Polar targeting of Shigella virulence factor IcsA in Enterobacteriacae and Vibrio

Shigella virulence protein VirG is a broadly protective antigen and vaccine candidate

Diarrhea caused by Shigella has been associated with high morbidity and mortality in young children worldwide. There are no licensed vaccines, and those clinically advanced have restricted coverage as they elicit serotype-specific immunity while disease is caused by multiple circulating serotypes. Our group had previously reported a close association between serum antibodies to the Shigella virulence factor VirG (or IcsA) and clinical protection in infected individuals. VirG is highly conserved among Shigella strains and appealing as a broad-spectrum vaccine candidate. In this study, we investigated the immunogenicity and protective capacity of VirG as a subunit vaccine in mice. The surface-exposed alpha (α) domain of VirG (VirGα) was produced as a recombinant protein. This region has almost identical immune reactivity to full-length VirG. Administered intramuscularly with alum, VirGα elicited robust immune responses and high protective efficacy against S. flexneri 2a and S. sonnei. Almost complete protection was afforded by VirGα given intranasally with the E. coli double mutant heat-labile toxin (dmLT). VirGα-specific antibodies recognized VirG expressed on live Shigella, and blocked Shigella adhesion and invasion to human colonic cells. These results show for the first time that VirGα is a promising cross-protective vaccine candidate to prevent Shigella infection.

Physical properties of the bacterial outer membrane

It has long been appreciated that the Gram-negative outer membrane acts as a permeability barrier, but recent studies have uncovered a more expansive and versatile role for the outer membrane in cellular physiology and viability. Owing to recent developments in microfluidics and microscopy, the structural, rheological and mechanical properties of the outer membrane are becoming apparent across multiple scales. In this Review, we discuss experimental and computational studies that have revealed key molecular factors and interactions that give rise to the spatial organization, limited diffusivity and stress-bearing capacity of the outer membrane. These physical properties suggest broad connections between cellular structure and physiology, and we explore future prospects for further elucidation of the implications of outer membrane construction for cellular fitness and survival.

Mechanistic insight into bacterial entrapment by septin cage reconstitution

Septins are cytoskeletal proteins that assemble into hetero-oligomeric complexes and sense micron-scale membrane curvature. During infection with Shigella flexneri, an invasive enteropathogen, septins restrict actin tail formation by entrapping bacteria in cage-like structures. Here, we reconstitute septin cages in vitro using purified recombinant septin complexes (SEPT2-SEPT6-SEPT7), and study how these recognize bacterial cells and assemble on their surface. We show that septin complexes recognize the pole of growing Shigella cells. An amphipathic helix domain in human SEPT6 enables septins to sense positively curved membranes and entrap bacterial cells. Shigella strains lacking lipopolysaccharide components are more efficiently entrapped in septin cages. Finally, cryo-electron tomography of in vitro cages reveals how septins assemble as filaments on the bacterial cell surface.

Septins are cytoskeletal proteins that assemble into complexes and contribute to immunity by entrapping intracellular bacteria in cage-like structures. Here, Lobato-Márquez et al. reconstitute septin cages in vitro using purified recombinant complexes, and study how these recognize bacterial cells and assemble as filaments on their surface.

Direct binding of polymeric GBP1 to LPS disrupts bacterial cell envelope functions

In the outer membrane of gram‐negative bacteria, O‐antigen segments of lipopolysaccharide (LPS) form a chemomechanical barrier, whereas lipid A moieties anchor LPS molecules. Upon infection, human guanylate binding protein‐1 (hGBP1) colocalizes with intracellular gram‐negative bacterial pathogens, facilitates bacterial killing, promotes activation of the lipid A sensor caspase‐4, and blocks actin‐driven dissemination of the enteric pathogen Shigella. The underlying molecular mechanism for hGBP1's diverse antimicrobial functions is unknown. Here, we demonstrate that hGBP1 binds directly to LPS and induces “detergent‐like” LPS clustering through protein polymerization. Binding of polymerizing hGBP1 to the bacterial surface disrupts the O‐antigen barrier, thereby unmasking lipid A, eliciting caspase‐4 recruitment, enhancing antibacterial activity of polymyxin B, and blocking the function of the Shigella outer membrane actin motility factor IcsA. These findings characterize hGBP1 as an LPS‐binding surfactant that destabilizes the rigidity of the outer membrane to exert pleiotropic effects on the functionality of gram‐negative bacterial cell envelopes.

Assembly and Subcellular Localization of Bacterial Type VI Secretion Systems

Bacteria need to deliver large molecules out of the cytosol to the extracellular space or even across membranes of neighboring cells to influence their environment, prevent predation, defeat competitors, or communicate. A variety of protein-secretion systems have evolved to make this process highly regulated and efficient. The type VI secretion system (T6SS) is one of the largest dynamic assemblies in gram-negative bacteria and allows for delivery of toxins into both bacterial and eukaryotic cells. The recent progress in structural biology and live-cell imaging shows the T6SS as a long contractile sheath assembled around a rigid tube with associated toxins anchored to a cell envelope by a baseplate and membrane complex. Rapid sheath contraction releases a large amount of energy used to push the tube and toxins through the membranes of neighboring target cells. Because reach of the T6SS is limited, some bacteria dynamically regulate its subcellular localization to precisely aim at their targets and thus increase efficiency of toxin translocation.

Evolutionary Perspectives on the Moonlighting Functions of Bacterial Factors That Support Actin-Based Motility

Various bacterial pathogens display an intracellular lifestyle and spread from cell to cell through actin-based motility (ABM). ABM requires actin polymerization at the bacterial pole and is mediated by the expression of bacterial factors that hijack the host cell actin nucleation machinery or exhibit intrinsic actin nucleation properties.

Various bacterial pathogens display an intracellular lifestyle and spread from cell to cell through actin-based motility (ABM). ABM requires actin polymerization at the bacterial pole and is mediated by the expression of bacterial factors that hijack the host cell actin nucleation machinery or exhibit intrinsic actin nucleation properties. It is increasingly recognized that bacterial ABM factors, in addition to having a crucial task during the intracellular phase of infection, display “moonlighting” adhesin functions, such as bacterial aggregation, biofilm formation, and host cell adhesion/invasion. Here, we review our current knowledge of ABM factors and their additional functions, and we propose that intracellular ABM functions have evolved from ancestral, extracellular adhesin functions.

Taming the Triskelion: Bacterial Manipulation of Clathrin

The entry of pathogens into nonphagocytic host cells has received much attention in the past three decades, revealing a vast array of strategies employed by bacteria and viruses. A method of internalization that has been extensively studied in the context of viral infections is the use of the clathrin-mediated pathway. More recently, a role for clathrin in the entry of some intracellular bacterial pathogens was discovered. Classically, clathrin-mediated endocytosis was thought to accommodate internalization only of particles smaller than 150 nm; however, this was challenged upon the discovery that Listeria monocytogenes requires clathrin to enter eukaryotic cells. Now, with discoveries that clathrin is required during other stages of some bacterial infections, another paradigm shift is occurring. There is a more diverse impact of clathrin during infection than previously thought. Much of the recent data describing clathrin utilization in processes such as bacterial attachment, cell-to-cell spread and intracellular growth may be due to newly discovered divergent roles of clathrin in the cell. Not only does clathrin act to facilitate endocytosis from the plasma membrane, but it also participates in budding from endosomes and the Golgi apparatus and in mitosis. Here, the manipulation of clathrin processes by bacterial pathogens, including its traditional role during invasion and alternative ways in which clathrin supports bacterial infection, is discussed. Researching clathrin in the context of bacterial infections will reveal new insights that inform our understanding of host-pathogen interactions and allow researchers to fully appreciate the diverse roles of clathrin in the eukaryotic cell.

