Altered signaling surrounding the C-lobe of cardiac troponin C in myofilaments containing an α-tropomyosin mutation linked to familial hypertrophic cardiomyopathy

A region of interaction between the near N-terminal of cardiac troponin I (cTnI) and the C-lobe of troponin C (cTnC), where troponin T (cTnT) binds, appears to be critical in regulation of myofilament Ca²⁺-activation. We probed whether functional consequences of modulation of this interface influence the function of tropomyosin (Tm) in thin filament activation. We modified the C-lobe of cTnC directly by addition of the Ca ²⁺-sensitizer, EMD 57033, and indirectly by replacing native cTnI with cTnI-containing Glu residues at Ser-43 and Ser-45 (cTnI-S43E/S45E) in myofilaments from hearts of non-transgenic (NTG) and transgenic (TG) mice expressing a point mutation on α-Tm (E180G) linked to familial hypertrophic cardiomyopathy. Introduction of cTnI-S43E/S45E induced a significantly greater reduction in tension in TG myofilaments compared to NTG controls. Furthermore, the effect of EMD 57033 to restore Ca ²⁺-sensitivity was higher in TG compared to NTG fiber bundles containing cTnI-S43E/S45E and compared to TG or NTG fiber bundles containing native TnI. Our results indicate that alterations in regions of interaction among the N-terminal of cTnI, the C-lobe of cTnC, and the C-terminus of cTnT are important in the regulation of myofilament activity. Although levels of phosphorylation at protein kinase C-dependent sites were the same in TG and NTG myofilaments, our data indicate that the effects of phosphorylation were more depressive in TG hearts.