Dilated cardiomyopathy mutation (E525K) in human beta-cardiac myosin enhances actin-activated phosphate release but stabilizes the auto-inhibited super-relaxed state

Mutations in beta-cardiac myosin (M2β) are a common cause of inherited cardiomyopathies including dilated (DCM) and hypertrophic cardiomyopathy (HCM). We investigated the impact of a DCM mutation (E525K) in the human M2β motor domain, which has been shown to stabilize both the interacting heads motif (IHM) and auto-inhibited super-relaxed (SRX) state via head-tail electrostatic interactions in 15 heptad heavy meromyosin (M2β HMM). These findings suggest E525K may reduce muscle force and power by triggering an increase in the IHM/SRX state. However, it is unclear how E525K impacts the intrinsic motor properties of M2β. Thus, we introduced the mutation into single-headed human cardiac myosin subfragment 1 (M2β-S1), unable to form the IHM, and examined its impact on the ATPase mechanism. We revealed that E525K induces a 2.8-fold increase in maximum steady-state actin-activated ATPase activity (kcat), a 6-fold decrease in the actin concentration at which ATPase is half maximal (KATPase), and a 3.4-fold increase in the fast phase of actin-activated phosphate release. Glutamate 525 is located in the conserved activation loop that is proposed to trigger phosphate release by interacting with the negatively charged N-terminus of actin. We propose the mutation may enhance the rate of rotation of the lower 50-kDa domain, movement of the relay helix/switch II, and lever arm rotation associated with the power stroke. Thus, we will directly examine the impact on the actin-activated power stroke utilizing a FRET approach that measures lever arm rotation. Our results suggest the E525K mutation triggers an increase in intrinsic motor properties, while its ability to stabilize the SRX/IHM state in the myosin dimer likely dominates the molecular mechanism that leads to decreased muscle force and power and DCM pathogenesis.