Streptococcus mutans, a facultatively aerobic and gram-positive bacterium, has been implicated as the most important cariogenic bacterium among the bacterial species present in oral flora which cause dental caries. The virulence factors ofS. mutans which contribute to dental caries include carbohydrate metabolism, tolerance and production of acids, synthesis of cell polysaccharides and adhesion ability.
Reports on aromatic amino acid aminotransferase (AroAT) participating in nitrogen and carbon metabolism have been published for years. In this study, S. mutans AroAT was recombinantly expressed in Escherichia coli. An effective purification protocol was established. Bioactivity assay indicated that AroAT had aminotransferase activity. Far-UV circular dichroism (CD) spectroscopic analysis showed that the protein was α-helical in structure under physiological conditions, and stable at pH 7.0 and 8.0 at temperature ≤40°C. The recombinant protein was crystallized using the hanging-drop vapour-diffusion method with PEG 3350 as the primary precipitant. The structure of AroAT was solved at 2.15 Å resolution by molecular replacement method. Comparison with other structures of homologous proteins enables us to identify conserved structural elements that might play a role in substrate binding. This work has provided structural information for understanding the pathogenic mechanism of S. mutans and for developing vaccine against S. mutans.