Non-specific porins of Gram-negative bacteria as proteins containing intrinsically disordered regions with amyloidogenic potential

Formation of Amyloid-Like Conformational States of β-Structured Membrane Proteins on the Example of OMPF Porin from the Yersinia pseudotuberculosis Outer Membrane

The work presents results of the in vitro and in silico study of formation of amyloid-like structures under harsh denaturing conditions by non-specific OmpF porin of Yersinia pseudotuberculosis (YpOmpF), a membrane protein with β-barrel conformation. It has been shown that in order to obtain amyloid-like porin aggregates, preliminary destabilization of its structure in a buffer solution with acidic pH at elevated temperature followed by long-term incubation at room temperature is necessary. After heating at 95°C in a solution with pH 4.5, significant conformational rearrangements are observed in the porin molecule at the level of tertiary and secondary structure of the protein, which are accompanied by the increase in the content of total β-structure and sharp decrease in the value of characteristic viscosity of the protein solution. Subsequent long-term exposure of the resulting unstable intermediate YpOmpF at room temperature leads to formation of porin aggregates of various shapes and sizes that bind thioflavin T, a specific fluorescent dye for the detection of amyloid-like protein structures. Compared to the initial protein, early intermediates of the amyloidogenic porin pathway, oligomers, have been shown to have increased toxicity to the Neuro-2aCCL-131™ mouse neuroblastoma cells. The results of computer modeling and analysis of the changes in intrinsic fluorescence during protein aggregation suggest that during formation of amyloid-like aggregates, changes in the structure of YpOmpF affect not only the areas with an internally disordered structure corresponding to the external loops of the porin, but also main framework of the molecule, which has a rigid spatial structure inherent to β-barrel.

New insights into xenobiotic tolerance of Antarctic bacteria: transcriptomic analysis of Pseudomonas sp. TNT3 during 2,4,6-trinitrotoluene biotransformation

The xenobiotic 2,4,6-trinitrotoluene (TNT) is a highly persistent environmental contaminant, whose biotransformation by microorganisms has attracted renewed attention. In previous research, we reported the discovery of Pseudomonas sp. TNT3, the first described Antarctic bacterium with the ability to biotransform TNT. Furthermore, through genomic analysis, we identified distinctive features in this isolate associated with the biotransformation of TNT and other xenobiotics. However, the metabolic pathways and genes active during TNT exposure in this bacterium remained unexplored. In the present transcriptomic study, we used RNA-sequencing to investigate gene expression changes in Pseudomonas sp. TNT3 exposed to 100 mg/L of TNT. The results showed differential expression of 194 genes (54 upregulated and 140 downregulated), mostly encoding hypothetical proteins. The most highly upregulated gene (> 1000-fold) encoded an azoreductase enzyme not previously described. Other significantly upregulated genes were associated with (nitro)aromatics detoxification, oxidative, thiol-specific, and nitrosative stress responses, and (nitro)aromatic xenobiotic tolerance via efflux pumps. Most of the downregulated genes were involved in the electron transport chain, pyrroloquinoline quinone (PQQ)-related alcohol oxidation, and motility. These findings highlight a complex cellular response to TNT exposure, with the azoreductase enzyme likely playing a crucial role in TNT biotransformation. Our study provides new insights into the molecular mechanisms of TNT biotransformation and aids in developing effective TNT bioremediation strategies. To the best of our knowledge, this report is the first transcriptomic response analysis of an Antarctic bacterium during TNT biotransformation.

OmpC and OmpF Outer Membrane Proteins of Escherichia coli and Salmonella enterica Form Bona Fide Amyloids

Outer membrane proteins (Omps) of Gram-negative bacteria represent porins involved in a wide range of virulence- and pathogenesis-related cellular processes, including transport, adhesion, penetration, and the colonization of host tissues. Most outer membrane porins share a specific spatial structure called the β-barrel that provides their structural integrity within the membrane lipid bilayer. Recent data suggest that outer membrane proteins from several bacterial species are able to adopt the amyloid state alternative to their β-barrel structure. Amyloids are protein fibrils with a specific spatial structure called the cross-β that gives them an unusual resistance to different physicochemical influences. Various bacterial amyloids are known to be involved in host-pathogen and host-symbiont interactions and contribute to colonization of host tissues. Such an ability of outer membrane porins to adopt amyloid state might represent an important mechanism of bacterial virulence. In this work, we investigated the amyloid properties of the OmpC and OmpF porins from two species belonging to Enterobacteriaceae family, Escherichia coli, and Salmonella enterica. We demonstrated that OmpC and OmpF of E. coli and S. enterica form toxic fibrillar aggregates in vitro. These aggregates exhibit birefringence upon binding Congo Red dye and show characteristic reflections under X-ray diffraction. Thus, we confirmed amyloid properties for OmpC of E. coli and demonstrated bona fide amyloid properties for three novel proteins: OmpC of S. enterica and OmpF of E. coli and S. enterica in vitro. All four studied porins were shown to form amyloid fibrils at the surface of E. coli cells in the curli-dependent amyloid generator system. Moreover, we found that overexpression of recombinant OmpC and OmpF in the E. coli BL21 strain leads to the formation of detergent- and protease-resistant amyloid-like aggregates and enhances the birefringence of bacterial cultures stained with Congo Red. We also detected detergent- and protease-resistant aggregates comprising OmpC and OmpF in S. enterica culture. These data are important in the context of understanding the structural dualism of Omps and its relation to pathogenesis.