Micro-plasticity of genomes as illustrated by the evolution of glutathione transferases in 12 Drosophila species

Structural Analysis of the Drosophila melanogaster GSTome

Glutathione transferase (GST) is a superfamily of ubiquitous enzymes, multigenic in numerous organisms and which generally present homodimeric structures. GSTs are involved in numerous biological functions such as chemical detoxification as well as chemoperception in mammals and insects. GSTs catalyze the conjugation of their cofactor, reduced glutathione (GSH), to xenobiotic electrophilic centers. To achieve this catalytic function, GSTs are comprised of a ligand binding site and a GSH binding site per subunit, which is very specific and highly conserved; the hydrophobic substrate binding site enables the binding of diverse substrates. In this work, we focus our interest in a model organism, the fruit fly Drosophila melanogaster (D. mel), which comprises 42 GST sequences distributed in six classes and composing its GSTome. The goal of this study is to describe the complete structural GSTome of D. mel to determine how changes in the amino acid sequence modify the structural characteristics of GST, particularly in the GSH binding sites and in the dimerization interface. First, we predicted the 3D atomic structures of each GST using the AlphaFold (AF) program and compared them with X-ray crystallography structures, when they exist. We also characterized and compared their global and local folds. Second, we used multiple sequence alignment coupled with AF-predicted structures to characterize the relationship between the conservation of amino acids in the sequence and their structural features. Finally, we applied normal mode analysis to estimate thermal B-factors of all GST structures of D. mel. Particularly, we extracted flexibility profiles of GST and identify key residues and motifs that are systematically involved in the ligand binding/dimerization processes and thus playing a crucial role in the catalytic function. This methodology will be extended to guide the in silico design of synthetic GST with new/optimal catalytic properties for detoxification applications.

Role of Insect and Mammal Glutathione Transferases in Chemoperception

Glutathione transferases (GSTs) are ubiquitous key enzymes with different activities as transferases or isomerases. As key detoxifying enzymes, GSTs are expressed in the chemosensory organs. They fulfill an essential protective role because the chemosensory organs are located in the main entry paths of exogenous compounds within the body. In addition to this protective function, they modulate the perception process by metabolizing exogenous molecules, including tastants and odorants. Chemosensory detection involves the interaction of chemosensory molecules with receptors. GST contributes to signal termination by metabolizing these molecules. By reducing the concentration of chemosensory molecules before receptor binding, GST modulates receptor activation and, therefore, the perception of these molecules. The balance of chemoperception by GSTs has been shown in insects as well as in mammals, although their chemosensory systems are not evolutionarily connected. This review will provide knowledge supporting the involvement of GSTs in chemoperception, describing their localization in these systems as well as their enzymatic capacity toward odorants, sapid molecules, and pheromones in insects and mammals. Their different roles in chemosensory organs will be discussed in light of the evolutionary advantage of the coupling of the detoxification system and chemosensory system through GSTs.

Shortened lifespan induced by a high-glucose diet is associated with intestinal immune dysfunction in Drosophila sechellia

Organisms can generally be divided into two nutritional groups: generalists that consume various types of food and specialists that consume specific types of food. However, it remains unclear how specialists adapt to only limited nutritional conditions in nature. In this study, we addressed this question by focusing on Drosophila fruit flies. The generalist Drosophila melanogaster can consume a wide variety of foods that contain high glucose levels. In contrast, the specialist Drosophila sechellia consumes only the Indian mulberry, known as noni (Morinda citrifolia), which contains relatively little glucose. We showed that the lifespan of D. sechellia was significantly shortened under a high-glucose diet, but this effect was not observed for D. melanogaster. In D. sechellia, a high-glucose diet induced disorganization of the gut epithelia and visceral muscles, which was associated with abnormal digestion and constipation. RNA-sequencing analysis revealed that many immune-responsive genes were suppressed in the gut of D. sechellia fed a high-glucose diet compared with those fed a control diet. Consistent with this difference in the expression of immune-responsive genes, high glucose-induced phenotypes were restored by the addition of tetracycline or scopoletin, a major nutritional component of noni, each of which suppresses gut bacterial growth. We propose that, in D. sechellia, a high-glucose diet impairs gut immune function, which leads to a change in gut microbiota, disorganization of the gut epithelial structure and a shortened lifespan.