Shigella MreB promotes polar IcsA positioning for actin tail formation

Pathogenic Shigella bacteria are a paradigm to address key issues of cell and infection biology. Polar localisation of the Shigella autotransporter protein IcsA is essential for actin tail formation, which is necessary for the bacterium to travel from cell-to-cell; yet how proteins are targeted to the bacterial cell pole is poorly understood. The bacterial actin homologue MreB has been extensively studied in broth culture using model organisms including Escherichia coli, Bacillus subtilis and Caulobacter crescentus, but has never been visualised in rod-shaped pathogenic bacteria during infection of host cells. Here, using single-cell analysis of intracellular Shigella, we discover that MreB accumulates at the cell pole of bacteria forming actin tails, where it colocalises with IcsA. Pharmacological inhibition of host cell actin polymerisation and genetic deletion of IcsA is used to show, respectively, that localisation of MreB to the cell poles precedes actin tail formation and polar localisation of IcsA. Finally, by exploiting the MreB inhibitors A22 and MP265, we demonstrate that MreB polymerisation can support actin tail formation. We conclude that Shigella MreB promotes polar IcsA positioning for actin tail formation, and suggest that understanding the bacterial cytoskeleton during host–pathogen interactions can inspire development of new therapeutic regimes for infection control.

This article has an associated First Person interview with the first author of the paper.

Summary: The pathogen Shigella forms actin tails to move through the cytosol of infected cells. We show that the bacterial actin homologue MreB can help to position the autotransporter protein IcsA for such actin tail formation.

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.

Cardiolipin Synthesis and Outer Membrane Localization Are Required for Shigella flexneri Virulence

Cardiolipin, an anionic phospholipid that resides at the poles of the inner and outer membranes, is synthesized primarily by the putative cardiolipin synthase ClsA in Shigella flexneri. An S. flexneri clsA mutant had no cardiolipin detected within its membrane, grew normally in vitro, and invaded cultured epithelial cells, but it failed to form plaques in epithelial cell monolayers, indicating that cardiolipin is required for virulence. The clsA mutant was initially motile within the host cell cytoplasm but formed filaments and lost motility during replication and failed to spread efficiently to neighboring cells. Mutation of pbgA, which encodes the transporter for cardiolipin from the inner membrane to the outer membrane, also resulted in loss of plaque formation. The S. flexneri pbgA mutant had normal levels of cardiolipin in the inner membrane, but no cardiolipin was detected in the outer membrane. The pbgA mutant invaded and replicated normally within cultured epithelial cells but failed to localize the actin polymerization protein IcsA properly on the bacterial surface and was unable to spread to neighboring cells. The clsA mutant, but not the pbgA mutant, had increased phosphatidylglycerol in the outer membrane. This appeared to compensate partially for the loss of cardiolipin in the outer membrane, allowing some IcsA localization in the outer membrane of the clsA mutant. We propose a dual function for cardiolipin in S. flexneri pathogenesis. In the inner membrane, cardiolipin is essential for proper cell division during intracellular growth. In the outer membrane, cardiolipin facilitates proper presentation of IcsA on the bacterial surface.

The human pathogen Shigella flexneri causes bacterial dysentery by invading colonic epithelial cells, rapidly multiplying within their cytoplasm, and then spreading intercellularly to neighboring cells. Worldwide, Shigella spp. infect hundreds of millions of people annually, with fatality rates up to 15%. Antibiotic treatment of Shigella infections is compromised by increasing antibiotic resistance, and there is no approved vaccine to prevent future infections. This has created a growing need to understand Shigella pathogenesis and identify new targets for antimicrobial therapeutics. Here we show a previously unknown role of phospholipids in S. flexneri pathogenesis. We demonstrate that cardiolipin is required in the outer membrane for proper surface localization of IcsA and in the inner membrane for cell division during growth in the host cell cytoplasm.

Super-Resolution Imaging of Protein Secretion Systems and the Cell Surface of Gram-Negative Bacteria

Gram-negative bacteria have a highly evolved cell wall with two membranes composed of complex arrays of integral and peripheral proteins, as well as phospholipids and glycolipids. In order to sense changes in, respond to, and exploit their environmental niches, bacteria rely on structures assembled into or onto the outer membrane. Protein secretion across the cell wall is a key process in virulence and other fundamental aspects of bacterial cell biology. The final stage of protein secretion in Gram-negative bacteria, translocation across the outer membrane, is energetically challenging so sophisticated nanomachines have evolved to meet this challenge. Advances in fluorescence microscopy now allow for the direct visualization of the protein secretion process, detailing the dynamics of (i) outer membrane biogenesis and the assembly of protein secretion systems into the outer membrane, (ii) the spatial distribution of these and other membrane proteins on the bacterial cell surface, and (iii) translocation of effector proteins, toxins and enzymes by these protein secretion systems. Here we review the frontier research imaging the process of secretion, particularly new studies that are applying various modes of super-resolution microscopy.

Structural insights into the architecture of the Shigella flexneri virulence factor IcsA/VirG and motifs involved in polar distribution and secretion

IcsA/VirG is a key virulence factor of the human pathogen Shigella flexneri, acting as both an adhesin and actin-polymerizing factor during infection. We identified a soluble expression construct of the IcsA/VirG α-domain using the ESPRIT library screening system and determined its structure to 1.9Å resolution. In addition to the previously characterized autochaperone domain, our structure reveals a new domain, which shares a common fold with the autochaperone domains of various autotransporters. We further provide insight into the previously structurally uncharacterized β-helix domain that harbors the polar targeting motif and passenger-associated transport repeat. This structure is the first of any member of the recently identified passenger-associated transport repeat-containing autotransporters. Thus, it provides new insights into the overall architecture of this class of autotransporters, the function of the identified additional autochaperone domain and the structural properties of motifs involved in polar targeting and secretion of the Shigella flexneri virulence factor IcsA/VirG.

Molecular and Cellular Mechanisms of Shigella flexneri Dissemination

The intracellular pathogen Shigella flexneri is the causative agent of bacillary dysentery in humans. The disease is characterized by bacterial invasion of intestinal cells, dissemination within the colonic epithelium through direct spread from cell to cell, and massive inflammation of the intestinal mucosa. Here, we review the mechanisms supporting S. flexneri dissemination. The dissemination process primarily relies on actin assembly at the bacterial pole, which propels the pathogen throughout the cytosol of primary infected cells. Polar actin assembly is supported by polar expression of the bacterial autotransporter family member IcsA, which recruits the N-WASP/ARP2/3 actin assembly machinery. As motile bacteria encounter cell-cell contacts, they form plasma membrane protrusions that project into adjacent cells. In addition to the ARP2/3-dependent actin assembly machinery, protrusion formation relies on formins and myosins. The resolution of protrusions into vacuoles occurs through the collapse of the protrusion neck, leading to the formation of an intermediate membrane-bound compartment termed vacuole-like protrusions (VLPs). VLP formation requires tyrosine kinase and phosphoinositide signaling in protrusions, which relies on the integrity of the bacterial type 3 secretion system (T3SS). The T3SS is also required for escaping double membrane vacuoles through the activity of the T3SS translocases IpaB and IpaC, and the effector proteins VirA and IcsB. Numerous factors supporting envelope biogenesis contribute to IcsA exposure and maintenance at the bacterial pole, including LPS synthesis, membrane proteases, and periplasmic chaperones. Although less characterized, the assembly and function of the T3SS in the context of bacterial dissemination also relies on factors supporting envelope biogenesis. Finally, the dissemination process requires the adaptation of the pathogen to various cellular compartments through transcriptional and post-transcriptional mechanisms.

Protein-protein interactions and the spatiotemporal dynamics of bacterial outer membrane proteins

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We discuss spatiotemporal patterning in the bacterial outer membrane.

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Promiscuous interactions between outer membrane proteins govern their behaviour.

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Turnover and biogenesis of outer membrane proteins linked to formation of clusters.

