Abstract
Detoxification is a fundamental cellular stress defense mechanism, which allows an organism to survive or even thrive in the presence of environmental toxins and/or pollutants. The glutathione S-transferase (GST) superfamily is a set of enzymes involved in the detoxification process. This highly diverse protein superfamily is characterized by multiple gene duplications, with over 40 GST genes reported in some insects. However, less is known about the GST superfamily in marine organisms, including crustaceans. The availability of two de novo transcriptomes for the copepod, Calanus finmarchicus, provided an opportunity for an in depth study of the GST superfamily in a marine crustacean. The transcriptomes were searched for putative GST-encoding transcripts using known GST proteins from three arthropods as queries. The identified transcripts were then translated into proteins, analyzed for structural domains, and annotated using reciprocal BLAST analysis. Mining the two transcriptomes yielded a total of 41 predicted GST proteins belonging to the cytosolic, mitochondrial or microsomal classes. Phylogenetic analysis of the cytosolic GSTs validated their annotation into six different subclasses. The predicted proteins are likely to represent the products of distinct genes, suggesting that the diversity of GSTs in C. finmarchicus exceeds or rivals that described for insects. Analysis of relative gene expression in different developmental stages indicated low levels of GST expression in embryos, and relatively high expression in late copepodites and adult females for several cytosolic GSTs. A diverse diet and complex life history are factors that might be driving the multiplicity of GSTs in C. finmarchicus, as this copepod is commonly exposed to a variety of natural toxins. Hence, diversity in detoxification pathway proteins may well be key to their survival.
Highlights
The activation of multiple cellular stress defense mechanisms, including an increase in the activity of detoxification enzymes, is key to an organism’s ability to survive, and sometimes even thrive, in environments characterized by the presence of toxins and/or pollutants [1]
A total of 39 putative glutathione S-transferase (GST)-encoding transcripts were retrieved from the Gulf of Maine C. finmarchicus transcriptome using known GSTs from the crustaceans T. japonicus and D. pulex and the insect D. melanogaster as queries (Table 1)
We found a similar pattern in C. finmarchicus, where even cytosolic GSTs with identical top hits were quite different from each other, with amino acid identity ranging from 27%-60%, supporting the conclusion that each of the 34 cytosolic GSTs represents a transcript from a separate gene
Summary
The activation of multiple cellular stress defense mechanisms, including an increase in the activity of detoxification enzymes, is key to an organism’s ability to survive, and sometimes even thrive, in environments characterized by the presence of toxins and/or pollutants [1]. In the second phase (Phase II), the modified toxicant is enzymatically conjugated to a polar molecule. In the final phase of the detoxification process (Phase III), efflux transporters that recognize conjugated toxins remove the modified xenobiotic from the cell. Among the key enzymes for Phase II of the detoxification process are members of the glutathione S-transferase (GST) superfamily [3]. GSTs catalyze the conjugation of reduced glutathione (GSH) to hydrophobic xenobiotics, such as naturally occurring toxins and anthropogenically derived pharmaceuticals and pesticides [5]. The coupling of the xenobiotic to GSH increases the solubility of the toxin, facilitating its excretion [5]
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