Protein localization in Escherichia coli cells: comparison of the cytoplasmic membrane proteins ProP, LacY, ProW, AqpZ, MscS, and MscL

Manipulating subcellular protein localization to enhance target protein accumulation in minicells

BACKGROUND

Minicells are chromosome-free derivatives of bacteria formed through irregular cell division. Unlike simplified structures, minicells retain all cellular components of the parent cell except for the chromosome. This feature reduces immunogenic responses, making them advantageous for various biotechnological applications, including chemical production and drug delivery. To effectively utilize minicells, it is essential to ensure the accumulation of target proteins within them, enhancing their efficiency as delivery vehicles.

RESULTS

In this study, we engineered Escherichia coli by deleting the minCD genes, generating minicell-producing strains, and investigated strategies to enhance protein accumulation within the minicells. Comparative proteomic analysis revealed that minicells retain most parent-cell proteins but exhibit an asymmetric proteome distribution, leading to selective protein enrichment. We demonstrated that heterologous proteins, such as GFP and RFP, accumulate more abundantly in minicells than in parent cells, regardless of expression levels. To further enhance this accumulation, we manipulated protein localization by fusing target proteins to polar localization signals. While proteins fused with PtsI and Tsr exhibited 2.6-fold and 2.8-fold increases in accumulation, respectively, fusion with the heterologous PopZ protein resulted in a remarkable 15-fold increase in protein concentration under low induction conditions.

CONCLUSIONS

These findings highlight the critical role of spatial protein organization in enhancing the cargo-loading capabilities of minicells. By leveraging polar localization signals, this work provides a robust framework for optimizing minicells as efficient carriers for diverse applications, from therapeutic delivery to industrial biomanufacturing.

Long-term impacts of egg quiescence and Wolbachia infection on lipid profiles in Aedes aegypti: Ovarian roles in lipid synthesis during reproduction

Wolbachia, an endosymbiotic bacterium, relies on nutrients from its host to complete its life cycle. The presence of Wolbachia strain wAlbB in the mosquito Aedes aegypti during egg or larval stages affects the host's development, leading to the absence of developed and visible ovaries in adult mosquito females. In this study, we investigated the impacts of egg quiescence and Wolbachia infection on lipid profiles of adult Ae. aegypti females, and discerned the role of ovaries in lipid synthesis in the reproductive process. The lipidomes of Wolbachia infected and uninfected female individuals at various developmental stages were quantitatively analyzed by LC-MS/MS. Lipidomic change patterns were systematically further investigated in wAlbB-infected fertile females and infertile females following blood feeding. Prolonged egg quiescence induced a shortage of acyl-carnitine (CAR) and potentially impacted some molecules of diacyl-phospholipid (diacyl-PL) and sphingolipid (SL) in young adult mosquitoes. After the first gonotrophic cycle, infertile females accumulated more CAR and lyso-phospholipid (lyso-PL) than fertile females. Then in the second gonotrophic cycle, the patterns of different lipid groups remained similar between fertile and infertile females. Only a small proportion of molecules of triglyceride (TG), phospholipid (lyso-PL and diacyl-PL) and ceramide (Cer) increased exclusively in fertile females from 0 h to 16 h post blood meal, suggesting that the generation or prescence of these lipids rely on ovaries. In addition, we found cardiolipins (CL) might be impacted by Wolbachia infection at the egg stage, and infected mosquitoes also showed distinct patterns between fertile and infertile females at their second gonotrophic cycle. Our study provides new insights into the long-term influence of Wolbachia on lipid profiles throughout various life stages of mosquitoes. Additionally, it suggests a role played by ovaries in lipid synthesis during mosquito reproduction.

Dynamic structure of E. coli cytoplasm: supramolecular complexes and cell aging impact spatial distribution and mobility of proteins

Protein diffusion is a critical factor governing the functioning and organization of a cell's cytoplasm. In this study, we investigate the influence of (poly)ribosome distribution, cell aging, protein aggregation, and biomolecular condensate formation on protein mobility within the E. coli cytoplasm. We employ nanoscale single-molecule displacement mapping (SMdM) to determine the spatial distribution of the proteins and to meticulously track their diffusion. We show that the distribution of polysomes does not impact the lateral diffusion coefficients of proteins. However, the degradation of mRNA induced by rifampicin treatment leads to an increase in protein mobility within the cytoplasm. Additionally, we establish a significant correlation between cell aging, the asymmetric localization of protein aggregates and reduced diffusion coefficients at the cell poles. Notably, we observe variations in the hindrance of diffusion at the poles and the central nucleoid region for small and large proteins, and we reveal differences between the old and new pole of the cell. Collectively, our research highlights cellular processes and mechanisms responsible for spatially organizing the bacterial cytoplasm into domains with different structural features and apparent viscosity.

Building complex membranes with Martini 3

The Martini model is a popular force field for coarse-grained simulations. Membranes have always been at the center of its development, with the latest version, Martini 3, showing great promise in capturing more and more realistic behavior. In this chapter we provide a step-by-step tutorial on how to construct starting configurations, run initial simulations and perform dedicated analysis for membrane-based systems of increasing complexity, including leaflet asymmetry, curvature gradients and embedding of membrane proteins.

Bacterial membrane dynamics: Compartmentalization and repair

In every bacterial cell, the plasma membrane plays a key role in viability as it forms a selective barrier between the inside of the cell and its environment. This barrier function depends on the physical state of the lipid bilayer and the proteins embedded or associated with the bilayer. Over the past decade or so, it has become apparent that many membrane-organizing proteins and principles, which were described in eukaryote systems, are ubiquitous and play important roles in bacterial cells. In this minireview, we focus on the enigmatic roles of bacterial flotillins in membrane compartmentalization and bacterial dynamins and ESCRT-like systems in membrane repair and remodeling.

Activity modulation of the Escherichia coli F1FO ATP synthase by a designed antimicrobial peptide via cardiolipin sequestering

Most antimicrobial peptides (AMPs) exert their microbicidal activity through membrane permeabilization. The designed AMP EcDBS1R4 has a cryptic mechanism of action involving the membrane hyperpolarization of Escherichia coli, suggesting that EcDBS1R4 may hinder processes involved in membrane potential dissipation. We show that EcDBS1R4 can sequester cardiolipin, a phospholipid that interacts with several respiratory complexes of E. coli. Among these, F₁F_O ATP synthase uses membrane potential to fuel ATP synthesis. We found that EcDBS1R4 can modulate the activity of ATP synthase upon partition to membranes containing cardiolipin. Molecular dynamics simulations suggest that EcDBS1R4 alters the membrane environment of the transmembrane F_O motor, impairing cardiolipin interactions with the cytoplasmic face of the peripheral stalk that binds the catalytic F₁ domain to the F_O domain. The proposed mechanism of action, targeting membrane protein function through lipid reorganization may open new venues of research on the mode of action and design of other AMPs.

