A localized multimeric anchor attaches the Caulobacter holdfast to the cell pole

Phase separation modulates the assembly and dynamics of a polarity-related scaffold-signaling hub

Asymmetric cell division (ACD) produces morphologically and behaviorally distinct cells and is the primary way to generate cell diversity. In the model bacterium Caulobacter crescentus, the polarization of distinct scaffold-signaling hubs at the swarmer and stalked cell poles constitutes the basis of ACD. However, mechanisms involved in the formation of these hubs remain elusive. Here, we show that a swarmer-cell-pole scaffold, PodJ, forms biomolecular condensates both in vitro and in living cells via phase separation. The coiled-coil 4-6 and the intrinsically disordered regions are the primary domains that contribute to biomolecular condensate generation and signaling protein recruitment in PodJ. Moreover, a negative regulation of PodJ phase separation by the stalked-cell-pole scaffold protein SpmX is revealed. SpmX impedes PodJ cell-pole accumulation and affects its recruitment ability. Together, by modulating the assembly and dynamics of scaffold-signaling hubs, phase separation may serve as a general biophysical mechanism that underlies the regulation of ACD in bacteria and other organisms.

What Glues the Glue to the Cell Surface?

In the Caulobacterales , a highly adhesive polysaccharide called the holdfast mediates attachment to exogenous surfaces. The mechanism by which this polysaccharide is anchored to the cell envelope is not well defined.

HfaE Is a Component of the Holdfast Anchor Complex That Tethers the Holdfast Adhesin to the Cell Envelope

Bacteria use adhesins to colonize different surfaces and form biofilms. The species of the Caulobacterales order use a polar adhesin called holdfast, composed of polysaccharides, proteins, and DNA, to irreversibly adhere to surfaces. In Caulobacter crescentus, a freshwater Caulobacterales species, the holdfast is anchored at the cell pole via the holdfast anchor (Hfa) proteins HfaA, HfaB, and HfaD. HfaA and HfaD colocalize with holdfast and are thought to form amyloid-like fibers that anchor holdfast to the cell envelope. HfaB, a lipoprotein, is required for the translocation of HfaA and HfaD to the cell surface. Deletion of the anchor proteins leads to a severe defect in adherence resulting from holdfast not being properly attached to the cell and shed into the medium. This phenotype is greater in a ΔhfaB mutant than in a ΔhfaA ΔhfaD double mutant, suggesting that HfaB has other functions besides the translocation of HfaA and HfaD. Here, we identify an additional HfaB-dependent holdfast anchoring protein, HfaE, which is predicted to be a secreted protein. HfaE is highly conserved among Caulobacterales species, with no predicted function. In planktonic culture, hfaE mutants produce holdfasts and rosettes similar to those produced by the wild type. However, holdfasts from hfaE mutants bind to the surface but are unable to anchor cells, similarly to other anchor mutants. We showed that fluorescently tagged HfaE colocalizes with holdfast and that HfaE forms an SDS-resistant high-molecular-weight species consistent with amyloid fiber formation. We propose that HfaE is a novel holdfast anchor protein and that HfaE functions to link holdfast material to the cell envelope. IMPORTANCE For surface attachment and biofilm formation, bacteria produce adhesins that are composed of polysaccharides, proteins, and DNA. Species of the Caulobacterales produce a specialized polar adhesin, holdfast, which is required for permanent attachment to surfaces. In this study, we evaluate the role of a newly identified holdfast anchor protein, HfaE, in holdfast anchoring to the cell surface in two different members of the Caulobacterales with drastically different environments. We show that HfaE plays an important role in adhesion and biofilm formation in the Caulobacterales. Our results provide insights into bacterial adhesins and how they interact with the cell envelope and surfaces.

Dual adhesive unipolar polysaccharides synthesized by overlapping biosynthetic pathways in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a member of the Alphaproteobacteria that pathogenises plants and associates with biotic and abiotic surfaces via a single cellular pole. A. tumefaciens produces the unipolar polysaccharide (UPP) at the site of surface contact. UPP production is normally surface-contact inducible, but elevated levels of the second messenger cyclic diguanylate monophosphate (cdGMP) bypass this requirement. Multiple lines of evidence suggest that the UPP has a central polysaccharide component. Using an A. tumefaciens derivative with elevated cdGMP and mutationally disabled for other dispensable polysaccharides, a series of related genetic screens have identified a large number of genes involved in UPP biosynthesis, most of which are Wzx-Wzy-type polysaccharide biosynthetic components. Extensive analyses of UPP production in these mutants have revealed that the UPP is composed of two genetically, chemically, and spatially discrete forms of polysaccharide, and that each requires a specific Wzy-type polymerase. Other important biosynthetic, processing, and regulatory functions for UPP production are also revealed, some of which are common to both polysaccharides, and a subset of which are specific to each type. Many of the UPP genes identified are conserved among diverse rhizobia, whereas others are more lineage specific.

A polysaccharide deacetylase enhances bacterial adhesion in high-ionic-strength environments

Differences in ionic strength, pH, temperature, shear forces, and other environmental factors impact adhesion, and organisms have evolved various strategies to optimize their adhesins for their specific environmental conditions. Many species of Alphaproteobacteria, including members of the order Caulobacterales, use a polar adhesin, called holdfast, for surface attachment and subsequent biofilm formation in both freshwater and marine environments. Hirschia baltica, a marine member of Caulobacterales, produces a holdfast adhesin that tolerates a drastically higher ionic strength than the holdfast produced by its freshwater relative, Caulobacter crescentus. In this work, we show that the holdfast polysaccharide deacetylase HfsH plays an important role in adherence in high-ionic-strength environments. We show that increasing expression of HfsH improves holdfast binding in high-ionic-strength environments. We conclude that HfsH plays a role in modulating holdfast binding at high ionic strength and hypothesize that this modulation occurs through varied deacetylation of holdfast polysaccharides.

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The polysaccharide deacetylase HfsH is required for H. baltica adhesion

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Holdfast polysaccharides in H. baltica ΔhfsH lack cohesive and adhesive properties

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HfsH expression correlates positively with holdfast binding in high ionic strength

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HfsH is an important factor for adherence in high-ionic-strength environments

Plasmid-encoded H-NS controls extracellular matrix composition in a modern Acinetobacter baumannii urinary isolate

Acinetobacter baumannii is emerging as a multidrug-resistant (MDR) nosocomial pathogen of increasing threat to human health worldwide. The recent MDR urinary isolate UPAB1 carries the plasmid pAB5, a member of a family of large conjugative plasmids (LCP). LCP encode several antibiotic resistance genes and repress the type VI secretion system (T6SS) to enable their dissemination, employing two TetR transcriptional regulators. Furthermore, pAB5 controls the expression of additional chromosomally encoded genes, impacting UPAB1 virulence. Here we show that a pAB5-encoded H-NS transcriptional regulator represses the synthesis of the exopolysaccharide PNAG and the expression of a previously uncharacterized three-gene cluster that encodes a protein belonging to the CsgG/HfaB family. Members of this protein family are involved in amyloid or polysaccharide formation in other species. Deletion of the CsgG homolog abrogated PNAG production and CUP pili formation, resulting in a subsequent reduction in biofilm formation. Although this gene cluster is widely distributed in Gram-negative bacteria, it remains largely uninvestigated. Our results illustrate the complex cross-talks that take place between plasmids and the chromosomes of their bacterial host, which in this case can contribute to the pathogenesis of Acinetobacter. IMPORTANCE The opportunistic human pathogen Acinetobacter baumannii displays the highest reported rates of multidrug resistance among Gram-negative pathogens. Many A. baumannii strains carry large conjugative plasmids like pAB5. In recent years, we have witnessed an increase in knowledge about the regulatory cross-talks between plasmids and bacterial chromosomes. Here we show that pAB5 controls the composition of the bacterial extracellular matrix, resulting in a drastic reduction in biofilm formation. The association between biofilm formation, virulence, and antibiotic resistance is well-documented. Therefore, understanding the factors involved in the regulation of biofilm formation in Acinetobacter has remarkable therapeutic potential.

