Intramolecular chaperone-mediated secretion of an Rhs effector toxin by a type VI secretion system

Abstract

Bacterial Rhs proteins containing toxic domains are often secreted by type VI secretion systems (T6SSs) through unclear mechanisms. Here, we show that the T6SS Rhs-family effector TseI of Aeromonas dhakensis is subject to self-cleavage at both the N-and the Cterminus, releasing the middle Rhs core and two VgrG-interacting domains (which we name VIRN and VIRC). VIRC is an endonuclease, and the immunity protein TsiI protects against VIRC toxicity through direct interaction. Proteolytic release of VIRC and VIRN is mediated, respectively, by an internal aspartic protease activity and by two conserved glutamic residues in the Rhs core. Mutations abolishing self-cleavage do not block secretion, but reduce TseI toxicity. Deletion of VIRN or the Rhs core abolishes secretion. TseI homologs from Pseudomonas syringae, P. aeruginosa, and Vibrio parahaemolyticus are also self-cleaved. VIRN and VIRC interact with protein VgrG1, while the Rhs core interacts with protein TecI. We propose that VIRN and the Rhs core act as T6SS intramolecular chaperones to facilitate toxin secretion and function.

Structural basis for effector transmembrane domain recognition by type VI secretion system chaperones

“…While previous work has relied on genetic context to identify the cognate effector of an Eag chaperone ( Whitney et al, 2015 ; Alcoforado Diniz and Coulthurst, 2015 ), our use of the prePAAR motif as an effector discovery tool enables the identification of these effectors in any genetic context. Other families of chaperones, such as the DUF4123 or DUF2169 protein families, have also been shown to affect the stability and/or export of their cognate effectors ( Burkinshaw et al, 2018 ; Bondage et al, 2016 ; Pei et al, 2020 ). However, little is known about the specificity of these chaperones for their effector targets, which do not contain predicted N-terminal TMDs.…”

Activity, delivery, and diversity of Type VI secretion effectors

“…Most of the T6SS DNase effector (Tde) toxins belong to the HNH superfamily, also known as His‐Me finger endonucleases (Alcoforado Diniz & Coulthurst, 2015; Koskiniemi et al, 2013; Pei et al, 2020; Pissaridou et al, 2018; Zhang et al, 2012). A His‐Me finger is a short 30‐amino acid motif formed by a β‐hairpin followed by an α‐helix (ββα) that constitute a binding site for a divalent metal ion (Jablonska et al, 2017).…”

Role of Recipient Susceptibility Factors During Contact-Dependent Interbacterial Competition

“…Besides DsbA, involvement of the ClpAP protease in T6SS susceptibility (Lin et al, 2020) also suggested that various types of proteases may be used for activating toxin activity by either cleaving full-length toxin proteins into more active truncated forms or degrading proteins that may inhibit toxin activity. A recent report showed that self-cleavage at both the N-and C-termini of an Rhs-family T6SS toxin TseI is not required for secretion but critical for its toxin activity (Pei et al, 2020). This finding also suggests that protease cleavage could be a strategy used for toxin activation in the recipient cell.…”