Identification of a new effector-immunity pair of Aeromonas hydrophila type VI secretion system

Shuiyan Ma,Nannan Wang,Jin Liu,Yongjie Liu,Chengping Lu,Yuhao Dong

doi:10.1186/s13567-020-00794-w

Shuiyan Ma, Nannan Wang + Show 4 more

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https://doi.org/10.1186/s13567-020-00794-w

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Abstract

The type VI secretion system (T6SS) is a multiprotein weapon that kills eukaryotic predators or prokaryotic competitors by delivering toxic effectors. Despite the importance of T6SS in bacterial environmental adaptation, it is still challenging to systematically identify T6SS effectors because of their high diversity and lack of conserved domains. In this report, we discovered a putative effector gene, U876-17730, in the whole genome of Aeromonas hydrophila NJ-35 based on the reported conservative domain DUF4123 (domain of unknown function), with two cognate immunity proteins encoded downstream. Phylogenetic tree analysis of amino acids indicates that AH17730 belongs to the Tle1 (type VI lipase effector) family, and therefore was named Tle1AH. The deletion of tle1AH resulted in significantly decreased biofilm formation, antibacterial competition ability and virulence in zebrafish (Danio rerio) when compared to the wild-type strain. Only when the two immunity proteins coexist can bacteria protect themselves from the toxicity of Tle1AH. Further study shows that Tle1AH is a kind of phospholipase that possesses a conserved lipase motif, Gly-X-Ser-X-Gly (X is for any amino acid). Tle1AH is secreted by T6SS, and this secretion requires its interaction with an associated VgrG (valine-glycine repeat protein G). In conclusion, we identified a T6SS effector-immunity pair and verified its function, which lays the foundation for future research on the role of T6SS in the pathogenic mechanism of A. hydrophila.

Highlights

Aeromonas hydrophila is a prevalent agent of aquatic infections, mainly causing motile aeromonad septicemia (MAS)
We report the identification of a novel T6SS effector, Tle1AH, based on the conserved domain DUF4123, that plays a critical role in the successful colonization of A. hydrophila in the host
A potential T6SS effector is predicted in A. hydrophila NJ‐35 Based on the conserved domain DUF4123 [26], we searched potential T6SS effectors in A. hydrophila NJ-35

Summary

Introduction

Aeromonas hydrophila is a prevalent agent of aquatic infections, mainly causing motile aeromonad septicemia (MAS). Many virulence factors of A. hydrophila have been investigated, including motility [2], toxins [3], tissue-destructive enzymes [4] and S-layer [5]. Protein secretion systems are essential for virulence and competition with nearby microorganisms. Nine types of bacterial secretion systems, T6SS was discovered as early as 2006 in Pseudomonas aeruginosa [9] and Vibrio cholerae [10], and approximately 25% of all Gram-negative bacteria have highly conserved T6SS gene clusters based on bioinformatics analysis [11]. The T6SS apparatus is composed of thirteen highly conserved “core” proteins and is believed to resemble the structure of the contractile tail of Escherichia coli bacteriophage T4 [14, 15]. TssB and TssC make up the bacteriophage

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