Three-dimensional structural analysis and development of new inhibitors of glucan synthetase produced by cariogenic oral streptococci

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Insoluble glucan synthetase (GTF-I) of Streptococcus sobrinus, a member of oral streptococci, is an enzyme that splits sucrose into glucose and fructose and synthesizes water-insoluble glucan. For structure-function studies of this enzyme, we constructed overexpression systems of the GTF-I and its deletion mutants, established efficient purification methods, and investigated optimum conditions for crystallization. The GTF-I is composed two domains, N-terminal catalytic domain (GS) and C-terminal dextran-binding domain (DXB). Prior to crystallization, we constructed the Escherichia coli expression systems of three polypeptides, the complete peptide GTF-I, the N-terminal 83 amino acids-deleted GS-6R, and the only catalytic domain GS. Each polypeptide was highly purified with high efficiencies by combinations of ammonium sulfate precipitation, ion-exchange chromatography, affinity chromatography, and gel-filtration chromatography. Optimum conditions (pH, salt concentrations and the presence of precipitant agents) for crystallization were screened for each polypeptide using hanging-drop vapor diffusion method. In the complete GTF-I, micro-crystals were obtained in a pH range of pH 6-8 and in salt concentrations of 0-0.2 M. However, crystal formation of GS-6R protein was not observed in any conditions tested. In GS, many micro-crystals were formed in similar conditions to those in GTF-I. Furthermore, to reduce the effects of the flexibility of DXB domain, crystallization was performed at low temperature or at the presence of dextran. However, the quality of crystals was not improved. Although crystals suitable for X-ray structural analysis have not yet been obtained, further screening will lead to achieve high-quality crystals of this enzyme.