title: Crystal structure of the C-terminal region of Streptococcus mutans antigen I/II and characterization of salivary agglutinin adherence domains.
abstract: The Streptococcus mutans antigen I/II (AgI/II) is a cell surface-localized protein that adheres to salivary components and extracellular matrix molecules. Here we report the 2.5 Å resolution crystal structure of the complete C-terminal region of AgI/II. The C-terminal region is comprised of three major domains: C(1), C(2), and C(3). Each domain adopts a DE-variant IgG fold, with two β-sheets whose A and F strands are linked through an intramolecular isopeptide bond. The adherence of the C-terminal AgI/II fragments to the putative tooth surface receptor salivary agglutinin (SAG), as monitored by surface plasmon resonance, indicated that the minimal region of binding was contained within the first and second DE-variant-IgG domains (C(1) and C(2)) of the C terminus. The minimal C-terminal region that could inhibit S. mutans adherence to SAG was also confirmed to be within the C(1) and C(2) domains. Competition experiments demonstrated that the C- and N-terminal regions of AgI/II adhere to distinct sites on SAG. A cleft formed at the intersection between these C(1) and C(2) domains bound glucose molecules from the cryo-protectant solution, revealing a putative binding site for its highly glycosylated receptor SAG. Finally, electron microscopy images confirmed the elongated structure of AgI/II and enabled building a composite tertiary model that encompasses its two distinct binding regions.
authors.affiliation: Department of Physiology and Biophysics, University of Alabama, Birmingham, Alabama 35294, USA.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Larson
authors.forename: Kanagalaghatta R
authors.initials: KR
authors.lastname: Rajashankar
authors.forename: Paula J
authors.initials: PJ
authors.lastname: Crowley
authors.forename: Charles
authors.initials: C
authors.lastname: Kelly
authors.forename: Tim J
authors.initials: TJ
authors.lastname: Mitchell
authors.forename: L Jeannine
authors.initials: LJ
authors.lastname: Brady
authors.forename: Champion
authors.initials: C
authors.lastname: Deivanayagam
mesh_terms: D018829:Adhesins, Bacterial / Q000737:chemistry; D000373:Agglutinins / Q000737:chemistry*; D000942:Antigens, Bacterial / Q000737:chemistry*; D001422:Bacterial Adhesion; D001665:Binding Sites; D018360:Crystallography, X-Ray / Q000379:methods; D017886:Durapatite / Q000737:chemistry; D006801:Humans; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012463:Saliva / Q000378:metabolism*; D013295:Streptococcus mutans / Q000378:metabolism*; D020349:Surface Plasmon Resonance
chemical_list: D018829:Adhesins, Bacterial; D000373:Agglutinins; D000942:Antigens, Bacterial; D017886:Durapatite