Structure of a bacterial Rhs effector exported by the type VI secretion system

Patrick Günther,John C Whitney,Stefan Raunser,Shehryar Ahmad,Kartik Sachar,Dennis Quentin,Christos Gatsogiannis,Karla J.F Satchell

doi:10.1371/journal.ppat.1010182

Patrick Günther, John C Whitney + Show 6 more

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https://doi.org/10.1371/journal.ppat.1010182

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Abstract

The type VI secretion system (T6SS) is a widespread protein export apparatus found in Gram-negative bacteria. The majority of T6SSs deliver toxic effector proteins into competitor bacteria. Yet, the structure, function, and activation of many of these effectors remains poorly understood. Here, we present the structures of the T6SS effector RhsA from Pseudomonas protegens and its cognate T6SS spike protein, VgrG1, at 3.3 Å resolution. The structures reveal that the rearrangement hotspot (Rhs) repeats of RhsA assemble into a closed anticlockwise β-barrel spiral similar to that found in bacterial insecticidal Tc toxins and in metazoan teneurin proteins. We find that the C-terminal toxin domain of RhsA is autoproteolytically cleaved but remains inside the Rhs 'cocoon' where, with the exception of three ordered structural elements, most of the toxin is disordered. The N-terminal 'plug' domain is unique to T6SS Rhs proteins and resembles a champagne cork that seals the Rhs cocoon at one end while also mediating interactions with VgrG1. Interestingly, this domain is also autoproteolytically cleaved inside the cocoon but remains associated with it. We propose that mechanical force is required to remove the cleaved part of the plug, resulting in the release of the toxin domain as it is delivered into a susceptible bacterial cell by the T6SS.

Highlights

One way that bacteria interact with their environment is by secreting toxic molecules into their surroundings
One system widely used by Gram-negative bacteria is the T6 secretion system which delivers a plethora of effectors into competing bacterial cells
We found that RhsA forms a closed cocoon similar to that found in bacterial toxin complex (Tc) toxins and metazoan teneurin proteins

Summary

Introduction

One way that bacteria interact with their environment is by secreting toxic molecules into their surroundings. The tail-like complex consists of a contractile sheath that surrounds an inner tube comprised of many copies of stacked ring-shaped hexameric hemolysin co-regulated protein (Hcp) [6,7]. This Hcp tube is capped with a member of the homotrimeric valine-glycine repeat protein G (VgrG) spike protein family, which typically interacts with a cone-shaped proline-alanine-alanine-arginine (PAAR) domain-containing protein to form a complete T6SS tail tube-spike complex [8]. Self-protection from antibacterial T6SS effectors is accomplished via effector coexpression with cognate immunity proteins, which neutralize effector toxicity by occluding the effector active site or by the hydrolysis of effector generated toxic products [10,11]

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