Acid-induced loss of functional properties of bacterial cell division protein FtsZ: evidence for an alternative conformation at acidic pH

The lack of the cell division protein FtsZ induced generation of giant cells under acidic stress in cyanobacterium Synechocystis sp. PCC6803

Bacteria exposed to environmental stresses often exhibit superior acclimation abilities to environmental change. Acid treatment causes an increase in the cell length of the cyanobacterium Synechocystis sp. PCC6803 under light conditions. We aimed to elucidate the relationship between acidic stress and cell enlargement. After being synchronized under dark conditions, the cells were cultivated at different pH (pH 8.0 or pH 6.0) levels under light conditions. Synechocystis 6803 cells exhibited only cell growth occurred (cell volume expansion) and slow proliferation under the acidic condition. In the recovery experiment of the enlarged cells, they proliferated normally at pH 8.0, and the cell lengths decreased to the normal cell size under light conditions. Inhibition of cell division might be caused by acidic stress. To understand the effect of acidic stress on cell division, we evaluated the expression of FtsZ via Western blotting. The FtsZ concentration in cells was lower at pH 6.0 than at pH 8.0 and was not sufficient for cell division in the photoautotrophic conditions. ClpXP is well known as a regulator of the Z-ring dynamics in E. coli. The transcriptional level of four clpXP genes was upregulated approximately threefold at pH 6.0 after 24 h compared with that in cells grown at pH 8.0. The lack of FtsZ may be caused by the upregulation of clpXP expression under acidic condition. Therefore, ClpXP may participate in the degradation of FtsZ and be involved in the regulation of cell division via FtsZ under acidic stress in Synechocystis 6803.

Potential Probiotic Pediococcus pentosaceus M41 Modulates Its Proteome Differentially for Tolerances Against Heat, Cold, Acid, and Bile Stresses

Probiotics containing functional food confer health benefits in addition to their nutritional properties. In this study, we have evaluated the differential proteomic responses of a potential novel probiotic Pediococcus pentosaceus M41 under heat, cold, acid, and bile stress conditions. We identified stress response proteins that could provide tolerances against these stresses and could be used as probiotic markers for evaluating stress tolerance. Pediococcus pentosaceus M41 was exposed for 2 h to each condition: 50°C (heat stress), 4°C (cold stress), pH 3.0 (acid stress) and 0.05% bile (bile stress). Proteomic analysis was carried out using 2D-IEF SDS PAGE and LC-MS/MS. Out of 60 identified proteins, 14 upregulated and 6 downregulated proteins were common among all the stress conditions. These proteins were involved in different biological functions such as translation-related proteins, carbohydrate metabolism (phosphoenolpyruvate phosphotransferase), histidine biosynthesis (imidazole glycerol phosphate synthase) and cell wall synthesis (tyrosine-protein kinase CapB). Proteins such as polysaccharide deacetylase, lactate oxidase, transcription repressor NrdR, dihydroxyacetone kinase were upregulated under three out of the four stress conditions. The differential expression of these proteins might be responsible for tolerance and protection of P. pentosaceus M41 against different stress conditions.

Thermodesulfobium sp. strain 3baa, an acidophilic sulfate reducing bacterium forming biofilms triggered by mineral precipitation

Sulfate reducing prokaryotes are promising candidates for the remediation of acidic metal-rich waste waters. However, only few acidophilic species have been described to date. Chemolithoautotrophic strain 3baa was isolated from sediments of an acidic mine pit lake. Based on its 16S-rRNA gene sequence it belongs to the genus Thermodesulfobium. It was identified as an acidophile growing in artificial pore water medium in the range of pH 2.6-6.6. Though the highest sulfate reduction rates were obtained at the lower end of this range, elongated cells and extended lag phases demonstrated acid stress. Sulfate reduction at low pH was accompanied by the formation of mineral precipitates strongly adhering to solid surfaces. A structural investigation by laser scanning microscopy, electron microscopy and X-ray microanalysis revealed the formation of Al hydroxides and Fe sulfides which were densely populated by cells. Al hydroxides precipitated first, enabling initial cell attachment. Colonization of solid surfaces coincided with increased sulfate reducing activity indicating more favourable growth conditions within biofilms compared with free-living cells. These findings point out the importance of cell-mineral interaction for biofilm formation and contribute to our understanding how sulfate reducing prokaryotes thrive in both natural and engineered systems at low pH.

ZipA binds to FtsZ with high affinity and enhances the stability of FtsZ protofilaments

A bacterial membrane protein ZipA that tethers FtsZ to the membrane is known to promote FtsZ assembly. In this study, the binding of ZipA to FtsZ was monitored using fluorescence spectroscopy. ZipA was found to bind to FtsZ with high affinities at three different (6.0, 6.8 and 8.0) pHs, albeit the binding affinity decreased with increasing pH. Further, thick bundles of FtsZ protofilaments were observed in the presence of ZipA under the pH conditions used in this study indicating that ZipA can promote FtsZ assembly and stabilize FtsZ polymers under unfavorable conditions. Bis-ANS, a hydrophobic probe, decreased the interaction of FtsZ and ZipA indicating that the interaction between FtsZ and ZipA is hydrophobic in nature. ZipA prevented the dilution induced disassembly of FtsZ polymers suggesting that it stabilizes FtsZ protofilaments. Fluorescein isothiocyanate-labeled ZipA was found to be uniformly distributed along the length of the FtsZ protofilaments indicating that ZipA stabilizes FtsZ protofilaments by cross-linking them.