Generating asymmetry in a changing environment: cell cycle regulation in dimorphic alphaproteobacteria

While many bacteria divide by symmetric binary fission, some alphaproteobacteria have strikingly asymmetric cell cycles, producing offspring that differs significantly in their morphology and reproductive state. To establish this asymmetry, these species employ a complex cell cycle regulatory pathway based on two-component signaling cascades. At the center of this network is the essential DNA-binding response regulator CtrA, which acts as a transcription factor controlling numerous genes with cell cycle-relevant functions as well as a regulator of chromosome replication. The DNA-binding activity of CtrA is controlled at the level of both protein phosphorylation and stability, dependent on an intricate network of regulatory proteins, whose function is tightly coordinated in time and space. CtrA is differentially activated in the two (developing) offspring, thereby establishing distinct transcriptional programs that ultimately determine their distinct cell fates. Phase-separated polar microdomains of changing composition sequester proteins involved in the (in-)activation and degradation of CtrA specifically at each pole. In this review, we summarize the current knowledge of the CtrA pathway and discuss how it has evolved to regulate the cell cycle of morphologically distinct alphaproteobacteria.

In Situ Structure of an Intact Lipopolysaccharide-Bound Bacterial Surface Layer

Most bacterial and all archaeal cells are encapsulated by a paracrystalline, protective, and cell-shape-determining proteinaceous surface layer (S-layer). On Gram-negative bacteria, S-layers are anchored to cells via lipopolysaccharide. Here, we report an electron cryomicroscopy structure of the Caulobacter crescentus S-layer bound to the O-antigen of lipopolysaccharide. Using native mass spectrometry and molecular dynamics simulations, we deduce the length of the O-antigen on cells and show how lipopolysaccharide binding and S-layer assembly is regulated by calcium. Finally, we present a near-atomic resolution in situ structure of the complete S-layer using cellular electron cryotomography, showing S-layer arrangement at the tip of the O-antigen. A complete atomic structure of the S-layer shows the power of cellular tomography for in situ structural biology and sheds light on a very abundant class of self-assembling molecules with important roles in prokaryotic physiology with marked potential for synthetic biology and surface-display applications.

Comparative Analysis of Ionic Strength Tolerance between Freshwater and Marine Caulobacterales Adhesins

Bacterial adhesion is affected by environmental factors, such as ionic strength, pH, temperature, and shear forces. Therefore, marine bacteria must have developed adhesins with different compositions and structures than those of their freshwater counterparts to adapt to their natural environment. The dimorphic alphaproteobacterium Hirschia baltica is a marine budding bacterium in the clade Caulobacterales H. baltica uses a polar adhesin, the holdfast, located at the cell pole opposite the reproductive stalk, for surface attachment and cell-cell adhesion. The holdfast adhesin has been best characterized in Caulobacter crescentus, a freshwater member of the Caulobacterales, and little is known about holdfast compositions and properties in marine Caulobacterales Here, we use H. baltica as a model to characterize holdfast properties in marine Caulobacterales We show that freshwater and marine Caulobacterales use similar genes in holdfast biogenesis and that these genes are highly conserved among the species in the two genera. We determine that H. baltica produces a larger holdfast than C. crescentus and that the holdfasts have different chemical compositions, as they contain N-acetylglucosamine and galactose monosaccharide residues and proteins but lack DNA. Finally, we show that H. baltica holdfasts tolerate higher ionic strength than those of C. crescentus We conclude that marine Caulobacterales holdfasts have physicochemical properties that maximize binding in high-ionic-strength environments.IMPORTANCE Most bacteria spend a large part of their life spans attached to surfaces, forming complex multicellular communities called biofilms. Bacteria can colonize virtually any surface, and therefore, they have adapted to bind efficiently in very different environments. In this study, we compare the adhesive holdfasts produced by the freshwater bacterium C. crescentus and a relative, the marine bacterium H. baltica We show that H. baltica holdfasts have a different morphology and chemical composition and tolerate high ionic strength. Our results show that the H. baltica holdfast is an excellent model to study the effect of ionic strength on adhesion and provides insights into the physicochemical properties required for adhesion in the marine environment.

A Multiprotein Complex Anchors Adhesive Holdfast at the Outer Membrane of Caulobacter crescentus

Adhesion allows microbes to colonize surfaces and is the first stage in biofilm formation. Stable attachment of the freshwater alphaproteobacterium Caulobacter crescentus to surfaces requires an adhesive polysaccharide called holdfast, which is synthesized at a specific cell pole and ultimately found at the tip of cylindrical extensions of the cell envelope called stalks. Secretion and anchoring of holdfast to the cell surface are governed by proteins HfsDAB and HfaABD, respectively. The arrangement and organization of these proteins with respect to each other and the cell envelope, and the mechanism by which the holdfast is anchored on cells, are unknown. In this study, we have imaged a series of C. crescentus mutants using electron cryotomography, revealing the architecture and arrangement of the molecular machinery involved in holdfast anchoring in cells. We found that the holdfast is anchored to cells by a defined complex made up of the HfaABD proteins and that the HfsDAB secretion proteins are essential for proper assembly and localization of the HfaABD anchor. Subtomogram averaging of cell stalk tips showed that the HfaABD complex spans the outer membrane. The anchor protein HfaB is the major component of the anchor complex located on the periplasmic side of the outer membrane, while HfaA and HfaD are located on the cell surface. HfaB is the critical component of the complex, without which no HfaABD complex was observed in cells. These results allow us to propose a working model of holdfast anchoring, laying the groundwork for further structural and cell biological investigations.IMPORTANCE Adhesion and biofilm formation are fundamental processes that accompany bacterial colonization of surfaces, which are of critical importance in many infections. Caulobacter crescentus biofilm formation proceeds via irreversible adhesion mediated by a polar polysaccharide called holdfast. Mechanistic and structural details of how the holdfast is secreted and anchored on cells are still lacking. Here, we have assigned the location and described the arrangement of the holdfast anchor complex. This work increases our knowledge of the relatively underexplored field of polysaccharide-mediated adhesion by identifying structural elements that anchor polysaccharides to the cell envelope, which is important in a variety of bacterial species.

Composition of the Holdfast Polysaccharide from Caulobacter crescentus

Surface colonization is central to the lifestyles of many bacteria. Exploiting surface niches requires sophisticated systems for sensing and attaching to solid materials. Caulobacter crescentus synthesizes a polysaccharide-based adhesin known as the holdfast at one of its cell poles, which enables tight attachment to exogenous surfaces. The genes required for holdfast biosynthesis have been analyzed in detail, but difficulties in isolating analytical quantities of the adhesin have limited efforts to characterize its chemical structure. In this report, we describe a method to extract the holdfast from C. crescentus cultures and present a survey of its carbohydrate content. Glucose, 3-O-methylglucose, mannose, N-acetylglucosamine, and xylose were detected in our extracts. Our results provide evidence that the holdfast contains a 1,4-linked backbone of glucose, mannose, N-acetylglucosamine, and xylose that is decorated with branches at the C-6 positions of glucose and mannose. By defining the monosaccharide components in the polysaccharide, our work establishes a framework for characterizing enzymes in the holdfast pathway and provides a broader understanding of how polysaccharide adhesins are built.IMPORTANCE To colonize solid substrates, bacteria often deploy dedicated adhesins that facilitate attachment to surfaces. Caulobacter crescentus initiates surface colonization by secreting a carbohydrate-based adhesin called the holdfast. Because little is known about the chemical makeup of the holdfast, the pathway for its biosynthesis and the physical basis for its unique adhesive properties are poorly understood. This study outlines a method to extract the C. crescentus holdfast and describes the monosaccharide components contained within the adhesive matrix. The composition analysis adds to our understanding of the chemical basis for holdfast attachment and provides missing information needed to characterize enzymes in the biosynthetic pathway.

