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 Ca2+-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 alpha-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 Ca2+-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. (C) 2003 Elsevier Ltd. All rights reserved.