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Implications of spatiotemporal patterning for bacterial physiology discussed.

It has until recently been unclear whether outer membrane proteins (OMPs) of Gram-negative bacteria are organized or distributed randomly. Studies now suggest promiscuous protein–protein interactions (PPIs) between β-barrel OMPs in Escherichia coli govern their local and global dynamics, engender spatiotemporal patterning of the outer membrane into micro-domains and are the basis of β-barrel protein turnover. We contextualize these latest advances, speculate on areas of bacterial cell biology that might be influenced by the organization of OMPs into supramolecular assemblies, and highlight the new questions and controversies this revised view of the bacterial outer membrane raises.

Type V Secretion: the Autotransporter and Two-Partner Secretion Pathways

The autotransporter and two-partner secretion (TPS) pathways are used by E. coli and many other Gram-negative bacteria to delivervirulence factors into the extracellular milieu.Autotransporters arecomprised of an N-terminal extracellular ("passenger") domain and a C-terminal β barrel domain ("β domain") that anchors the protein to the outer membrane and facilitates passenger domain secretion. In the TPS pathway, a secreted polypeptide ("exoprotein") is coordinately expressed with an outer membrane protein that serves as a dedicated transporter. Bothpathways are often grouped together under the heading "type V secretion" because they have many features in common and are used for the secretion of structurally related polypeptides, but it is likely that theyhave distinct evolutionary origins. Although it was proposed many years ago that autotransporterpassenger domains are transported across the outer membrane through a channel formed by the covalently linked β domain, there is increasing evidence that additional factors are involved in the translocation reaction. Furthermore, details of the mechanism of protein secretion through the TPS pathway are only beginning to emerge. In this chapter I discussour current understanding ofboth early and late steps in the biogenesis of polypeptides secreted through type V pathways and current modelsofthe mechanism of secretion.

A small conserved motif supports polarity augmentation of Shigella flexneri IcsA

The rod-shaped enteric intracellular pathogen Shigella flexneri and other Shigella species are the causative agents of bacillary dysentery. S. flexneri are able to spread within the epithelial lining of the gut, resulting in lesion formation, cramps and bloody stools. The outer membrane protein IcsA is essential for this spreading process. IcsA is the initiator of an actin-based form of motility whereby it allows the formation of a filamentous actin 'tail' at the bacterial pole. Importantly, IcsA is specifically positioned at the bacterial pole such that this process occurs asymmetrically. The mechanism of IcsA polarity is not completely understood, but it appears to be a multifactorial process involving factors intrinsic to IcsA and other regulating factors. In this study, we further investigated IcsA polarization by its intramolecular N-terminal and central polar-targeting (PT) regions (nPT and cPT regions, respectively). The results obtained support a role in polar localization for the cPT region and contend the role of the nPT region. We identified single IcsA residues that have measurable impacts on IcsA polarity augmentation, resulting in decreased S. flexneri sprading efficiency. Intriguingly, regions and residues involved in PT clustered around a highly conserved motif which may provide a functional scaffold for polarity-augmenting residues. How these results fit with the current model of IcsA polarity determination is discussed.

Multi-color imaging of the bacterial nucleoid and division proteins with blue, orange, and near-infrared fluorescent proteins

Studies of the spatiotemporal protein dynamics within live bacterial cells impose a strong demand for multi-color imaging. Despite the increasingly large collection of fluorescent protein (FP) variants engineered to date, only a few of these were successfully applied in bacteria. Here, we explore the performance of recently engineered variants with the blue (TagBFP), orange (TagRFP-T, mKO2), and far-red (mKate2) spectral colors by tagging HU, LacI, MinD, and FtsZ for visualizing the nucleoid and the cell division process. We find that, these FPs outperformed previous versions in terms of brightness and photostability at their respective spectral range, both when expressed as cytosolic label and when fused to native proteins. As this indicates that their folding is sufficiently fast, these proteins thus successfully expand the applicable spectra for multi-color imaging in bacteria. A near-infrared protein (eqFP670) is found to be the most red-shifted protein applicable to bacteria so far, with brightness and photostability that are advantageous for cell-body imaging, such as in microfluidic devices. Despite the multiple advantages, we also report the alarming observation that TagBFP directly interacts with TagRFP-T, causing interference of localization patterns between their fusion proteins. Our application of diverse FPs for endogenous tagging provides guidelines for future engineering of fluorescent fusions in bacteria, specifically: (1) The performance of newly developed FPs should be quantified in vivo for their introduction into bacteria; (2) spectral crosstalk and inter-variant interactions between FPs should be carefully examined for multi-color imaging; and (3) successful genomic fusion to the 5^′-end of a gene strongly depends on the translational read-through of the inserted coding sequence.

A polar-localized iron-binding protein determines the polar targeting of Burkholderia BimA autotransporter and actin tail formation

Intracellular bacterial pathogens including Shigella, Listeria, Mycobacteria, Rickettsia and Burkholderia spp. deploy a specialized surface protein onto one pole of the bacteria to induce filamentous actin tail formation for directional movement within host cytosol. The mechanism underlying polar targeting of the actin tail proteins is unknown. Here we perform a transposon screen in Burkholderia thailandensis and identify a conserved bimC that is required for actin tail formation mediated by BimA from B. thailandensis and its closely related pathogenic species B. pseudomallei and B. mallei. bimC is located upstream of bimA in the same operon. Loss of bimC results in even distribution of BimA on the outer membrane surface, where actin polymerization still occurs. BimC is targeted to the same bacterial pole independently of BimA. BimC confers polar targeting of BimA prior to BimA translocation across bacterial inner membrane. BimC is an iron-binding protein, requiring a four-cysteine cluster at the carboxyl terminus. Mutation of the cysteine cluster disrupts BimC polar localization. Truncation analyses identify the transmembrane domain in BimA being responsible for its polar targeting. Consistently, BimC can interact with BimA transmembrane domain in an iron binding-dependent manner. Our study uncovers a new mechanism that determines the polar distribution of bacteria-induced actin tail in infected host cells.

Delineation of polar localization domains of Agrobacterium tumefaciens type IV secretion apparatus proteins VirB4 and VirB11

Agrobacterium tumefaciens transfers DNA and proteins to a plant cell through a type IV secretion apparatus assembled by the VirB proteins. All VirB proteins localized to a cell pole, although these conclusions are in dispute. To study subcellular location of the VirB proteins and to identify determinants of their subcellular location, we tagged two proteins, VirB4 and VirB11, with the visual marker green fluorescent protein (GFP) and studied localization of the fusion proteins by epifluorescence microscopy. Both GFP-VirB4 and GFP-VirB11 fusions localized to a single cell pole. GFP-VirB11 was also functional in DNA transfer. To identify the polar localization domains (PLDs) of VirB4 and VirB11, we analyzed fusions of GFP with smaller segments of the two proteins. Two noncontiguous regions in VirB4, residues 236–470 and 592–789, contain PLDs. The VirB11 PLD mapped to a 69 amino acid segment, residues 149–217, in the central region of the protein. These domains are probably involved in interactions that target the two proteins to a cell pole.

Polar localization of Escherichia coli chemoreceptors requires an intact Tol-Pal complex

Subcellular biomolecular localization is critical for the metabolic and structural properties of the cell. The functional implications of the spatiotemporal distribution of protein complexes during the bacterial cell cycle have long been acknowledged; however, the molecular mechanisms for generating and maintaining their dynamic localization in bacteria are not completely understood. Here we demonstrate that the trans-envelope Tol-Pal complex, a widely conserved component of the cell envelope of Gram-negative bacteria, is required to maintain the polar positioning of chemoreceptor clusters in Escherichia coli. Localization of the chemoreceptors was independent of phospholipid composition of the membrane and the curvature of the cell wall. Instead, our data indicate that chemoreceptors interact with components of the Tol-Pal complex and that this interaction is required to polarly localize chemoreceptor clusters. We found that disruption of the Tol-Pal complex perturbs the polar localization of chemoreceptors, alters cell motility, and affects chemotaxis. We propose that the E. coli Tol-Pal complex restricts mobility of the chemoreceptor clusters at the cell poles and may be involved in regulatory mechanisms that co-ordinate cell division and segregation of the chemosensory machinery.

