The conserved core component EsaA mediates bacterial killing by the type VIIb secretion system

Abstract

Type VII secretion systems (T7SS) are employed by Gram-positive bacteria to infect the host or to intoxicate bacterial competitors. Bacterial killing mediated by other secretion systems involves the assembly of large, extracellular structures promoting toxin delivery into bacterial competitors. However, such extracellular structures are not known for T7SS nor is it known whether the secreted toxins have cell-penetrating properties. Here, we used a multi-drug resistant Staphylococcus aureus, one of the most relevant hospital-associated bacterial pathogens, to study the mechanism of bacterial killing by the T7SSb. We report the proteolytic resistant, extracellular structure of the EsaA core component, which is an integral part of the membrane-embedded T7SSb secretion machine. The extracellular structure of EsaA has an elongated antenna-like shape providing an explanation how the T7SSb spans the staphylococcal cell wall. We demonstrate that the extracellular structure of EsaA has an essential function in secretion and contributes to the killing of prey bacteria. We show that the bactericidal activity of EsaA can be attributed to a membrane-damaging activity that leads to the permeabilization of phospholipid bilayers and could facilitate the delivery of antibacterial toxin into bacterial competitors. The structural similarity of the EsaA tip domain to bacterial lectins and human integrins suggests an additional function of EsaA in cell adhesion during infection or in targeting of bacterial competitors.

The Antibacterial Type VII Secretion System of Bacillus subtilis: Structure and Interactions of the Pseudokinase YukC/EssB

“…The polytopic membrane subunit EsaA presents a large extracellular domain. The structures of the Streptococcus gallolyticus and S. aureus EsaA ( Sg EsaA and Sa EsaA, respectively) extracellular domains demonstrate an elongated conformation, capable of spanning the peptidoglycan layer ( 40 , 41 ). While Sg EsaA was proposed to control effector translocation through the cell wall, the tip structure of Sa EsaA suggested a role in cell-cell contacts.…”