PAAR proteins act as the ‘sorting hat’ of the type VI secretion system

Alain Filloux,Thomas E. Wood,Sophie A. Howard,Sarah Wettstadt

doi:10.1099/mic.0.000842

Alain Filloux, Thomas E. Wood

Open Access

https://doi.org/10.1099/mic.0.000842

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Abstract

Bacteria exist in polymicrobial environments and compete to prevail in a niche. The type VI secretion system (T6SS) is a nanomachine employed by Gram-negative bacteria to deliver effector proteins into target cells. Consequently, T6SS-positive bacteria produce a wealth of antibacterial effector proteins to promote their survival among a prokaryotic community. These toxins are loaded onto the VgrG–PAAR spike and Hcp tube of the T6SS apparatus and recent work has started to document the specificity of effectors for certain spike components. Pseudomonas aeruginosa encodes several PAAR proteins, whose roles have been poorly investigated. Here we describe a phospholipase family antibacterial effector immunity pair from Pseudomonas aeruginosa and demonstrate that a specific PAAR protein is necessary for the delivery of the effector and its cognate VgrG. Furthermore, the PAAR protein appears to restrict the delivery of other phospholipase effectors that utilise distinct VgrG proteins. We provide further evidence for competition for PAAR protein recruitment to the T6SS apparatus, which determines the identities of the delivered effectors.

Highlights

Bacteria must influence their surroundings and compete with other micro-organisms for nutrients and space
We report the valine–glycine repeat protein G (VgrG) and proline–alanine–alanine–arginine repeat (PAAR) dependence of a new antibacterial effector of the H2-T6SS of P. aeruginosa and begin to define the competition between spike subassemblies for this secretion system
The Tle3 toxin belongs to the phospholipase family of antibacterial effectors and its detrimental activity in the periplasm is neutralised by its cognate immunity protein Tli3, which is predicted to localise to this compartment

Summary

Introduction

Bacteria must influence their surroundings and compete with other micro-organisms for nutrients and space. The T6SS is a prevalent Gram-negative bacterial virulence factor and antibacterial apparatus, delivering effector proteins directly into target cells or into the local environment [1,2,3]. Many effector proteins exhibit antibacterial activities, such as nucleases, phospholipases and peptidoglycan hydrolases, to induce stasis or lysis of the target bacterium [4]. The T6SS is a contractile apparatus that propels a lance-like structure decorated with effectors into neighbouring bacteria in a contact-dependent manner. Effectors exist as either domains covalently linked to the lance constituents, called ‘evolved’ structural components, or cargo proteins that bind Hcp, VgrG or PAAR proteins in a non-covalent but specific manner [7]

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