This domain is found in bacterial cell surface proteins such SusD Swiss:Q8A1G2 and SusD-like proteins Swiss:A7LXT5 as as well RagB, outer membrane surface receptor antigen. Bacteroidetes, one of the two dominant bacterial phyla in the human gut, are Gram-negative saccharolytic microorganisms that utilize a diverse array of glycans. Hence, they express starch-utilization system (Sus) for glycan uptake. SusD has 551 amino acids, and is almost entirely alpha-helical, with 22 alpha-helices, eight of which form 4 tetra-trico peptide repeats (TPRs: helix-turn-helix motifs involved in protein-protein interactions). The four TPRs pack together to create a right-handed super-helix. This is predicted to mediate the formation of SusD and SusC porin complex at the cell surface. The interaction between SusC and TPR1/TPR2 region of SusD is predicted to be of functional importance since it allows SusD to be in position for oligosaccharide capture from other Sus lipoproteins and delivery of these glycans to the SusC porin. The non-TPR containing portion of SusD is where starch binding occurs. The binding site is a shallow surface cavity located on top of TPR1. SusD homologs such as SusD-like proteins have a critical role in carbohydrate acquisition. Both SusD and its homologs, contain 15-20 residues at the N-terminus that might be a flexible linker region, anchoring the protein to the membrane and the glycan-binding domain [1]. Other homologs to SusD have been examined in Porphyromonas gingivalis such as RagB, an immunodominant outer-membrane surface receptor antigen. Structural characterization of RagB shows substantial similarity with Bacteroides thetaiotaomicron SusD (i.e alpha-helices and TPR regions). Based on this structural similarity, functional studies suggest that, RagB binding of glycans occurs at pockets on the molecular surface that are distinct from those of SusD [2].

1: Glycan recognition by the Bacteroidetes Sus-like systems. Bolam DN, Koropatkin NM; Curr Opin Struct Biol. 2012;22:563-569. PMID:22819666

2: Structure of RagB, a major immunodominant outer-membrane surface receptor antigen of Porphyromonas gingivalis. Goulas T, Ferrer IG, Hutcherson JA, Potempa BA, Potempa J, Scott DA, Gomis-Ruth FX; Mol Oral Microbiol. 2015; [Epub ahead of print] PMID:26441291