Preliminary X-ray crystallographic analysis of SMU.2055 protein from the caries pathogen Streptococcus mutans

The SMU.2055 gene from the major caries pathogen Streptococcus mutans is annotated as a putative acetyltransferase with 163 amino‐acid residues. In order to identify its function via structural studies, the SMU.2055 gene was cloned into the expression vector pET28a. Native and SeMet‐labelled SMU.2055 proteins with a His6 tag at the N‐terminus were expressed at a high level in Escherichia coli strain BL21 (DE3) and purified to homogeneity by Ni2+‐chelating affinity chromatography. Diffraction‐quality crystals of SeMet‐labelled SMU.2055 were obtained using the sitting‐drop vapour‐diffusion method and diffracted to a resolution of 2.5 Å on beamline BL17A at the Photon Factory, Tsukuba, Japan. The crystals belong to the orthorhombic space group C2221, with unit‐cell parameters a = 92.0, b = 95.0, c = 192.2 Å. The asymmetric unit contained four molecules, with a solvent content of 57.1%.