Large-scale purification and in vitro characterization of the assembly of MreB from Leptospira interrogans

Changes of Ex Vivo Cervical Epithelial Cells Due to Electroporation with JMY

The ionic environment within the nucleoplasm might diverge from the conditions found in the cytoplasm, potentially playing a role in the cellular stress response. As a result, it is conceivable that interactions of nuclear actin and actin-binding proteins (ABPs) with apoptosis factors may differ in the nucleoplasm and cytoplasm. The primary intracellular stress response is Ca2+ influx. The junctional mediating and regulating Y protein (JMY) is an actin-binding protein and has the capability to interact with the apoptosis factor p53 in a Ca2+-dependent manner, forming complexes that play a regulatory role in cytoskeletal remodelling and motility. JMY's presence is observed in both the cytoplasm and nucleoplasm. Here, we show that ex vivo ectocervical squamous cells subjected to electroporation with JMY protein exhibited varying morphological alterations. Specifically, the highly differentiated superficial and intermediate cells displayed reduced nuclear size. In inflamed samples, nuclear enlargement and simultaneous cytoplasmic reduction were observable and showed signs of apoptotic processes. In contrast, the less differentiated parabasal and metaplastic cells showed increased cytoplasmic activity and the formation of membrane protrusions. Surprisingly, in severe inflammation, vaginosis or ASC-US (Atypical Squamous Cells of Undetermined Significance), JMY appears to influence only the nuclear and perinuclear irregularities of differentiated cells, and cytoplasmic abnormalities still existed after the electroporation. Our observations can provide an appropriate basis for the exploration of the relationship between cytopathologically relevant morphological changes of epithelial cells and the function of ABPs. This is particularly important since ABPs are considered potential diagnostic and therapeutic biomarkers for both cancers and chronic inflammation.

On the role of nucleotides and lipids in the polymerization of the actin homolog MreB from a Gram-positive bacterium

In vivo, bacterial actin MreB assembles into dynamic membrane-associated filamentous structures that exhibit circumferential motion around the cell. Current knowledge of MreB biochemical and polymerization properties in vitro remains limited and is mostly based on MreB proteins from Gram-negative species. In this study, we report the first observation of organized protofilaments by electron microscopy and the first 3D-structure of MreB from a Gram-positive bacterium. We show that Geobacillus stearothermophilus MreB forms straight pairs of protofilaments on lipid surfaces in the presence of ATP or GTP, but not in the presence of ADP, GDP or non-hydrolysable ATP analogs. We demonstrate that membrane anchoring is mediated by two spatially close short hydrophobic sequences while electrostatic interactions also contribute to lipid binding, and show that the population of membrane-bound protofilament doublets is in steady-state. In solution, protofilament doublets were not detected in any condition tested. Instead, MreB formed large sheets regardless of the bound nucleotide, albeit at a higher critical concentration. Altogether, our results indicate that both lipids and ATP are facilitators of MreB polymerization, and are consistent with a dual effect of ATP hydrolysis, in promoting both membrane binding and filaments assembly/disassembly.

Assembly properties of Spiroplasma MreB involved in swimming motility

Bacterial actin MreB forms filaments formed of antiparallel double strand units. The wall-less helical bacterium Spiroplasma has five MreB homologs (MreB1-5), some of which are involved in an intra-cellular ribbon for driving the bacterium's swimming motility. Although the interaction between MreB units is important for understanding Spiroplasma swimming, the interaction modes of each ribbon component are unclear. Here, we examined the assembly properties of Spiroplasma eriocheiris MreB5 (SpeMreB5), one of the ribbon component proteins that forms sheets. Electron microscopy (EM) revealed that sheet formation was inhibited under acidic conditions and bundle structures were formed under acidic and neutral conditions with low ionic strength. We also used solution assays and identified four properties of SpeMreB5 bundles as follows: (I) bundle formation followed sheet formation; (II) electrostatic interactions were required for bundle formation; (III) the positively charged and unstructured C-terminal region contributed to promoting lateral interactions for bundle formation; and (IV) bundle formation required Mg²⁺ at neutral pH but was inhibited by divalent cations under acidic pH conditions. During these studies, we also characterized two aggregation modes of SpeMreB5 with distinct responses to ATP. These properties will shed light on SpeMreB5 assembly dynamics at the molecular level.

Solubility and Thermal Stability of Thermotoga maritima MreB

The basis of MreB research is the study of the MreB protein from the Thermotoga maritima species, since it was the first one whose crystal structure was described. Since MreB proteins from different bacterial species show different polymerisation properties in terms of nucleotide and salt dependence, we conducted our research in this direction. For this, we performed measurements based on tryptophan emission, which were supplemented with temperature-dependent and chemical denaturation experiments. The role of nucleotide binding was studied through the fluorescent analogue TNP-ATP. These experiments show that Thermotoga maritima MreB is stabilised in the presence of low salt buffer and ATP. In the course of our work, we developed a new expression and purification procedure that allows us to obtain a large amount of pure, functional protein.

ATP-dependent polymerization dynamics of bacterial actin proteins involved in Spiroplasma swimming

MreB is a bacterial protein belonging to the actin superfamily. This protein polymerizes into an antiparallel double-stranded filament that determines cell shape by maintaining cell wall synthesis. Spiroplasma eriocheiris, a helical wall-less bacterium, has five MreB homologous (SpeMreB1-5) that probably contribute to swimming motility. Here, we investigated the structure, ATPase activity and polymerization dynamics of SpeMreB3 and SpeMreB5. SpeMreB3 polymerized into a double-stranded filament with possible antiparallel polarity, while SpeMreB5 formed sheets which contained the antiparallel filament, upon nucleotide binding. SpeMreB3 showed slow P_i release owing to the lack of an amino acid motif conserved in the catalytic centre of MreB family proteins. Our SpeMreB3 crystal structures and analyses of SpeMreB3 and SpeMreB5 variants showed that the amino acid motif probably plays a role in eliminating a nucleophilic water proton during ATP hydrolysis. Sedimentation assays suggest that SpeMreB3 has a lower polymerization activity than SpeMreB5, though their polymerization dynamics are qualitatively similar to those of other actin superfamily proteins, in which pre-ATP hydrolysis and post-P_i release states are unfavourable for them to remain as filaments.

Intracellular ion concentrations and cation-dependent remodelling of bacterial MreB assemblies

Here, we measured the concentrations of several ions in cultivated Gram-negative and Gram-positive bacteria, and analyzed their effects on polymer formation by the actin homologue MreB. We measured potassium, sodium, chloride, calcium and magnesium ion concentrations in Leptospira interrogans, Bacillus subtilis and Escherichia coli. Intracellular ionic strength contributed from these ions varied within the 130–273 mM range. The intracellular sodium ion concentration range was between 122 and 296 mM and the potassium ion concentration range was 5 and 38 mM. However, the levels were significantly influenced by extracellular ion levels. L. interrogans, Rickettsia rickettsii and E. coli MreBs were heterologously expressed and purified from E. coli using a novel filtration method to prepare MreB polymers. The structures and stability of Alexa-488 labeled MreB polymers, under varying ionic strength conditions, were investigated by confocal microscopy and MreB polymerization rates were assessed by measuring light scattering. MreB polymerization was fastest in the presence of monovalent cations in the 200–300 mM range. MreB filaments showed high stability in this concentration range and formed large assemblies of tape-like bundles that transformed to extensive sheets at higher ionic strengths. Changing the calcium concentration from 0.2 to 0 mM and then to 2 mM initialized rapid remodelling of MreB polymers.