Pseudomonas aeruginosa responds to altered membrane phospholipid composition by adjusting the production of two-component systems, proteases and iron uptake proteins

Membrane protein and phospholipid (PL) composition changes in response to environmental cues and during infections. To achieve these, bacteria use adaptation mechanisms involving covalent modification and remodelling of the acyl chain length of PLs. However, little is known about bacterial pathways regulated by PLs. Here, we investigated proteomic changes in the biofilm of P. aeruginosa phospholipase mutant (∆plaF) with altered membrane PL composition. The results revealed profound alterations in the abundance of many biofilm-related two-component systems (TCSs), including accumulation of PprAB, a key regulator of the transition to biofilm. Furthermore, a unique phosphorylation pattern of transcriptional regulators, transporters and metabolic enzymes, as well as differential production of several proteases, in ∆plaF, indicate that PlaF-mediated virulence adaptation involves complex transcriptional and posttranscriptional response. Moreover, proteomics and biochemical assays revealed the depletion of pyoverdine-mediated iron uptake pathway proteins in ∆plaF, while proteins from alternative iron-uptake systems were accumulated. These suggest that PlaF may function as a switch between different iron-acquisition pathways. The observation that PL-acyl chain modifying and PL synthesis enzymes were overproduced in ∆plaF reveals the interconnection of degradation, synthesis and modification of PLs for proper membrane homeostasis. Although the precise mechanism by which PlaF simultaneously affects multiple pathways remains to be elucidated, we suggest that alteration of PL composition in ∆plaF plays a role for the global adaptive response in P. aeruginosa mediated by TCSs and proteases. Our study revealed the global regulation of virulence and biofilm by PlaF and suggests that targeting this enzyme may have therapeutic potential.

Insights into the Formation of Intermolecular Complexes of Fluorescent Probe 10-N-Nonyl Acridine Orange with Cardiolipin and Phosphatidylglycerol in Bacterial Plasma Membrane by Molecular Modeling

In this article, we used molecular dynamics (MD), one of the most common methods for simulations of membranes, to study the interaction of fluorescent membranotropic biological probe 10-N-nonyl acridine orange (NAO) with the bilayer, mimicking a plasma membrane of Gram-negative bacteria. Fluorescent probes serve as an effective tool to study the localization of different components in biological membranes. Revealing the molecular details of their interaction with membrane phospholipids is important both for the interpretation of experimental results and future design of lipid-specific stains. By means of coarse-grained (CG) MD, we studied the interactions of NAO with a model membrane, imitating the plasma membrane of Gram-negative bacteria. In our simulations, we detected different NAO forms: monomers, dimers, and stacks. NAO dimers had the central cardiolipin (CL) molecule in a sandwich-like structure. The stacks were formed by NAO molecules interlayered with anionic lipids, predominantly CL. Use of the CG approach allowed to confirm the ability of NAO to bind to both major negatively charged phospholipids, phosphatidylglycerol (PG) and CL, and to shed light on the exact structure of previously proposed NAO-lipid complexes. Thus, CG modeling can be useful for the development of new effective and highly specific molecular probes.

How Functional Lipids Affect the Structure and Gating of Mechanosensitive MscS-like Channels

The ability to cope with and adapt to changes in the environment is essential for all organisms. Osmotic pressure is a universal threat when environmental changes result in an imbalance of osmolytes inside and outside the cell which causes a deviation from the normal turgor. Cells have developed a potent system to deal with this stress in the form of mechanosensitive ion channels. Channel opening releases solutes from the cell and relieves the stress immediately. In bacteria, these channels directly sense the increased membrane tension caused by the enhanced turgor levels upon hypoosmotic shock. The mechanosensitive channel of small conductance, MscS, from Escherichia coli is one of the most extensively studied examples of mechanically stimulated channels. Different conformational states of this channel were obtained in various detergents and membrane mimetics, highlighting an intimate connection between the channel and its lipidic environment. Associated lipids occupy distinct locations and determine the conformational states of MscS. Not all these features are preserved in the larger MscS-like homologues. Recent structures of homologues from bacteria and plants identify common features and differences. This review discusses the current structural and functional models for MscS opening, as well as the influence of certain membrane characteristics on gating.

New Roles for HAMP Domains: the Tri-HAMP Region of Pseudomonas aeruginosa Aer2 Controls Receptor Signaling and Cellular Localization

The Aer2 chemoreceptor from Pseudomonas aeruginosa is an O₂ sensor involved in stress responses, virulence, and tuning the behavior of the chemotaxis (Che) system. Aer2 is the sole receptor of the Che2 system. It is soluble, but membrane associated, and forms complexes at the cell pole during stationary phase. The domain arrangement of Aer2 is unusual, with a PAS sensing domain sandwiched between five HAMP domains, followed by a C-terminal kinase-control output domain. The first three HAMP domains form a poly-HAMP chain N-terminal to the PAS sensing domain. HAMP domains are often located between signal input and output domains, where they transduce signals. Given that HAMP1 to 3 reside N-terminal to the input-output pathway, we undertook a systematic examination of their function in Aer2. We found that HAMP1 to 3 influence PAS signaling over a considerable distance, as the majority of HAMP1, 2 and 3 mutations, and deletions of helical phase stutters, led to nonresponsive signal-off or off-biased receptors. PAS signal-on lesions that mimic activated Aer2 also failed to override N-terminal HAMP signal-off replacements. This indicates that HAMP1 to 3 are critical coupling partners for PAS signaling and likely function as a cohesive unit and moveable scaffold to correctly orient and poise PAS dimers for O₂-mediated signaling in Aer2. HAMP1 additionally controlled the clustering and polar localization of Aer2 in P. aeruginosa. Localization was not driven by HAMP1 charge, and HAMP1 signal-off mutants still localized. Employing HAMP as a clustering and localization determinant, as well as a facilitator of PAS signaling, are newly recognized roles for HAMP domains. IMPORTANCE P. aeruginosa is an opportunistic pathogen that interprets environmental stimuli via 26 chemoreceptors that signal through 4 distinct chemosensory systems. The second chemosensory system, Che2, contains a receptor named Aer2 that senses O₂ and mediates stress responses and virulence and tunes chemotactic behavior. Aer2 is membrane associated, but soluble, and has three N-terminal HAMP domains (HAMP1 to 3) that reside outside the signal input-output pathway of Aer2. In this study, we determined that HAMP1 to 3 facilitate O₂-dependent signaling from the PAS sensing domain and that HAMP1 controls the formation of Aer2-containing polar foci in P. aeruginosa. Both of these are newly recognized roles for HAMP domains that may be applicable to other non-signal-transducing HAMP domains and poly-HAMP chains.

