatory Light Chain

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Cardiac muscle myosin is comprised of two heavy chains (MHCs), two essential light chains, and two regulatory light chains (RLCs). The MHC contains both the ATPase and actin binding domains. It has been shown that the contractile properties of myosin can be tuned by the MHC isoform and that the MHC isoform distribution in the human heart changes during heart failure from predominantly beta isoform to all beta. One cause of heart failure is familial hypertrophic cardiomyopathy (FHC) which is triggered by mutation of sarcomeric proteins including the RLC. Although the RLC is spatially separated from the myosin active site, it appears to have a role in tuning myosin kinetics. In order to examine how two RLC mutations implicated in FHC, N47K and R58Q, affect the kinetic and mechanical properties of beta isoform myosin, we exchanged porcine cardiac RLC with recombinant mutant RLC. We examined the contractile properties of these mutants using the in vitro motility assay and compared these results to our earlier results with mutant RLCs on the alpha -MHC background. Regardless of MHC isoform, the mutations cause reductions in force and power output. However, on the alpha MHC backbone, R58Q shows differences in calcium handling and an elevated ATPase rate which is not seen on the beta backbone. Also, both mutants show increases in duty cycle on the alpha MHC but not the beta. These data suggest that small changes in the myosin structure, far from the active site, can disrupt the contractile properties of the motor depending on the MHC isoform.