E93R substitution of Escherichia coli FtsZ induces bundling of protofilaments, reduces GTPase activity, and impairs bacterial cytokinesis

Recently, we found that divalent calcium has no detectable effect on the assembly of Mycobacterium tuberculosis FtsZ (MtbFtsZ), whereas it strongly promoted the assembly of Escherichia coli FtsZ (EcFtsZ). While looking for potential calcium binding residues in EcFtsZ, we found a mutation (E93R) that strongly promoted the assembly of EcFtsZ. The mutation increased the stability and bundling of the FtsZ protofilaments and produced a dominating effect on the assembly of the wild type FtsZ (WT-FtsZ). Although E93R-FtsZ was found to bind to GTP similarly to the WT-FtsZ, it displayed lower GTPase activity than the WT-FtsZ. E93R-FtsZ complemented for its wild type counterpart as observed by a complementation test using JKD7-1/pKD3 cells. However, the bacterial cells became elongated upon overexpression of the mutant allele. We modeled the structure of E93R-FtsZ using the structures of MtbFtsZ/Methanococcus jannaschi FtsZ (MjFtsZ) dimers as templates. The MtbFtsZ-based structure suggests that the Arg(93)-Glu(138) salt bridge provides the additional stability, whereas the effect of mutation appears to be indirect (allosteric) if the EcFtsZ dimer is similar to that of MjFtsZ. The data presented in this study suggest that an increase in the stability of the FtsZ protofilaments is detrimental for the bacterial cytokinesis.

Dissecting the alternatively folded state of the antibody Fab fragment

Intact antibodies and antigen binding fragments (Fab) have been previously shown to form an alternatively folded state (AFS) at low pH. This state consists primarily of secondary structure interactions, with reduced tertiary structure content. The AFS can be distinguished from the molten globule state by the formation of nonnative structure and, in particular, its high stability. In this study, the isolated domains of the MAK33 (murine monoclonal antibody of the subtype kappa/IgG1) Fab fragment were investigated under conditions that have been reported to induce the AFS. Surprising differences in the ability of individual domains to form the AFS were observed, despite the similarities in their native structures. All Fab domains were able to adopt the AFS, but only for V(H) (variable domain of the heavy chain) could a significant amount of tertiary structure be detected and different conditions were needed to induce the AFS. V(H), the least stable of the domains under physiological conditions, was the most stable in the AFS, yet all domains showed significant stability against thermal and chemical unfolding in their AFS. Formation of the AFS was found to generally proceed via the unfolded state, with similar rates for most of the domains. Taken together, our data reveal striking differences in the biophysical properties of the AFS of individual antibody domains that reflect the variation possible for domains of highly homologous native structures. Furthermore, they allow individual domain contributions to be dissected from specific oligomer effects in the AFS of the antibody Fab fragment.

Structural and functional model for ionic (K(+)/Na(+)) and pH dependence of GTPase activity and polymerization of FtsZ, the prokaryotic ortholog of tubulin

Bacterial cell division occurs through the formation of a protein ring (division ring) at the site of division, with FtsZ being its main component in most bacteria. FtsZ is the prokaryotic ortholog of eukaryotic tubulin; it shares GTPase activity properties and the ability to polymerize in vitro. To study the mechanism of action of FtsZ, we used molecular dynamics simulations of the behavior of the FtsZ dimer in the presence of GTP-Mg(2+) and monovalent cations. The presence of a K(+) ion at the GTP binding site allows the positioning of one water molecule that interacts with catalytic residues Asp235 and Asp238, which are also involved in the coordination sphere of K(+). This arrangement might favor dimer stability and GTP hydrolysis. Contrary to this, Na(+) destabilizes the dimer and does not allow the positioning of the catalytic water molecule. Protonation of the GTP gamma-phosphate, simulating low pH, excludes both monovalent cations and the catalytic water molecule from the GTP binding site and stabilizes the dimer. These molecular dynamics predictions were contrasted experimentally by analyzing the GTPase and polymerization activities of purified Methanococcus jannaschii and Escherichia coli FtsZ proteins in vitro.

An analysis of FtsZ assembly using small angle X-ray scattering and electron microscopy

Small angle X-ray scattering (SAXS) was used for the first time to study the self-assembly of the bacterial cell division protein, FtsZ, with three different additives: calcium chloride, monosodium glutamate and DEAE-dextran hydrochloride in solution. The SAXS data were analyzed assuming a model form factor and also by a model-independent analysis using the pair distance distribution function. Transmission electron microscopy (TEM) was used for direct observation of the FtsZ filaments. By sectioning and negative staining with glow discharged grids, very high bundling as well as low bundling polymers were observed under different assembly conditions. FtsZ polymers formed different structures in the presence of different additives and these additives were found to increase the bundling of FtsZ protofilaments by different mechanisms. The combined use of SAXS and TEM provided us a significant insight of the assembly of FtsZ and microstructures of the assembled FtsZ polymers.