# The structure of Streptococcus gordonii surface protein SspB in complex with TEV peptide provides clues to oral streptococcal adherence to salivary agglutinin

## TL;DR

This paper identifies a key binding site in a bacterial protein that helps oral bacteria stick to saliva proteins, offering new targets for preventing harmful bacterial growth.

## Contribution

The study reports the first identified binding site in V-domains of oral streptococci for salivary agglutinin, with potential therapeutic applications.

## Key findings

- The structure of SspB’s V-domain bound to TEV peptide reveals a conserved binding interface with Gp340.
- The synthetic peptide PepCD1SRCR inhibits biofilm formation by Streptococcus mutans in a dose-dependent manner.
- Mutational analysis confirms the importance of conserved residues in adhesion to salivary agglutinin.

## Abstract

Streptococcus gordonii is a commensal bacterium in the oral cavity and has many surface adhesins that have been well characterized. SspA/B belongs to the Antigen I/II-like family of proteins, which are well known for their multifunctional adherence capabilities. Most AgI/II-like proteins adhere to salivary agglutinin (also known as glycoprotein 340, Gp340). In an effort to identify the putative binding site on the AgI/II-like family of proteins, we conducted structural studies to determine the V-domain of SspB. In this paper, we report the structure of SspB’s V-domain in complex with a TEV-peptide that was inserted to cleave the histidine tag at the C-terminus after purification. This peptide shared sequence and structural homology with a helical region on the scavenger receptor cysteine-rich (SRCR) domain of Gp340. Our studies with the synthetic peptide PepCD1SRCR show that it inhibits the Streptococcus mutans biofilm formation in a dose-dependent manner. A comprehensive comparative analysis of this site with the corresponding sites in the homologous V-domains of S. mutans AgI/II and GbpC established that most of these interface residues were conserved. Based on the structural data, mutational analysis was initiated to study the effect of binding-interface residues on the ability of each of these V-domains from S. mutans and S. gordonii to adhere to salivary agglutinin. Here, we report for the first time the binding site for the V-regions that are distinct among oral streptococci, which provides potential opportunities for therapeutic intervention of pathogenic species.