Francisella tularensis IglG Belongs to a Novel Family of PAAR-Like T6SS Proteins and Harbors a Unique N-terminal Extension Required for Virulence.
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.
CIRI, International Center for Infectiology Research, Inserm U1111, CNRS, UMR5308, Lyon, France.|University of Lyon, Lyon, France.|Ecole Normale Supérieure de Lyon, Lyon, France.
Mélanie
M
Rigard
Department of Clinical Microbiology, Clinical Bacteriology, and Laboratory for Molecular Infection Medicine Sweden (MIMS), Umeå University, Umeå, Sweden.
Jeanette E
JE
Bröms
CIRI, International Center for Infectiology Research, Inserm U1111, CNRS, UMR5308, Lyon, France.|University of Lyon, Lyon, France.|Ecole Normale Supérieure de Lyon, Lyon, France.
Amandine
A
Mosnier
Institut des Sciences Analytiques, CNRS, UMR 5280, Université de Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, Villeurbanne, France.
Maggy
M
Hologne
CIRI, International Center for Infectiology Research, Inserm U1111, CNRS, UMR5308, Lyon, France.|University of Lyon, Lyon, France.|Ecole Normale Supérieure de Lyon, Lyon, France.
Amandine
A
Martin
Department of Clinical Microbiology, Clinical Bacteriology, and Laboratory for Molecular Infection Medicine Sweden (MIMS), Umeå University, Umeå, Sweden.
Lena
L
Lindgren
CIRI, International Center for Infectiology Research, Inserm U1111, CNRS, UMR5308, Lyon, France.|University of Lyon, Lyon, France.|Ecole Normale Supérieure de Lyon, Lyon, France.
Claire
C
Punginelli
CIRI, International Center for Infectiology Research, Inserm U1111, CNRS, UMR5308, Lyon, France.|University of Lyon, Lyon, France.|Ecole Normale Supérieure de Lyon, Lyon, France.
Claire
C
Lays
Institut des Sciences Analytiques, CNRS, UMR 5280, Université de Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, Villeurbanne, France.
Olivier
O
Walker
Université Paris Descartes, Sorbonne Paris Cité, Bâtiment Leriche, Paris, France.|Institut Necker-Enfants Malades, Equipe 11: Pathogénie des Infections Systémiques, Paris, France.
Alain
A
Charbit
University of Lyon, Lyon, France.|Laboratoire de Geologie de Lyon; Ecole Normale Supérieure de Lyon, Lyon, France.
Philippe
P
Telouk
National Research Council Canada, Human Health Therapeutics Portfolio, Ottawa, Ontario, Canada.
Wayne
W
Conlan
Molecular Microbiology and Structural Biochemistry, UMR 5086, CNRS-Université de Lyon, Institut de Biologie et Chimie des Protéines, Lyon, France.
Laurent
L
Terradot
Department of Clinical Microbiology, Clinical Bacteriology, and Laboratory for Molecular Infection Medicine Sweden (MIMS), Umeå University, Umeå, Sweden.
Anders
A
Sjöstedt
CIRI, International Center for Infectiology Research, Inserm U1111, CNRS, UMR5308, Lyon, France.|University of Lyon, Lyon, France.|Ecole Normale Supérieure de Lyon, Lyon, France.
Thomas
T
Henry
D000595:Amino Acid Sequence; D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D005604:Francisella tularensis / Q000235:genetics* / Q000276:immunology / Q000472:pathogenicity; D015964:Gene Expression Regulation, Bacterial; D044404:Genomic Islands / Q000235:genetics*; D008264:Macrophages / Q000378:metabolism; D008958:Models, Molecular; D016415:Sequence Alignment; D017384:Sequence Deletion; D014406:Tularemia / Q000276:immunology / Q000382:microbiology*; D000069376:Type VI Secretion Systems / Q000235:genetics* / Q000378:metabolism; D014774:Virulence; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism*
D001426:Bacterial Proteins; D000069376:Type VI Secretion Systems; D037521:Virulence Factors