Display of a novel carboxylesterase CarCby on Escherichia coli cell surface for carbaryl pesticide bioremediation

Background

Carbamate pesticides have been widely used in agricultural and forestry pest control. The large-scale use of carbamates has caused severe toxicity in various systems because of their toxic environmental residues. Carbaryl is a representative carbamate pesticide and hydrolase/carboxylesterase is the initial and critical enzyme for its degradation. Whole-cell biocatalysts have become a powerful tool for environmental bioremediation. Here, a whole cell biocatalyst was constructed by displaying a novel carboxylesterase/hydrolase on the surface of Escherichia coli cells for carbaryl bioremediation.

Results

The carCby gene, encoding a protein with carbaryl hydrolysis activity was cloned and characterized. Subsequently, CarCby was displayed on the outer membrane of E. coli BL21(DE3) cells using the N-terminus of ice nucleation protein as an anchor. The surface localization of CarCby was confirmed by SDS–PAGE and fluorescence microscopy. The optimal temperature and pH of the engineered E. coli cells were 30 °C and 7.5, respectively, using pNPC4 as a substrate. The whole cell biocatalyst exhibited better stability and maintained approximately 8-fold higher specific enzymatic activity than purified CarCby when incubated at 30 °C for 120 h. In addition, ~ 100% and 50% of the original activity was retained when incubated with the whole cell biocatalyst at 4 ℃ and 30 °C for 35 days, respectively. However, the purified CarCby lost almost 100% of its activity when incubated at 30 °C for 134 h or 37 °C for 96 h, respectively. Finally, approximately 30 mg/L of carbaryl was hydrolyzed by 200 U of the engineered E. coli cells in 12 h.

Conclusions

Here, a carbaryl hydrolase-containing surface-displayed system was first constructed, and the whole cell biocatalyst displayed better stability and maintained its catalytic activity. This surface-displayed strategy provides a new solution for the cost-efficient bioremediation of carbaryl and could also have the potential to be used to treat other carbamates in environmental bioremediation.

Supplementary information

The online version contains supplementary material available at 10.1186/s12934-022-01821-5.

Biodegradation of λ-cyhalothrin through cell surface display of bacterial carboxylesterase

Pyrethroids are the third widespread used insecticides globally which have been extensively applied in agricultural or household environments. Due to continuous applications, pyrethroids have been detected both in living cells and environments. The permanent exposure to pyrethroids have caused substantial health risks and ecosystem concerns. In this work, a λ-cyhalothrin (one kind of pyrethroid insecticides) degrading bacterium Bacillus velezensis sd was isolated and a carboxylesterase gene, CarCB2 was characterized. A whole cell biocatalyst was developed for λ-cyhalothrin biodegradation by displaying CarCB2 on the surface of Escherichia coli cells. CarCB2 was successfully displayed and functionally expressed on E. coli cells with optimal pH and temperature of 7.5 and 30 °C, using p-NPC₄ as substrate, respectively. The whole cell biocatalyst exhibited better stability than the purified CarCB2, and approximately 120%, 60% or 50% of its original activity at 4 °C, 30 °C or 37 °C over a period of 35 d was retained, respectively. No enzymatic activity was detected when incubated the purified CarCB2 at 30 °C for 120 h, or 37 °C for 72 h, respectively. Additionally, 30 mg/L of λ-cyhalothrin was degraded in citrate-phosphate buffer by 10 U of the whole cell biocatalyst in 150 min. This work reveals that the whole cell biocatalyst affords a promising approach for efficient biodegradation of λ-cyhalothrin, and might have the potential to be applied in further environmental bioremediation of other different kinds of pyrethroid insecticides.

Subcellular Localization Defects Characterize Ribose-Binding Mutant Proteins with New Ligand Properties in Escherichia coli

Periplasmic binding proteins have been previously proclaimed as a general scaffold to design sensor proteins with new recognition specificities for nonnatural compounds. Such proteins can be integrated in bacterial bioreporter chassis with hybrid chemoreceptors to produce a concentration-dependent signal after ligand binding to the sensor cell. However, computationally designed new ligand-binding properties ignore the more general properties of periplasmic binding proteins, such as their periplasmic translocation, dynamic transition of open and closed forms, and interactions with membrane receptors. In order to better understand the roles of such general properties in periplasmic signaling behavior, we studied the subcellular localization of ribose-binding protein (RbsB) in Escherichia coli in comparison to a recently evolved set of mutants designed to bind 1,3-cyclohexanediol. As proxies for localization, we calibrated and deployed C-terminal end mCherry fluorescent protein fusions. Whereas RbsB-mCherry coherently localized to the periplasmic space and accumulated in (periplasmic) polar regions depending on chemoreceptor availability, mutant RbsB-mCherry expression resulted in high fluorescence cell-to-cell variability. This resulted in higher proportions of cells devoid of clear polar foci and of cells with multiple fluorescent foci elsewhere, suggesting poorer translocation, periplasmic autoaggregation, and mislocalization. Analysis of RbsB mutants and mutant libraries at different stages of directed evolution suggested overall improvement to more RbsB-wild-type-like characteristics, which was corroborated by structure predictions. Our results show that defects in periplasmic localization of mutant RbsB proteins partly explain their poor sensing performance. Future efforts should be directed to predicting or selecting secondary mutations outside computationally designed binding pockets, taking folding, translocation, and receptor interactions into account. IMPORTANCE Biosensor engineering relies on transcription factors or signaling proteins to provide the actual sensory functions for the target chemicals. Since for many compounds there are no natural sensory proteins, there is a general interest in methods that could unlock routes to obtaining new ligand-binding properties. Bacterial periplasmic binding proteins (PBPs) form an interesting family of proteins to explore for this purpose, because there is a large natural variety suggesting evolutionary trajectories to bind new ligands. PBPs are conserved and amenable to accurate computational binding pocket predictions. However, studying ribose-binding protein in Escherichia coli, we discovered that designed variants have defects in their proper localization in the cell, which can impair appropriate sensor signaling. This indicates that functional sensing capacity of PBPs cannot be obtained solely through computational design of the ligand-binding pocket but must take other properties of the protein into account, which are currently very difficult to predict.

Evolutionarily conserved mechanism for membrane recognition from bacteria to mitochondria

The mechanisms controlling membrane recognition by proteins with one hydrophobic stretch at their carboxyl terminus (tail anchor, TA) are poorly defined. The Escherichia coli TAs of ElaB and YqjD, which share sequential and structural similarity with the Saccharomyces cerevisiae TA of Fis1, were shown to localize to mitochondria. We show that YqjD and ElaB are directed by their TAs to bacterial cell poles. Fis1(TA) expressed in E. coli localizes like the endogenous TAs. The yeast and bacterial TAs are inserted in the E. coli inner membrane, and they all show affiliation to phosphatidic acid (PA), found in the membrane of the bacterial cell poles and of the yeast mitochondria. Our results suggest a mechanism for TA membrane recognition conserved from bacteria to mitochondria and raise the possibility that through their interaction with PA, and TAs play a role across prokaryotes and eukaryotes in controlling cell/organelle fate.

