Crystal Structure of the V-region of Streptococcus mutans Antigen I/II at 2.4 Å Resolution Suggests a Sugar Preformed Binding Site

Antigens I/II are large multifunctional adhesins from oral viridans streptococci that exert immunomodulatory effects on human cells and play important roles in inflammatory disorders. Among them, Streptococcus mutans plays a major role in the initiation of dental caries. The structure of the V-region (SrV+, residues 464–840) of the antigen I/II of S. mutans has been determined using the multiwavelength anomalous diffraction phasing technique with seleno-methionine-substituted recombinant protein and subsequently refined at 2.4 Å resolution. The crystal structure of SrV+ revealed a lectin-like fold that displays a putative preformed carbohydrate-binding site stabilized by a metal ion. Inhibition of this binding site may confer to humans a protection against dental caries and dissemination of the bacteria to extra-oral sites involved in life-threatening inflammatory diseases. This crystal structure constitutes a first step in understanding the structure–function relationship of antigens I/II and may help in delineating new preventive or therapeutic strategies against colonization of the host by oral streptococci.