Recombinant Phospholipase A1 of the Outer Membrane of Psychrotrophic Yersinia pseudotuberculosis: Expression, Purification, and Characterization

Studies on the Structure and Properties of Membrane Phospholipase A1 Inclusion Bodies Formed at Low Growth Temperatures Using GFP Fusion Strategy

The effect of cultivation temperatures (37, 26, and 18 °C) on the conformational quality of Yersinia pseudotuberculosis phospholipase A₁ (PldA) in inclusion bodies (IBs) was studied using green fluorescent protein (GFP) as a folding reporter. GFP was fused to the C-terminus of PldA to form the PldA-GFP chimeric protein. It was found that the maximum level of fluorescence and expression of the chimeric protein is observed in cells grown at 18 °C, while at 37 °C no formation of fluorescently active forms of PldA-GFP occurs. The size, stability in denaturant solutions, and enzymatic and biological activity of PldA-GFP IBs expressed at 18 °C, as well as the secondary structure and arrangement of protein molecules inside the IBs, were studied. Solubilization of the chimeric protein from IBs in urea and SDS is accompanied by its denaturation. The obtained data show the structural heterogeneity of PldA-GFP IBs. It can be assumed that compactly packed, properly folded, proteolytic resistant, and structurally less organized, susceptible to proteolysis polypeptides can coexist in PldA-GFP IBs. The use of GFP as a fusion partner improves the conformational quality of PldA, but negatively affects its enzymatic activity. The PldA-GFP IBs are not toxic to eukaryotic cells and have the property to penetrate neuroblastoma cells. Data presented in the work show that the GFP-marker can be useful not only as target protein folding indicator, but also as a tool for studying the molecular organization of IBs, their morphology, and localization in E. coli, as well as for visualization of IBs interactions with eukaryotic cells.

Phosphatidylinositol 4,5-Bisphosphate-Dependent Oligomerization of the Pseudomonas aeruginosa Cytotoxin ExoU

The Pseudomonas aeruginosa type III secretion system delivers effector proteins directly into target cells, allowing the bacterium to modulate host cell functions. ExoU is the most cytotoxic of the known effector proteins and has been associated with more severe infections in humans. ExoU is a patatin-like A₂ phospholipase requiring the cellular host factors phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and ubiquitin for its activation in vitro We demonstrated that PI(4,5)P₂ also induces the oligomerization of ExoU and that this PI(4,5)P₂-mediated oligomerization does not require ubiquitin. Single amino acid substitutions in the C-terminal membrane localization domain of ExoU reduced both its activity and its ability to form higher-order complexes in transfected cells and in vitro Combining inactive truncated ExoU proteins partially restored phospholipase activity and cytotoxicity, indicating that ExoU oligomerization may have functional significance. Our results indicate that PI(4,5)P₂ induces the oligomerization of ExoU, which may be a mechanism by which this coactivator enhances the phospholipase activity of ExoU.

Effects of elevated growth temperature and heat shock on the lipid composition of the inner and outer membranes of Yersinia pseudotuberculosis

Differences in the distribution of individual phospholipids between the inner (IM) and outer membranes (OM) of gram-negative bacteria have been detected in mesophilic Escherichia, Erwinia and Salmonella species but have never been investigated in the psychrotrophic Yersinia genus. Therefore, the influence of an elevated growth temperature and heat shock on the phospholipid and fatty acid (FA) compositions of the fractionated Yersinia pseudotuberculosis envelope was investigated. The shift of the growth temperature from 8 °C to 37 °C to mimic the switch from saprophytic to parasitic growth of this bacteria and the exposure of the cells to heat shock, which was induced by a sharp increase in the temperature from 8 °C to 45 °C, increased the lysophosphatidylethanolamine content from zero and 1% to 6% and 10% in the IM and OM, respectively. These changes were accompanied by a decrease in the phosphatidylethanolamine (PE) content and a drastic increase (up to 3-fold higher) in the phosphatidylglycerol (PG) level in the OM of the bacteria, which increases the net negative charge of the cell envelope. The levels of the predominant saturated palmitic (16:0) and cyclopropane FAs were approximately 1.5- and 7.5-fold higher, respectively, but the content of the predominant unsaturated palmitoleic (16:1n-7) and cis-vaccenic (18:1n-7) FAs was approximately 10-30-fold lower in both membranes that were isolated from the cells grown at elevated temperatures. Due to these changes, reflecting the process of "homeoviscous adaptation", the ratio between the unsaturated and saturated FAs decreased but remained higher in the IM than that in the OM. Simultaneously, no significant changes were observed in the FA composition of cells subjected to heat shock, demonstrating a difference between the responses of the heat-shocked and heat-adapted Y. pseudotuberculosis. The unique ability of Y. pseudotuberculosis to reciprocally regulate the ratio of anionic PG and net neutral PE and therefore adjust the negative charge of the OM may be a common strategy used by pathogenic bacteria to promote the barrier function of the OM.