Polar localization of PhoN2, a periplasmic virulence-associated factor of Shigella flexneri, is required for proper IcsA exposition at the old bacterial pole

Proper protein localization is critical for bacterial virulence. PhoN2 is a virulence-associated ATP-diphosphohydrolase (apyrase) involved in IcsA-mediated actin-based motility of S. flexneri. Herein, by analyzing a ΔphoN2 mutant of the S. flexneri strain M90T and by generating phoN2::HA fusions, we show that PhoN2, is a periplasmic protein that strictly localizes at the bacterial poles, with a strong preference for the old pole, the pole where IcsA is exposed, and that it is required for proper IcsA exposition. PhoN2-HA was found to be polarly localized both when phoN2::HA was ectopically expressed in a Escherichia coli K-12 strain and in a S. flexneri virulence plasmid-cured mutant, indicating a conserved mechanism of PhoN2 polar delivery across species and that neither IcsA nor the expression of other virulence-plasmid encoded genes are involved in this process. To assess whether PhoN2 and IcsA may interact, two-hybrid and cross-linking experiments were performed. While no evidence was found of a PhoN2-IcsA interaction, unexpectedly the outer membrane protein A (OmpA) was shown to bind PhoN2-HA through its periplasmic-exposed C-terminal domain. Therefore, to identify PhoN2 domains involved in its periplasmic polar delivery as well as in the interaction with OmpA, a deletion and a set of specific amino acid substitutions were generated. Analysis of these mutants indicated that neither the (183)PAPAP(187) motif of OmpA, nor the N-terminal polyproline (43)PPPP(46) motif and the Y155 residue of PhoN2 are involved in this interaction while P45, P46 and Y155 residues were found to be critical for the correct folding and stability of the protein. The relative rapid degradation of these amino acid-substituted recombinant proteins was found to be due to unknown S. flexneri-specific protease(s). A model depicting how the PhoN2-OmpA interaction may contribute to proper polar IcsA exposition in S. flexneri is presented.

How do bacteria localize proteins to the cell pole?

It is now well appreciated that bacterial cells are highly organized, which is far from the initial concept that they are merely bags of randomly distributed macromolecules and chemicals. Central to their spatial organization is the precise positioning of certain proteins in subcellular domains of the cell. In particular, the cell poles - the ends of rod-shaped cells - constitute important platforms for cellular regulation that underlie processes as essential as cell cycle progression, cellular differentiation, virulence, chemotaxis and growth of appendages. Thus, understanding how the polar localization of specific proteins is achieved and regulated is a crucial question in bacterial cell biology. Often, polarly localized proteins are recruited to the poles through their interaction with other proteins or protein complexes that were already located there, in a so-called diffusion-and-capture mechanism. Bacteria are also starting to reveal their secrets on how the initial pole 'recognition' can occur and how this event can be regulated to generate dynamic, reproducible patterns in time (for example, during the cell cycle) and space (for example, at a specific cell pole). Here, we review the major mechanisms that have been described in the literature, with an emphasis on the self-organizing principles. We also present regulation strategies adopted by bacterial cells to obtain complex spatiotemporal patterns of protein localization.

Fusion of HPV L1 into Shigella surface IcsA: a new approach in developing live attenuated Shigella-HPV vaccine

Despite the success of L1 virus-like particles (VLPs) vaccines in prevention of high-risk human papillomavirus (HPV) infection and cervical cancer, extraordinary high cost for the complete vaccination has impeded widespread use of the vaccine in resource-poor countries, where cervical cancers impose greater challenge. Presentation of HPV L1 protein by attenuated pathogenic bacteria through natural infection provides a promising low-cost and convenient alternative. Here, we describe the construction and characterization of attenuated L1-expressing Shigella vaccine candidate, by fusion of L1 into the autotransporter of Shigella sonnei, IcsA, an essential virulence factor responsible for actin-based motility. The functional α domain of IcsA was replaced by codon-optimized L1 gene with independent open reading frames (ORFs) facilitated by suicide vector pJCB12. The L1 gene was stabilized in the genome of recombinant S. sonnei with protein expression and assembly of VLPs in the bacterial cytoplasm. Through conjunctival route vaccination in guinea pigs, L1-containing S. sonnei was able to elicit specific immune response to HPV16 L1 VLP as well as bacterial antigens. The results demonstrated the feasibility of the novel stratagem to develop prophylactic Shigella-HPV vaccines.

Biogenesis of YidC cytoplasmic membrane substrates is required for positioning of autotransporter IcsA at future poles

Localization of proteins to specific sites within bacterial cells is often critical to their function. In rod-shaped bacteria, proteins involved in diverse and important cell processes localize to the cell poles. The molecular mechanisms by which these proteins are targeted to the pole, however, are poorly understood. The Shigella autotransporter protein IcsA, which is localized to the pole on the surface of the bacterium, is targeted to the pole in the cytoplasm by a mechanism that is conserved across multiple Gram-negative bacterial species and has thus served as an important and informative model for studying polar localization. We present evidence that in Escherichia coli, the establishment of polar positional information recognized by IcsA requires the activity of the cytoplasmic membrane protein insertase YidC. We show that the role of YidC in IcsA localization is independent of the cell septation and cytokinesis proteins FtsQ and FtsEX. FtsQ is required for polar localization of IcsA and, based on cross-linking studies, is inserted in the vicinity of YidC, but, we find, is not dependent on YidC for membrane insertion. FtsEX is a YidC substrate, but we find that it is not required for polar localization of IcsA. These findings indicate that polar positional information recognized by IcsA depends on one or more membrane proteins that require YidC for proper membrane insertion.

PBP deletion mutants of Escherichia coli exhibit irregular distribution of MreB at the deformed zones

MreB is a cytoskeletal protein, which is responsible for maintaining proper cellular morphology and is essential for cell survival. Likewise, penicillin-binding protein 5 (PBP5) helps in maintaining cell shape, though non-essential for survival. The contradicting feature of these two proteins paves the way for this study, wherein we attempt to draw a relation on the nature of distribution of MreB in PBP deletion mutants. The study revealed that the uniform MreB helices/patches were destabilized/disturbed at the zone of deformities of the PBP mutants, whereas the helical patterns were retained at the regions maintaining a rod shape. We interpret that MreB remains functional irrespective of its distribution being misguided by the aberrant shapes of PBP mutants.

The intracellular citrus huanglongbing bacterium, 'Candidatus Liberibacter asiaticus' encodes two novel autotransporters

Proteins secreted by the type V secretion system (T5SS), known as autotransporters, are large extracellular virulence proteins localized to the bacterial poles. In this study, we characterized two novel autotransporter proteins of 'Candidatus Liberibacter asiaticus' (Las), and redesignated them as LasAI and LasAII in lieu of the previous names HyvI and HyvII. As a phloem-limited, intracellular bacterial pathogen, Las has a significantly reduced genome and causes huanglongbing (HLB), a devastating disease of citrus worldwide. Bioinformatic analyses revealed that LasAI and LasAII share the structural features of an autotransporter family containing large repeats of a passenger domain and a unique C-terminal translocator domain. When fused to the GFP gene and expressed in E. coli, the LasAI C-terminus and the full length LasAII were localized to the bacterial poles, similar to other members of autotransporter family. Despite the absence of a typical signal peptide, LasAI was found to localize at the cell surface by immuno-dot blot using a monoclonal antibody against the partial LasAI protein. Its surface localization was also confirmed by the removal of the LasAI antigen using a proteinase K treatment of the intact bacterial cells. When co-inoculated with a P19 gene silencing suppressor and transiently expressed in tobacco leaves, the GFP-LasAI translocator targeted to the mitochondria. This is the first report that Las encodes novel autotransporters that target to mitochondria when expressed in the plants. These findings may lead to a better understanding of the pathogenesis of this intracellular bacterium.

