Heart contraction is a highly regulated process powered by β-cardiac myosin. This molecule can adopt an inactive state, the so-called “interacting-heads motif” ( car IHM), in which the motor heads fold back on the proximal tail (S2). The proportion of motors adopting the IHM is precisely regulated and can be activated when exertion increases. The number of inactive motors is finely tuned but can be altered by mutations that cause inherited cardiac diseases such as hypertrophic or dilated cardiomyopathies (HCM or DCM). However, the molecular mechanisms underlying the stability and regulation of car IHM remain unclear. In this work, we combine cryo-EM processing methods and molecular dynamics to explore the dynamics of car IHM. High-resolution structures of the wild-type car IHM demonstrate that the blocked head (BH)/S2 interface is dynamic and that S2 adopts multiple orientations, explaining the low stability of car IHM. The cryo-EM structure of car IHM carrying the DCM-causing mutation E525K shows that this mutation drastically reduces the dynamics of the motif, favoring a single conformation of the S2. These results, coupled with all-atom molecular dynamics, enabled our investigation of the effects of cardiomyopathy-causing mutations, revealing how they alter specific clusters of residues and the overall stability of car IHM. Finally, we built a near-atomic resolution model of the human cardiac thick filament and performed all-atoms molecular dynamics on it. The human cardiac thick filament is pseudo-symmetric with three distinct car IHMs—each with its own unique environment. Our findings demonstrate how the plasticity of the BH/S2 interface enables car IHM to dock differently in these environments. Overall, these results provide deeper insights into the regulation of cardiac contraction and offer a framework for predicting the impact of point mutations responsible for inherited cardiomyopathies.
Auguin et al. (Sun,) studied this question.