ROLE OF THE C-TERMINUS MOBILE DOMAIN OF CARDIAC TROPONIN I IN THE REGULATION OF THIN FILAMENT ACTIVATION IN SKINNED PAPILLARY MUSCLE STRIPS

The C-terminus mobile domain of cTnI (cTnI-MD) is a highly conserved region which stabilizes the actin-cTnI interaction during the diastole. Upon Ca2+-binding to cTnC, cTnI-MD participates in a regulatory switching that involves cTnI to switch from interacting with actin toward interacting with the Ca-regulatory domain of cTnC. Despite many studies targeting the cTnI-MD, the role of this region in the length-dependent activation of cardiac contractility is yet to be determined. The present study investigated the functional consequences of losing the entire cTnI-MD in cTnI(1-167) truncation mutant, as it was exchanged for endogenous cTnI in skinned rat papillary muscle fibers. The influence of cTnI-MD truncation on the extent of the N-domain of cTnC hydrophobic cleft opening and the steady-state force as a function of sarcomere length (SL), cross-bridge state, and [Ca2+] was assessed using the simultaneous in-situ time-resolved FRET and force measurements at short (1.8 µm) and long (2.2µm) SLs. Our results suggest that the cTnI-MD governs the equilibrium position of tropomyosin on actin filament at both relaxed and activated states and mediates the level of thin filament activation. Our results also suggest that cTnI-MD transmits the effects of SL change to the core of troponin complex. 1