# Functional synergy of heteropentameric B subunits underlies virulence in a Salmonella A2B5 toxin

## TL;DR

This study reveals how two B subunits in a Salmonella toxin work together to increase its toxicity and how this could help develop new treatments.

## Contribution

The study identifies a novel heteropentameric assembly mechanism of B subunits in an A2B5 toxin from Salmonella diarizonae.

## Key findings

- PltBd1 and PltBd2 B subunits form a 3:2 heteropentameric complex essential for toxin function.
- PltBd1 aids toxin secretion while PltBd2 targets host cells, showing functional synergy.
- Cryo-EM reveals the structural basis of the heteropentameric holotoxin stabilized by the PltA subunit.

## Abstract

AB5-type toxins are critical virulence factors in bacterial pathogenesis. Despite the identification of various B subunits in AB5 toxins across different pathogens, their assembly mechanisms and biological significance remain poorly understood. In this study, we identified and characterized a typhoid toxin-like A2B5 toxin, designated diarizonae toxin (DiaT), as a key virulence factor in Salmonella diarizonae. The DiaT genomic islet encodes two distinct B subunits, PltBd1 and PltBd2, which exhibit unique functional roles. Through genetic and functional analyses, we demonstrate that the heteropentameric assembly of PltBd1 and PltBd2 is essential for cytotoxicity, with our data suggesting PltBd1 facilitates toxin secretion and PltBd2 mediates host cell targeting. Cryo-EM structural analysis of endogenously expressed DiaT reveals a heteropentameric holotoxin with a 3:2 stoichiometry of PltBd1 to PltBd2, potentially stabilized by the PltA subunit. These findings uncover a novel assembly mechanism and synergistic functionality between distinct B subunits, advancing our understanding of the evolutionary diversity and functional complexity of AB5 toxins. This work provides new insights into bacterial pathogenesis and highlights potential targets for therapeutic intervention.

AB5 toxins are critical virulence factors in bacterial pathogens, but the assembly mechanisms and functional roles of their B subunits remain poorly understood. Our study reveals a novel heteropentameric toxin assembly mechanism in Salmonella diarizonae, where two distinct B subunits (PltBd1 and PltBd2) exhibit functional synergy to mediate toxin secretion and host cell targeting. Structural and functional analyses demonstrate that this cooperative interaction enhances cytotoxicity and underscores the evolutionary diversity of AB5 toxins. Our findings provide new insights into bacterial pathogenesis and highlight potential therapeutic targets for combating infections caused by AB5 toxin-producing pathogens.