# Identification of receptor-binding domains of Bacteroidales antibacterial pore-forming toxins

## TL;DR

This study identifies the part of a bacterial toxin that allows it to specifically target and kill other bacteria, which could lead to new antibacterial applications.

## Contribution

The study reveals that the C-terminal domain of BSAP-1 is responsible for receptor recognition and specificity in Bacteroidales antibacterial toxins.

## Key findings

- The C-terminal domain (CTD) of BSAP-1 is essential for receptor binding and bactericidal activity.
- Grafting the CTD onto unrelated proteins allows them to interact with the BSAP-1 receptor.
- Electrostatic interactions drive the BSAP-1–receptor interaction, as revealed by structural and mutational analysis.

## Abstract

Bacteroidales are abundant Gram-negative bacteria present in the gut microbiota of most animals, including humans, where they carry out vital functions for host health. To thrive in this competitive environment, Bacteroidales use sophisticated weapons to outmatch competitors. Among these, Bacteroidales secreted antimicrobial proteins (BSAPs) represent a novel class of bactericidal pore-forming toxins that are highly specific to their receptor, typically targeting only a single membrane protein or lipopolysaccharide. The molecular determinants conferring this high selectivity remain unknown. In this study, we therefore investigated the model protein BSAP-1 and determined which of its domains is involved in providing receptor specificity. We demonstrate that receptor recognition is entirely driven by the C-terminal domain (CTD) of BSAP-1 using a combination of in vivo competition assays, in vitro protein binding studies, and mutational analysis. Specifically, we show that deletion of the CTD abrogates BSAP-1 bactericidal activity by preventing receptor binding, whereas grafting the CTD to unrelated carrier proteins enables CTD-driven interaction with the BSAP-1 receptor. Combining structural investigation of a BSAP-1–receptor complex with mutational analysis, we unveil that this interaction is driven by electrostatic interactions. Building upon this discovery, we show that BSAPs can be categorized according to the structure of their CTD, suggesting a strong CTD structure–receptor type correlation. In summary, our research demonstrates that BSAP receptor recognition is driven by their CTD and paves the way for future applications.