ATP synthase: Evolution, energetics, and membrane interactions

The synthesis of ATP, life’s “universal energy currency,” is the most prevalent chemical reaction in biological systems and is responsible for fueling nearly all cellular processes, from nerve impulse propagation to DNA synthesis. ATP synthases, the family of enzymes that carry out this endless task, are nearly as ubiquitous as the energy-laden molecule they are responsible for making. The F-type ATP synthase (F-ATPase) is found in every domain of life and has facilitated the survival of organisms in a wide range of habitats, ranging from the deep-sea thermal vents to the human intestine. Accordingly, there has been a large amount of work dedicated toward understanding the structural and functional details of ATP synthases in a wide range of species. Less attention, however, has been paid toward integrating these advances in ATP synthase molecular biology within the context of its evolutionary history. In this review, we present an overview of several structural and functional features of the F-type ATPases that vary across taxa and are purported to be adaptive or otherwise evolutionarily significant: ion channel selectivity, rotor ring size and stoichiometry, ATPase dimeric structure and localization in the mitochondrial inner membrane, and interactions with membrane lipids. We emphasize the importance of studying these features within the context of the enzyme’s particular lipid environment. Just as the interactions between an organism and its physical environment shape its evolutionary trajectory, ATPases are impacted by the membranes within which they reside. We argue that a comprehensive understanding of the structure, function, and evolution of membrane proteins—including ATP synthase—requires such an integrative approach.

Eugene P. Kennedy's Legacy: Defining Bacterial Phospholipid Pathways and Function

In the 1950's and 1960's Eugene P. Kennedy laid out the blueprint for phospholipid biosynthesis in somatic cells and Escherichia coli, which have been coined the Kennedy Pathways for phospholipid biosynthesis. His research group continued to make seminal contributions in the area of phospholipids until his retirement in the early 1990's. During these years he mentored many young scientists that continued to build on his early discoveries and who also mentored additional scientists that continue to make important contributions in areas related to phospholipids and membrane biogenesis. This review will focus on the initial E. coli Kennedy Pathways and how his early contributions have laid the foundation for our current understanding of bacterial phospholipid genetics, biochemistry and function as carried on by his scientific progeny and others who have been inspired to study microbial phospholipids.

Molecular interactions and their predictive roles in cell pole determination in bacteria

Bacterial cell cycle is divided into well-coordinated phases; chromosome duplication and segregation, cell elongation, septum formation, and cytokinesis. The temporal separation of these phases depends upon the growth rates and doubling time in different bacteria. The entire process of cell division starts with the assembly of divisome complex at mid-cell position followed by constriction of the cell wall and septum formation. In the mapping of mid-cell position for septum formation, the gradient of oscillating Min proteins across the poles plays a pivotal role in several bacteria genus. The cues in the cell that defines the poles and plane of cell division are not fully characterized in cocci. Recent studies have shed some lights on molecular interactions at the poles and the underlying mechanisms involved in pole determination in non-cocci. In this review, we have brought forth recent findings on these aspects together, which would suggest a model to explain the mechanisms of pole determination in rod shaped bacteria and could be extrapolated as a working model in cocci.

A genomic analysis of osmotolerance in Staphylococcus aureus

A key phenotypic characteristic of the Gram-positive bacterial pathogen, Staphylococcus aureus, is its ability to grow in low a_w environments. A homology transfer based approach, using the well characterised osmotic stress response systems of Bacillus subtilis and Escherichia coli, was used to identify putative osmotolerance loci in Staphylococcus aureus ST772-MRSA-V. A total of 17 distinct putative hyper and hypo-osmotic stress response systems, comprising 78 genes, were identified. The ST772-MRSA-V genome exhibits significant degeneracy in terms of the osmotic stress response; with three copies of opuD, two copies each of nhaK and mrp/mnh, and five copies of opp. Furthermore, regulation of osmotolerance in ST772-MRSA-V appears to be mediated at the transcriptional, translational, and post-translational levels.

Bringing rafts to life: Lessons learned from lipid organization across diverse biological membranes

The ability of lipids to drive lateral organization is a remarkable feature of membranes and has been hypothesized to underlie the architecture of cells. Models for lipid rafts and related domains were originally based on the mammalian plasma membrane, but the nature of heterogeneity in this system is still not fully resolved. However, the concept of lipid-driven organization has been highly influential across biology, and has led to discoveries in organisms that feature a diversity of lipid chemistries and physiological needs. Here we review several emerging and instructive cases of membrane organization in non-mammalian systems. In bacteria, several types of membrane domains that act in metabolism and signaling have been elucidated. These widen our view of what constitutes a raft, but also introduce new questions about the relationship between organization and function. In yeast, observable membrane organization is found in both the plasma membrane and the vacuole. The latter serves as the best example of classic membrane phase partitioning in a living system to date, suggesting that internal organelles are important membranes to investigate across eukaryotes. Finally, we highlight plants as powerful model systems for complex membrane interactions in multicellular organisms. Plant membranes are organized by unique glycosphingolipids, supporting the importance of carbohydrate interactions in organizing lateral domains. These examples demonstrate that membrane organization is a potentially universal phenonenon in biology and argue for the continued broadening of lipid physical chemistry research into a wide range of systems.

Charged pore-lining residues are required for normal channel kinetics in the eukaryotic mechanosensitive ion channel MSL1

Mechanosensitive (MS) ion channels are widespread mechanisms for cellular mechanosensation that can be directly activated by increasing membrane tension. The well-studied MscS family of MS ion channels is found in bacteria, archaea, and plants. MscS-Like (MSL)1 is localized to the inner mitochondrial membrane of Arabidopsis thaliana, where it is required for normal mitochondrial responses to oxidative stress. Like Escherichia coli MscS, MSL1 has a pore-lining helix that is kinked. However, in MSL1 this kink is comprised of two charged pore-lining residues, R326 and D327. Using single-channel patch-clamp electrophysiology in E. coli, we show that altering the size and charge of R326 and D327 leads to dramatic changes in channel kinetics. Modest changes in gating pressure were also observed while no effects on channel rectification or conductance were detected. MSL1 channel variants had differing physiological function in E. coli hypoosmotic shock assays, without clear correlation between function and particular channel characteristics. Taken together, these results demonstrate that altering pore-lining residue charge and size disrupts normal channel state stability and gating transitions, and led us to propose the “sweet spot” model. In this model, the transition to the closed state is facilitated by attraction between R326 and D327 and repulsion between R326 residues of neighboring monomers. In the open state, expansion of the channel reduces inter-monomeric repulsion, rendering open state stability influenced mainly by attractive forces. This work provides insight into how unique charge-charge interactions can be combined with an otherwise conserved structural feature to help modulate MS channel function.

