Cardiac myosin filaments are directly regulated by calcium

Abstract

Contraction of cardiac and skeletal muscle is initiated by calcium (Ca2+) binding to regulatory proteins on actin-containing thin filaments. During muscle contraction, structural changes in Ca2+-dependent thin filament allow the binding of myosin motors to drive muscle contraction1. The dynamic switching between the resting off states and the active on states of myosin is also critical in regulating muscle contractility2–4. However, the molecular switch on the myosin-containing thick filament that drives this process is not understood. Here we show that cardiac thick filaments are directly Ca2+-regulated. We find that Ca2+ progressively moves the myosin heads from ordered off states close to the thick filament backbone to disordered on states closer to the thin filaments. This Ca2+-dependent structural shift of myosin is accompanied by a biochemically defined transition from the inactive super-relaxed state(s) to the active disordered relaxed state(s)3. Furthermore, we find that this Ca2+-mediated molecular switching is an intrinsic property of cardiac myosin but only when assembled into thick filaments. This novel concept of Ca2+ as a regulatory modulator of the thick filament provides a fresh perspective on cardiac muscle regulation, which may be particularly valuable for devising restorative treatments for pathologies altering the Ca2+ sensitivity of the sarcomere.