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

Published online by Cambridge University Press:  01 July 2000

CHANG-SHUNG TUNG
Affiliation:
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
MICHAEL E. WALL
Affiliation:
Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
STEPHEN C. GALLAGHER
Affiliation:
Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
JILL TREWHELLA
Affiliation:
Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
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Abstract

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 β-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 β-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 β-hairpin is an important structural motif that communicates the Ca2+-activated troponin regulatory signal to actin.

Type
Research Article
Copyright
2000 The Protein Society

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