Structural changes in myosin affect chemo-mechanical properties of the myosin-actin interaction

Mutations in the genes encoding striated muscle myosins lead to a variety of diseases including hypertrophic (HCM) and dilated (DCM) cardiomyopathies (MYH7) and distal arthrogryposis syndromes (MYH3 and MYH8). We have used a muscle myoblast cell line to express and purify recombinant human beta-cardiac myosin and human embryonic skeletal muscle myosins, comparing the WT versions of these proteins with an array of disease-causing variants. HCM-causing mutations in the beta-cardiac motor domain often destabilize a folded back, autoinhibited state of myosin, leading to an increase in the number of heads available within the sarcomere to interact with actin and produce force, which in turn results in the hypercontractility characteristic of HCM. Destabilization is seen for mutations that alter residues involved in the regions of myosin important for forming the intramolecular contacts in the folded back state. However, mutations affecting residues remote from these interfaces can also increase the number of available heads by destabilizing the pre-power stroke state of myosin that enables formation of the folded back state. HCM mutations that affect only myosin motor function without affecting the stability of the folded back state are less common. In contrast, we have studied a variety of DCM-causing mutations and found that most DCM-mutant myosins exhibit decreases in one or more aspects of motor function, including actin-activated ATPase activity and in vitro gliding velocity. Current efforts focus on correlating the effects of distal arthrogryposis syndrome-causing mutations on MYH3 biomechanical function and contrasting the effects of these mutations with paralogous mutations in MYH7.