C1 IG-Domain of Myosin Binding Protein-C Activates Cardiac Thin Filament by Means of Thethering Tropomyosin to the Subdomain-1 of Actin

Abstract

Cardiac muscle contraction is based on the interaction between two filamentous systems: the thick filament, which is comprised of myosin and its accessory protein, cardiac myosin binding protein C (cMyBP-C) and the thin filament (TF), which is comprised of troponin (Tn), tropomyosin (Tpm), and filamentous actin (F-actin). Mutations in the gene encoding cMyBP-C are one of the most common causes of hypertrophic cardiomyopathy (HCM), a disease that affects approximately 1 in 500 people. The interaction between thick and thin filaments is regulated through translocation of Tpm cable by the Tn-complex in response to Ca2+ followed by additional Tpm azimuthal movement upon binding of rigor myosin cross-bridges to F-actin. Therefore, activation of the TF is a two-step process which depends on both Ca2+-induced translocation of the Tpm and subsequent binding of rigor myosin heads. The N-terminal domain of cMyBP-C is comprised of three Ig-domains (C0, C1 and C2) and a regulatory linker (M-domain). We show that C1 Ig-domain is the only Ig-domain that can activate the TFs at low Ca2+. 3D-reconstruction of frozen hydrated cardiac TFs decorated with C1 Ig-domain shows that the C1 Ig-domain on its own can translocate the Tpm cable on the surface of F-actin to the same extent as the combination of Ca2+ and rigor myosin by tethering the Tpm cable to the subdomain-1 (SD1) of actin. Disruption of the interaction of the C1 Ig-domain interaction with either the SD1 of actin or Tpm cable by point mutations in C1 Ig-domain results in a complete loss of the C1-induced TF activation at low Ca2+. These data suggest how cMyBP-C can modulate the activation of the TF in heart muscle.