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;
12:e1005821. [PMID:
27602570 PMCID:
PMC5014421 DOI:
10.1371/journal.ppat.1005821]
[Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2016] [Accepted: 07/20/2016] [Indexed: 12/12/2022] Open
Abstract
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.
Francisella tularensis is a highly pathogenic bacterium causing tularemia. Its ability to cause disease is linked to its ability to replicate in the macrophage cytosol. The intracellular life cycle of Francisella is controlled by a type VI secretion system (T6SS), which is thought to inject effectors into the host cell to allow bacterial escape into the host cytosol. The molecular mechanisms behind this process are still largely unclear. In this work, we identify IglG as a protein with two important domains, one conserved in proteins from more than 250 bacterial species (DUF4280, renamed here as PAAR-like domain) and one specific for the Francisella genus. Using protein sequence analysis and three-dimensional structure predictions, comparative modeling and biochemistry approaches, our data demonstrate that IglG is a metal-binding protein that based on its PAAR-like domain might cap the VgrG spike of the T6SS and act as a membrane-puncturing protein. Furthermore, we identified that the Francisella-specific domain is directly involved in forming a protein complex with another virulence protein, IglF. This work, in addition to enhancing the molecular understanding of the Francisella T6SS, defines the features of the conserved DUF4280, a novel PAAR-like domain involved in type VI secretion (T6S) of many bacterial species.
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