Feedback regulation of Caulobacter crescentus holdfast synthesis by flagellum assembly via the holdfast inhibitor HfiA

To permanently attach to surfaces, Caulobacter crescentusproduces a strong adhesive, the holdfast. The timing of holdfast synthesis is developmentally regulated by cell cycle cues. When C. crescentusis grown in a complex medium, holdfast synthesis can also be stimulated by surface sensing, in which swarmer cells rapidly synthesize holdfast in direct response to surface contact. In contrast to growth in complex medium, here we show that when cells are grown in a defined medium, surface contact does not trigger holdfast synthesis. Moreover, we show that in a defined medium, flagellum synthesis and regulation of holdfast production are linked. In these conditions, mutants lacking a flagellum attach to surfaces over time more efficiently than either wild-type strains or strains harboring a paralyzed flagellum. Enhanced adhesion in mutants lacking flagellar components is due to premature holdfast synthesis during the cell cycle and is regulated by the holdfast synthesis inhibitor HfiA. hfiA transcription is reduced in flagellar mutants and this reduction is modulated by the diguanylate cyclase developmental regulator PleD. We also show that, in contrast to previous predictions, flagella are not necessarily required for C. crescentus surface sensing in the absence of flow, and that arrest of flagellar rotation does not stimulate holdfast synthesis. Rather, our data support a model in which flagellum assembly feeds back to control holdfast synthesis via HfiA expression in a c-di-GMP-dependent manner under defined nutrient conditions.

Second messenger-mediated tactile response by a bacterial rotary motor

When bacteria encounter surfaces, they respond with surface colonization and virulence induction. The mechanisms of bacterial mechanosensation and downstream signaling remain poorly understood. Here, we describe a tactile sensing cascade in Caulobacter crescentus in which the flagellar motor acts as sensor. Surface-induced motor interference stimulated the production of the second messenger cyclic diguanylate by the motor-associated diguanylate cyclase DgcB. This led to the allosteric activation of the glycosyltransferase HfsJ to promote rapid synthesis of a polysaccharide adhesin and surface anchoring. Although the membrane-embedded motor unit was essential for surface sensing, mutants that lack external flagellar structures were hypersensitive to mechanical stimuli. Thus, the bacterial flagellar motor acts as a tetherless sensor reminiscent of mechanosensitive channels.

Layered Structure and Complex Mechanochemistry Underlie Strength and Versatility in a Bacterial Adhesive

While designing synthetic adhesives that perform in aqueous environments has proven challenging, microorganisms commonly produce bioadhesives that efficiently attach to a variety of substrates, including wet surfaces. The aquatic bacterium Caulobacter crescentus uses a discrete polysaccharide complex, the holdfast, to strongly attach to surfaces and resist flow. The holdfast is extremely versatile and has impressive adhesive strength. Here, we used atomic force microscopy in conjunction with superresolution microscopy and enzymatic assays to unravel the complex structure of the holdfast and to characterize its chemical constituents and their role in adhesion. Our data support a model whereby the holdfast is a heterogeneous material organized as two layers: a stiffer nanoscopic core layer wrapped into a sparse, far-reaching, flexible brush layer. Moreover, we found that the elastic response of the holdfast evolves after surface contact from initially heterogeneous to more homogeneous. From a composition point of view, besides N-acetyl-d-glucosamine (NAG), the only component that had been identified to date, our data show that the holdfast contains peptides and DNA. We hypothesize that, while polypeptides are the most important components for adhesive force, the presence of DNA mainly impacts the brush layer and the strength of initial adhesion, with NAG playing a primarily structural role within the core. The unanticipated complexity of both the structure and composition of the holdfast likely underlies its versatility as a wet adhesive and its distinctive strength. Continued improvements in understanding of the mechanochemistry of this bioadhesive could provide new insights into how bacteria attach to surfaces and could inform the development of new adhesives.IMPORTANCE There is an urgent need for strong, biocompatible bioadhesives that perform underwater. To strongly adhere to surfaces and resist flow underwater, the bacterium Caulobacter crescentus produces an adhesive called the holdfast, the mechanochemistry of which remains undefined. We show that the holdfast is a layered structure with a stiff core layer and a polymeric brush layer and consists of polysaccharides, polypeptides, and DNA. The DNA appears to play a role in the structure of the brush layer and initial adhesion, the peptides in adhesive strength, and the polysaccharides in the structure of the core. The complex, multilayer organization and diverse chemistry described here underlie the distinctive adhesive properties of the holdfast and will provide important insights into the mechanisms of bacterial adhesion and bioadhesive applications.

Mutations in Sugar-Nucleotide Synthesis Genes Restore Holdfast Polysaccharide Anchoring to Caulobacter crescentus Holdfast Anchor Mutants

Attachment is essential for microorganisms to establish interactions with both biotic and abiotic surfaces. Stable attachment of Caulobacter crescentus to surfaces requires an adhesive polysaccharide holdfast, but the exact composition of the holdfast is unknown. The holdfast is anchored to the cell envelope by outer membrane proteins HfaA, HfaB, and HfaD. Holdfast anchor gene mutations result in holdfast shedding and reduced cell adherence. Translocation of HfaA and HfaD to the cell surface requires HfaB. The Wzx homolog HfsF is predicted to be a bacterial polysaccharide flippase. An hfsF deletion significantly reduced the amount of holdfast produced per cell and slightly reduced adherence. A ΔhfsF ΔhfaD double mutant was completely deficient in adherence. A suppressor screen that restored adhesion in the ΔhfsF ΔhfaD mutant identified mutations in three genes: wbqV, rfbB, and rmlA Both WbqV and RfbB belong to a family of nucleoside-diphosphate epimerases, and RmlA has similarity to nucleotidyltransferases. The loss of wbqV or rfbB in the ΔhfsF ΔhfaD mutant reduced holdfast shedding but did not restore holdfast synthesis to parental levels. Loss of wbqV or rfbB did not restore adherence to a ΔhfsF mutant but did restore adherence and holdfast anchoring to a ΔhfaD mutant, confirming that suppression occurs through restoration of holdfast anchoring. The adherence and holdfast anchoring of a ΔhfaA ΔhfaD mutant could be restored by wbqV or rfbB mutation, but such mutations could not suppress these phenotypes in the ΔhfaB mutant. We hypothesize that HfaB plays an additional role in holdfast anchoring or helps to translocate an unknown factor that is important for holdfast anchoring.IMPORTANCE Biofilm formation results in increased resistance to both environmental stresses and antibiotics. Caulobacter crescentus requires an adhesive holdfast for permanent attachment and biofilm formation, but the exact mechanism of polysaccharide anchoring to the cell and the holdfast composition are unknown. Here we identify novel polysaccharide genes that affect holdfast anchoring to the cell. We identify a new role for the holdfast anchor protein HfaB. This work increases our specific knowledge of the polysaccharide adhesin involved in Caulobacter attachment and the general knowledge regarding production and anchoring of polysaccharide adhesins by bacteria. This work also explores the interactions between different polysaccharide biosynthesis and secretion systems in bacteria.

