A model of troponin-I in complex with troponin-C using hybrid experimentaldata: 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 effort s by multiple groups for more than a decade, attempts to crystallize tropon in-C in complex with troponin-I, or in the ternary troponin complex, have n ot yet delivered a high-resolution structure. Many groups have pursued diff erent experimental strategies, such as X-ray crystallography NMR, small-ang le scattering chemical cross-linking and fluorescent resonance energy trans fer (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 se quence was fit to each of two alternate neutron scattering envelopes: one t hat winds about TnC in a left-handed sense (Model L), and another that wind s about TnC in a right-handed sense (Model R). Information from crystallogr aphy and NMR experiments was used to define segments of the models. Tests s how that both models are consistent with available cross-linking and FRET d ata. The inhibitory region TnI(95-114) is modeled as a flexible beta-hairpi n, and in both models it is localized to the same region on the central hel ix of TnC. The sequence of the inhibitory region is similar to that of a be ta-hairpin region of the actin-binding protein profilin. This similarity su pports our model and suggests the possibility of using an available profili n/actin crystal structure to model TnI/actin interaction. We propose that t he beta-hairpin is an important structural motif that communicates the Ca2-activated troponin regulatory signal to actin.