Preparation, crystallization and preliminary X-ray analysis of the methionine synthase (MetE) from Streptococcus mutans

The Streptococcus mutans metE gene encodes methionine synthase (MetE), which catalyzes the direct transfer of a methyl group from methyltetrahydrofolate to homocysteine in the last step of methionine synthesis. metE was cloned into pET28a and the gene product was expressed at high levels in the Escherichia coli strain BL21 (DE3). MetE was purified to homogeneity using Ni²⁺-chelating chromatography followed by size-exclusion chromatography. Crystals of the protein were obtained by the hanging-drop vapour-diffusion method and diffracted to 2.2 Å resolution. The crystal belongs to space group P2₁, with unit-cell parameters a = 52.85, b = 99.48, c = 77.88 Å, β = 94.55°.