DETERMINATION OF A BINDING-SITE FOR A NONSUBSTRATE LIGAND IN MAMMALIAN CYTOSOLIC GLUTATHIONE S-TRANSFERASES BY MEANS OF FLUORESCENCE-RESONANCE ENERGY-TRANSFER

To determine the location of the non-substrate-ligand-binding region i n mammalian glutathione S-transferases. fluorescence-resonance energy transfer was used to calculate distances between tryptophan residues a nd protein-bound 8-anilinonaphthalene 1-sulphonate (an anionic ligand) in the human class-a glutathione S-transferase, and in a human Trp28- ->Phe mutant class-pi glutathione S-transferase. Distance values of 2. 21 nm and 1.82 nm were calculated for the class-alpha and class-pi enz ymes, respectively. Since glutathione S-transferases bind one non-subs trate ligand/protein dimer, the ligand-binding region, according to th e calculated distances, is found to be located in the dimer interface near the twofold ads. This region is the same as that in which the par asitic helminth Schistosoma japonicum glutathione S-transferase binds praziquantel, a non-substrate drug used to treat schistosomiasis [McTi gue, M. A., Williams, D. R. & Tainer, J. A. (1995) J. Mol. Biol. 246, 21-27]. Since the overall folding topology is conserved and certain fe atures at the dimer interface are similar throughout the superfamily, it is reasonable to expect that all cytosolic glutathione S-transferas es bind non-substrate ligands in the amphipathic groove at the dimer i nterface.