Outer membrane protein A (OmpA): a new player in shigella flexneri protrusion formation and inter-cellular spreading

Outer membrane protein A (OmpA) is a multifaceted predominant outer membrane protein of Escherichia coli and other Enterobacteriaceae whose role in the pathogenesis of various bacterial infections has recently been recognized. Here, the role of OmpA on the virulence of Shigella flexneri has been investigated. An ompA mutant of wild-type S. flexneri 5a strain M90T was constructed (strain HND92) and it was shown to be severely impaired in cell-to-cell spreading since it failed to plaque on HeLa cell monolayers. The lack of OmpA significantly reduced the levels of IcsA while the levels of cell associated and released IcsP-cleaved 95 kDa amino-terminal portion of the mature protein were similar. Nevertheless, the ompA mutant displayed IcsA exposed across the entire bacterial surface. Surprisingly, the ompA mutant produced proper F-actin comet tails, indicating that the aberrant IcsA exposition at bacterial lateral surface did not affect proper activation of actin-nucleating proteins, suggesting that the absence of OmpA likely unmasks mature or cell associated IcsA at bacterial lateral surface. Moreover, the ompA mutant was able to invade and to multiply within HeLa cell monolayers, although internalized bacteria were found to be entrapped within the host cell cytoplasm. We found that the ompA mutant produced significantly less protrusions than the wild-type strain, indicating that this defect could be responsible of its inability to plaque. Although we could not definitely rule out that the ompA mutation might exert pleiotropic effects on other S. flexneri genes, complementation of the ompA mutation with a recombinant plasmid carrying the S. flexneri ompA gene clearly indicated that a functional OmpA protein is required and sufficient for proper IcsA exposition, plaque and protrusion formation. Moreover, an independent ompA mutant was generated. Since we found that both mutants displayed identical virulence profile, these results further supported the findings presented in this study.

Analysis of surface protein expression reveals the growth pattern of the gram-negative outer membrane

The outer membrane (OM) of Gram-negative bacteria is a complex bilayer composed of proteins, phospholipids, lipoproteins, and lipopolysaccharides. Despite recent advances revealing the molecular pathways underlying protein and lipopolysaccharide incorporation into the OM, the spatial distribution and dynamic regulation of these processes remain poorly understood. Here, we used sequence-specific fluorescent labeling to map the incorporation patterns of an OM-porin protein, LamB, by labeling proteins only after epitope exposure on the cell surface. Newly synthesized LamB appeared in discrete puncta, rather than evenly distributed over the cell surface. Further growth of bacteria after labeling resulted in divergence of labeled LamB puncta, consistent with a spatial pattern of OM growth in which new, unlabeled material was also inserted in patches. At the poles, puncta remained relatively stationary through several rounds of division, a salient characteristic of the OM protein population as a whole. We propose a biophysical model of growth in which patches of new OM material are added in discrete bursts that evolve in time according to Stokes flow and are randomly distributed over the cell surface. Simulations based on this model demonstrate that our experimental observations are consistent with a bursty insertion pattern without spatial bias across the cylindrical cell surface, with approximately one burst of ∼10⁻² µm² of OM material per two minutes per µm². Growth by insertion of discrete patches suggests that stochasticity plays a major role in patterning and material organization in the OM.

All Gram-negative bacteria share common structural features, including an inner membrane, a stiff cell wall, and an outer membrane. Balancing growth in all three of these layers is critical for bacterial proliferation and survival, and malfunctions in growth often lead to cellular deformations and/or cell death. However, relatively little is known about how the incorporation of new material into the outer membrane is regulated in space and time. This work combines time-lapse microscopy with biophysical modeling and simulations to examine potential mechanisms by which new material is added to the outer membrane of the rod-shaped Gram-negative bacterium Escherichia coli. Our results indicate that the outer membrane grows in discrete bursts randomly distributed over the cylindrical cell surface. Each insertion event adds a random amount of new material, pushing old material into new locations and thus expanding the cell membrane. Using our biophysical model, we generated simulated fluorescence images and directly compared analyses of our experimental and computational results to constrain the rate and size of bursts of growth. Together, this indicates that growth of the outer membrane does not require spatial regulation, and the stochastic nature of insertion may contribute to the establishment of cellular patterning and asymmetry.

Peptidoglycan maturation enzymes affect flagellar functionality in bacteria

The flagellar machinery is a highly complex organelle composed of a free rotating flagellum and a fixed stator that converts energy into movement. The assembly of the flagella and the stator requires interactions with the peptidoglycan layer through which the organelle has to pass for externalization. Lytic transglycosylases are peptidoglycan degrading enzymes that cleave the sugar backbone of peptidoglycan layer. We show that an endogenous lytic transglycosylase is required for full motility of Helicobacter pylori and colonization of the gastric mucosa. Deficiency of motility resulted from a paralysed phenotype implying an altered ability to generate flagellar rotation. Similarly, another Gram-negative pathogen Salmonella typhimurium and the Gram-positive pathogen Listeria monocytogenes required the activity of lytic transglycosylases, Slt or MltC, and a glucosaminidase (Auto), respectively, for full motility. Furthermore, we show that in absence of the appropriate lytic transglycosylase, the flagellar motor protein MotB from H. pylori does not localize properly to the bacterial pole. We present a new model involving the maturation of the surrounding peptidoglycan for the proper anchoring and functionality of the flagellar motor.

Physical constraints on the establishment of intracellular spatial gradients in bacteria

Background

Bacteria dynamically regulate their intricate intracellular organization involving proteins that facilitate cell division, motility, and numerous other processes. Consistent with this sophisticated organization, bacteria are able to create asymmetries and spatial gradients of proteins by localizing signaling pathway components. We use mathematical modeling to investigate the biochemical and physical constraints on the generation of intracellular gradients by the asymmetric localization of a source and a sink.

Results

We present a systematic computational analysis of the effects of other regulatory mechanisms, such as synthesis, degradation, saturation, and cell growth. We also demonstrate that gradients can be established in a variety of bacterial morphologies such as rods, crescents, spheres, branched and constricted cells.

Conclusions

Taken together, these results suggest that gradients are a robust and potentially common mechanism for providing intracellular spatial cues.

BmaC, a novel autotransporter of Brucella suis, is involved in bacterial adhesion to host cells

Brucella is an intracellular pathogen responsible of a zoonotic disease called brucellosis. Brucella survives and proliferates within several types of phagocytic and non-phagocytic cells. Like in other pathogens, adhesion of brucellae to host surfaces was proposed to be an important step in the infection process. Indeed, Brucella has the capacity to bind to culture human cells and key components of the extracellular matrix, such as fibronectin. However, little is known about the molecular bases of Brucella adherence. In an attempt to identify bacterial genes encoding adhesins, a phage display library of Brucella suis was panned against fibronectin. Three fibronectin-binding proteins of B. suis were identified using this approach. One of the candidates, designated BmaC was a very large protein of 340 kDa that is predicted to belong to the type I (monomeric) autotransporter family. Microscopy studies showed that BmaC is located at one pole on the bacterial surface. The phage displaying the fibronectin-binding peptide of BmaC inhibited the attachment of brucellae to both, HeLa cells and immobilized fibronectin in vitro. In addition, a bmaC deletion mutant was impaired in the ability of B. suis to attach to immobilized fibronectin and to the surface of HeLa and A549 cells and was out-competed by the wild-type strain in co-infection experiments. Finally, anti-fibronectin or anti-BmaC antibodies significantly inhibited the binding of wild-type bacteria to HeLa cells. Our results highlight the role of a novel monomeric autotransporter protein in the adhesion of B. suis to the extracellular matrix and non-phagocytic cells via fibronectin binding.

