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A new factor controlling cell envelope integrity in Alphaproteobacteria in the context of cell cycle, stress response and infection

Abstract : The bacterial envelope is a remarkable and complex compartment of the prokaryotic cell in which many essential functions take place. The article by Herrou and collaborators (Herrou et al., in press), by a clever combination of structural analysis, genetics and functional characterization in free-living bacterial cells and during infection in animal models, elucidates a new factor, named EipA, that plays a major role in Brucella spp envelope biogenesis and cell division. The authors demonstrate a genetic connection between eipA and lipopolysaccharide synthesis , specifically genes involved in the synthesis of the O-antigen portion of lipopolysaccharide (LPS). Beyond its crucial role in Brucella physiology, the conservation of EipA in the class Alphaproteobacteria urges microbiologists to pursue future investigation of its homologs in other species belonging to this important group of bacteria. Cell envelope and division Bacteria are relatively simple organisms when compared to eukaryotes, in which cells are highly compartmentalized by the presence of internal membranes and organelles. However, at a deeper molecular level, bacteria present a complex physiology in which spatially localized functions can take place thanks to organized distribution of proteins within or along different regions of the cell membrane. Gram-negative bacteria present two membranes comprising a periplasmic space, in which selectively secreted proteins are involved in multiple functions, such as the transport of various substrates, energy production, and also cell growth and division. A plethora of proteins predicted to be involved in the physiology of the periplasmic space remains undefined or unstudied. In the article by Herrou and collaborators (Herrou et al., in press), an important new protein, named EipA, has been characterized in Brucella species which are members of the Gram-negative class Alphaproteobacteria. The authors provide evidence for a model in which Brucella EipA, O-polysaccharide bio-synthesis and cell division are connected. The class of Alphaproteobacteria comprises several important species including plant symbionts (e.g. Rhizobia) (Andrews and Andrews, 2017) and pathogens of plants (Agrobacterium) (Gelvin, 2017) and animals (e.g. Rickettsia, Brucella and Bartonella) (Di Russo Case and Samuel, 2016). Other well-studied model species belonging to the alphaproteobacterial class include the bacterial cell cycle and differentiation model Caulobacter crescentus (Lasker et al., 2016), the photosynthetic bacterium Rhodobacter sphaeroides (Porter et al., 2008) and the magnetotactic bacterium Magnetospirillum magneti-cum (Uebe and Schüler, 2016). As outlined above, the cell envelope of Gram-negative bacteria has inner and outer membranes that are separated by a space called the periplasm and a protective layer, the cell wall, mainly composed of peptidoglycan (Silhavy et al., 2010). The external part of the outer membrane presents a lipopolysaccharide layer (LPS), which is composed of a glycolipid component and a sac-charide component that provides a major contact point between the cell and the environment. The LPS saccha-ride has a variable core structure and a variable, often dispensable, outer component (named O-specific), whose presence defines what is known as 'smooth' LPS. A lack of this O-specific polysaccharide results in an
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Wanassa Beroual, Emanuele Biondi. A new factor controlling cell envelope integrity in Alphaproteobacteria in the context of cell cycle, stress response and infection. Molecular Microbiology, Wiley, 2019, 111 (3), pp.553-555. ⟨10.1111/mmi.14201⟩. ⟨hal-02186945⟩

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