Function and Regulation of Agrobacterium tumefaciens Cell Surface Structures that Promote Attachment

Agrobacterium tumefaciens attaches stably to plant host tissues and abiotic surfaces. During pathogenesis, physical attachment to the site of infection is a prerequisite to infection and horizontal gene transfer to the plant. Virulent and avirulent strains may also attach to plant tissue in more benign plant associations, and as with other soil microbes, to soil surfaces in the terrestrial environment. Although most A. tumefaciens virulence functions are encoded on the tumor-inducing plasmid, genes that direct general surface attachment are chromosomally encoded, and thus this process is not obligatorily tied to virulence, but is a more fundamental capacity. Several different cellular structures are known or suspected to contribute to the attachment process. The flagella influence surface attachment primarily via their propulsive activity, but control of their rotation during the transition to the attached state may be quite complex. A. tumefaciens produces several pili, including the Tad-type Ctp pili, and several plasmid-borne conjugal pili encoded by the Ti and At plasmids, as well as the so-called T-pilus, involved in interkingdom horizontal gene transfer. The Ctp pili promote reversible interactions with surfaces, whereas the conjugal and T-pili drive horizontal gene transfer (HGT) interactions with other cells and tissues. The T-pilus is likely to contribute to physical association with plant tissues during DNA transfer to plants. A. tumefaciens can synthesize a variety of polysaccharides including cellulose, curdlan (β-1,3 glucan), β-1,2 glucan (cyclic and linear), succinoglycan, and a localized polysaccharide(s) that is confined to a single cellular pole and is called the unipolar polysaccharide (UPP). Lipopolysaccharides are also in the outer leaflet of the outer membrane. Cellulose and curdlan production can influence attachment under certain conditions. The UPP is required for stable attachment under a range of conditions and on abiotic and biotic surfaces. Other factors that have been reported to play a role in attachment include the elusive protein called rhicadhesin. The process of surface attachment is under extensive regulatory control and can be modulated by environmental conditions, as well as by direct responses to surface contact. Complex transcriptional and post-transcriptional control circuitry underlies much of the production and deployment of these attachment functions.

End-in-Sight: Cell Polarization by the Polygamic Organizer PopZ

Understanding how asymmetries in cellular constituents are achieved and how such positional information directs the construction of structures in a nonrandom fashion is a fundamental problem in cell biology. The recent identification of determinants that self-assemble into macromolecular complexes at the bacterial cell pole provides new insight into the underlying organizational principles in bacterial cells. Specifically, polarity studies in host-associated or free-living α-proteobacteria, a lineage of Gram-negative (diderm) bacteria, reveals that functional and cytological mono- and bipolarity is often conferred by the multivalent polar organizer PopZ, originally identified as a component of a polar chromosome anchor in the cell cycle model system Caulobacter crescentus. PopZ-dependent polarization appears to be widespread and also functional in obligate intracellular pathogens. Here, we discuss how PopZ polarization and the establishment of polar complexes occurs, and we detail the physiological roles of these complexes.

More than a Tad: spatiotemporal control of Caulobacter pili

The Type IV pilus (T4P) is a powerful and sophisticated bacterial nanomachine involved in numerous cellular processes, including adhesion, DNA uptake and motility. Aside from the well-described subtype T4aP of the Gram-negative genera, including Myxococcus, Pseudomonas and Neisseria, the Tad (tight adherence) pilus secretion system re-shuffles homologous parts from other secretion systems along with uncharacterized components into a new type of protein translocation apparatus. A representative of the Tad apparatus, the Caulobacter crescentus pilus assembly (Cpa) machine is built exclusively at the newborn cell pole once per cell cycle. Recent comprehensive genetic analyses unearthed a myriad of spatiotemporal determinants acting on the Tad/Cpa system, many of which are conserved in other α-proteobacteria, including obligate intracellular pathogens and symbionts.

Cohesive Properties of the Caulobacter crescentus Holdfast Adhesin Are Regulated by a Novel c-di-GMP Effector Protein

When encountering surfaces, many bacteria produce adhesins to facilitate their initial attachment and to irreversibly glue themselves to the solid substrate. A central molecule regulating the processes of this motile-sessile transition is the second messenger c-di-GMP, which stimulates the production of a variety of exopolysaccharide adhesins in different bacterial model organisms. In Caulobacter crescentus, c-di-GMP regulates the synthesis of the polar holdfast adhesin during the cell cycle, yet the molecular and cellular details of this control are currently unknown. Here we identify HfsK, a member of a versatile N-acetyltransferase family, as a novel c-di-GMP effector involved in holdfast biogenesis. Cells lacking HfsK form highly malleable holdfast structures with reduced adhesive strength that cannot support surface colonization. We present indirect evidence that HfsK modifies the polysaccharide component of holdfast to buttress its cohesive properties. HfsK is a soluble protein but associates with the cell membrane during most of the cell cycle. Coincident with peak c-di-GMP levels during the C. crescentus cell cycle, HfsK relocalizes to the cytosol in a c-di-GMP-dependent manner. Our results indicate that this c-di-GMP-mediated dynamic positioning controls HfsK activity, leading to its inactivation at high c-di-GMP levels. A short C-terminal extension is essential for the membrane association, c-di-GMP binding, and activity of HfsK. We propose a model in which c-di-GMP binding leads to the dispersal and inactivation of HfsK as part of holdfast biogenesis progression.

Exopolysaccharide (EPS) adhesins are important determinants of bacterial surface colonization and biofilm formation. Biofilms are a major cause of chronic infections and are responsible for biofouling on water-exposed surfaces. To tackle these problems, it is essential to dissect the processes leading to surface colonization at the molecular and cellular levels. Here we describe a novel c-di-GMP effector, HfsK, that contributes to the cohesive properties and stability of the holdfast adhesin in C. crescentus. We demonstrate for the first time that c-di-GMP, in addition to its role in the regulation of the rate of EPS production, also modulates the physicochemical properties of bacterial adhesins. By demonstrating how c-di-GMP coordinates the activity and subcellular localization of HfsK, we provide a novel understanding of the cellular processes involved in adhesin biogenesis control. Homologs of HfsK are found in representatives of different bacterial phyla, suggesting that they play important roles in various EPS synthesis systems.

TonB-Dependent Heme/Hemoglobin Utilization by Caulobacter crescentus HutA

Siderophore nutrition tests with Caulobacter crescentus strain NA1000 revealed that it utilized a variety of ferric hydroxamate siderophores, including asperchromes, ferrichromes, ferrichrome A, malonichrome, and ferric aerobactin, as well as hemin and hemoglobin. C. crescentus did not transport ferrioxamine B or ferric catecholates. Because it did not use ferric enterobactin, the catecholate aposiderophore was an effective agent for iron deprivation. We determined the kinetics and thermodynamics of [⁵⁹Fe]apoferrichrome and ⁵⁹Fe-citrate binding and transport by NA1000. Its affinity and uptake rate for ferrichrome (equilibrium dissociation constant [K_d ], 1 nM; Michaelis-Menten constant [K_M ], 0.1 nM; V_max, 19 pMol/10⁹ cells/min) were similar to those of Escherichia coli FhuA. Transport properties for ⁵⁹Fe-citrate were similar to those of E. coli FecA (K_M , 5.3 nM; V_max, 29 pMol/10⁹ cells/min). Bioinformatic analyses implicated Fur-regulated loci 00028, 00138, 02277, and 03023 as TonB-dependent transporters (TBDT) that participate in iron acquisition. We resolved TBDT with elevated expression under high- or low-iron conditions by SDS-PAGE of sodium sarcosinate cell envelope extracts, excised bands of interest, and analyzed them by mass spectrometry. These data identified five TBDT: three were overexpressed during iron deficiency (00028, 02277, and 03023), and 2 were overexpressed during iron repletion (00210 and 01196). CLUSTALW analyses revealed homology of putative TBDT 02277 to Escherichia coli FepA and BtuB. A Δ02277 mutant did not transport hemin or hemoglobin in nutrition tests, leading us to designate the 02277 structural gene as hutA (for heme/hemoglobin utilization).IMPORTANCE The physiological roles of the 62 putative TBDT of C. crescentus are mostly unknown, as are their evolutionary relationships to TBDT of other bacteria. We biochemically studied the iron uptake systems of C. crescentus, identified potential iron transporters, and clarified the phylogenetic relationships among its numerous TBDT. Our findings identified the first outer membrane protein involved in iron acquisition by C. crescentus, its heme/hemoglobin transporter (HutA).