Expression Patterns of Drosophila Melanogaster Glutathione Transferases

Glutathione transferases (GSTs) are ubiquitous enzymes that catalyze the conjugation of glutathione to various molecules. Among the 42 GSTs identified in Drosophila melanogaster, Delta and Epsilon are the largest classes, with 25 members. The Delta and Epsilon classes are involved in different functions, such as insecticide resistance and ecdysone biosynthesis. The insect GST number variability is due mainly to these classes. Thus, they are generally considered supports during the evolution for the adaptability of the insect species. To explore the link between Delta and Epsilon GST and their evolution, we analyzed the sequences using bioinformatic tools. Subgroups appear within the Delta and Epsilon GSTs with different levels of diversification. The diversification also appears in the sequences showing differences in the active site. Additionally, amino acids essential for structural stability or dimerization appear conserved in all GSTs. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the transcripts corresponding to these two classes are heterogeneously expressed within D. melanogaster. Some GSTs, such as GSTD1, are highly expressed in all tissues, suggesting their general function in detoxification. Conversely, some others, such as GSTD11 or GSTE4, are specifically expressed at a high level specifically in antennae, suggesting a potential role in olfaction.

Delta class glutathione S-transferase (TuGSTd01) from the two-spotted spider mite Tetranychus urticae is inhibited by abamectin

GSTs (Glutathione S-transferases) are known to catalyze the nucleophilic attack of the sulfhydryl group of reduced glutathione (GSH) on electrophilic centers of xenobiotic compounds, including insecticides and acaricides. Genome analyses of the polyphagous spider mite herbivore Tetranychus urticae (two-spotted spider mite) revealed the presence of a set of 32 genes that code for secreted proteins belonging to the GST family of enzymes. To better understand the role of these proteins in T. urticae, we have functionally characterized TuGSTd01. Moreover, we have modeled the structure of the enzyme in apo form, as well as in the form with bound inhibitor. We demonstrated that this protein is a glutathione S-transferase that can conjugate glutathione to 1-chloro-2,4-dinitrobenzene (CDNB). We have tested TuGSTd01 activity with a range of potential substrates such as cinnamic acid, cumene hydroperoxide, and allyl isothiocyanate; however, the enzyme was unable to process these compounds. Using mutagenesis, we showed that putative active site variants S11A, E66A, S67A, and R68A mutants, which were residues predicted to interact directly with GSH, have no measurable activity, and these residues are required for the enzymatic activity of TuGSTd01. There are several reports that associate some T. urticae acaricide resistance with increased activity of GSTs . However, we found that TuGSTd01 is not able to detoxify abamectin; in fact, the acaricide inhibits the enzyme with K_i = 101 μM. Therefore, we suggest that the increased GST activity observed in abamectin resistant T. urticae field populations is a part of the compensatory feedback loop. In this case, the increased production of GSTs and relatively high concentration of GSH in cells allow GSTs to maintain physiological functions despite the presence of the acaricide.

Phylogeny of the Genus Drosophila

Understanding phylogenetic relationships among taxa is key to designing and implementing comparative analyses. The genus Drosophila, which contains over 1600 species, is one of the most important model systems in the biological sciences. For over a century, one species in this group, Drosophila melanogaster, has been key to studies of animal development and genetics, genome organization and evolution, and human disease. As whole-genome sequencing becomes more cost-effective, there is increasing interest in other members of this morphologically, ecologically, and behaviorally diverse genus. Phylogenetic relationships within Drosophila are complicated, and the goal of this paper is to provide a review of the recent taxonomic changes and phylogenetic relationships in this genus to aid in further comparative studies.

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.

GC Content Heterogeneity Transition of Conserved Noncoding Sequences Occurred at the Emergence of Vertebrates

Conserved non-coding sequences (CNSs) of Eukaryotes are known to be significantly enriched in regulatory sequences. CNSs of diverse lineages follow different patterns in abundance, sequence composition, and location. Here, we report a thorough analysis of CNSs in diverse groups of Eukaryotes with respect to GC content heterogeneity. We examined 24 fungi, 19 invertebrates, and 12 non-mammalian vertebrates so as to find lineage specific features of CNSs. We found that fungi and invertebrate CNSs are predominantly GC rich as in plants we previously observed, whereas vertebrate CNSs are GC poor. This result suggests that the CNS GC content transition occurred from the ancestral GC rich state of Eukaryotes to GC poor in the vertebrate lineage due to the enrollment of GC poor transcription factor binding sites that are lineage specific. CNS GC content is closely linked with the nucleosome occupancy that determines the location and structural architecture of DNAs.

Epsilon glutathione transferases possess a unique class-conserved subunit interface motif that directly interacts with glutathione in the active site

Epsilon class glutathione transferases (GSTs) have been shown to contribute significantly to insecticide resistance. We report a new Epsilon class protein crystal structure from Drosophila melanogaster for the glutathione transferase DmGSTE6. The structure reveals a novel Epsilon clasp motif that is conserved across hundreds of millions of years of evolution of the insect Diptera order. This histidine-serine motif lies in the subunit interface and appears to contribute to quaternary stability as well as directly connecting the two glutathiones in the active sites of this dimeric enzyme.