Development of a whole-cell biocatalyst for diisobutyl phthalate degradation by functional display of a carboxylesterase on the surface of Escherichia coli

BACKGROUND

Phthalic acid esters (PAEs) are widely used as plasticizers or additives during the industrial manufacturing of plastic products. PAEs have been detected in both aquatic and terrestrial environments due to their overuse. Exposure of PAEs results in human health concerns and environmental pollution. Diisobutyl phthalate is one of the main plasticizers in PAEs. Cell surface display of recombinant proteins has become a powerful tool for biotechnology applications. In this current study, a carboxylesterase was displayed on the surface of Escherichia coli cells, for use as whole-cell biocatalyst in diisobutyl phthalate biodegradation.

RESULTS

A carboxylesterase-encoding gene (carEW) identified from Bacillus sp. K91, was fused to the N-terminal of ice nucleation protein (inpn) anchor from Pseudomonas syringae and gfp gene, and the fused protein was then cloned into pET-28a(+) vector and was expressed in Escherichia coli BL21(DE3) cells. The surface localization of INPN-CarEW/or INPN-CarEW-GFP fusion protein was confirmed by SDS-PAGE, western blot, proteinase accessibility assay, and green fluorescence measurement. The catalytic activity of the constructed E. coli surface-displayed cells was determined. The cell-surface-displayed CarEW displayed optimal temperature of 45 °C and optimal pH of 9.0, using p-NPC2 as substrate. In addition, the whole cell biocatalyst retained ~ 100% and ~ 200% of its original activity per OD600 over a period of 23 days at 45 °C and one month at 4 °C, exhibiting the better stability than free CarEW. Furthermore, approximately 1.5 mg/ml of DiBP was degraded by 10 U of surface-displayed CarEW cells in 120 min.

CONCLUSIONS

This work provides a promising strategy of cost-efficient biodegradation of diisobutyl phthalate for environmental bioremediation by displaying CarEW on the surface of E. coli cells. This approach might also provide a reference in treatment of other different kinds of environmental pollutants by displaying the enzyme of interest on the cell surface of a harmless microorganism.

Interaction between mechanosensitive channels embedded in lipid membrane

The study of the gating mechanism of mechanosensitive channels opens a window to the exploration of how different mechanical stimuli induce adaptive cellular behaviors of both the protein and the lipid, across different time and length scales. In this work, through a molecular dynamics-decorated finite element method (MDeFEM), the gating behavior of mechanosensitive channels of small conductance (MscS) in Escherichia coli (E. coli) is studied upon membrane stretch or global bending. The local membrane curvature around MscS is incorporated, as well as multiple MscL (mechanosensitive channels of large conductance) molecules in proximity to MscS. The local membrane curvature is found to delay MscS opening and diminishes moderately upon membrane stretching. Mimicking the insertion of lysophosphatidylcholine (LPC) molecules into the lipid, both downward and upward bending can active MscS, as long as the global membrane curvature radius reaches 34 nm. Based on the different MscS pore evolutions observed with the presence of one or more MscLs nearby, we propose that when coreconstituted, multiple MscL molecules tend to be located at the local membrane curvature zone around MscS. In another word, as MscL "swims around" in the lipid bilayer, it can be trapped by the membrane's local curvature. Collectively, the current study provides valuable insights into the interplay between mechanosensitive channels and lipid membrane at structural and physical levels, and specific predictions are proposed for further experimental investigations.

Loss of a Cardiolipin Synthase in Helicobacter pylori G27 Blocks Flagellum Assembly

Helicobacter pylori uses a cluster of polar, sheathed flagella for motility, which it requires for colonization of the gastric epithelium in humans. As part of a study to identify factors that contribute to localization of the flagella to the cell pole, we disrupted a gene encoding a cardiolipin synthase (clsC) in H. pylori strains G27 and B128. Flagellum biosynthesis was abolished in the H. pylori G27 clsC mutant but not in the B128 clsC mutant. Transcriptome sequencing analysis showed that flagellar genes encoding proteins needed early in flagellum assembly were expressed at wild-type levels in the G27 clsC mutant. Examination of the G27 clsC mutant by cryo-electron tomography indicated the mutant assembled nascent flagella that contained the MS ring, C ring, flagellar protein export apparatus, and proximal rod. Motile variants of the G27 clsC mutant were isolated after allelic exchange mutagenesis using genomic DNA from the B128 clsC mutant as the donor. Genome resequencing of seven motile G27 clsC recipients revealed that each isolate contained the flgI (encodes the P-ring protein) allele from B128. Replacing the flgI allele in the G27 clsC mutant with the B128 flgI allele rescued flagellum biosynthesis. We postulate that H. pylori G27 FlgI fails to form the P ring when cardiolipin levels in the cell envelope are low, which blocks flagellum assembly at this point. In contrast, H. pylori B128 FlgI can form the P ring when cardiolipin levels are low and allows for the biosynthesis of mature flagella.IMPORTANCE H. pylori colonizes the epithelial layer of the human stomach, where it can cause a variety of diseases, including chronic gastritis, peptic ulcer disease, and gastric cancer. To colonize the stomach, H. pylori must penetrate the viscous mucous layer lining the stomach, which it accomplishes using its flagella. The significance of our research is identifying factors that affect the biosynthesis and assembly of the H. pylori flagellum, which will contribute to our understanding of motility in H. pylori, as well as other bacterial pathogens that use their flagella for host colonization.

Curvature sensing by cardiolipin in simulated buckled membranes

Cardiolipin is a non-bilayer phospholipid with a unique dimeric structure. It localizes to negative curvature regions in bacteria and is believed to stabilize respiratory chain complexes in the highly curved mitochondrial membrane. Cardiolipin's localization mechanism remains unresolved, because important aspects such as the structural basis and strength for lipid curvature preferences are difficult to determine, partly due to the lack of efficient simulation methods. Here, we report a computational approach to study curvature preferences of cardiolipin by simulated membrane buckling and quantitative modeling. We combine coarse-grained molecular dynamics with simulated buckling to determine the curvature preferences in three-component bilayer membranes with varying concentrations of cardiolipin, and extract curvature-dependent concentrations and lipid acyl chain order parameter profiles. Cardiolipin shows a strong preference for negative curvatures, with a highly asymmetric chain order parameter profile. The concentration profiles are consistent with an elastic model for lipid curvature sensing that relates lipid segregation to local curvature via the material constants of the bilayers. These computations constitute new steps to unravel the molecular mechanism by which cardiolipin senses curvature in lipid membranes, and the method can be generalized to other lipids and membrane components as well.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

