Calcium-induced flexibility changes in the troponin C-troponin I complex

Conformational changes in the Ca2+-regulatory region from soybean calcium-dependent protein kinase-alpha: fluorescence resonance energy transfer studies

Calcium-dependent protein kinases are key proteins involved in plant and protozoal Ca2+ signaling. These unique molecules include a calcium regulatory calmodulin-like domain (CLD), which binds to another small regulatory domain named the junction domain (JD). Both CLD and JD are part of the same polypeptide as the protein kinase domain. The CLD consists of N- and C-terminal lobes, each having two helix-loop-helix Ca2+-binding motifs. In this study, fluorescence resonance energy transfer using a series of Trp and Cys site-directed mutants was undertaken to probe the relative motions of the two lobes of CLD between the apo- and Ca2+-saturated forms, as well as bound to a peptide encoding the JD sequence. Using an IAEDANS-modified Cys, a total of 15 Trp --> Cys distances were measured in these three states from the five donor-acceptor combinations F334W-Cys436, L371W-Cys436, L403W-Cys436, F334W-L403C, and L371W-L403C. Consistent with recently reported NMR diffusion measurements and with 1H,15N heteronuclear correlation NMR spectra, the distances derived from fluorescence resonance energy transfer measurements in apoCLD indicate partial unfolding and a subsequent contraction on binding Ca2+, which is maintained on addition of the JD peptide. Interpretation of the distances suggests that the Ca2+-saturated form is open with the two lobes sitting side-by-side although highly flexible. The transition to the JD-CLD state appears to be accompanied by a rotation of the N- and C-terminal domains with respect to each other inducing a slightly more closed overall complex. The observed differences between the behavior of CLD and the well studied related protein calmodulin are likely because of different physiological requirements for activation in vivo.

Calcium-dependent protein-protein interactions induce changes in proximity relationships of Cys48 and Cys64 in chicken skeletal troponin I

The goal of this study was to relate conformational changes in the N-terminal domain of chicken troponin I (TnI) to Ca2+ activation of the actin-myosin interaction. The two cysteine residues in this region (Cys48 and Cys64) were labeled with two sulfhydryl-reactive pyrene-containing fluorophores [N-(1-pyrene)maleimide, and N-(1-pyrene)iodoacetamide]. The labeled TnI showed a typical fluorescence spectrum: two sharp peaks of monomer fluorescence and a broad peak of excimer fluorescence arising from the formation of an excited dimer (excimer). Results obtained show that forming a binary complex of labeled TnI with skeletal TnC (sTnC) in the absence of Ca2+ decreases the excimer fluorescence, indicating a separation of the two residues. This reduction in excimer fluorescence does not occur when labeled TnI is complexed with cardiac TnC (cTnC). The latter causes only partial activation of the Ca2+-dependent myofibrillar ATPase. The binding of Ca2+ to the two N-terminal sites of sTnC causes a significant decrease in excimer fluorescence and an increase in monomer fluorescence in complexes of labeled TnI with skeletal TnC or TnC/TnT, while Ca2+ binding to site II of cTnC only causes an increase in monomer fluorescence but no change in excimer fluorescence. Thus a conformational change in the N-terminal region of TnI may be necessary for full activation of muscle contraction.

Structure of the inhibitory region of troponin by site directed spin labeling electron paramagnetic resonance

Site-directed spin labeling EPR (SDSL-EPR) was used to determine the structure of the inhibitory region of TnI in the intact cardiac troponin ternary complex. Maeda and collaborators have modeled the inhibitory region of TnI (skeletal 96-112: the structural motif that communicates the Ca(2+) signal to actin) as a kinked alpha-helix [Vassylyev, D., Takeda, S., Wakatsuki, S., Maeda, K. & Maeda, Y. (1998) Proc. Natl. Acad. Sci. USA 95, 4847-4852), whereas Trewhella and collaborators have proposed the same region to be a flexible beta-hairpin [Tung, C. S., Wall, M. E., Gallagher, S. C. & Trewhella, J. (2000) Protein Sci. 9, 1312-1326]. To distinguish between the two models, residues 129-145 of cardiac TnI were mutated sequentially to cysteines and labeled with the extrinsic spin probe, MTSSL. Sequence-dependent solvent accessibility was measured as a change in power saturation of the spin probe in the presence of the relaxation agent. In the ternary complex, the 129-137 region followed a pattern characteristic of a regular 3.6 residues/turn alpha-helix. The following region, residues 138-145, showed no regular pattern in solvent accessibility. Measurements of 4 intradomain distances within the inhibitory sequence, using dipolar EPR, were consistent with an alpha-helical structure. The difference in side-chain mobility between the ternary (C.I.T) and binary (C.I) complexes revealed a region of interaction of TnT located at the N-terminal end of the inhibitory sequence, residues 130-135. The above findings for the troponin complex in solution do not support either of the computational models of the binary complex; however, they are in very good agreement with a preliminary report of the x-ray structure of the cardiac ternary complex [Takeda, S. Yamashita, A., Maeda, K. & Maeda, Y. (2002) Biophys. J. 82, 832].

A model of troponin-I in complex with troponin-C using hybrid experimental data: the inhibitory region is a beta-hairpin

We present a model for the skeletal muscle troponin-C (TnC)/troponin-I (TnI) interaction, a critical molecular switch that is responsible for calcium-dependent regulation of the contractile mechanism. Despite concerted efforts by multiple groups for more than a decade, attempts to crystallize troponin-C in complex with troponin-I, or in the ternary troponin-complex, have not yet delivered a high-resolution structure. Many groups have pursued different experimental strategies, such as X-ray crystallography, NMR, small-angle scattering, chemical cross-linking, and fluorescent resonance energy transfer (FRET) to gain insights into the nature of the TnC/TnI interaction. We have integrated the results of these experiments to develop a model of the TnC/TnI interaction, using an atomic model of TnC as a scaffold. The TnI sequence was fit to each of two alternate neutron scattering envelopes: one that winds about TnC in a left-handed sense (Model L), and another that winds about TnC in a right-handed sense (Model R). Information from crystallography and NMR experiments was used to define segments of the models. Tests show that both models are consistent with available cross-linking and FRET data. The inhibitory region TnI(95-114) is modeled as a flexible beta-hairpin, and in both models it is localized to the same region on the central helix of TnC. The sequence of the inhibitory region is similar to that of a beta-hairpin region of the actin-binding protein profilin. This similarity supports our model and suggests the possibility of using an available profilin/actin crystal structure to model the TnI/actin interaction. We propose that the beta-hairpin is an important structural motif that communicates the Ca2+-activated troponin regulatory signal to actin.