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 Ca-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.

Functional significance of C-terminal mobile domain of cardiac troponin I.

Ca-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive.

Sarcomere length dependent effects on the interaction between cTnC and cTnI in skinned papillary muscle strips.

Sarcomere length dependent activation (LDA) of myocardial force development is the cellular basis underlying the Frank-Starling law of the heart, but it is still elusive how the sarcomeres detect the length changes and convert them into altered activation of thin filament. In this study we investigated how the C-domain of cardiac troponin I (cTnI) functionally and structurally responds to the comprehensive effects of the Ca(2+), crossbridge, and sarcomere length of chemically skinned myocardial preparations. Using our in situ technique which allows for simultaneous measurements of time-resolved FRET and mechanical force of the skinned myocardial preparations, we measured changes in the FRET distance between cTnI(167C) and cTnC(89C), labeled with FRET donor and acceptor, respectively, as a function of [Ca(2+)], crossbridge state and sarcomere length of the skinned muscle preparations.