Protein polarization driven by nucleoid exclusion of DnaK(HSP70)-substrate complexes

Many bacterial proteins require specific subcellular localization for function. How Escherichia coli proteins localize at one pole, however, is still not understood. Here, we show that the DnaK (HSP70) chaperone controls unipolar localization of the Shigella IpaC type III secretion substrate. While preventing the formation of lethal IpaC aggregates, DnaK promoted the incorporation of IpaC into large and dynamic complexes (LDCs) restricted at the bacterial pole through nucleoid occlusion. Unlike stable polymers and aggregates, LDCs show dynamic behavior indicating that nucleoid occlusion also applies to complexes formed through transient interactions. Fluorescence recovery after photobleaching analysis shows DnaK-IpaC exchanges between opposite poles and DnaKJE-mediated incorporation of immature substrates in LDCs. These findings reveal a key role for LDCs as reservoirs of functional DnaK-substrates that can be rapidly mobilized for secretion triggered upon bacterial contact with host cells.

Many bacterial proteins exhibit spatially defined localization important for function. Here the authors show that the polar localization of Shigella IpaC type III secretion substrate is mediated by its interaction with the DnaK chaperone and occlusion by the bacterial nucleoid.

The Gram-positive model organism Bacillus subtilis does not form microscopically detectable cardiolipin-specific lipid domains

Rather than being homogenous diffusion-dominated structures, biological membranes can exhibit areas with distinct composition and characteristics, commonly termed as lipid domains. Arguably the most comprehensively studied examples in bacteria are domains formed by cardiolipin, which have been functionally linked to protein targeting, the cell division process and the mode of action of membrane-targeting antimicrobials. Cardiolipin domains were originally identified in the Gram-negative model organism Escherichia coli based on preferential staining by the fluorescent membrane dye nonylacridine orange (NAO), and later reported to also exist in other Gram-negative and -positive bacteria. Recently, the lipid-specificity of NAO has been questioned based on studies conducted in E. coli. This prompted us to reanalyse cardiolipin domains in the Gram-positive model organism Bacillus subtilis. Here we show that logarithmically growing B. subtilis does not form microscopically detectable cardiolipin-specific lipid domains, and that NAO is not a specific stain for cardiolipin in this organism.

Perspective: challenges and opportunities for the study of cardiolipin, a key player in bacterial cell structure and function

Cardiolipin (CL) is a key player in bacterial cell biology. CL accumulates at the poles of rod-shaped cells; the polar localization and function of diverse bacterial proteins are CL-dependent. Cardiolipin (CL) is an unusual phospholipid comprised of a glycerol headgroup coupled with two phosphatidate moieties. CL-rich membrane domains are often visualized with the fluorescent indicator 10-N-nonyl-acridine orange (NAO). Recent data show that NAO can also indicate phosphatidylglycerol localization under different experimental conditions, in the absence of CL. The formation of CL-rich membrane domains at bacterial cell poles was predicted to occur spontaneously, by lipid microphase separation arising from the conical CL shape. New data reveal that membrane-anchored cardiolipin synthase A is targeted to the cytoplasmic membrane surface at bacterial cell poles. Thus, localized CL synthesis, interaction of CL with ClsA, and membrane curvature could all contribute to retention of CL at cell poles. These observations provide new insight regarding the mechanism for assembly of CL-rich membrane domains in prokaryotes and eukaryotes.

Cardiolipin synthase A colocalizes with cardiolipin and osmosensing transporter ProP at the poles of Escherichia coli cells

Osmosensing by transporter ProP is modulated by its cardiolipin (CL)-dependent concentration at the poles of Escherichia coli cells. Other contributors to this phenomenon were sought with the BACterial Two-Hybrid System (BACTH). The BACTH-tagged variants T18-ProP and T25-ProP retained ProP function and localization. Their interaction confirmed the ProP homo-dimerization previously established by protein crosslinking. YdhP, YjbJ and ClsA were prominent among the putative ProP interactors identified by the BACTH system. The functions of YdhP and YjbJ are unknown, although YjbJ is an abundant, osmotically induced, soluble protein. ClsA (CL Synthase A) had been shown to determine ProP localization by mediating CL synthesis. Unlike a deletion of clsA, deletion of ydhP or yjbJ had no effect on ProP localization or function. All three proteins were concentrated at the cell poles, but only ClsA localization was CL-dependent. ClsA was shown to be N-terminally processed and membrane-anchored, with dual, cytoplasmic, catalytic domains. Active site amino acid replacements (H224A plus H404A) inactivated ClsA and compromised ProP localization. YdhP and YjbJ may be ClsA effectors, and interactions of YdhP, YjbJ and ClsA with ProP may reflect their colocalization at the cell poles. Targeted CL synthesis may contribute to the polar localization of CL, ClsA and ProP.

Bacterial Mechanosensors

Bacteria represent one of the most evolutionarily successful groups of organisms to inhabit Earth. Their world is awash with mechanical cues, probably the most ancient form of which are osmotic forces. As a result, they have developed highly robust mechanosensors in the form of bacterial mechanosensitive (MS) channels. These channels are essential in osmoregulation, and in this setting, provide one of the simplest paradigms for the study of mechanosensory transduction. We explore the past, present, and future of bacterial MS channels, including the alternate mechanosensory roles that they may play in complex microbial communities. Central to all of these functions is their ability to change conformation in response to mechanical stimuli. We discuss their gating according to the force-from-lipids principle and its applicability to eukaryotic MS channels. This includes the new paradigms emerging for bilayer-mediated channel mechanosensitivity and how this molecular detail may provide advances in both industry and medicine.

Using transposition to introduce eGFP fusions in Sinorhizobium meliloti: A tool to analyze protein localization patterns in bacteria

Conventional methods used for the in vivo analysis of subcellular protein localizations and their spatio-temporal dynamics in prokaryotes are based on either the engineering of N(amino)- or C(carboxy)-terminal fusions of fluorescent proteins with the protein of interest, or involved probing internal sites for tag integration. In addition, the use of inducible or constitutive promoters for the expression of fluorescent fusion proteins can lead to overexpression and result in localization artifacts. Here, we describe a method for the synthesis of fluorescent fusion proteins using transposable elements, which can randomly integrate in the internal sections of the protein coding sequence to produce full-length fluorescent fusion proteins expressed at endogenous levels. The established method was used for investigating subcellular localization of proteins in the soil bacterium and plant symbiont Sinorhizobium meliloti. Two constructs for transposition-based insertion of the enhanced green fluorescent protein (eGFP), as well as for in vivo excision of the selection marker for the production of full-length proteins were engineered. Conjugation with pHB14 plasmid and induction of the transposition in S. meliloti produced approx. 3.22×10⁴ transconjugant colonies harboring the fluorescent marker with the transposition efficiency of 0.8%. Sixteen randomly targeted proteins of diverse functions, fused to the eGFP were identified and analyzed in living cells by epifluorescence microscopy, demonstrating the suitability of the novel tool for massive, random production of fluorescent proteins and for following of these proteins with different localizations inside the prokaryotic cell.

