Structural alterations in a type IV pilus subunit protein result in concurrent defects in multicellular behaviour and adherence to host tissue

Deep mutational scanning of the Neisseria meningitidis major pilin reveals the importance of pilus tip-mediated adhesion

Type IV pili (TFP) are multifunctional micrometer-long filaments expressed at the surface of many prokaryotes. In Neisseria meningitidis, TFP are crucial for virulence. Indeed, these homopolymers of the major pilin PilE mediate interbacterial aggregation and adhesion to host cells. However, the mechanisms behind these functions remain unclear. Here, we simultaneously determined regions of PilE involved in pilus display, auto-aggregation, and adhesion by using deep mutational scanning and started mining this extensive functional map. For auto-aggregation, pili must reach a minimum length to allow pilus-pilus interactions through an electropositive cluster of residues centered around Lys140. For adhesion, results point to a key role for the tip of the pilus. Accordingly, purified pili interacting with host cells initially bind via their tip-located major pilin and then along their length. Overall, these results identify functional domains of PilE and support a direct role of the major pilin in TFP-dependent aggregation and adhesion.

Transcriptional initiation of a small RNA, not R-loop stability, dictates the frequency of pilin antigenic variation in Neisseria gonorrhoeae

Neisseria gonorrhoeae, the sole causative agent of gonorrhea, constitutively undergoes diversification of the Type IV pilus. Gene conversion occurs between one of the several donor silent copies located in distinct loci and the recipient pilE gene, encoding the major pilin subunit of the pilus. A guanine quadruplex (G4) DNA structure and a cis-acting sRNA (G4-sRNA) are located upstream of the pilE gene and both are required for pilin antigenic variation (Av). We show that the reduced sRNA transcription lowers pilin Av frequencies. Extended transcriptional elongation is not required for Av, since limiting the transcript to 32 nt allows for normal Av frequencies. Using chromatin immunoprecipitation (ChIP) assays, we show that cellular G4s are less abundant when sRNA transcription is lower. In addition, using ChIP, we demonstrate that the G4-sRNA forms a stable RNA:DNA hybrid (R-loop) with its template strand. However, modulating R-loop levels by controlling RNase HI expression does not alter G4 abundance quantified through ChIP. Since pilin Av frequencies were not altered when modulating R-loop levels by controlling RNase HI expression, we conclude that transcription of the sRNA is necessary, but stable R-loops are not required to promote pilin Av.

Bacterial autoaggregation

Many bacteria, both environmental and pathogenic, exhibit the property of autoaggregation. In autoaggregation (sometimes also called autoagglutination or flocculation), bacteria of the same type form multicellular clumps that eventually settle at the bottom of culture tubes. Autoaggregation is generally mediated by self-recognising surface structures, such as proteins and exopolysaccharides, which we term collectively as autoagglutinins. Although a widespread phenomenon, in most cases the function of autoaggregation is poorly understood, though there is evidence to show that aggregating bacteria are protected from environmental stresses or host responses. Autoaggregation is also often among the first steps in forming biofilms. Here, we review the current knowledge on autoaggregation, the role of autoaggregation in biofilm formation and pathogenesis, and molecular mechanisms leading to aggregation using specific examples.

Fimbriae: Classification and Biochemistry

Proteinaceous, nonflagellar surface appendages constitute a variety of structures, including those known variably as fimbriae or pili. Constructed by distinct assembly pathways resulting in diverse morphologies, fimbriae have been described to mediate functions including adhesion, motility, and DNA transfer. As these structures can represent major diversifying elements among Escherichia and Salmonella isolates, multiple fimbrial classification schemes have been proposed and a number of mechanistic insights into fimbrial assembly and function have been made. Herein we describe the classifications and biochemistry of fimbriae assembled by the chaperone/usher, curli, and type IV pathways.

Pili-Induced Clustering of N. gonorrhoeae Bacteria

Type IV pili (Tfp) are prokaryotic retractable appendages known to mediate surface attachment, motility, and subsequent clustering of cells. Tfp are the main means of motility for Neisseria gonorrhoeae, the causative agent of gonorrhea. Tfp are also involved in formation of the microcolonies, which play a crucial role in the progression of the disease. While motility of individual cells is relatively well understood, little is known about the dynamics of N. gonorrhoeae aggregation. We investigate how individual N. gonorrhoeae cells, initially uniformly dispersed on flat plastic or glass surfaces, agglomerate into spherical microcolonies within hours. We quantify the clustering process by measuring the area fraction covered by the cells, number of cell aggregates, and their average size as a function of time. We observe that the microcolonies are also able to move but their mobility rapidly vanishes as the size of the colony increases. After a certain critical size they become immobile. We propose a simple theoretical model which assumes a pili-pili interaction of cells as the main clustering mechanism. Numerical simulations of the model quantitatively reproduce the experimental data on clustering and thus suggest that the agglomeration process can be entirely explained by the Tfp-mediated interactions. In agreement with this hypothesis mutants lacking pili are not able to form colonies. Moreover, cells with deficient quorum sensing mechanism show similar aggregation as the wild-type bacteria. Therefore, our results demonstrate that pili provide an essential mechanism for colony formation, while additional chemical cues, for example quorum sensing, might be of secondary importance.

