CRYSTAL-STRUCTURES OF A SCHISTOSOMAL DRUG AND VACCINE TARGET - GLUTATHIONE-S-TRANSFERASE FROM SCHISTOSOMA-JAPONICA AND ITS COMPLEX WITH THELEADING ANTISCHISTOSOMAL DRUG PRAZIQUANTEL

Glutathione S-transferase (GST), an essential detoxification enzyme in parasitic helminths, is a major vaccine target and an attractive drug target against schistosomiasis and other helminthic diseases. Crystal structures of the 26 kDa GST from the helminth Schistosoma japonica ( SjGST) have been determined for the unligated enzyme (resolution = 2.4 Angstrom, R-factor = 19.7%) and for the enzyme bound to the leading a ntischistosomal drug praziquantel (resolution = 2.6 Angstrom, R-factor = 21.2%). The protein, recombinantly expressed using the Pharamacia P GEX-3X vector for production of GST fusion proteins, contains all 218 residues of SjGST and an additional 13 residues at the C terminus. The structure of unligated SjGST shows that the glutathione binding site pre-exists unchanged in the ligand-free enzyme and is conserved betwee n parasitic and the mammalian class mu enzymes. At therapeutic concent rations the leading antischistosomal drug praziquantel (PZQ) binds one drug per enzyme homodimer in the dimer interface groove adjoining the two catalytic sites. This establishes a protein target for PZQ, ident ifies the GST non-substrate ligand transport site, and implicates PZQ in steric inhibition of SjGST catalytic and transport for large ligand s. Thus, increased expression or mutagenesis of SjGST by the parasite may confer resistance to PZQ. Differences in the xenobiotic binding re gion between parasitic and mammalian GSTs reveal a distinct substrate repertoire for SjGST and, together with the newly identified PZQ bindi ng site, provide the basis for design of novel antischistosomal drugs. Due to the widespread use expression systems based on SjGST fusions, the atomic structure of SjGST should also provide an important tool fo r phasing fusion protein structures by molecular replacement.