Different motifs on Salivary Agglutinin mediate IgA and bacteria binding

Salivary agglutinin (SAG) is encoded by the DMBT-1 gene (Deleted in Malignant Brain Tumors-1 gene) and is identical to gp-340 from the lungs. It is a member of the SRCR (Scavenger Receptor Cysteine Rich) superfamily, a group of immune related proteins with SRCR domains. In saliva SAG binds to bacteria and partially complexes with IgA. In previous studies was shown that a 16 amino acid sequence (peptide SRCRP2; QGRVEVLYRGSWGTVC) is involved in binding to both bacteria and IgA (Ligtenberg et al. 2004). The minimal bacteria binding site on the SRCRP2 was narrowed down to an 11 amino acid peptide (GRVEVLYRGSW) with VEVL and W being the critical residues in this motif (Bikker et al. 2004). Objective: to identify the minimal IgA binding site on the SRCRP2 peptide of SAG. Methods: peptides were coated on microplates and tested for IgA binding in an Enzyme Linked Immuno Sorbent Assay. Results: using overlapping peptides was shown that on SRCRP2, and thus SAG, the same 11 amino acid motif required for bacteria binding binds IgA (GRVEVLYRGSW). An alanine substitution scan revealed that VVLY and W are critical residues in this motif. Conclusions: bacteria and IgA both share the minimal binding site on the SRCR domain of SAG but differ in the residues that are critical for binding in this motif (VEVLxxxxW versus VxVLYxxxW). This project was supported by the Moss International Research Scholarship at Columbia University and by the Netherlands Institute for Dental Sciences (IOT).