Francisella tularensis IglG Belongs to a Novel Family of PAAR-Like T6SS Proteins and Harbors a Unique N-terminal Extension Required for Virulence

PLoS Pathog. 2016 Sep 7;12(9):e1005821. doi: 10.1371/journal.ppat.1005821. eCollection 2016 Sep.


The virulence of Francisella tularensis, the etiological agent of tularemia, relies on an atypical type VI secretion system (T6SS) encoded by a genomic island termed the Francisella Pathogenicity Island (FPI). While the importance of the FPI in F. tularensis virulence is clearly established, the precise role of most of the FPI-encoded proteins remains to be deciphered. In this study, using highly virulent F. tularensis strains and the closely related species F. novicida, IglG was characterized as a protein featuring a unique α-helical N-terminal extension and a domain of unknown function (DUF4280), present in more than 250 bacterial species. Three dimensional modeling of IglG and of the DUF4280 consensus protein sequence indicates that these proteins adopt a PAAR-like fold, suggesting they could cap the T6SS in a similar way as the recently described PAAR proteins. The newly identified PAAR-like motif is characterized by four conserved cysteine residues, also present in IglG, which may bind a metal atom. We demonstrate that IglG binds metal ions and that each individual cysteine is required for T6SS-dependent secretion of IglG and of the Hcp homologue, IglC and for the F. novicida intracellular life cycle. In contrast, the Francisella-specific N-terminal α-helical extension is not required for IglG secretion, but is critical for F. novicida virulence and for the interaction of IglG with another FPI-encoded protein, IglF. Altogether, our data suggest that IglG is a PAAR-like protein acting as a bi-modal protein that may connect the tip of the Francisella T6SS with a putative T6SS effector, IglF.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Francisella tularensis / genetics*
  • Francisella tularensis / immunology
  • Francisella tularensis / pathogenicity
  • Gene Expression Regulation, Bacterial
  • Genomic Islands / genetics*
  • Macrophages / metabolism
  • Models, Molecular
  • Sequence Alignment
  • Sequence Deletion
  • Tularemia / immunology
  • Tularemia / microbiology*
  • Type VI Secretion Systems / genetics*
  • Type VI Secretion Systems / metabolism
  • Virulence
  • Virulence Factors / genetics
  • Virulence Factors / metabolism*


  • Bacterial Proteins
  • Type VI Secretion Systems
  • Virulence Factors

Grants and funding

This work is supported by a FINOVI grant to TH, an ANR grant to TH and AC (ANR-12-ASTR-0018); Direction Générale de l’Armemen (DGA) and Fondation pour la recherche médicale (FRM) (#FDT20150532668) fellowships to MR and a collaborative grant from the Swedish Research Agency K2013-4581 and K2013-8621. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.