Crystal Structure of the Minor Pilin CofB, the Initiator of CFA/III Pilus Assembly in Enterotoxigenic Escherichia coli

Type IV pili are extracellular polymers of the major pilin subunit. These subunits are held together in the pilus filament by hydrophobic interactions among their N-terminal α-helices, which also anchor the pilin subunits in the inner membrane prior to pilus assembly. Type IV pilus assembly involves a conserved group of proteins that span the envelope of Gram-negative bacteria. Among these is a set of minor pilins, so named because they share their hydrophobic N-terminal polymerization/membrane anchor segment with the major pilins but are much less abundant. Minor pilins influence pilus assembly and retraction, but their precise functions are not well defined. The Type IV pilus systems of enterotoxigenic Escherichia coli and Vibrio cholerae are among the simplest of Type IV pilus systems and possess only a single minor pilin. Here we show that the enterotoxigenic E. coli minor pilins CofB and LngB are required for assembly of their respective Type IV pili, CFA/III and Longus. Low levels of the minor pilins are optimal for pilus assembly, and CofB can be detected in the pilus fraction. We solved the 2.0 Å crystal structure of N-terminally truncated CofB, revealing a pilin-like protein with an extended C-terminal region composed of two discrete domains connected by flexible linkers. The C-terminal region is required for CofB to initiate pilus assembly. We propose a model for CofB-initiated pilus assembly with implications for understanding filament growth in more complex Type IV pilus systems as well as the related Type II secretion system.

Single-bacterium nanomechanics in biomedicine: unravelling the dynamics of bacterial cells

The use of the atomic force microscope (AFM) in microbiology has progressed significantly throughout the years since its first application as a high-resolution imaging instrument. Modern AFM setups are capable of characterizing the nanomechanical behaviour of bacterial cells at both the cellular and molecular levels, where elastic properties and adhesion forces of single bacterium cells can be examined under different experimental conditions. Considering that bacterial and biofilm-mediated infections continue to challenge the biomedical field, it is important to understand the biophysical events leading towards bacterial adhesion and colonization on both biological and non-biological substrates. The purpose of this review is to present the latest findings concerning the field of single-bacterium nanomechanics, and discuss future trends and applications of nanoindentation and single-cell force spectroscopy techniques in biomedicine.

Type IV pilus assembly proficiency and dynamics influence pilin subunit phospho-form macro- and microheterogeneity in Neisseria gonorrhoeae

The PilE pilin subunit protein of the gonococcal Type IV pilus (Tfp) colonization factor undergoes multisite, covalent modification with the zwitterionic phospho-form modification phosphoethanolamine (PE). In a mutant lacking the pilin-like PilV protein however, PilE is modified with a mixture of PE and phosphocholine (PC). Moreover, intrastrain variation of PilE PC modification levels have been observed in backgrounds that constitutively express PptA (the protein phospho-form transferase A) required for both PE and PC modification. The molecular basis underlying phospho-form microheterogeneity in these instances remains poorly defined. Here, we examined the effects of mutations at numerous loci that disrupt or perturb Tfp assembly and observed that these mutants phenocopy the pilV mutant vis a vis phospho-form modification status. Thus, PC modification appears to be directly or indirectly responsive to the efficacy of pilin subunit interactions. Despite the complexity of contributing factors identified here, the data favor a model in which increased retention in the inner membrane may act as a key signal in altering phospho-form modification. These results also provide an alternative explanation for the variation in PilE PC levels observed previously and that has been assumed to be due to phase variation of pptA. Moreover, mass spectrometry revealed evidence for mono- and di-methylated forms of PE attached to PilE in mutants deficient in pilus assembly, directly implicating a methyltransferase-based pathway for PC synthesis in N. gonorrhoeae.

Neisseria gonorrhoeae RecQ helicase HRDC domains are essential for efficient binding and unwinding of the pilE guanine quartet structure required for pilin antigenic variation