Genetic reporter system for positioning of proteins at the bacterial pole

Spatial organization within bacteria is fundamental to many cellular processes, although the basic mechanisms underlying localization of proteins to specific sites within bacteria are poorly understood. The study of protein positioning has been limited by a paucity of methods that allow rapid large-scale screening for mutants in which protein positioning is altered. We developed a genetic reporter system for protein localization to the pole within the bacterial cytoplasm that allows saturation screening for mutants in Escherichia coli in which protein localization is altered. Utilizing this system, we identify proteins required for proper positioning of the Shigella autotransporter IcsA. Autotransporters, widely distributed bacterial virulence proteins, are secreted at the bacterial pole. We show that the conserved cell division protein FtsQ is required for localization of IcsA and other autotransporters to the pole. We demonstrate further that this system can be applied to the study of proteins other than autotransporters that display polar positioning within bacterial cells.

Many proteins localize to specific sites within bacterial cells, and localization to these sites is frequently critical to proper protein function. The mechanisms that underlie protein localization are incompletely understood, in part because of the paucity of methods that allow saturation screening for mutants in which protein localization is altered. We developed a genetic reporter assay that enables screening of bacterial populations for changes in localization of proteins to the bacterial pole, and we demonstrate the utility of the system in identifying factors required for proper localization of the polar Shigella autotransporter protein IcsA. Using this method, we identify the conserved cell division protein FtsQ as being required for positioning of IcsA to the bacterial pole. We demonstrate further that the requirement for FtsQ for polar positioning applies to other autotransporters and that the method can be applied to polar proteins other than autotransporters.

IcsA autotransporter passenger promotes increased fusion protein expression on the cell surface

Background

Autotransporters are attractive cell surface display vehicles as they lack complex adaptor proteins necessary for protein export. Recent reports have suggested that the native effector domain (α domain) and translocation domain (β domain) interact with each other to drive translocation of the effector domain to the outer membrane. In this report we compared the expression, surface localisation and folding of TEM-1 β-lactamase (Bla) and maltose binding protein (MalE or MBP) fused to either full length Shigella flexneri IcsA (IcsA) autotransporter or to the β domain alone (IcsA_β) to determine the contribution of the native IcsA α domain in presenting the fusion proteins on the surface of E. coli K-12 UT5600 (ΔompT).

Results

Expression of IcsA-Bla was greater than IcsA_β-Bla. High levels of IcsA-MalE were detected but IcsA_β-MalE was not expressed. All fusion proteins other than IcsA_β-MalE were localised to the outer membrane and were detected on the surface of UT5600 via immunofluorescence microscopy. All bacteria expressing IcsA-MalE were labelled with both α-IcsA and α-MBP. UT5600 expressing IcsA_β-MalE was not labelled with α-MBP. A third of UT5600 expressing IcsA-Bla were detectable with α-Bla but only 5% of UT5600 (IcsA_β-Bla) were labelled with α-Bla. The correct folding of the Bla moiety when fused to IcsA and IcsA_β was also retained as UT5600 expressing either fusion protein exhibited a decreased zone of inhibition in the presence of ampicillin. UT5600 expressing IcsA-Bla was more resistant compared to UT5600 expressing IcsA_β-Bla.

Conclusions

The export mechanism of autotransporters is not well understood but accumulating evidence suggest a critical role for the native effector or α domain in facilitating its own export via interactions with the translocation or β domain. This is the first report directly comparing expression of heterologous proteins fused to the full length IcsA autotransporter and fusion to the β domain alone. Protein expression and surface presentation of the fusion proteins were dramatically improved when fused to IcsA rather than IcsA_β. Future studies involved in designing autotransporters as cell surface display vehicles would benefit from including the native α domain. This work also provides further evidence for a key interaction between the autotransporter α and β domains.

The iron-responsive Fur/RyhB regulatory cascade modulates the Shigella outer membrane protease IcsP

Actin-based motility is central to the pathogenicity of the intracellular bacterial pathogen Shigella. Two Shigella outer membrane proteins, IcsA and IcsP, are required for efficient actin-based motility in the host cell cytoplasm, and the genes encoding both proteins are carried on the large virulence plasmid. IcsA triggers actin polymerization on the surface of the bacterium, leading to the formation of an actin tail that allows both intra- and intercellular spread. IcsP, an outer membrane protease, modulates the amount and distribution of the IcsA protein on the bacterial surface through proteolytic cleavage of IcsA. Transcription of icsP is increased in the presence of VirB, a DNA-binding protein that positively regulates many genes carried on the large virulence plasmid. In Shigella dysenteriae, the small regulatory RNA RyhB, which is a member of the iron-responsive Fur regulon, suppresses several virulence-associated phenotypes by downregulating levels of virB in response to iron limitation. Here we show that the Fur/RyhB regulatory pathway downregulates IcsP levels in response to low iron concentrations in Shigella flexneri and that this occurs at the level of transcription through the RyhB-dependent regulation of VirB. These observations demonstrate that in Shigella species the Fur/RyhB regulatory pathway provides a mechanism to finely tune the expression of icsP in response to the low concentrations of free iron predicted to be encountered within colonic epithelial cells.

Single-gene tuning of Caulobacter cell cycle period and noise, swarming motility, and surface adhesion

We established that the sensor histidine kinase DivJ has an important role in the regulation of C. crescentus cell cycle period and noise. This was accomplished by designing and conducting single-cell experiments to probe the dependence of cell cycle noise on divJ expression and constructing a simplified cell cycle model that captures the dependence of cell cycle noise on DivJ with molecular details.

In addition to its role in regulating the cell cycle, DivJ also affects polar cell development in C. crescentus, regulating swarming motility and surface adhesion. We propose that pleiotropic control of polar cell development by the DivJ–DivK–PleC signaling pathway underlies divJ-dependent tuning of cell swarming and adhesion behaviors.

We have integrated the study of single-cell fluorescence dynamics with a kinetic model simulation to provide direct quantitative evidence that the DivJ histidine kinase is localized to the cell pole through a dynamic diffusion-and-capture mechanism during the C. crescentus cell cycle.

Temporally-coordinated localization of various structural and signaling proteins is critical for proper cell cycle regulation and polar cell development in the bacterium, Caulobacter crescentus. Included among these dynamically-localized regulatory proteins is the sensor histidine kinase, DivJ (Wheeler and Shapiro, 1999). Co-localized with DivJ in the early stalked phase is the phosphorylated response regulator DivK∼P (Jacobs et al, 2001), and the protease ClpXP (McGrath et al, 2006), which degrades the master cell cycle regulator, CtrA (Jenal and Fuchs, 1998). Recent single-cell measurements of surface attached C. crescentus cells have revealed an intriguing role for DivJ in the control of noise in cell division period (Siegal-Gaskins and Crosson, 2008). The noise of the cell cycle increases significantly upon disruption of the divJ gene, with a relatively small accompanying increase in the mean cell cycle time. The deterministic nature of the existing cell cycle models (Li et al, 2008, 2009; Shen et al, 2008) cannot explain the measured increase in cell cycle period and noise in a divJ null strain. Moreover, mechanistic descriptions of how DivJ and its signaling partners are localized and how these proteins underlie the control of polar cell development and cell adhesion in C. crescentus remain immature.

