A novel molluscan sigma-like glutathione S-transferase from Manila clam, Ruditapes philippinarum: Cloning, characterization and transcriptional profiling

Glutathione S-transferases (GSTs) are versatile enzymes, act as primary intracellular detoxifiers and contribute to a broad range of physiological processes including cellular defense. In this study, a full-length cDNA representing a novel sigma-like GST was identified from Manila clam, Ruditapes philippinarum (RpGST sigma). RpGST sigma (884 bp) was found to possess an open reading frame of 609 bp. The encoded polypeptide (203 amino acids) had a predicted molecular mass of 23.21 kDa and an isoelectric point of 7.64. Sequence analysis revealed two conserved GST domain profiles in N- and C-termini. Alignment studies revealed that the identity between deduced peptides of RpGST sigma and known GST sigma members was relatively low (<35%), except a previously identified Manila clam GST sigma isoform (87.2%). Phylogenetic analysis indicated that RpGST sigma clustered together with molluscan GST sigma homologs, which were closely related to insect GST sigma s. The RpGST sigma was subsequently cloned and expressed as recombinant protein, in order to characterize its biological activity. The recombinant RpGST sigma exhibited characteristic glutathione conjugating catalytic activity toward 1-chloro-2,4-dinitrobenzene, 3,4-dichloronitrobenzene and ethacrynic acid. It had an optimal pH and temperature of 8.0 and 35 therefore C, respectively. Expression profiles under normal conditions and in response to lipopolysaccharide-, poly I:C- and Vibrio tapetis-challenges were also investigated. RpGST sigma demonstrated a differential tissue distribution with robust transcription in gills of normal animals. We explored potential association of GST sigma in cellular defense during bacterial infection and found that in challenged clams, RpGST sigma gene was significantly induced in internal and external tissues, in conjunction with manganese- as well as copper-zinc superoxide dismutase (MnSOD and CuZnSOD) genes. Moreover, the induction was remarkably higher in hemocytes than in gill. Collectively, our findings suggested that RpGST alpha could play a significant role in cellular defense against oxidative stress caused by bacteria, in conjunction with other antioxidant enzymes, such as SODs. 2012 Elsevier Inc. All rights reserved.