Protein preparation, crystallization and preliminary X-ray crystallographic analysis of Smu.260 from Streptococcus mutans -: a cariogenic dental pathogen

Smu.260 encodes a putative protein of 200 residues in Streptococcus mutans, a primary pathogen for human dental caries. Smu.260 was cloned into expression vector pET28a and expressed in good amount from the E.coli strain BL21 (DE3). Smu.260 protein was purified to homogeneity in a two-step procedure of Ni2+ chelating and size exclusion chromatography. The purified protein exists in two forms, a dimer form about 46 ku with yellow color and a tetramer form without apparent color. Crystals were obtained from the dimer protein by hanging-drop vapor-diffusion method. The crystals diffracted to about 2.3 angstrom resolution and belong to orthorhombic space group P2(1)2(1)2(1) with cell dimensions of a = 89.88 angstrom, b = 90.91 angstrom, c = 105.17 angstrom. The asymmetric unit is expected to contain two dimers with solvent content of 53%.