C0 Ig-Domain of Cardiac Myosin Binding Protein-C Interacts with the Regulatory Light Chain of Myosin-S1 Bound to the Native Cardiac Thin Filament

Abstract

Muscle contraction, relies on sliding of myosin-based thick filaments and thin filaments comprised of actin, tropomyosin and troponin. Cardiac myosin binding protein-C (cMyBP-C), an intrinsic protein of the thick filament, is a key regulator of actomyosin interactions, essential for both normal cardiac contraction and for increased cardiac contractility in response to inotropic stimuli. Mutations in MYBPC3, the gene encoding cMyBP-C, are the single most common genetic cause of hypertrophic cardiomyopathy (HCM). It has been proposed that the interaction of the N-terminal domains (NTDs) of cMyBP-C (e.g. C0, C1, M and C2) with myosin heads may stabilize the super-relaxed state (SRX-state) of the thick filament, while NMR experiments directly confirmed the binding of the C0 Ig-domain of cMyBP-C to the isolated regulatory light chain (RLC) of myosin. We recently demonstrated that C1 can activate the thin filament to the same extent as rigor myosin-S1, while C0 significantly enhances the activating effect of C1. Our data also show that C0 competes with myosin-S1 for binding to the thin filament. In order to understand how C0 interacts with the thick and thin filaments upon active cross-bridge formation we used cryo electron microscopy and image analysis of native cardiac thin filaments decorated with rigor myosin-S1 and the C0-domain of cMyBP-C. We show that C0 readily binds to the RLC of myosin-S1 bound to the thin filament in rigor state. Taking into account that the C0-domain is specific to the cardiac muscle and is absent in the skeletal MyBP-C we suggest a model of how the C0-domain of cMyBP-C participates in the regulation of cardiac contraction.