Two intramolecular isopeptide bonds are identified in the crystal structure of the Streptococcus gordonii SspB C-terminal domain.
Streptococcus gordonii is a primary colonizer and is involved in the formation of dental plaque. This bacterium expresses several surface proteins. One of them is the adhesin SspB, which is a member of the Antigen I/II family of proteins. SspB is a large multi-domain protein that has interactions with surface molecules on other bacteria and on host cells, and is thus a key factor in the formation of biofilms. Here, we report the crystal structure of a truncated form of the SspB C-terminal domain, solved by single-wavelength anomalous dispersion to 1.5 A resolution. The structure represents the first of a C-terminal domain from a streptococcal Antigen I/II protein and is comprised of two structurally related beta-sandwich domains, C2 and C3, both with a Ca(2+) bound in equivalent positions. In each of the domains, a covalent isopeptide bond is observed between a lysine and an asparagine, a feature that is believed to be a common stabilization mechanism in Gram-positive surface proteins. S. gordonii biofilms contain attachment sites for the periodontal pathogen Porphyromonas gingivalis and the SspB C-terminal domain has been shown to have one such recognition motif, the SspB adherence region. The motif protrudes from the protein, and serves as a handle for attachment. The structure suggests several additional putative binding surfaces, and other binding clefts may be created when the full-length protein is folded.
Forsgren
Nina
N
Department of Odontology, Umeå University, Umeå, Sweden.
Lamont
Richard J
RJ
Persson
Karina
K
D018829:Adhesins, Bacterial / Q000737:chemistry*; D000595:Amino Acid Sequence; D002118:Calcium / Q000378:metabolism; D018360:Crystallography, X-Ray; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation / Q000235:genetics; D010446:Peptide Fragments / Q000737:chemistry*; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D054773:Streptococcus gordonii / Q000737:chemistry*
D018829:Adhesins, Bacterial; D010446:Peptide Fragments; C068465:salivary agglutinin receptor, Streptocococcus; D002118:Calcium