Sumario: |
Brucella virulence is linked to components of the cell envelope and tightly connected to the function of
the BvrR/BvrS sensory-regulatory system. To quantify the impact of BvrR/BvrS on cell envelope proteins,
we performed a label-free mass spectrometry-based proteomic analysis of spontaneously released
outer membrane fragments from four strains of Brucella abortus (wild type virulent, avirulent bvrR and bvrS- mutants as well as reconstituted virulent bvrR+ (bvrR-/pbvrR+)). We identified 167 differentially
expressed proteins, of which 25 were assigned to the outer membrane. Approximately half of the outer
membrane proteins decreased in abundance, whereas half increased. Notably, expression of five Omp3
family proteins decreased whereas five lipoproteins increased in the mutant strains. In the periplasmic
space, by contrast, approximately 80% of the 60 differentially expressed proteins were increased in at
least one avirulent mutant. Periplasmic proteins are primarily involved in substrate uptake and transport,
and a uniform increase in this class may indicate a nutritional stress response, possibly a consequence
of defective outer membrane function. Virtually all proteins reverted to wild type levels in the
reconstituted virulent bvrR+ strain. We propose that the wide changes in cell envelope protein expression
relate to the markedly avirulent phenotype of bvrR- and bvrS- mutants and that Brucella virulence
depends on regulatory networks involving cell envelope and metabolism rather than on discrete
virulence factors. This model may be relevant to other R-Proteobacteria harboring BvrR/BvrS orthologous
systems known to be essential for parasitism or endosymbiosis.
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