Structural studies of the surface adhesin SspB from Streptococcus gordonii

Surface proteins on microorganisms that build up the oral biofilm are key players in the formation of the biofilm. Antigen I/II proteins are surface adhesins found on virtually all oral streptococci and share a conserved multi-domain architecture. These adhesins bind surface components on other bacteria and on host cells. Thus, they are crucial for the development of the biofilm. The objective of this thesis work is the structural characterization of the large multi-domain Antigen I/II protein SspB from the primary colonizing commensal bacterium Streptococcus gordonii. The crystal structure of the variable domain of SspB was determined to 2.3 Å resolution. The domain comprises a β-supersandwich and a putative binding cleft stabilized by a calcium ion. Despite high similarity in the overall structure, the cleft within SspB is significantly smaller than the cleft within the homologous protein from Streptococcus mutans, indicating that different substrates may bind in the clefts. A screen for carbohydrate binding resulted in no hits for interaction with the SspB variable domain suggesting that the cleft may not be suitable for binding sugars. This thesis also presents the high resolution 1.5 Å structure of a truncated C-terminal domain of SspB, the first of an Antigen I/II C-domain. The structure contains two structurally related domains, each containing one calcium ion and one intramolecular isopeptide bond. The SspB protein shares the feature of intramoleular isopeptide bonds with other surface proteins from Gram positive bacteria, such as pili from Streptococcus pyogenes and Corynebacterium diphtheriae. Intramolecular isopeptide bonds are suggested to be a common feature for retaining stability in a harsh environment. The SspB adherence region, shown to be the recognition motif for Porphyromonas gingivalis attachment to S. gordonii, protrudes from the core protein as a handle available for recognition. In conclusion, this thesis work has provided new knowledge about the SspB protein and increased the understanding of the common structure of AgI/II proteins