The single-cell experiments and analysis presented herein reveal that C. crescentus cell cycle period and noise can be tuned by DivJ (Figure 2). Specifically, in the case of low (or no) divJ expression the cell cycle is perturbed, and this is quantified by way of the (measured) noise in the cell cycle period. The level of noise is readily controlled through regulated expression of the divJ gene (Figure 2B). A simplified protein interaction network of stalked C. crescentus cell cycle regulation involving minimal components (CtrA, CtrA∼P, DivK, DivK∼P, and DivJ) was constructed to explore such tunability at the molecular level. The agreement of our model with our (and other) experiments suggests this simplified protein regulatory network is sufficient to explain the major features of the C. crescentus cell cycle. Indeed, stochastic simulations of this model using the Gillespie method (Gillespie, 1976) establish the importance of robust DivJ-mediated phosphorylation of its cognate receiver protein, DivK, in regulating the variance of cell cycle oscillations. Increased variability in the concentration of DivK∼P at the single cell level under divJ depletion subsequently leads to increased noise in the regulation of CtrA phosphorylation and degradation. Our experiments and simulations provide evidence that the steady state level of DivK∼P at the single-cell level (as maintained by DivJ) is essential in maintaining regular timing of the cell division period in C. crescentus.

In addition to its role in regulating cell cycle, divJ expression also affects polar cell development in C. crescentus. Specifically, the capacity of swarmer cells to adhere to a glass surface is suppressed at high levels of divJ expression. The effect of elevated divJ expression on the adhesive capacity of the cell is reflected in a reduced rate of two-dimensional biofilm formation. This effect is quantitatively captured by our mathematical model that relates single-cell surface adhesion physiology and biofilm formation dynamics. This result, and our observation that divJ expression tunes swarming motility in semi-solid growth medium, suggests a model in which increased DivJ concentration in the swarmer compartment (due to constitutive overexpression) ultimately results in improper development of polar organelles that are required for adhesion of swarming motility.

Despite the appreciated significance of protein localization for bacterial physiological functions, the molecular mechanism of how polar protein localization is achieved has only been tested in a few cases (Shapiro et al, 2002; Thanbichler and Shapiro, 2008). Mechanisms such as the polar insertion model and diffusion-and-capture have been proposed but the community's knowledge is limited to very few examples (Charles et al, 2001; Rudner et al, 2002). We provide direct evidence from experiments and simulations that the DivJ histidine kinase becomes localized to the cell pole through a dynamic diffusion-and-capture mechanism during the C. crescentus cell cycle (Figure 7). We show that a kinetic model based on a Langmuir adsorption/desorption relationship (Figure 7D) is sufficient to explain the time evolution of the single cell fluorescence time traces (Figure 7C and E) and allows establishing quantitative correspondences between the simulated dynamics and experimentally determined DivJ–EGFP dynamics. This localization mechanism is consistent with a diffusion-and-capture model. In short, the model posits that proteins are randomly distributed and are freely diffusing until they are captured at the site where they ultimately reside (Rudner et al, 2002; Shapiro et al, 2002; Bardy and Maddock, 2007). With a diffusion-and-capture pathway, it has been argued that proteins can be adsorbed either dynamically or statically (Shapiro et al, 2009). Our analysis of DivJ–EGFP in single cells supports a dynamic diffuse-and-capture mechanism for DivJ localization.

Sensor histidine kinases underlie the regulation of a range of physiological processes in bacterial cells, from chemotaxis to cell division. In the gram-negative bacterium Caulobacter crescentus, the membrane-bound histidine kinase, DivJ, is a polar-localized regulator of cell cycle progression and development. We show that DivJ localizes to the cell pole through a dynamic diffusion and capture mechanism rather than by active localization. Analysis of single C. crescentus cells in microfluidic culture demonstrates that controlled expression of divJ permits facile tuning of both the mean and noise of the cell division period. Simulations of the cell cycle that use a simplified protein interaction network capture previously measured oscillatory protein profiles, and recapitulate the experimental observation that deletion of divJ increases the cell cycle period and noise. We further demonstrate that surface adhesion and swarming motility of C. crescentus in semi-solid media can also be tuned by divJ expression. We propose a model in which pleiotropic control of polar cell development by the DivJ–DivK–PleC signaling pathway underlies divJ-dependent tuning of cell swarming and adhesion behaviors.

Crystal structure of the autochaperone region from the Shigella flexneri autotransporter IcsA

The IcsA (intracellular spread gene A) autotransporter from Shigella flexneri is a key virulence factor. We identified a stable fragment comprising residues 591 to 758, which corresponds to the autochaperone region of the IcsA passenger domain. We showed that thermal unfolding of the autochaperone region is reversible and determined its crystal structure at 2.0-Å resolution.

Identification and characterization of ZapC, a stabilizer of the FtsZ ring in Escherichia coli

In Escherichia coli, spatiotemporal control of cell division occurs at the level of the assembly/disassembly process of the essential cytoskeletal protein FtsZ. A number of regulators interact with FtsZ and modulate the dynamics of the assembled FtsZ ring at the midcell division site. In this article, we report the identification of an FtsZ stabilizer, ZapC (Z-associated protein C), in a protein localization screen conducted with E. coli. ZapC colocalizes with FtsZ at midcell and interacts directly with FtsZ, as determined by a protein-protein interaction assay in yeast. Cells lacking or overexpressing ZapC are slightly elongated and have aberrant FtsZ ring morphologies indicative of a role for ZapC in FtsZ regulation. We also demonstrate the ability of purified ZapC to promote lateral bundling of FtsZ in a sedimentation reaction visualized by transmission electron microscopy. While ZapC lacks sequence similarity with other nonessential FtsZ regulators, ZapA and ZapB, all three Zap proteins appear to play an important role in FtsZ regulation during rapid growth. Taken together, our results suggest a key role for lateral bundling of the midcell FtsZ polymers in maintaining FtsZ ring stability during division.

A spindle-like apparatus guides bacterial chromosome segregation

Until recently, a dedicated mitotic apparatus that segregates newly replicated chromosomes into daughter cells was believed to be unique to eukaryotic cells. Here we demonstrate that the bacterium Caulobacter crescentus segregates its chromosome using a partitioning (Par) apparatus that has surprising similarities to eukaryotic spindles. We show that the C. crescentus ATPase ParA forms linear polymers in vitro and assembles into a narrow linear structure in vivo. The centromere-binding protein ParB binds to and destabilizes ParA structures in vitro. We propose that this ParB-stimulated ParA depolymerization activity moves the centromere to the opposite cell pole through a burnt bridge Brownian ratchet mechanism. Finally, we identify the pole-specific TipN protein as a new component of the Par system that is required to maintain the directionality of DNA transfer towards the new cell pole. Our results elucidate a bacterial chromosome segregation mechanism that features basic operating principles similar to eukaryotic mitotic machines, including a multivalent protein complex at the centromere that stimulates the dynamic disassembly of polymers to move chromosomes into daughter compartments.

Protein subcellular localization in bacteria

Like their eukaryotic counterparts, bacterial cells have a highly organized internal architecture. Here, we address the question of how proteins localize to particular sites in the cell and how they do so in a dynamic manner. We consider the underlying mechanisms that govern the positioning of proteins and protein complexes in the examples of the divisome, polar assemblies, cytoplasmic clusters, cytoskeletal elements, and organelles. We argue that geometric cues, self-assembly, and restricted sites of assembly are all exploited by the cell to specifically localize particular proteins that we refer to as anchor proteins. These anchor proteins in turn govern the localization of a whole host of additional proteins. Looking ahead, we speculate on the existence of additional mechanisms that contribute to the organization of bacterial cells, such as the nucleoid, membrane microdomains enriched in specific lipids, and RNAs with positional information.

