On the Functional Assessment of Hypertrophic Cardiomyopathy-Causing Mutations in Human β-Cardiac Myosin and the Role of Myosin Binding Protein-C

Abstract

Human β-cardiac myosin and cardiac myosin binding protein-C (MyBP-C) harbor the majority of hypertrophic cardiomyopathy (HCM)-causing point mutations which lead to a hypercontractile heart with systolic and diastolic defects. Previous studies looking at the effects of HCM mutations on the force, velocity and ATPase activity of the catalytic domain of human β-cardiac myosin have not shown clear trends that establish a causal link to hypercontractility. However, recently, a regulatory, off-state of myosin with its heads folded back onto its own coiled-coil tail (probably the interacting heads motif or IHM described by others) has been experimentally demonstrated to be one of the primary states that yields the very slow turnover of ATP (called the super relaxed state or SRX) typically observed in muscle fibers. This regulatory off-state is in equilibrium with the on-state of myosin which is available to interact with actin and generate force. Our model posits that HCM mutations destabilize this SRX state, thus leading to a premature release of myosin heads from the thick filament backbone causing clinically observed hypercontractility. Using IHM as the model for the folded-back state, we have previously presented binding data demonstrating that HCM mutations at the head-tail interface weaken this interaction and possibly destabilize the IHM state. Here we present functional data showing that six separate HCM mutations located at the myosin head-tail and head-head interfaces of the IHM lead to a significant increase in the number of heads functionally accessible for interaction with actin. Interestingly, one of the mutations also ablates the binding of myosin with the N-terminal C0C7 fragment of MyBP-C hinting at a role of MyBP-C in regulating the availability of myosin heads for contraction of the cardiac muscle.