Members of the RNase III family are post-transcriptional regulators of gene expression. They are double-stranded (ds) RNA-specific endoribonucleases characterized by a signature motif in their active centers and a two-nucleotide 3' overhang in their products. While Dicer is currently the focus of intense interest, the structurally simpler bacterial RNase III serves as a paradigm for the entire family. After providing the first glimpse at the active center of RNase III in 2001 and revealing the architecture of its noncatalytic complexes with dsRNA over the years, we have recently determined the structure of RNase III-product complex. The structural information of protein-dsRNA interactions and the mechanism of dsRNA processing by RNase III can be extrapolated to other family members, including Rnt1p, Drosha, and Dicer. Stringent starvation protein A (SspA) is a transcriptional activator for the lytic development of phage P1 and is essential for stationary phase-induced acid tolerance in Escherichia coli. We have determined the crystal structure of Yersinia pestis SspA (Yp-SspA), which is 83% identical to E. coli SspA (Ec-SspA) in amino acid sequence and is functionally complementary in supporting the lytic growth of phage P1 and acid resistance of an Ec-SspA mutant. The structure reveals that SspA assumes the characteristic fold of glutathione S-transferase (GST). However, SspA lacks GST activity and does not bind glutathione. It appears that a surface pocket of SspA is important for its function. The size, shape, and property of the pocket suggest that it mediates protein-protein interactions. Glutathione S-transferase (GST) catalyzes glutathione conjugation with electrophilic compounds. In preneoplastic and neoplastic cells, specific forms of GST are expressed at high levels and to participate in the cells' resistance to anticancer drugs. Class pi GST (GSTP) is of particular importance in biological resistance to alkylating agents. Therapeutic strategies aimed at inhibiting GSTP to extend the efficacy of alkylating agents have been unsuccessful, whereas GSTP-activated prodrugs have shown great potential. We have developed a new type of anticancer agents, which function by releasing nitric oxide inside cancer cells. Using structure-based approach, GSTP specificity has been achieved with two structural modifications of a lead compound. Other molecular targets/agents in the pipeline include folate and shikimate pathway enzymes and virus-neutralizing molecules.
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