BPS2026 – The structure of the native cardiac cross-bridge revealed by cryo-electron microscopy

Every twitch of the myocardium depends on force-generating interactions between the thick (myosin) and thin (actin) filaments, which are regulated by Ca 2+ ions binding to the thin filaments (TFs). TF consists of individual regulatory units (RUs) made up of seven actin protomers, a tropomyosin cable (Tm), and three troponin (Tn) subunits—Ca 2+ -binding TnC, inhibitory TnI, and Tm-binding TnT—that control Tm’s position on actin and thus regulate its ability to bind myosin heads. Striated thick filaments are formed from myosin dimers, each comprising the N-terminal part of the heavy chains containing actin-binding domains, an α-helical coiled-coil region that interweaves to form the filament’s backbone, and associated essential light chains (ELCs) and regulatory light chains (RLCs) that have regulatory roles. Consequently, each myosin molecule has two actin-binding heads. All resolved actomyosin complexes to date involve helically averaged actin filaments fully decorated with single-headed myosin constructs, which lack information about how dual-headed myosin molecules interact with each other during binding to the TF and how they interact with the TF components. Here, we present the near-atomic structure of heavy meromyosin (HMM) complexed with native cardiac thin filament at systolic Ca 2+ levels. We show that HMM prefers a specific binding site on the TF and that the two heads communicate with each other and the Tn complex during the formation of an active cross-bridge. We demonstrate that interactions between myosin loop-4 and Tm, as well as the extent of Tm activation, depend on the site of HMM binding on the RU. Finally, we provide novel insights into the interactions of ELCs with the motor domain and the arrangement of the interacting RLCs. Our work has a direct and immediate impact on our understanding of cardiac contractility.