A new class of glutathione transferases has been discovered by analysis of the expressed sequence tag data base and sequence alignment. Glutathione S-transferases (GSTs) of the new class, named Omega, exist in several mammalian species and Caenorhabditis elegans. In humans, GSTO 1-1 is expressed in most tissues and exhibits glutathione-dependent thiol transferase and dehydroascorbate reductase activities characteristic of the glutaredoxins. The structure of GSTO 1-1 has been determined at 2.0-Å resolution and has a characteristic GST fold (Protein Data Bank entry code 1eem). The Omega class GSTs exhibit an unusual N-terminal extension that abuts the C terminus to form a novel structural unit. Unlike other mammalian GSTs, GSTO 1-1 appears to have an active site cysteine that can form a disulfide bond with glutathione.

Sequence alignment and phylogenetic analysis has identified a new subgroup of glutathione S-transferase (GST)-like proteins from a range of species extending from plants to humans. This group has been termed the Zeta class. An atomic model of the N-terminal domain suggests that the members of the Zeta class have a similar structure to that of other GSTs, binding glutathione in a similar orientation in the G site. Recombinant human GSTZ1-1 has been expressed in Escherichia coli and characterized. The protein is a dimer composed of 24.2 kDa subunits and has minimal glutathione-conjugating activity with ethacrynic acid and 7-chloro-4-nitrobenz-2-oxa-1, 3-diazole. Although low in comparison with other GSTs, GSTZ1-1 has glutathione peroxidase activity with t-butyl and cumene hydroperoxides. The members of the Zeta class have been conserved over a long evolutionary period, suggesting that they might have a role in the metabolism of a compound that is common in many living cells.

The ubiquitous glutathione transferases (GSTs) catalyze glutathione conjugation to many compounds and have other diverse functions that continue to be discovered. We noticed sequence similarities between Omega class GSTs and a nuclear chloride channel, NCC27 (CLIC1), and show here that NCC27 belongs to the GST structural family. The structural homology prompted us to investigate whether the human Omega class glutathione transferase GSTO1-1 forms or modulates ion channels. We find that GSTO1-1 modulates ryanodine receptors (RyR), which are calcium channels in the endoplasmic reticulum of various cells. Cardiac RyR2 activity was inhibited by GSTO1-1, whereas skeletal muscle RyR1 activity was potentiated. An enzymatically active conformation of GSTO1-1 was required for inhibition of RyR2, and mutation of the active site cysteine (Cys-32 3 Ala) abolished the inhibitory activity. We propose a novel role for GSTO1-1 in protecting cells containing RyR2 from apoptosis induced by Ca 2؉ mobilization from intracellular stores.Glutathione transferases (GSTs) 1 are a family of ubiquitous intracellular enzymes that catalyze the conjugation of glutathione to many exogenous and endogenous compounds (1). GSTs are known to have other functions including the binding of bilirubin and carcinogens (2), the isomerization of maleylacetoacetate (3), and the regulation of stress kinases (4), with presumably further roles yet to be discovered. New members of the GST structural family with novel catalytic activities and functions have recently been discovered (5-7). For example, the Omega class glutathione transferase GSTO1-1 has a typical glutathione transferase fold but little enzymatic activity with many conventional substrates (7). Unlike other mammalian glutathione transferases that have active site tyrosine or serine residues (8), GSTO1-1 has a novel active site cysteine that participates in weak thiol transferase reactions. Although the intracellular function of the Omega class GSTs is unknown, a member of the Omega class is over-expressed in a radiationresistant mouse lymphoma cell line (9).We used BLAST searches (10) to identify additional members of the glutathione transferase structural family and were impressed by sequence similarities between GSTO1-1 and the chloride intracellular channel (CLIC) family of proteins, which are thought to form chloride channels in intracellular membranes or to be chloride channel modulators (11, 12). We therefore compared the structure of the CLIC proteins and GSTO1-1 in more detail and found that NCC27 (CLIC1) belongs to the GST structural family. Because of the structural similarity, we also examined the ability of GSTO1-1 to form or modulate ion channels. We find that GSTO1-1 modulates ryanodine receptors (RyRs), which are the calcium release channels in skeletal and cardiac sarcoplasmic reticulum (SR). There is evidence that GSTO1-1 is present in skeletal and cardiac muscle (7) and is thus colocalized with RyRs. RyRs are also located in intracellular membranes of a variety of cells (13) and...

hGST T2-2 shares less than 15% sequence identity with other GST classes, yet adopts a similar three-dimensional fold. The C-terminal extension that blocks the active site is not disordered in either the apo or complexed forms of the enzyme, but nevertheless catalysis occurs in the crystalline state. A narrow tunnel leading from the active site to the surface may provide a pathway for the entry of substrates and the release of products. The results suggest a molecular basis for the unique sulfatase activity of this GST.

The high level of polymorphism in major histocompatibility complex (MHC) molecules leads to many allele-specific peptide binding repertoires that can generally be characterized by sequence motifs. Such motifs have previously been elucidated experimentally for several MHC molecules and shown to bind in specificity pockets in the antigen binding cleft. Here, a new and less restrictive description of the traditional antigen binding pockets is derived. These regions are referred to as peptide binding environments and are defined as those residues in a fixed neighborhood of the peptide residues in known crystal structure complexes. By examining the antigen binding environments from MHC molecules with known motifs, we made predictions as to likely motifs for other MHC molecules which share the same environments. The predictions are presented in the form of Tables and are pertinent to class I HLA-A, HLA-B, and HLA-C MHC sequences, and are shown to correlate well with experiments.