Identification of the hydroxyapatite-binding domain of salivary agglutinin

The salivary agglutinin glycoprotein (SAG) is present in saliva but is also part of the salivary pellicle, playing a seemingly paradoxical role with regard to bacterial homeostasis. On the one hand, SAG aggregates bacteria in solution, thereby preventing bacterial colonization. On the other hand, when bound to the tooth surface, SAG facilitates bacterial colonization and microbial growth. The protein part of SAG is predominantly composed of conserved scavenger receptor cysteine-rich (SRCR) domains. Previously it was found that bacterial binding and aggregation is mediated via a single peptide loop, designated SRCRP2 (P2), within the SRCR domains of SAG. The current data suggest that the SRCR domains also harbour a hydroxyapatite (HA)-binding moiety, SRCRP3 (P3). The observation that P2 and P3 individually play unique roles in the function of SAGs contributes to our understanding of the dual role of SAGs in bacterial binding. Inspired by the bacterial-modulating capacity of SAGs, we created a P3-polyethylene glycol (PEG) conjugate. It was found that a P3 coating resulted in an increased antifouling activity of 20% compared with the uncoated surface in vitro. An additional PEG moiety resulted in an antifouling activity of up to 40% and 30% for Streptococcus mutans and Staphylococcus epidermidis, respectively.