Curvature and Torsion of Protein Main Chain as Local Order Parameters of Protein Unfolding.

Thermal protein unfolding resembles a global (two-state) phase transition. At the local scale, protein unfolding is, however, heterogeneous and probe dependent. Here, we consider local order parameters defined by the local curvature and torsion of the protein main chain. Because chemical shifts (CS's) measured by NMR spectroscopy are extremely sensitive to the local atomic environment, CS has served as a local probe of thermal unfolding of proteins by varying the position of the atomic isotope along the amino acid sequence. The variation of the CS of each Cα atom along the sequence as a function of the temperature defines a local heat-induced denaturation curve. We demonstrate that these local heat-induced denaturation curves mirror the local protein nativeness defined by the free energy landscape of the local curvature and torsion of the protein main chain described by the Cα-Cα virtual bonds. Comparison between molecular dynamics simulations and CS data of the gpW protein demonstrates that some local native states defined by the local curvature and torsion of the main chain, mainly located in secondary structures, are coupled to each other whereas others, mainly located in flexible protein segments, are not. Consequently, CS's of some residues are faithful reporters of global protein unfolding, with heat-induced denaturation curves similar to the average global one, whereas other residues remain silent about the protein unfolded state. For the latter, the local deformation of the protein main chain, characterized by its local curvature and torsion, is not cooperatively coupled to global unfolding.

Statistical Model To Decipher Protein Folding/Unfolding at a Local Scale.

Protein folding/unfolding can be analyzed experimentally at a local scale by monitoring the physical properties of local probes as a function of the temperature, for example, the distance between fluorophores or the values of chemical shifts of backbone atoms. Here, the analytical Lifson-Roig model for the helix-coil transition is modified to analyze local thermal unfolding of the fast-folder W protein of bacteriophage lambda (gpW) simulated by all-atom molecular dynamics (MD) simulations in explicit solvent at 15 different temperatures. The protein structure is described by the coarse-grained dihedral angles (γ) and bond angles (θ) built between successive Cα-Cα virtual bonds. Each (γ,θ) pair serves as a local probe of protein unfolding. Local native/non-native states are defined for each pair of (γ,θ) angles by analyzing the free-energy landscapes Δ G(γ,θ) computed from MD trajectories. The three local elementary equilibrium constants of the model are extracted for each (γ,θ) pair along the sequence from MD simulations, and the model predictions are compared to the MD data. Using only the local equilibrium constants as an input, we show that the local denaturation curves computed from the model partition function fit their MD simulated counterparts in a satisfying manner without any adjustment. In the model and MD simulations, gpW unfolds gradually between 320 and 340 K, with an average native percentage decreasing from 0.8 (320 K) to 0.2 (340 K). In the prism of the model, there is no stable structure at the local scale in this 20 K unfolding temperature range. The enthalpy change upon local unfolding computed from the model and from MD trajectories suggests that the unfolded state between 320 and 340 K corresponds to a dynamical equilibrium between a large ensemble of constantly changing structures. The present results confirm the downhill unfolding of gpW, which does not obey a two-state global folding/unfolding model, and shed light on the interpretation of local denaturation curves.

Data on the expression of GSTE1 and GSTE7 in Drosophila chemosensory organs after isothiocyanate exposure.

The data presented in this article are related to the research article entitled "Characterization of a Drosophila glutathione transferase involved in isothiocyanate detoxification." (Gonzalez et al., 2018) [1]. This article includes the expression level of Drosophila melanogaster GSTE1 and GSTE7 in chemosensory male tissues and the expression level of the mRNAs coding for the same enzymes after a PEITC exposure in food.

Characterization of a Drosophila glutathione transferase involved in isothiocyanate detoxification.

Glutathione transferases (GSTs) are ubiquitous key enzymes that catalyse the conjugation of glutathione to xenobiotic compounds in the detoxification process. GSTs have been proposed to play a dual role in the signal termination of insect chemodetection by modifying odorant and tasting molecules and by protecting the chemosensory system. Among the 40 GSTs identified in Drosophila melanogaster, the Delta and Epsilon groups are insect-specific. GSTs Delta and Epsilon may have evolved to serve in detoxification, and have been associated with insecticide resistance. Here, we report the heterologous expression and purification of the D. melanogaster GST Delta 2 (GSTD2). We investigated the capacity of GSTD2 to bind tasting molecules. Among them, we found that isothiocyanates (ITC), insecticidal compounds naturally present in cruciferous plant and perceived as bitter, are good substrates for GSTD2. The X-ray structure of GSTD2 was solved, showing the absence of the classical Ser catalytic residue, conserved in the Delta and Epsilon GSTs. Using molecular dynamics, the interaction of ITC with the GSTD2 three-dimensional structure is analysed and discussed. These findings allow us to consider a biological role for GSTD2 in chemoperception, considering GSTD2 expression in the chemosensory organs and the potential consequences of insect exposure to ITC.