A genome-scale proteomic screen identifies a role for DnaK in chaperoning of polar autotransporters in Shigella

Autotransporters are outer membrane proteins that are widely distributed among gram-negative bacteria. Like other autotransporters, the Shigella autotransporter IcsA, which is required for actin assembly during infection, is secreted at the bacterial pole. In the bacterial cytoplasm, IcsA localizes to poles and potential cell division sites independent of the cell division protein FtsZ. To identify bacterial proteins involved in the targeting of IcsA to the pole in the bacterial cytoplasm, we screened a genome-scale library of Escherichia coli proteins tagged with green fluorescent protein (GFP) for those that displayed a localization pattern similar to that of IcsA-GFP in cells that lack functional FtsZ using a strain carrying a temperature-sensitive ftsZ allele. For each protein that mimicked the localization of IcsA-GFP, we tested whether IcsA localization was dependent on the presence of the protein. Although these approaches did not identify a polar receptor for IcsA, the cytoplasmic chaperone DnaK both mimicked IcsA localization at elevated temperatures as a GFP fusion and was required for the localization of IcsA to the pole in the cytoplasm of E. coli. DnaK was also required for IcsA secretion at the pole in Shigella flexneri. The localization of DnaK-GFP to poles and potential cell division sites was dependent on elevated growth temperature and independent of the presence of IcsA or functional FtsZ; native DnaK was found to be enhanced at midcell and the poles. A second Shigella autotransporter, SepA, also required DnaK for secretion, consistent with a role of DnaK more generally in the chaperoning of autotransporter proteins in the bacterial cytoplasm.

E. coli transports aggregated proteins to the poles by a specific and energy-dependent process

Aggregation of proteins due to failure of quality control mechanisms is deleterious to both eukaryotes and prokaryotes. We found that in Escherichia coli, protein aggregates are delivered to the pole and form a large polar aggregate (LPA). The formation of LPAs involves two steps: the formation of multiple small aggregates and the delivery of these aggregates to the pole to form an LPA. Formation of randomly distributed aggregates, their delivery to the poles, and LPA formation are all energy-dependent processes. The latter steps require the proton motive force, activities of the DnaK and DnaJ chaperones, and MreB. About 90 min after their formation, the LPAs are dissolved in a process that is dependent upon ClpB, DnaK, and energy. Our results confirm and substantiate the notion that the formation of LPAs allows asymmetric inheritance of the aggregated proteins to a small number of daughter cells, enabling their rapid elimination from most of the bacterial population. Moreover, the results show that the processing of aggregated proteins by the protein quality control system is a multi-step process with distinct spatial and temporal controls.

Contribution of the periplasmic chaperone Skp to efficient presentation of the autotransporter IcsA on the surface of Shigella flexneri

IcsA is an outer membrane protein in the autotransporter family that is required for Shigella flexneri pathogenesis. Following its secretion through the Sec translocon, IcsA is incorporated into the outer membrane in a process that depends on YaeT, a component of an outer membrane beta-barrel insertion machinery. We investigated the role of the periplasmic chaperone Skp in IcsA maturation. Skp is required for the presentation of the mature amino terminus (alpha-domain) of IcsA on the bacterial surface and contributes to cell-to-cell spread of S. flexneri in cell culture. A mutation in skp does not prevent the insertion of the beta-barrel into the outer membrane, suggesting that the primary role of Skp is the folding of the IcsA alpha-domain. In addition, the requirement for skp can be partially bypassed by disrupting icsP, an ortholog of Escherichia coli ompT, which encodes the protease that processes IcsA between the mature amino terminus and the beta-barrel outer membrane anchor. These findings are consistent with a model in which Skp plays a critical role in the chaperoning of the alpha-domain of IcsA during transit through the periplasm.

Group II intron protein localization and insertion sites are affected by polyphosphate

Mobile group II introns consist of a catalytic intron RNA and an intron-encoded protein with reverse transcriptase activity, which act together in a ribonucleoprotein particle to promote DNA integration during intron mobility. Previously, we found that the Lactococcus lactis Ll.LtrB intron-encoded protein (LtrA) expressed alone or with the intron RNA to form ribonucleoprotein particles localizes to bacterial cellular poles, potentially accounting for the intron's preferential insertion in the oriC and ter regions of the Escherichia coli chromosome. Here, by using cell microarrays and automated fluorescence microscopy to screen a transposon-insertion library, we identified five E. coli genes (gppA, uhpT, wcaK, ynbC, and zntR) whose disruption results in both an increased proportion of cells with more diffuse LtrA localization and a more uniform genomic distribution of Ll.LtrB-insertion sites. Surprisingly, we find that a common factor affecting LtrA localization in these and other disruptants is the accumulation of intracellular polyphosphate, which appears to bind LtrA and other basic proteins and delocalize them away from the poles. Our findings show that the intracellular localization of a group II intron-encoded protein is a major determinant of insertion-site preference. More generally, our results suggest that polyphosphate accumulation may provide a means of localizing proteins to different sites of action during cellular stress or entry into stationary phase, with potentially wide physiological consequences.

Group II introns are bacterial mobile elements thought to be ancestors of introns—genetic material that is discarded from messenger RNA transcripts—and retroelements—genetic elements and viruses that replicate via reverse transcription—in higher organisms. They propagate by forming a complex consisting of the catalytically active intron RNA and an intron-encoded reverse transcriptase (which converts the RNA to DNA, which can then be reinserted in the host genome). The Ll.LtrB group II intron-encoded protein (LtrA) was found previously to localize to bacterial cellular poles, potentially accounting for the preferential insertion of Ll.LtrB in the replication origin (oriC) and terminus (ter) regions of the Escherichia coli chromosome, which are located near the poles during much of the cell cycle. Here, we identify E. coli genes whose disruption leads both to more diffuse LtrA localization and a more uniform chromosomal distribution of Ll.LtrB-insertion sites, proving that the location of the LtrA protein contributes to insertion-site preference. Surprisingly, we find that LtrA localization in the disruptants is affected by the accumulation of intracellular polyphosphate, which appears to bind basic proteins and delocalize them away from the cellular poles. Thus, polyphosphate, a ubiquitous but enigmatic molecule in prokaryotes and eukaryotes, can localize proteins to different sites of action, with potentially wide physiological consequences.

A novel cell microarray method uncovers connections between group II intron mobility, cell stress, and polyphosphate metabolism, including the finding that polyphosphate can influence intracellular protein localization.

Bacteriophage infection is targeted to cellular poles

The poles of bacteria exhibit several specialized functions related to the mobilization of DNA and certain proteins. To monitor the infection of Escherichia coli cells by light microscopy, we developed procedures for the tagging of mature bacteriophages with quantum dots. Surprisingly, most of the infecting phages were found attached to the bacterial poles. This was true for a number of temperate and virulent phages of E. coli that use widely different receptors and for phages infecting Yersinia pseudotuberculosis and Vibrio cholerae. The infecting phages colocalized with the polar protein marker IcsA-GFP. ManY, an E. coli protein that is required for phage lambda DNA injection, was found to localize to the bacterial poles as well. Furthermore, labelling of lambda DNA during infection revealed that it is injected and replicated at the polar region of infection. The evolutionary benefits that lead to this remarkable preference for polar infections may be related to lambda's developmental decision as well as to the function of poles in the ability of bacterial cells to communicate with their environment and in gene regulation.

Mutagenesis of the Shigella flexneri autotransporter IcsA reveals novel functional regions involved in IcsA biogenesis and recruitment of host neural Wiscott-Aldrich syndrome protein

The IcsA (VirG) protein of Shigella flexneri is a polarly localized, outer membrane protein that is essential for virulence. Within host cells, IcsA activates the host actin regulatory protein, neural Wiskott-Aldrich syndrome protein (N-WASP), which in turn recruits the Arp2/3 complex, which nucleates host actin to form F-actin comet tails and initiate bacterial motility. Linker insertion mutagenesis was undertaken to randomly introduce 5-amino-acid in-frame insertions within IcsA. Forty-seven linker insertion mutants were isolated and expressed in S. flexneri Delta icsA strains. Mutants were characterized for IcsA protein production, cell surface expression and localization, intercellular spreading, F-actin comet tail formation, and N-WASP recruitment. Using this approach, we have identified a putative autochaperone region required for IcsA biogenesis, and our data suggest an additional region, not previously identified, is required for N-WASP recruitment.