Conformation and Dynamics of the Troponin I C-Terminal Domain

Abstract

The troponin complex acts as a molecular switch in striated muscle cells to regulate myosin attachment to and isomerization on actin filaments. Transitions between the inactive and active states of the cardiac thin filament require extensive domain movements and binding exchanges involving the C-terminal domain of cardiac troponin I (TnI-C, residues 130-210). Mutations in TnI-C are associated with hypertrophic cardiomyopathy, highlighting the importance of this domain in regulating cardiac contraction. However, the conformation and dynamics of TnI-C are poorly characterized due to order/disorder transitions between binding states, leaving mechanistic details undetermined for both wild-type and diseased complexes. TnI-C is likely disordered in solution and the unbound state [1], coupling binding and folding as it fly-casts onto its actin/tropomyosin binding site to block myosin access to actin [2].Here we use a combination of computational and experimental techniques to probe the conformation and dynamics of TnI-C in its multiple binding states, using pairwise fluorescence measurements applicable to both the solution-state and the thin filament-docked complex. Our molecular dynamics simulations are consistent with TnI-C disorder in the unbound state; simulations beginning from a helical model unfold regardless of starting orientation or the presence of the rest of the complex, and there is no consensus structure in the disordered state over multiple 50 ns simulations of the free domain. Remarkably, the presence of the N-terminal globular head of troponin C affects TnI-C conformation in silico, emphasizing the importance of interdomain and intermolecular interactions in the troponin complex.1. Julien, O., Mercier, P., Allen, C.N., Fisette, O., Ramos, C.H.I, et al. (2011) Proteins 79: 1240-1250.2. Zhou, Z., Li, K-L, Riek, D. Ouyang, Y., Chandra, M., Dong, W.J. (2012) J. Biol. Chem. 287(10): 7661-7674.