A Rhizobiales-Specific Unipolar Polysaccharide Adhesin Contributes to Rhodopseudomonas palustris Biofilm Formation across Diverse Photoheterotrophic Conditions

Bacteria predominantly exist as members of surfaced-attached communities known as biofilms. Many bacterial species initiate biofilms and adhere to each other using cell surface adhesins. This is the case for numerous ecologically diverse Alphaprotebacteria, which use polar exopolysaccharide adhesins for cell-cell adhesion and surface attachment. Here, we show that Rhodopseudomonas palustris, a metabolically versatile member of the alphaproteobacterial order Rhizobiales, contains a functional unipolar polysaccharide (UPP) biosynthesis gene cluster. Deletion of genes predicted to be critical for UPP biosynthesis and export abolished UPP production. We also found that R. palustris uses UPP to mediate biofilm formation across diverse photoheterotrophic growth conditions, wherein light and organic substrates are used to support growth. However, UPP was less important for biofilm formation during photoautotrophy, where light and CO₂ support growth, and during aerobic respiration with organic compounds. Expanding our analysis beyond R. palustris, we examined the phylogenetic distribution and genomic organization of UPP gene clusters among Rhizobiales species that inhabit diverse niches. Our analysis suggests that UPP is a conserved ancestral trait of the Rhizobiales but that it has been independently lost multiple times during the evolution of this clade, twice coinciding with adaptation to intracellular lifestyles within animal hosts.

IMPORTANCE

Bacteria are ubiquitously found as surface-attached communities and cellular aggregates in nature. Here, we address how bacterial adhesion is coordinated in response to diverse environments using two complementary approaches. First, we examined how Rhodopseudomonas palustris, one of the most metabolically versatile organisms ever described, varies its adhesion to surfaces in response to different environmental conditions. We identified critical genes for the production of a unipolar polysaccharide (UPP) and showed that UPP is important for adhesion when light and organic substrates are used for growth. Looking beyond R. palustris, we performed the most comprehensive survey to date on the conservation of UPP biosynthesis genes among a group of closely related bacteria that occupy diverse niches. Our findings suggest that UPP is important for free-living and plant-associated lifestyles but dispensable for animal pathogens. Additionally, we propose guidelines for classifying the adhesins produced by various Alphaprotebacteria, facilitating future functional and comparative studies.

Scaling down for a broader understanding of underwater adhesives - a case for the Caulobacter crescentus holdfast

The adhesion of two materials in the presence of water is greatly impeded by a boundary layer of water between the adhesive and the adherend, resulting in adhesive failure of most synthetic adhesives; however, life evolved first in water and there are many aquatic organisms that have to overcome this impediment to underwater adhesion. For example, multicellular aquatic organisms like the mussel, sandcastle worm and the caddisfly larva employ well-studied adhesive mechanisms for sticking in the presence of water. Unicellular organisms such as bacteria also make use of various means for attaching to surfaces, within similar environmental conditions. Prominent among them is the aquatic bacteria, Caulobacter crescentus which utilizes a unique adhesive secretion, the holdfast, to adhere strongly in the presence of water. Here we review the attachment mechanisms of some multicellular aquatic organisms and compare the similarities and differences in the composition and structure of the C. crescentus holdfast, which holds promise as a potential source for bio-inspired synthetic underwater adhesives with prospective applications in medicine, engineering and biomimetics.

Proper Control of Caulobacter crescentus Cell Surface Adhesion Requires the General Protein Chaperone DnaK

Growth in a surface-attached bacterial community, or biofilm, confers a number of advantages. However, as a biofilm matures, high-density growth imposes stresses on individual cells, and it can become less advantageous for progeny to remain in the community. Thus, bacteria employ a variety of mechanisms to control attachment to and dispersal from surfaces in response to the state of the environment. The freshwater oligotroph Caulobacter crescentus can elaborate a polysaccharide-rich polar organelle, known as the holdfast, which enables permanent surface attachment. Holdfast development is strongly inhibited by the small protein HfiA; mechanisms that control HfiA levels in the cell are not well understood. We have discovered a connection between the essential general protein chaperone, DnaK, and control of C. crescentus holdfast development. C. crescentus mutants partially or completely lacking the C-terminal substrate binding "lid" domain of DnaK exhibit enhanced bulk surface attachment. Partial or complete truncation of the DnaK lid domain increases the probability that any single cell will develop a holdfast by 3- to 10-fold. These results are consistent with the observation that steady-state levels of an HfiA fusion protein are significantly diminished in strains that lack the entire lid domain of DnaK. While dispensable for growth, the lid domain of C. crescentus DnaK is required for proper chaperone function, as evidenced by observed dysregulation of HfiA and holdfast development in strains expressing lidless DnaK mutants. We conclude that DnaK is an important molecular determinant of HfiA stability and surface adhesion control.

IMPORTANCE

Regulatory control of cell adhesion ensures that bacterial cells can transition between free-living and surface-attached states. We define a role for the essential protein chaperone, DnaK, in the control of Caulobacter crescentus cell adhesion. C. crescentus surface adhesion is mediated by an envelope-attached organelle known as the holdfast. Holdfast development is tightly controlled by HfiA, a small protein inhibitor that directly interacts with a WecG/TagA-family glycosyltransferase required for holdfast biosynthesis. We demonstrate that the C-terminal lid domain of DnaK is not essential for growth but is necessary for proper control of HfiA levels in the cell and for control of holdfast adhesin development.

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

Timescales and Frequencies of Reversible and Irreversible Adhesion Events of Single Bacterial Cells

In the environment, most bacteria form surface-attached cell communities called biofilms. The attachment of single cells to surfaces involves an initial reversible stage typically mediated by surface structures such as flagella and pili, followed by a permanent adhesion stage usually mediated by polysaccharide adhesives. Here, we determine the absolute and relative timescales and frequencies of reversible and irreversible adhesion of single cells of the bacterium Caulobacter crescentus to a glass surface in a microfluidic device. We used fluorescence microscopy of C. crescentus expressing green fluorescent protein to track the swimming behavior of individual cells prior to adhesion, monitor the cell at the surface, and determine whether the cell reversibly or irreversibly adhered to the surface. A fluorescently labeled lectin that binds specifically to polar polysaccharides, termed holdfast, discriminated irreversible adhesion events from reversible adhesion events where no holdfast formed. In wild-type cells, the holdfast production time for irreversible adhesion events initiated by surface contact (23 s) was 30-times faster than the holdfast production time that occurs through developmental regulation (13 min). Irreversible adhesion events in wild-type cells (3.3 events/min) are 15-times more frequent than in pilus-minus mutant cells (0.2 events/min), indicating the pili are critical structures in the transition from reversible to irreversible surface-stimulated adhesion. In reversible adhesion events, the dwell time of cells at the surface before departing was the same for wild-type cells (12 s) and pilus-minus mutant cells (13 s), suggesting the pili do not play a significant role in reversible adhesion. Moreover, reversible adhesion events in wild-type cells (6.8 events/min) occur twice as frequently as irreversible adhesion events (3.3 events/min), demonstrating that most cells contact the surface multiple times before transitioning from reversible to irreversible adhesion.

The essential features and modes of bacterial polar growth

Polar growth represents a surprising departure from the canonical dispersed cell growth model. However, we know relatively little of the underlying mechanisms governing polar growth or the requisite suite of factors that direct polar growth. Underscoring how classic doctrine can be turned on its head, the peptidoglycan layer of polar-growing bacteria features unusual crosslinks and in some species the quintessential cell division proteins FtsA and FtsZ are recruited to the growing poles. Remarkably, numerous medically important pathogens utilize polar growth, accentuating the need for intensive research in this area. Here we review models of polar growth in bacteria based on recent research in the Actinomycetales and Rhizobiales, with emphasis on Mycobacterium and Agrobacterium species.

