Molecular identification of glutathione S-transferase gene and cDNAs of two isotypes from northern quahog (Mercenaria mercenaria)

Glutathione S-transferases (GSTs) are universally xenobiotic detoxifying enzymes which have been shown to play a unique detoxifying role in northern quahogs. GST consists of two distinct domains: N-terminal domain which contributes residues to form G-site (GSH-binding site) and C-terminal domain which provides residues to form a hydrophobic H-site (second substrate-binding site). In this study, glutathione S-transferases (GSTs) gene and cDNAs of two isotypes from the northern quahog (Mercenaria mercenaria) were cloned and characterized for the first time. Two full-length GST cDNAs were obtained and named as GST Pi-1 and Pi-2, respectively. Both cDNAs have an open reading frame of 624 bp, which encodes a 207-amino acid protein. Multiple sequences alignment analysis shows that the deduced amino acid sequences of GST Pi-1 and Pi-2 have high homology with other Pi class GSTs. The evolutionary relationship assessment indicates that the two deduced amino acid sequences are closely related to Pi class GSTs. The GST Pi-1 gene is an intronless gene so that it has the same sequence as the transcript. The digested peptide fragments of the purified northern quahog GSTs were analyzed by tandem mass spectrometry and the results confirm the existence of the translation products of the GST Pi-1 and Pi-2 transcripts. The predicted three-dimensional structures of GST Pi-1 and Pi-2 showed that both proteins have two domains, N-terminal domain and C-terminal domain. The conserved phenylalanine-48 serving as the ball in ball and socket style interface in all reported Pi class GSTs was found in both proteins. These results suggest that GST Pi-1 and Pi-2 from the northern quahog belong to the Pi class GSTs which may be involved in polychlorinated biphenyl (PCB) dechlorination.