Mu-class glutathione transferase from Xenopus laevis: molecular cloning, expression and site-directed mutagenesis

A cDNA encoding a Mu-class glutathione transferase (XlGSTM1-1) has been isolated from a Xenopus laevis liver library, and its nucleotide sequence has been determined. XlGSTM1-1 is composed of 219 amino acid residues with a calculated molecular mass of 25359 Da. Unlike many mammalian Mu-class GSTs, XlGSTM1-1 has a narrow spectrum of substrate specificity and it is also less effective in conjugating 1-chloro-2,4-dinitrobenzene. A notable structural feature of XlGSTM1-1 is the presence of the Cys-139 residue in place of the Glu-139, as well as the absence of the Cys-114 residue, present in other Mu-class GSTs, which is replaced by Ala. Site-directed mutagenesis experiments indicate that Cys-139 is not involved in the catalytic mechanism of XlGSTM1-1 but may be in part responsible for its structural instability, and experiments in vivo confirmed the role of this residue in stability. Evidence indicating that Arg-107 is essential for the 1-chloro-2,4-dinitrobenzene conjugation capacity of XlGSTM1-1 is also presented.