Curli biogenesis: order out of disorder

Many bacteria assemble extracellular amyloid fibers on their cell surface. Secretion of proteins across membranes and the assembly of complex macromolecular structures must be highly coordinated to avoid the accumulation of potentially toxic intracellular protein aggregates. Extracellular amyloid fiber assembly poses an even greater threat to cellular health due to the highly aggregative nature of amyloids and the inherent toxicity of amyloid assembly intermediates. Therefore, temporal and spatial control of amyloid protein secretion is paramount. The biogenesis and assembly of the extracellular bacterial amyloid curli is an ideal system for studying how bacteria cope with the many challenges of controlled and ordered amyloid assembly. Here, we review the recent progress in the curli field that has made curli biogenesis one of the best-understood functional amyloid assembly pathways. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Identification of essential alphaproteobacterial genes reveals operational variability in conserved developmental and cell cycle systems

The cell cycle of Caulobacter crescentus is controlled by a complex signalling network that co-ordinates events. Genome sequencing has revealed many C. crescentus cell cycle genes are conserved in other Alphaproteobacteria, but it is not clear to what extent their function is conserved. As many cell cycle regulatory genes are essential in C. crescentus, the essential genes of two Alphaproteobacteria, Agrobacterium tumefaciens (Rhizobiales) and Brevundimonas subvibrioides (Caulobacterales), were elucidated to identify changes in cell cycle protein function over different phylogenetic distances as demonstrated by changes in essentiality. The results show the majority of conserved essential genes are involved in critical cell cycle processes. Changes in component essentiality reflect major changes in lifestyle, such as divisome components in A. tumefaciens resulting from that organism's different growth pattern. Larger variability of essentiality was observed in cell cycle regulators, suggesting regulatory mechanisms are more customizable than the processes they regulate. Examples include variability in the essentiality of divJ and divK spatial cell cycle regulators, and non-essentiality of the highly conserved and usually essential DNA methyltransferase CcrM. These results show that while essential cell functions are conserved across varying genetic distance, much of a given organism's essential gene pool is specific to that organism.

Mechanisms and regulation of surface interactions and biofilm formation in Agrobacterium

For many pathogenic bacteria surface attachment is a required first step during host interactions. Attachment can proceed to invasion of host tissue or cells or to establishment of a multicellular bacterial community known as a biofilm. The transition from a unicellular, often motile, state to a sessile, multicellular, biofilm-associated state is one of the most important developmental decisions for bacteria. Agrobacterium tumefaciens genetically transforms plant cells by transfer and integration of a segment of plasmid-encoded transferred DNA (T-DNA) into the host genome, and has also been a valuable tool for plant geneticists. A. tumefaciens attaches to and forms a complex biofilm on a variety of biotic and abiotic substrates in vitro. Although rarely studied in situ, it is hypothesized that the biofilm state plays an important functional role in the ecology of this organism. Surface attachment, motility, and cell division are coordinated through a complex regulatory network that imparts an unexpected asymmetry to the A. tumefaciens life cycle. In this review, we describe the mechanisms by which A. tumefaciens associates with surfaces, and regulation of this process. We focus on the transition between flagellar-based motility and surface attachment, and on the composition, production, and secretion of multiple extracellular components that contribute to the biofilm matrix. Biofilm formation by A. tumefaciens is linked with virulence both mechanistically and through shared regulatory molecules. We detail our current understanding of these and other regulatory schemes, as well as the internal and external (environmental) cues mediating development of the biofilm state, including the second messenger cyclic-di-GMP, nutrient levels, and the role of the plant host in influencing attachment and biofilm formation. A. tumefaciens is an important model system contributing to our understanding of developmental transitions, bacterial cell biology, and biofilm formation.

A cell cycle and nutritional checkpoint controlling bacterial surface adhesion

In natural environments, bacteria often adhere to surfaces where they form complex multicellular communities. Surface adherence is determined by the biochemical composition of the cell envelope. We describe a novel regulatory mechanism by which the bacterium, Caulobacter crescentus, integrates cell cycle and nutritional signals to control development of an adhesive envelope structure known as the holdfast. Specifically, we have discovered a 68-residue protein inhibitor of holdfast development (HfiA) that directly targets a conserved glycolipid glycosyltransferase required for holdfast production (HfsJ). Multiple cell cycle regulators associate with the hfiA and hfsJ promoters and control their expression, temporally constraining holdfast development to the late stages of G1. HfiA further functions as part of a 'nutritional override' system that decouples holdfast development from the cell cycle in response to nutritional cues. This control mechanism can limit surface adhesion in nutritionally sub-optimal environments without affecting cell cycle progression. We conclude that post-translational regulation of cell envelope enzymes by small proteins like HfiA may provide a general means to modulate the surface properties of bacterial cells.

Regulated proteolysis in bacterial development

Bacteria use proteases to control three types of events temporally and spatially during the processes of morphological development. These events are the destruction of regulatory proteins, activation of regulatory proteins, and production of signals. While some of these events are entirely cytoplasmic, others involve intramembrane proteolysis of a substrate, transmembrane signaling, or secretion. In some cases, multiple proteolytic events are organized into pathways, for example turnover of a regulatory protein activates a protease that generates a signal. We review well-studied and emerging examples and identify recurring themes and important questions for future research. We focus primarily on paradigms learned from studies of model organisms, but we note connections to regulated proteolytic events that govern bacterial adaptation, biofilm formation and disassembly, and pathogenesis.

Physiochemical properties of Caulobacter crescentus holdfast: a localized bacterial adhesive

To colonize surfaces, the bacterium Caulobacter crescentus employs a polar polysaccharide, the holdfast, located at the end of a thin, long stalk protruding from the cell body. Unlike many other bacteria which adhere through an extended extracellular polymeric network, the holdfast footprint area is tens of thousands times smaller than that of the total bacterium cross-sectional surface, making for some very demanding adhesion requirements. At present, the mechanism of holdfast adhesion remains poorly understood. We explore it here along three lines of investigation: (a) the impact of environmental conditions on holdfast binding affinity, (b) adhesion kinetics by dynamic force spectroscopy, and (c) kinetic modeling of the attachment process to interpret the observed time-dependence of the adhesion force at short and long time scales. A picture emerged in which discrete molecular units called adhesins are responsible for initial holdfast adhesion, by acting in a cooperative manner.

Proteomic profiling of the surface-exposed cell envelope proteins of Caulobacter crescentus

Biotinylation of intact cells, avidin enrichment of derivatized peptides, and shotgun proteomics were employed to reveal the composition of the surface-exposed proteome of the aquatic bacterium, Caulobacter crescentus. Ninety-one unique proteins were identified with the majority originating from the outer membrane, periplasm, and inner membrane, subcellular regions that comprise the Gram-negative bacterium cell envelope. Many of these proteins were described as 'conserved hypothetical protein' or 'hypothetical protein'; and so, the actual expression of these gene products was confirmed. Others did not have any known function or lacked annotation. However, this investigation of the Caulobacter surfaceome did reveal the unanticipated presence of a number of enzymes involved in protein degradation.

BIOLOGICAL SIGNIFICANCE

The results presented here can provide a starting point for hypothesis-driven research projects focused on this bacterium in particular and centered on understanding Gram-negative cell architecture and outer membrane biogenesis broadly. The detected protein degradation enzymes anchored on or located within the outer membrane suggest that Caulobacter has nutrient sources larger than small molecules and/or further processes surface proteins once secreted to this location. Additionally, confirmation of outer membrane residency of those proteins predicted to be periplasmic or whose location prediction was not definitive could potentially elucidate the identities of Gram-negative specific anchorless surface proteins. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.

Effect of a ctrA promoter mutation, causing a reduction in CtrA abundance, on the cell cycle and development of Caulobacter crescentus

Background

Polar development during the alphaproteobacterium Caulobacter crescentus cell cycle is integrated to the point that individual mutations can have pleiotropic effects on the synthesis of polar organelles. Disruption of the genes encoding the histidine kinase PleC, or its localization factor PodJ, disrupts synthesis or functionality of pili, flagella and adhesive holdfast. However, the mechanism by which these mutations affect polar development is not well understood. The aim of this study was to identify new regulators that control multiple aspects of polar organelle development.

