Type IV secretion and Brucella virulence
Section snippets
Type IV secretion and bacterial virulence
Type IV secretion systems (TFSS) are a recently described family of multi-protein complex which serve to secrete macromolecules (Christie and Vogel, 2000, Lai and Kado, 2000, Baron et al., 2002). The first described members serve primarily to transfer nucleoprotein complexes during bacterial conjugation or the transfer of T-DNA from Agrobacterium to plant cells. More recently, additional members of the family have been discovered which play a role in the secretion of virulence factors by
Identification of the Brucella virB region
The first virB mutant was identified by screening a bank of Brucella suis TnBlaM insertion mutants for their ability to survive and multiply in either Hela cells or macrophages. In one attenuated mutant, the transposon was found to be inserted in a region containing genes with sequence homology to virB9 and virB10 from A. tumefaciens and to ptlG and ptlH from Bordetella pertussis. The flanking DNA was sequenced on a cosmid clone showing the presence of a 12 kb region encoding homologues of all
virB mutants are attenuated
The virB region is essential for intracellular survival and multiplication in B. suis, B. abortus and B. melitensis (O’Callaghan et al., 1999, Sieira et al., 2000, Hong et al., 2000, Delrue et al., 2001). Mutants have been constructed or isolated in all of the genes of the region and all have lost the ability to multiply intracellularly (O’Callaghan et al., 1999, Foulongne et al., 2000, Boschiroli et al., 2002; our unpublished data). In a recent screen for attenuated mutants, approximately 25%
Sequence analysis and operon structure
The DNA sequences are now available for the virB regions of three Brucella species. When the B. suis virB region was first identified, the most homologous type IV secretion systems were those of Agrobacterium, B. pertussis and, to a lesser extent the incN plasmid pKM101. Genome sequencing projects are identifying a growing number of TFSS in a wide number of bacterial species. At the time of writing, the most similar systems, with surprisingly high levels of identity, are those found in the
Regulation of the virB operon
The genus Brucella does not naturally contain plasmids and it is therefore probable that the TFSS encoded by the virB genes are involved in protein secretion rather than conjugation. A possible role in virulence is to inject effector molecules, which help with the establishment of the replication niche, into the host cell. The bacteria will only need the injection system and the effector molecules for a limited period during the infectious process and their expression is likely to be tightly
What does the VirB system do?
The exact role of the Brucella VirB system in virulence is still unclear. Two studies have shown that virB mutants of both B. abortus and B. melitensis have lost their ability to modulate intracellular trafficking. Brucella has been shown to have the ability to modulate the endosomal pathway in Hela cells and create an unique intracellular niche in which it multiplies (Pizzaro-Cerda et al., 2000). Virulent strains inhibit phagosome lysosome fusion in an active manner (Naroeni et al., 2001).
Unanswered questions
We are only just beginning to understand the role of the VirB system in Brucella virulence, and many questions remain unanswered. The first question is ‘what is the identity of the effector molecules secreted by the VirB system?’ This question is closely followed by ‘how do they effect the cell biology and allow Brucella to establish the replication niche?’ Other more fundamental questions concerning TFSS in general can be posed concerning the process secretion; what are the signals in the
Acknowledgements
We thank Jean Pierre Gorvel for providing the anti CIMPR antiserum. Our laboratory is supported by INSERM and the EEC (BIO4 CT960144 and QLK2-CT-2001-01200) Maria Laura Boschiroli was supported by fellowships from INSERM, CONICET and the Fondation pour la Recherché Médicale. Chantal Cazevieille is supported by a fellowship from Vaincre le Mucoviscidose.
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