Structural transition of the inhibitory region of troponin I within the regulated cardiac thin filament

Contraction and relaxation of cardiac muscle are regulated by the inhibitory and regulatory regions of troponin I (cTnI). Our previous FRET studies showed that the inhibitory region of cTnI in isolated troponin experiences a structural transition from a β-turn/coil motif to an extended conformation upon Ca2+ activation. During the relaxation process, the kinetics of the reversal of this conformation is coupled to the closing of the Ca2+-induced open conformation of the N-domain of troponin C (cTnC) and an interaction between cTnC and cTnI in their interface. We have since extended the structural kinetic study of the inhibitory region to fully regulated thin filament. Single-tryptophan and single-cysteine mutant cTnI(L129W/S151C) was labeled with 1,5-IAEDANS at Cys151, and the tryptophan-AEDANS pair served as a donor–acceptor pair. Labeled cTnI mutant was used to prepare regulated thin filaments. Ca2+-induced conformational changes in the segment of Trp129-Cys151 of cTnI were monitored by FRET sensitized acceptor (AEDANS) emission in Ca2+ titration and stopped-flow measurements. Control experiments suggested energy transfer from endogenous tryptophan residues of actin and myosin S1 to AEDANS attached to Cys151 of cTnI was very small and Ca2+ independent. The present results show that the rate of Ca2+-induced structural transition and Ca2+ sensitivity of the inhibitory region of cTnI were modified by (1) thin filament formation, (2) the presence of strongly bound S1, and (3) PKA phosphorylation of the N-terminus of cTnI. Ca2+ sensitivity was not significantly changed by the presence of cTm and actin. However, the cTn–cTm interaction decreased the cooperativity and kinetics of the structural transition within cTnI, while actin filaments elicited opposite effects. The strongly bound S1 significantly increased the Ca2+ sensitivity and slowed down the kinetics of structural transition. In contrast, PKA phosphorylation of cTnI decreased the Ca2+ sensitivity and accelerated the structural transition rate of the inhibitory region of cTnI on thin filaments. These results support the idea of a feedback mechanism by strong cross-bridge interaction with actin and provide insights on the molecular basis for the fine tuning of cardiac function by β-adrenergic stimulation.