Results

To identify mutants with pleiotropic polar organelle synthesis defects, transposon mutagenesis was performed and mutants were selected based resistance to the pili-tropic bacteriophage ΦCbK. Mutants were then screened for defects in motility and holdfast production. Only a single podJ/pleC-independent mutant was isolated which had defects in all three phenotypes. Directed phage assays confirmed the phage resistance phenotype, while the strain demonstrated a similar dispersal radius as a podJ mutant in swarm agar, and treatment with a fluorescent lectin that labels the holdfast showed no staining for this mutant. The transposon had inserted into the promoter region of ctrA, a gene encoding a master transcriptional regulator of the cell cycle, disrupting native transcription but still allowing reduced transcriptional activity and protein production of this essential protein. Transcriptional fusions showed that essential genes controlled by CtrA exhibited minor to moderate changes in expression in the ctrA promoter mutant, while the pilA gene, encoding the subunit of the pilus filament, had a drastic decrease in gene expression. Introduction of a plasmid-born copy of ctrA under its native promoter complemented the phage resistance and holdfast defects, as well as a moderate cell morphology defect, but not the swarming defect.

Conclusions

A mutation was identified that caused pleiotropic defects in polar organelle synthesis, and revealed the surprising result that some CtrA-dependent promoters are more sensitive to changes in CtrA concentration than others. However, the fact that no pleiotropic mutations were found in new regulators suggests that downstream signaling of PleC/PodJ is either essential, redundant, or branching such that all three phenotypes were not simultaneously affected.

Holdfast spreading and thickening during Caulobacter crescentus attachment to surfaces

Background

Adhesion to surfaces facilitates many crucial functions of microbes in their natural habitats. Thus understanding the mechanism of microbial adhesion is of broad interest to the microbiology research community.

Results

We report a study by fluorescence imaging and atomic force microscopy on the growth in size and thickness of the holdfast of synchronized Caulobacter crescentus cells as they attach to a glass surface. We found that the holdfast undergoes a two-stage process of spreading and thickening during its morphogenesis. The holdfast first forms a thin plate on the surface. The diameter of the holdfast plate reaches its final average value of 360 nm by the cell age of ~ 30 min, while its thickness further increases until the age of ~ 60 min. Our AFM analysis indicates that the holdfast is typically thicker in the middle, with gradual falloff in thickness towards the outer edge.

Conclusions

We propose that the newly secreted holdfast substance is fluid-like. It has strong affinity to the surface and cures to form a plate-like holdfast capable of supporting strong and permanent adhesion.

Bypassing the need for subcellular localization of a polysaccharide export-anchor complex by overexpressing its protein subunits

Subcellular protein localization is thought to promote protein-protein interaction by increasing the effective concentration and enabling spatial co-ordination and proper segregation of proteins. We found that protein overexpression allowed the assembly of a productive polysaccharide biosynthesis-export-anchoring complex in the absence of polar localization in Caulobacter crescentus. Polar localization of the holdfast export protein, HfsD, depends on the presence of the other export proteins, HfsA and HfsB, and on the polar scaffold protein PodJ. The holdfast deficiency of hfsB and podJ mutants is suppressed by the overexpression of export proteins. Restored holdfasts are randomly positioned and colocalize with a holdfast anchor protein in these strains, indicating that functional complexes can form at non-polar sites. Therefore, overexpression of export proteins surpasses a concentration threshold necessary for holdfast synthesis. Restoration of holdfast synthesis at non-polar sites reduces surface adhesion, consistent with the need to spatially co-ordinate the holdfast synthesis machinery with the flagellum and pili. These strains lack the cell-specific segregation of the holdfast, resulting in the presence of holdfasts in motile daughter cells. Our results highlight the fact that multiple facets of subcellular localization can be coupled to improve the phenotypic outcome of a protein assembly.

The adhesive and cohesive properties of a bacterial polysaccharide adhesin are modulated by a deacetylase

Bacterial exopolysaccharide synthesis is a prevalent and indispensible activity in many biological processes, including surface adhesion and biofilm formation. In Caulobacter crescentus, surface attachment and subsequent biofilm growth depend on the ability to synthesize an adhesive polar polysaccharide known as the holdfast. In this work, we show that polar polysaccharide synthesis is a conserved phenomenon among Alphaproteobacterial species closely related to C. crescentus. Among them, mutagenesis of Asticcacaulis biprosthecum showed that disruption of the hfsH gene, which encodes a putative polysaccharide deacetylase, leads to accumulation of holdfast in the culture supernatant. Examination of the hfsH deletion mutant in C. crescentus revealed that this strain synthesizes holdfast; however, like the A. biprosthecum hfsH mutant, the holdfasts are shed into the medium and have decreased adhesiveness and cohesiveness. Site-directed mutagenesis at the predicted catalytic site of C. crescentus HfsH phenocopied the ΔhfsH mutant and abolished the esterase activity of HfsH. In contrast, overexpression of HfsH increased cell adherence without increasing holdfast synthesis. We conclude that the polysaccharide deacetylase activity of HfsH is required for the adhesive and cohesive properties of the holdfast, as well as for the anchoring of the holdfast to the cell envelope.

The conserved polarity factor podJ1 impacts multiple cell envelope-associated functions in Sinorhizobium meliloti

Although diminutive in size, bacteria possess highly diverse and spatially confined cellular structures. Two related alphaproteobacteria, Sinorhizobium meliloti and Caulobacter crescentus, serve as models for investigating the genetic basis of morphological variations. S. meliloti, a symbiont of leguminous plants, synthesizes multiple flagella and no prosthecae, whereas C. crescentus, a freshwater bacterium, has a single polar flagellum and stalk. The podJ gene, originally identified in C. crescentus for its role in polar organelle development, is split into two adjacent open reading frames, podJ1 and podJ2, in S. meliloti. Deletion of podJ1 interferes with flagellar motility, exopolysaccharide production, cell envelope integrity, cell division and normal morphology, but not symbiosis. As in C. crescentus, the S. meliloti PodJ1 protein appears to act as a polarity beacon and localizes to the newer cell pole. Microarray analysis indicates that podJ1 affects the expression of at least 129 genes, the majority of which correspond to observed mutant phenotypes. Together, phenotypic characterization, microarray analysis and suppressor identification suggest that PodJ1 controls a core set of conserved elements, including flagellar and pili genes, the signalling proteins PleC and DivK, and the transcriptional activator TacA, while alternative downstream targets have evolved to suit the distinct lifestyles of individual species.

The scaffolding and signalling functions of a localization factor impact polar development

In the differentiating alphaproteobacterium Caulobacter crescentus, organelle synthesis at cell poles is critical to forming different progeny after cell division. Co-ordination of polar organelle synthesis, including pili and holdfast, and flagellum ejection, is mediated in part by the scaffolding protein PodJ. At the time of cell division, PodJ undergoes regulated processing to a short form that persists at the flagellar pole of swarmer cells. This study analyses how PodJ's role in structural and signalling protein localization impacts organelle synthesis. A PodJ mutant with an internal deletion exhibits reduced sensitivity to pili-tropic phage ΦCbK, resulting from reduced pilA gene expression, which can be linked to altered signalling protein localization. The phage sensitivity defect of a ΔpodJ mutant can be partially suppressed by ectopic pilA expression. Induction of PodJ processing, by manipulation of podJ itself or controlled perP expression, resulted in decreased pilus biogenesis and, when coupled with a podJ mutation that reduced pilA expression, led to complete loss of phage sensitivity. As a whole, the results show that PodJ's scaffolding role for structural and signalling proteins both contribute to flagellar pole organelle development.

