Abstact

At the molecular level, clinical hypercontractility associated with many hypertrophic cardiomyopathy (HCM)-causing mutations in β-cardiac myosin appears to be driven by their disruptive effect on the energy-conserving, folded-back 'OFF'-state of myosin, which results in increased number of heads free to interact with actin and produce force. While many characterized mutations likely act by directly perturbing intramolecular interfaces stabilizing the OFF-state, others may function allosterically by altering conformational states of the myosin motor. We investigate two such allosteric HCM mutations, Y115H (Transducer) and E497D (Relay helix), which do not directly contact OFF-state interfaces. Biochemical analyses and high-resolution crystallography reveal that both mutations increase active myosin head availability likely by destabilizing the pre-powerstroke conformation required for OFF-state formation. We propose that destabilization of the folded-back state of myosin, either directly or allosterically, represents a common molecular mechanism underlying hypercontractility in HCM across a broader spectrum of pathogenic mutations than previously recognized.