Crystal structure of the variable domain of the Streptococcus gordonii surface protein SspB

Abstract

The Antigen I/II (AgI/II) family of proteins are cell wall anchored adhesins expressed on the surface of oral streptococci. The AgI/II proteins interact with molecules on other bacteria, on the surface of host cells, and with salivary proteins. Streptococcus gordonii is a commensal bacterium, and one of the primary colonizers that initiate the formation of the oral biofilm. S. gordonii expresses two AgI/II proteins, SspA and SspB that are closely related. One of the domains of SspB, called the variable (V-) domain, is significantly different from corresponding domains in SspA and all other AgI/II proteins. As a first step to elucidate the differences among these proteins, we have determined the crystal structure of the V-domain from S. gordonii SspB at 2.3 Å resolution. The domain comprises a b-supersandwich with a putative binding cleft stabilized by a metal ion. The overall structure of the SspB V-domain is similar to the previously reported V-domain of the Streptococcus mutans protein SpaP, despite their low sequence similarity. In spite of the conserved architecture of the binding cleft, the cavity is significantly smaller in SspB, which may provide clues about the difference in ligand specificity. We also verified that the metal in the binding cleft is a calcium ion, in concurrence with previous biological data. It was previously suggested that AgI/II V-domains are carbohydrate binding. However, we tested that hypothesis by screening the SspB V-domain for binding to over 400 glycoconjucates and found that the domain does not interact with any of the carbohydrates.

Structural and Functional Analysis of Cell Wall-anchored Polypeptide Adhesin BspA in Streptococcus agalactiae

“…BspA-V adopts a fold that is distinct from those reported for other AgI/II family polypeptide V domains (11,12). This consists of two anti-parallel ␤-sheets, S1 and S2, comprising 5 and 7 strands, respectively (Fig.…”

“…The N terminus forms a stabilizing scaffold by wrapping behind the base of the stalk (10). Crystal structures of the V regions of SpaP and SspB have revealed a common architecture consisting of a lectin-like fold with a putative binding cleft (11,12). The structure of the C-terminal subdomains C1, C2, and C3 has also been elucidated by x-ray crystallography and found to consist of ␤-sandwich domains stabilized by isopeptide bonds (13)(14)(15)(16).…”

Interaction of Candida albicans Cell Wall Als3 Protein with Streptococcus gordonii SspB Adhesin Promotes Development of Mixed-Species Communities

“…Interestingly, the SspB polypeptide sequence also contains three potential ␤-aggregation sequences, predicted by the TANGO algorithm (15). One of these (742 to 746 amino acids [aa]) is within the central V region of the protein and, on the basis of the three-dimensional (3D) crystal structure (4), is predicted to be at the opening of the receptor-binding pocket (16). It remains to be determined if these self-propagating sequences are involved in interactions of Als3p with SspB.…”

Stick to Your Gums

“…1). Highresolution crystallography of the C-region of S. gordonii SspB (Forsgren et al, 2009) showed this to form two distinct domains, each containing a covalent isopeptide bond between a lysine and an asparagine residue. Such intra-molecular cross-links have been reported in Gram-positive bacterial pili (Kang et al, 2007) and are thought to stabilize elongated structures, potentially enabling streptococci to better resist detachment from oral surfaces.…”