Tat transport in Escherichia coli requires zwitterionic phosphatidylethanolamine but no specific negatively charged phospholipid

Translocation of folded proteins by the Tat system of Escherichia coli is believed to rely on the presence of phosphatidylethanolamine (PE) and the negatively charged phospholipids cardiolipin (CL) and phosphatidylglycerol (PG). Here, we show that while PE is indeed essential for activity, the Tat system is fully functional in a clsA/clsB/clsC deletion strain lacking CL, and in a pgsA deletion strain lacking both PG and CL during aerobic growth on complex media. In contrast to early studies that relied on strains with reduced lipid levels, this study therefore demonstrates that PG and CL are dispensable for Tat transport. The lack of these lipids may be compensated by other anionic phospholipids such as phosphatidic acid, CDP-diacylglycerol or N-acyl-PE.

Bacterial Membranes: Structure, Domains, and Function

The bacterial cytoplasmic membrane is composed of roughly equal proportions of lipids and proteins. The main lipid components are phospholipids, which vary in acyl chain length, saturation, and branching and carry head groups that vary in size and charge. Phospholipid variants determine membrane properties such as fluidity and charge that in turn modulate interactions with membrane-associated proteins. We summarize recent advances in understanding bacterial membrane structure and function, focusing particularly on the possible existence and significance of specialized membrane domains. We review the role of membrane curvature as a spatial cue for recruitment and regulation of proteins involved in morphogenic functions, especially elongation and division. Finally, we examine the role of the membrane, especially regulation of synthesis and fluid properties, in the life cycle of cell wall-deficient L-form bacteria.

Polar localization of MreB actin is inhibited by anionic phospholipids in the rod-shaped bacterium Escherichia coli

Bacterial actin MreB is required for the maintenance of cell polarity. MreB is located underneath the cell membrane and mainly localizes at a central cylindrical part of the cell. In addition, it has recently been found that anionic phospholipids (aPLs: phosphatidylglycerol and cardiolipin) play a crucial role in excluding MreB from the cell poles. Subcellular localization of MreB is positively and negatively regulated by membrane curvature and aPLs, respectively.

Exclusion of assembled MreB by anionic phospholipids at cell poles confers cell polarity for bidirectional growth

Cell polarity determines the direction of cell growth in bacteria. MreB actin spatially regulates peptidoglycan synthesis to enable cells to elongate bidirectionally. MreB densely localizes in the cylindrical part of the rod cell and not in polar regions in Escherichia coli. When treated with A22, which inhibits MreB polymerization, rod-shaped cells became round and MreB was diffusely distributed throughout the cytoplasmic membrane. A22 removal resulted in restoration of the rod shape. Initially, diffuse MreB started to re-assemble, and MreB-free zones were subsequently observed in the cytoplasmic membrane. These MreB-free zones finally became cell poles, allowing the cells to elongate bidirectionally. When MreB was artificially located at the cell poles, an additional pole was created, indicating that artificial localization of MreB at the cell pole induced local peptidoglycan synthesis. It was found that the anionic phospholipids (aPLs), phosphatidylglycerol and cardiolipin, which were enriched in cell poles preferentially interact with monomeric MreB compared with assembled MreB in vitro. MreB tended to localize to cell poles in cells lacking both aPLs, resulting in production of Y-shaped cells. Their findings indicated that aPLs exclude assembled MreB from cell poles to establish cell polarity, thereby allowing cells to elongate in a particular direction.

On the mobility, membrane location and functionality of mechanosensitive channels in Escherichia coli

Bacterial mechanosensitive channels protect cells from structural damage during hypoosmotic shock. MscS, MscL and MscK are the most abundant channels in E. coli and arguably the most important ones in osmoprotection. By combining physiological assays with quantitative photo-activated localization microscopy (qPALM), we find an almost linear relationship between channel abundance and cell survival. A minimum of 100 MscL (or MscS) channels is needed for protection when a single type of channel is expressed. Under native-like conditions MscL, MscS as well as MscK distribute homogeneously over the cytoplasmic membrane and the lateral diffusion of the channels is in accordance with their relative protein mass. However, we observe cluster formation and a reduced mobility of MscL when the majority of the subunits of the pentameric channel contain the fluorescent mEos3.2 protein. These data provide new insights into the quantitative biology of mechanosensitive channels and emphasizes the need for care in analysing protein complexes even when the fluorescent tag has been optimized for monomeric behaviour.

Three-Dimensional Distribution of Phospholipids in Gram Negative Bacteria

Exploration of the molecular structure of the bacterial cell envelope informs our understanding of its role in bacterial growth. This is crucial for research into both inhibiting and promoting bacterial growth as well as fundamental studies of cell cycle control. The spatial arrangement of the lipids in the cell envelope of Gram negative bacteria in particular has attracted considerable research attention in recent years. In this mini-review, we explore advances in understanding the spatial distribution of lipids in the model Gram negative prokaryote Escherichia coli. This includes the distribution of lipids in three dimensions, (a) lateral distribution within a monolayer, (b) asymmetry between bilayers and monolayers, and (c) distribution as a function of progress through membrane division (temporal shifts). We conclude that lipid distribution in E. coli and probably all bacteria is dynamic despite a narrow lipid profile and that the biophysical properties of the membrane are inhomogeneous as a result. Finally, we suggest that further work in this field may indicate how lipid distribution is controlled and what this means for bacterial growth and metabolism and even cell cycle control.

Physics of Intracellular Organization in Bacteria

With the realization that bacteria achieve exquisite levels of spatiotemporal organization has come the challenge of discovering the underlying mechanisms. In this review, we describe three classes of such mechanisms, each of which has physical origins: the use of landmarks, the creation of higher-order structures that enable geometric sensing, and the emergence of length scales from systems of chemical reactions coupled to diffusion. We then examine the diversity of geometric cues that exist even in cells with relatively simple geometries, and end by discussing both new technologies that could drive further discovery and the implications of our current knowledge for the behavior, fitness, and evolution of bacteria. The organizational strategies described here are employed in a wide variety of systems and in species across all kingdoms of life; in many ways they provide a general blueprint for organizing the building blocks of life.

