GLUTATHIONE-S-TRANSFERASE CLASS-KAPPA - CHARACTERIZATION BY THE CLONING OF RAT MITOCHONDRIAL GST AND IDENTIFICATION OF A HUMAN HOMOLOG

We have isolated a cDNA clone that encodes rat glutathione S-transfera se (GST) subunit 13, a GST originally isolated from rat liver mitochon drial matrix by Harris, Meyer, Coles and Ketterer [(1991) Biochem. J. 278, 137-141]. The 896 bp cDNA contains an open reading frame of 678 b p encoding a deduced protein sequence of which the first 33 residues ( excluding the initiation methionine residue) correspond to the N-termi nal sequence reported by Harris er al. Hence like many other nuclear-e ncoded, mitochondrially located proteins, there is no cleavable mitoch ondrial presequence at the N-terminus. GST subunit 13 was originally p laced into the Theta class of GSTs on the basis of sequence identity a t the N-terminus; however, this is the only identity with the Theta cl ass and in fact GST subunit 13 shows little sequence similarity to any of the known GST classes. Most importantly it lacks the SNAIL/TRAIL m otif that has so far been a characteristic of soluble GSTs, although i t does possess a second motif (FGXXXXVXXVDGXXXXXF) reported for GST-re lated proteins (Koonin, Mushegian, Tatusov, Altschul, Bryant, Pork and Valencia [(1994) Protein Sci. 3, 2045-2054]. Southern and Northern bl ot analyses of rat DNA and mRNA are consistent with GST subunit 13's b eing the product of a single hybridizing gene locus. Searches of EST d atabases identified numerous similar human DNA sequences and a single pig sequence. We have derived a human cDNA sequence from these EST seq uences which shows a high nucleotide similarity (77 %) to rat GST subu nit 13. The largest open reading frame is identical in length with sub unit 13 and yields a deduced protein sequence identity of 70%. Most un usually the 3' non-coding nucleotide sequence identity is also 77 %. W e conclude that these cDNAs belong to a novel GST class hereby designa ted Kappa, with the rat GST subunit 13 gene designated rGSTK1 and the human gene being called hGSTK1.