title: The Hcp proteins fused with diverse extended-toxin domains represent a novel pattern of antibacterial effectors in type VI secretion systems.
abstract: The type VI secretion system (T6SS) is a widespread molecular weapon deployed by many bacterial species to target eukaryotic host cells or rival bacteria. Using a dynamic injection mechanism, diverse effectors can be delivered by T6SS directly into recipient cells. Here, we report a new family of T6SS effectors encoded by extended Hcps carrying diverse toxin domains. Bioinformatic analyses revealed that these Hcps with C-terminal extension toxins, designated as Hcp-ET, exist widely in the Enterobacteriaceae. To verify our findings, Hcp-ET1 was tested for its antibacterial effect, and showed effective inhibition of target cell growth via the predicted HNH-DNase activity by T6SS-dependent delivery. Further studies showed that Hcp-ET2 mediated interbacterial antagonism via a Tle1 phospholipase (encoded by DUF2235 domain) activity. Notably, comprehensive analyses of protein homology and genomic neighborhoods revealed that Hcp-ET3-4 is fused with 2 toxin domains (Pyocin S3 and Colicin-DNase) C-terminally, and its encoding gene is followed 3 duplications of the cognate immunity genes. However, some bacteria encode a separated hcp-et3 and an orphan et4 (et4O1) genes caused by a termination-codon mutation in the fusion region between Pyocin S3 and Colicin-DNase encoding fragments. Our results demonstrated that both of these toxins had antibacterial effects. Further, all duplications of the cognate immunity protein contributed to neutralize the DNase toxicity of Pyocin S3 and Colicin, which has not been reported previously. In conclusion, we propose that Hcp-ET proteins are polymorphic T6SS effectors, and thus present a novel encoding pattern of T6SS effectors.
authors.affiliation: a Department of Veterinary Microbiology and Immunology, College of Veterinary Medicine, Nanjing Agricultural University , Nanjing , China.|b Key Lab of Animal Bacteriology, Ministry of Agriculture , Nanjing , China.
authors.forename: Jiale
authors.initials: J
authors.lastname: Ma
authors.affiliation: a Department of Veterinary Microbiology and Immunology, College of Veterinary Medicine, Nanjing Agricultural University , Nanjing , China.|b Key Lab of Animal Bacteriology, Ministry of Agriculture , Nanjing , China.
authors.forename: Zihao
authors.initials: Z
authors.lastname: Pan
authors.affiliation: a Department of Veterinary Microbiology and Immunology, College of Veterinary Medicine, Nanjing Agricultural University , Nanjing , China.|b Key Lab of Animal Bacteriology, Ministry of Agriculture , Nanjing , China.
authors.forename: Jinhu
authors.initials: J
authors.lastname: Huang
authors.affiliation: a Department of Veterinary Microbiology and Immunology, College of Veterinary Medicine, Nanjing Agricultural University , Nanjing , China.|b Key Lab of Animal Bacteriology, Ministry of Agriculture , Nanjing , China.
authors.forename: Min
authors.initials: M
authors.lastname: Sun
authors.affiliation: a Department of Veterinary Microbiology and Immunology, College of Veterinary Medicine, Nanjing Agricultural University , Nanjing , China.|b Key Lab of Animal Bacteriology, Ministry of Agriculture , Nanjing , China.
authors.forename: Chengping
authors.initials: C
authors.lastname: Lu
authors.affiliation: a Department of Veterinary Microbiology and Immunology, College of Veterinary Medicine, Nanjing Agricultural University , Nanjing , China.|b Key Lab of Animal Bacteriology, Ministry of Agriculture , Nanjing , China.
authors.forename: Huochun
authors.initials: H
authors.lastname: Yao
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D001419:Bacteria / Q000187:drug effects / Q000254:growth & development; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*; D001427:Bacterial Toxins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D004755:Enterobacteriaceae / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000072417:Protein Domains; D000069376:Type VI Secretion Systems / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D000069376:Type VI Secretion Systems
keywords: HNH-DNase; Hcp; Pyocin S3 and Colicin-DNase; T6SS; antibacterial effectors; extension toxin