Discovery and Confirmation of Ligand Binding Specificities of the Schistosoma japonicum Polarity Protein Scribble

BACKGROUND: Schistosomiasis is a chronic debilitating parasitic disease that afflicts more than 200 million individuals worldwide. Long-term administration of chemotherapy with the single available drug, praziquantel, has led to growing concerns about drug resistance. The PSD-95/Dlg/ZO-1 (PDZ) domain is an important module found in many scaffolding proteins, which has been recognized as promising targets for the development of novel drugs. However, the parasite-derived PDZ domains and their associated functions are still largely unknown. METHODOLOGY/PRINCIPAL FINDINGS: The gene encoding the Schistosoma japonicum Scribble protein (SjScrib) was identified by homologous search with the S. mansoni Scrib sequence. By screening an arbitrary peptide library in yeast two-hybrid (Y2H) assays, we identified and confirmed the ligand binding specificity for each of the four PDZ domains of SjScrib. Both SjScrib-PDZ1 and SjScrib-PDZ3 recognize type I C-terminal PDZ-domain binding motifs (PBMs), which can be deduced as consensus sequences of -[Φ][x][E][TS][x][ILF] and -[x][RKx][E(TS)][T][WΦ][ILV], respectively. SjScrib-PDZ2 prefers stringent type II C-terminal PBMs, which significantly differs from that of its human ortholog. SjScrib-PDZ4 binds to typical II C-terminal PBMs with a consensus sequence -[x][F(W)][x][LI][x][LIV], in which the aromatic residue Phe is predominantly selected at position -4. The irregular and unconventional internal ligand binding specificities for the PDZ domains of SjScrib were confirmed by point mutations of the key amino acids within the ligand binding motifs. We also compared the differences in ligand specificities between SjScrib-PDZs and hScrib-PDZs, and explored the structural basis for the ligand binding properties of SjScrib-PDZs. CONCLUSIONS/SIGNIFICANCE: In this study, we characterized and confirmed the ligand binding specificities of all four PDZ domains of SjScrib for the first time. We denoted the differential ligand binding specificities between SjScrib-PDZs and hScrib-PDZs as well as the structural basis for these properties. This work may provide a fundamental basis for the rational design of novel anti-schistosomal drugs.