A multi-site, steady-state Forster resonance energy transfer (FRET) approach was used to quantify Ca(2+)-induced changes in proximity between donor loci on human cardiac troponin I (cTnI), and acceptor loci on human cardiac tropomyosin (cTm) and F-actin within functional thin filaments. A fluorescent donor probe was introduced to unique and key cysteine residues on the C- and N-termini of cTnI. A FRET acceptor probe was introduced to one of three sites located on the inner or outer domain of F-actin, namely Cys-374 and the phalloidin-binding site on F-actin, and Cys-190 of cTm. Unlike earlier FRET analyses of protein dynamics within the thin filament, this study considered the effects of non-random distribution of dipoles for the donor and acceptor probes. The major conclusion drawn from this study is that Ca(2+) and myosin S1-binding to the thin filament results in movement of the C-terminal domain of cTnI from the outer domain of F-actin towards the inner domain, which is associated with the myosin-binding. A hinge-linkage model is used to best-describe the finding of a Ca(2+)-induced movement of the C-terminus of cTnI with a stationary N-terminus. This dynamic model of the activation of the thin filament is discussed in the context of other structural and biochemical studies on normal and mutant cTnI found in hypertrophic cardiomyopathies.
Wang, H., Chalovich, J. M., & Marriott, G. (2012). Structural Dynamics of Troponin I during Ca2+-Activation of Cardiac Thin Filaments: A Multi-Site Förster Resonance Energy Transfer Study. PLoS ONE, 7(12). https://doi.org/10.1371/journal.pone.0050420