Distribution and dynamics of OXPHOS complexes in the bacterial cytoplasmic membrane

Oxidative phosphorylation (OXPHOS) is an essential process for most living organisms mostly sustained by protein complexes embedded in the cell membrane. In order to thrive, cells need to quickly respond to changes in the metabolic demand or in their environment. An overview of the strategies that can be employed by bacterial cells to adjust the OXPHOS outcome is provided. Regulation at the level of gene expression can only provide a means to adjust the OXPHOS outcome to long-term trends in the environment. In addition, the actual view is that bioenergetic membranes are highly compartmentalized structures. This review discusses what is known about the spatial organization of OXPHOS complexes and the timescales at which they occur. As exemplified with the commensal gut bacterium Escherichia coli, three levels of spatial organization are at play: supercomplexes, membrane microdomains and polar assemblies. This review provides a particular focus on whether dynamic spatial organization can fine-tune the OXPHOS through the definition of specialized functional membrane microdomains. Putative mechanisms responsible for spatio-temporal regulation of the OXPHOS complexes are discussed. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

E. coli MG1655 modulates its phospholipid composition through the cell cycle

This paper describes a study of the phospholipid profile of Escherichia coli MG1655 cultures at the B and D periods of the cell cycle. The results indicate that the phosphatidyl glycerol fraction grows relatively rapidly and that the size of the cardiolipin (CL) fraction does not grow at all during cell elongation. This is consistent with observations that CL is located preferentially at the poles of E. coli. It also suggests that lipid production is controlled as a function of the cell cycle.

Energetics of gating MscS by membrane tension in azolectin liposomes and giant spheroplasts

Mechanosensitive (MS) ion channels are molecular sensors that detect and transduce signals across prokaryotic and eukaryotic cell membranes arising from external mechanical stimuli or osmotic gradients. They play an integral role in mechanosensory responses including touch, hearing, and proprioception by opening or closing in order to facilitate or prevent the flow of ions and organic osmolytes. In this study we use a linear force model of MS channel gating to determine the gating membrane tension (γ) and the gating area change (ΔA) associated with the energetics of MscS channel gating in giant spheroplasts and azolectin liposomes. Analysis of Boltzmann distribution functions describing the dependence of MscS channel gating on membrane tension indicated that the gating area change (ΔA) was the same for MscS channels recorded in both preparations. The comparison of the membrane tension (γ) gating the channel, however, showed a significant difference between the MscS channel activities in these two preparations.

The membrane: transertion as an organizing principle in membrane heterogeneity

The bacterial membrane exhibits a significantly heterogeneous distribution of lipids and proteins. This heterogeneity results mainly from lipid-lipid, protein-protein, and lipid-protein associations which are orchestrated by the coupled transcription, translation and insertion of nascent proteins into and through membrane (transertion). Transertion is central not only to the individual assembly and disassembly of large physically linked groups of macromolecules (alias hyperstructures) but also to the interactions between these hyperstructures. We review here these interactions in the context of the processes in Bacillus subtilis and Escherichia coli of nutrient sensing, membrane synthesis, cytoskeletal dynamics, DNA replication, chromosome segregation, and cell division.

Cytoplasmic Domain of MscS Interacts with Cell Division Protein FtsZ: A Possible Non-Channel Function of the Mechanosensitive Channel in Escherichia Coli

Bacterial mechano-sensitive (MS) channels reside in the inner membrane and are considered to act as emergency valves whose role is to lower cell turgor when bacteria enter hypo-osmotic environments. However, there is emerging evidence that members of the Mechano-sensitive channel Small (MscS) family play additional roles in bacterial and plant cell physiology. MscS has a large cytoplasmic C-terminal region that changes its shape upon activation and inactivation of the channel. Our pull-down and co-sedimentation assays show that this domain interacts with FtsZ, a bacterial tubulin-like protein. We identify point mutations in the MscS C-terminal domain that reduce binding to FtsZ and show that bacteria expressing these mutants are compromised in growth on sublethal concentrations of β-lactam antibiotics. Our results suggest that interaction between MscS and FtsZ could occur upon inactivation and/or opening of the channel and could be important for the bacterial cell response against sustained stress upon stationary phase and in the presence of β-lactam antibiotics.

Negative and positive temperature dependence of potassium leak in MscS mutants: Implications for understanding thermosensitive channels

Bacterial mechanosensitive channel of small conductance (MscS) is a protein, whose activity is modulated by membrane tension, voltage and cytoplasmic crowding. MscS is a homoheptamer and each monomer consists of three transmembrane helices (TM1-3). Hydrophobic pore of the channel is made of TM3s surrounded by peripheral TM1/2s. MscS gating is a complex process, which involves opening and inactivation in response to the increase of membrane tension. A number of MscS mutants were isolated. Among them mutants affecting gating have been found including gain-of-function (GOF) and loss-of-function (LOF) that open at lower or at higher thresholds, respectively. Previously, using an in vivo screen we isolated multiple MscS mutants that leak potassium and some of them were GOF or LOF. Here we show that for a subset of these mutants K+ leak is negatively (NTD) or positively (PTD) temperature dependent. We show that temperature reliance of these mutants does not depend on how MS gating is affected by a particular mutation. Instead, we argue that NTD or PTD leak is due to the opposite allosteric coupling of the structures that determine the temperature dependence to the channel gate. In PTD mutants an increased hydration of the pore vestibule is directly coupled to the increase in the channel conductance. In NTD mutants, at higher temperatures an increased hydration of peripheral structures leads to complete separation of TM3 and a pore collapse.

Lipid-protein interactions: Lessons learned from stress

Biological membranes are essential for normal function and regulation of cells, forming a physical barrier between extracellular and intracellular space and cellular compartments. These physical barriers are subject to mechanical stresses. As a consequence, nature has developed proteins that are able to transpose mechanical stimuli into meaningful intracellular signals. These proteins, termed Mechanosensitive (MS) proteins provide a variety of roles in response to these stimuli. In prokaryotes these proteins form transmembrane spanning channels that function as osmotically activated nanovalves to prevent cell lysis by hypoosmotic shock. In eukaryotes, the function of MS proteins is more diverse and includes physiological processes such as touch, pain and hearing. The transmembrane portion of these channels is influenced by the physical properties such as charge, shape, thickness and stiffness of the lipid bilayer surrounding it, as well as the bilayer pressure profile. In this review we provide an overview of the progress to date on advances in our understanding of the intimate biophysical and chemical interactions between the lipid bilayer and mechanosensitive membrane channels, focusing on current progress in both eukaryotic and prokaryotic systems. These advances are of importance due to the increasing evidence of the role the MS channels play in disease, such as xerocytosis, muscular dystrophy and cardiac hypertrophy. Moreover, insights gained from lipid-protein interactions of MS channels are likely relevant not only to this class of membrane proteins, but other bilayer embedded proteins as well. This article is part of a Special Issue entitled: Lipid-protein interactions.