Sequence features, expression profiles and biochemical characteristics of a sigma class glutathione S-transferase gene (AiGSTσ) from bay scallop Argopecten irradians

Glutathione S-transferases (GSTs) are a class of enzymes that facilitate the detoxification of xenobiotics and also play important roles in innate immunity. In the present study, a novel sigma class GST gene (designated as AiGST sigma) was cloned from the bay scallop Argopecten irradians via rapid amplification of cDNA ends (RACE) technique. The complete cDNA sequence of AiGSTs consisted of a 5' untranslated regions (UTR) of 48 bp, a 3' UTR of 113 bp with a poly A tail and an open reading frame (ORF) of 618 bp. The ORF encoded a polypeptide of 205 amino acid residues with a calculated molecular mass of approximately 23.11 kDa and a theoretical isoelectric point of 5.354. The deduced amino acid sequence of AiGSTs contained a GST_N domain and a GST_C domain, and exhibited high similarity with other reported sigma class GSTs. In the phylogenetic tree, AiGSTs was located in the sigma class GSTs sub-branch. The AiGSTs mRNA transcripts were constitutively expressed in the tissues of hemocytes, muscle, mantle, gill, hepatopancreas and gonad, with the highest expression level in hemocytes, and the mRNA expression levels of AiGSTs were significantly up-regulated in hemocytes after various pathogen associated molecular patterns (PAMPs) stimulation. The purified recombinant AiGSTs protein exhibited catalytic activity against the common substrate 1-chloro-2, 4-dinitrobenzene (CDNB) with low thermal stability and narrow optimum pH spectrum. All these results indicated that AiGSTs was a fragile but efficient antioxidant enzyme and was potentially involved in the innate immune responses of scallop.