This repeated domain is found at the C-terminus of cell surface antigens [1-3]. In the Streptococcus mutans antigen I/II there are three repeats of this domain, a cleft between the first two of these forms a binding site for the human salivary agglutinin (SAG) [3].

1: A novel variant of the immunoglobulin fold in surface adhesins of Staphylococcus aureus: crystal structure of the fibrinogen-binding MSCRAMM, clumping factor A. Deivanayagam CC, Wann ER, Chen W, Carson M, Rajashankar KR, Hook M, Narayana SV; EMBO J. 2002;21:6660-6672. PMID:12485987

2: Two intramolecular isopeptide bonds are identified in the crystal structure of the Streptococcus gordonii SspB C-terminal domain. Forsgren N, Lamont RJ, Persson K; J Mol Biol. 2010;397:740-751. PMID:20138058

3: Crystal structure of the C-terminal region of Streptococcus mutans antigen I/II and characterization of salivary agglutinin adherence domains. Larson MR, Rajashankar KR, Crowley PJ, Kelly C, Mitchell TJ, Brady LJ, Deivanayagam C; J Biol Chem. 2011;286:21657-21666. PMID:21505225