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.
Theoretical Division, Los Alamos National Laboratory, New Mexico 87545, USA.
C S
CS
Tung
M E
ME
Wall
S C
SC
Gallagher
J
J
Trewhella
D000595:Amino Acid Sequence; D003198:Computer Simulation; D003285:Contractile Proteins; D003432:Cross-Linking Reagents / Q000737:chemistry; D008840:Microfilament Proteins / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D051304:Profilins; D011487:Protein Conformation; D017510:Protein Folding; D017386:Sequence Homology, Amino Acid; D019209:Troponin C / Q000737:chemistry* / Q000378:metabolism*; D019210:Troponin I / Q000737:chemistry* / Q000378:metabolism*
D003285:Contractile Proteins; D003432:Cross-Linking Reagents; D008840:Microfilament Proteins; D051304:Profilins; D019209:Troponin C; D019210:Troponin I