In Situ Structural Changes in Thick and Thin Filaments of Cardiac Muscle Induced by Fragments of Myosin Binding Protein C (MyBP-C)

Abstract

MyBP-C is a component of the thick filaments of striated muscle, and mutations in the cardiac MyBP-C gene are the second most common cause of hypertrophic cardiomyopathy. The C-terminus of MyBP-C is bound to the thick filament, but the N-terminus is believed to interact with both the thin filaments and the S2 domain of myosin in the thick filaments. These interactions are phosphorylation-dependent, and are thought to regulate cardiac contractility. In this study, polarized fluorescence was used to monitor structural changes of the cardiac myosin regulatory light chain (cRLC) in the thick filaments and cardiac troponin C (cTnC) in the thin filaments on activation of skinned cardiac muscle either by calcium or by exogenous N-terminal fragments of cardiac MyBP-C. The structural changes in both proteins produced by activation with 50 μM C1-C2 fragment of cMyBPC at sarcomere length 2.1 μm, 20°C, pCa 9 were similar to those produced by calcium activation (pCa 4.5). However, only the cTnC probes showed significant structural changes when contraction was inhibited by blebbistatin, suggesting that the activating effect of C1-C2 is mediated by binding to the thin filament.Supported by the British Heart Foundation