Glutathione S-transferase class Kappa: Characterization by the cloning of rat mitochondrial GST and identification of a human homologue

We have isolated a cDNA clone that encodes rat glutathione S-transferase (GST) subunit 13, a GST originally isolated from rat liver mitochondrial matrix by Harris, Meyer, Coles and Ketterer [(1991) Biochem. J. 278, 137-141]. The 896 bp cDNA contains an open reading frame of 678 bp encoding a deduced protein sequence of which the first 33 residues (excluding the initiation methionine residue) correspond to the N-terminal sequence reported by Harris er al. Hence like many other nuclear-encoded, mitochondrially located proteins, there is no cleavable mitochondrial presequence at the N-terminus. GST subunit 13 was originally placed into the Theta class of GSTs on the basis of sequence identity at the N-terminus; however, this is the only identity with the Theta class and in fact GST subunit 13 shows little sequence similarity to any of the known GST classes. Most importantly it lacks the SNAIL/TRAIL motif that has so far been a characteristic of soluble GSTs, although it does possess a second motif (FGXXXXVXXVDGXXXXXF) reported for GST-related proteins (Koonin, Mushegian, Tatusov, Altschul, Bryant, Pork and Valencia [(1994) Protein Sci. 3, 2045-2054]. Southern and Northern blot analyses of rat DNA and mRNA are consistent with GST subunit 13's being the product of a single hybridizing gene locus. Searches of EST databases identified numerous similar human DNA sequences and a single pig sequence. We have derived a human cDNA sequence from these EST sequences which shows a high nucleotide similarity (77 %) to rat GST subunit 13. The largest open reading frame is identical in length with subunit 13 and yields a deduced protein sequence identity of 70%. Most unusually the 3' non-coding nucleotide sequence identity is also 77 %. We conclude that these cDNAs belong to a novel GST class hereby designated Kappa, with the rat GST subunit 13 gene designated rGSTK1 and the human gene being called hGSTK1.