Functional characterization of UDP-glucose:undecaprenyl-phosphate glucose-1-phosphate transferases of Escherichia coli and Caulobacter crescentus

Escherichia coli K-12 WcaJ and the Caulobacter crescentus HfsE, PssY, and PssZ enzymes are predicted to initiate the synthesis of colanic acid (CA) capsule and holdfast polysaccharide, respectively. These proteins belong to a prokaryotic family of membrane enzymes that catalyze the formation of a phosphoanhydride bond joining a hexose-1-phosphate with undecaprenyl phosphate (Und-P). In this study, in vivo complementation assays of an E. coli K-12 wcaJ mutant demonstrated that WcaJ and PssY can complement CA synthesis. Furthermore, WcaJ can restore holdfast production in C. crescentus. In vitro transferase assays demonstrated that both WcaJ and PssY utilize UDP-glucose but not UDP-galactose. However, in a strain of Salmonella enterica serovar Typhimurium deficient in the WbaP O antigen initiating galactosyltransferase, complementation with WcaJ or PssY resulted in O-antigen production. Gas chromatography-mass spectrometry (GC-MS) analysis of the lipopolysaccharide (LPS) revealed the attachment of both CA and O-antigen molecules to lipid A-core oligosaccharide (OS). Therefore, while UDP-glucose is the preferred substrate of WcaJ and PssY, these enzymes can also utilize UDP-galactose. This unexpected feature of WcaJ and PssY may help to map specific residues responsible for the nucleotide diphosphate specificity of these or similar enzymes. Also, the reconstitution of O-antigen synthesis in Salmonella, CA capsule synthesis in E. coli, and holdfast synthesis provide biological assays of high sensitivity to examine the sugar-1-phosphate transferase specificity of heterologous proteins.

AGGRESCAN: method, application, and perspectives for drug design

Protein aggregation underlies the development of an increasing number of conformational human diseases of growing incidence, such as Alzheimer's and Parkinson's diseases. Furthermore, the accumulation of recombinant proteins as intracellular aggregates represents a critical obstacle for the biotechnological production of polypeptides. Also, ordered protein aggregates constitute novel and versatile nanobiomaterials. Consequently, there is an increasing interest in the development of methods able to forecast the aggregation properties of polypeptides in order to modulate their intrinsic solubility. In this context, we have developed AGGRESCAN, a simple and fast algorithm that predicts aggregation-prone segments in protein sequences, compares the aggregation properties of different proteins or protein sets and analyses the effect of mutations on protein aggregation propensities.

Complete genome sequence of Hirschia baltica type strain (IFAM 1418(T))

The family Hyphomonadaceae within the Alphaproteobacteria is largely comprised of bacteria isolated from marine environments with striking morphologies and an unusual mode of cell growth. Here, we report the complete genome sequence Hirschia baltica, which is only the second a member of the Hyphomonadaceae with a published genome sequence. H. baltica is of special interest because it has a dimorphic life cycle and is a stalked, budding bacterium. The 3,455,622 bp long chromosome and 84,492 bp plasmid with a total of 3,222 protein-coding and 44 RNA genes were sequenced as part of the DOE Joint Genome Institute Program CSP 2008.

Surface contact stimulates the just-in-time deployment of bacterial adhesins

The attachment of bacteria to surfaces provides advantages such as increasing nutrient access and resistance to environmental stress. Attachment begins with a reversible phase, often mediated by surface structures such as flagella and pili, followed by a transition to irreversible attachment, typically mediated by polysaccharides. Here we show that the interplay between pili and flagellum rotation stimulates the rapid transition between reversible and polysaccharide-mediated irreversible attachment. We found that reversible attachment of Caulobacter crescentus cells is mediated by motile cells bearing pili and that their contact with a surface results in the rapid pili-dependent arrest of flagellum rotation and concurrent stimulation of polar holdfast adhesive polysaccharide. Similar stimulation of polar adhesin production by surface contact occurs in Asticcacaulis biprosthecum and Agrobacterium tumefaciens. Therefore, single bacterial cells respond to their initial contact with surfaces by triggering just-in-time adhesin production. This mechanism restricts stable attachment to intimate surface interactions, thereby maximizing surface attachment, discouraging non-productive self-adherence, and preventing curing of the adhesive.

Dual amyloid domains promote differential functioning of the chaplin proteins during Streptomyces aerial morphogenesis

The chaplin proteins are functional amyloids found in the filamentous Streptomyces bacteria. These secreted proteins are required for the aerial development of Streptomyces coelicolor, and contribute to an intricate rodlet ultrastructure that decorates the surfaces of aerial hyphae and spores. S. coelicolor encodes eight chaplin proteins. Previous studies have revealed that only three of these proteins (ChpC, ChpE, and ChpH) are necessary for promoting aerial development, and of these three, ChpH is the primary developmental determinant. Here, we show that the model chaplin, ChpH, contains two amyloidogenic domains: one in the N terminus and one in the C terminus of the mature protein. These domains have different polymerization properties as determined using fluorescence spectroscopy, secondary structure analyses, and electron microscopy. We coupled these in vitro assays with in vivo genetic studies to probe the connection between ChpH amyloidogenesis and its biological function. Using mutational analyses, we demonstrated that both N- and C-terminal amyloid domains of ChpH were required for promoting aerial hypha formation, while the N-terminal domain was dispensable for assembly of the rodlet ultrastructure. These results suggest that there is a functional differentiation of the dual amyloid domains in the chaplin proteins.

Poles apart: prokaryotic polar organelles and their spatial regulation

While polar organelles hold the key to understanding the fundamentals of cell polarity and cell biological principles in general, they have served in the past merely for taxonomical purposes. Here, we highlight recent efforts in unraveling the molecular basis of polar organelle positioning in bacterial cells. Specifically, we detail the role of members of the Ras-like GTPase superfamily and coiled-coil-rich scaffolding proteins in modulating bacterial cell polarity and in recruiting effector proteins to polar sites. Such roles are well established for eukaryotic cells, but not for bacterial cells that are generally considered diffusion-limited. Studies on spatial regulation of protein positioning in bacterial cells, though still in their infancy, will undoubtedly experience a surge of interest, as comprehensive localization screens have yielded an extensive list of (polarly) localized proteins, potentially reflecting subcellular sites of functional specialization predicted for organelles.

A bacterial extracellular DNA inhibits settling of motile progeny cells within a biofilm

In natural systems, bacteria form complex, surface-attached communities known as biofilms. This lifestyle presents numerous advantages compared with unattached or planktonic life, such as exchange of nutrients, protection from environmental stresses and increased tolerance to biocides. Despite such benefits, dispersal also plays an important role in escaping deteriorating environments and in successfully colonizing favourable, unoccupied habitat patches. The α-proteobacterium Caulobacter crescentus produces a motile swarmer cell and a sessile stalked cell at each cell division. We show here that C. crescentus extracellular DNA (eDNA) inhibits the ability of its motile cell type to settle in a biofilm. eDNA binds to the polar holdfast, an adhesive structure required for permanent surface attachment and biofilm formation, thereby inhibiting cell attachment. Because stalked cells associate tightly with the biofilm through their holdfast, we hypothesize that this novel mechanism acts on swarmer cells born in a biofilm, where eDNA can accumulate to a sufficient concentration to inhibit their ability to settle. By targeting a specific cell type in a biofilm, this mechanism modulates biofilm development and promotes dispersal without causing a potentially undesirable dissolution of the existing biofilm.

Cell dispersal in biofilms: an extracellular DNA masks nature's strongest glue

Growth in biofilms provides bacterial species with many advantages over growth in suspension, e.g. colonization of nutrient-rich areas. In the α-proteobacterium Caulobacter crescentus biofilm formation is facilitated through its asymmetric cell division, where one daughter cell becomes a motile flagellated swarmer cell able to colonize new surfaces while the other remains as a stalked cell attached to the substrate through the adhesive holdfast. The Caulobacter biofilm consists of stalked cells arranged either in a monolayer or in a multicellular 'mushroom' structure. In this issue of Molecular Microbiology, Berne et al. demonstrate that extracellular DNA (eDNA) from lysed cells prevents biofilm maturation. eDNA masks the adhesive properties of newly synthesized holdfast to enable the escape of swarmer cells from the biofilm. By contrast, holdfasts on previously attached stalked cells remain unaffected by eDNA. Surprisingly, the inhibitory effect was genus-specific, as only DNA from Caulobacter, but not from other genera, could interfere with biofilm maturation. This study reveals a new role for DNA in biofilms, as a regulatory rather than a structural component, and a novel mechanism to facilitate the escape of cells from biofilms. A compelling case is made for the existence of a new type of genus-specific 'macromolecular language'.