Schematic representation of the antagonistic mechanism by which the HCM-linked A57G and RCM-associated E143K mutations in cardiac myosin ELC regulate the SRX state of myosin and myosin RLC phosphorylation. The HCM-A57G mutation promotes the SRX-to-DRX transition, and this process is coupled with a ∼40% increase in myosin RLC phosphorylation compared with the level of RLC phosphorylation in WT-ELC myocardium. The RCM-E143K model favors the SRX conformation, and its RLC phosphorylation is approximately twofold less than in WT-ELC hearts, as judged by Western blotting and RLC-specific antibodies. Mechanistically, it is proposed that N-ELC, which is missing in the Δ43 model, regulates the SRX-to-DRX transition and contributes to sterically blocking (RCM-E143K) or facilitating (HCM-A57G) myosin RLC phosphorylation. Fig. 5 was created with https://BioRender.com.