The strict human pathogen Neisseria gonorrhoeae utilizes homologous recombination to antigenically vary the pilus, thus evading the host immune response. High-frequency gene conversion reactions between many silent pilin loci and the expressed pilin locus (pilE) allow for numerous pilus variants per strain to be produced from a single strain. For pilin antigenic variation (Av) to occur, a guanine quartet (G4) structure must form upstream of pilE. The RecQ helicase is one of several recombination or repair enzymes required for efficient levels of pilin Av, and RecQ family members have been shown to bind to and unwind G4 structures. Additionally, the vast majority of RecQ helicase family members encode one "helicase and RNase D C-terminal" (HRDC) domain, whereas the N. gonorrhoeae RecQ helicase gene encodes three HRDC domains, which are critical for pilin Av. Here, we confirm that deletion of RecQ HRDC domains 2 and 3 causes a decrease in the frequency of pilin Av comparable to that obtained with a functional knockout. We demonstrate that the N. gonorrhoeae RecQ helicase can bind and unwind the pilE G4 structure. Deletion of the RecQ HRDC domains 2 and 3 resulted in a decrease in G4 structure binding and unwinding. These data suggest that the decrease in pilin Av observed in the RecQ HRDC domain 2 and 3 deletion mutant is a result of the enzyme's inability to efficiently bind and unwind the pilE G4 structure.

Transcription of a cis-acting, noncoding, small RNA is required for pilin antigenic variation in Neisseria gonorrhoeae

The strict human pathogen Neisseria gonorrhoeae can utilize homologous recombination to generate antigenic variability in targets of immune surveillance. To evade the host immune response, N. gonorrhoeae promotes high frequency gene conversion events between many silent pilin copies and the expressed pilin locus (pilE), resulting in the production of variant pilin proteins. Previously, we identified a guanine quartet (G4) structure localized near pilE that is required for the homologous recombination reactions leading to pilin antigenic variation (Av). In this work, we demonstrate that inactivating the promoter of a small non-coding RNA (sRNA) that initiates within the G4 forming sequence blocks pilin Av. The sRNA promoter is conserved in all sequenced gonococcal strains, and mutations in the predicted transcript downstream of the G4 forming sequence do not alter pilin Av. A mutation that produces a stronger promoter or substitution of the pilE G4-associated sRNA promoter with a phage promoter (when the phage polymerase was expressed) produced wild-type levels of pilin Av. Altering the direction and orientation of the pilE G4-associated sRNA disrupted pilin Av. In addition, expression of the sRNA at a distal site on the gonococcal chromosome in the context of a promoter mutant did not support pilin Av. We conclude that the DNA containing the G-rich sequence can only form the G4 structure during transcription of this sRNA, thus providing a unique molecular step for the initiation of programmed recombination events.

To evade the host immune response, pathogens have evolved mechanisms to provide genetic diversity in targets of immune surveillance. Organisms that express these diversification systems are under strong evolutionary pressure to provide subpopulations of preexisting variants and often rely on cellular recombination machinery to catalyze dedicated high-frequency reactions without disturbing genome integrity. Previously, we defined a guanine quartet (G4) structure in the strict human pathogen Neisseria gonorrhoeae that is required for initiating the homologous recombination reactions leading to pilin antigenic variation (Av). G4 structures have been implicated in many biological processes, however the mechanisms allowing their formation within a chromosome have not been elucidated. In this work, we show a direct link between transcription of a small RNA (sRNA) that initiates within the G4 structure forming sequence and pilin Av and conclude that the process of transcription is necessary for G4 structure formation. sRNAs have emerged as important regulatory molecules in both eukaryotes and prokaryotes, and this is a novel activity of a sRNA in a bacterium. We anticipate that the reliance of G4 structure formation on transcription is a mechanism used by other biological systems that rely on this alternative DNA structure.

Meningococcal PilV potentiates Neisseria meningitidis type IV pilus-mediated internalization into human endothelial and epithelial cells

The type IV pilus of Neisseria meningitidis is the major factor for meningococcal adhesion to host cells. In this study, we showed that a mutant of N. meningitidis pilV, a minor pilin protein, internalized less efficiently to human endothelial and epithelial cells than the wild-type strain. Matrix-assisted laser desorption ionization-time of flight mass spectrometry and electrospray ionization tandem mass spectrometry analyses showed that PilE, the major subunit of pili, was less glycosylated at its serine 62 residue (Ser62) in the ΔpilV mutant than in the pilV(+) strain, whereas phosphoglycerol at PilE Ser93 and phosphocholine at PilE Ser67 were not changed. Introduction of the pglL mutation, which results in complete loss of O-linked glycosylation from Ser62, slightly reduced N. meningitidis internalization into human brain microvascular endothelial cells, whereas the addition of the ΔpilV mutation greatly reduced N. meningitidis internalization. The accumulation of ezrin, which is part of the cytoskeleton ERM family, was observed with pilV(+), ΔpglL, and pilE(S62A) strains but not with the ΔpilV mutant. These results suggested that whereas N. meningitidis pilin originally had an adhesive activity that was less affected by minor pilin proteins, the invasive function evolved with incorporation of the PilV protein into the pili to promote the N. meningitidis internalization into human cells.

Structure of the Vibrio cholerae Type IVb Pilus and stability comparison with the Neisseria gonorrhoeae type IVa pilus

Type IV pili are multifunctional filaments displayed on many bacterial pathogens. Members of the Type IVa pilus subclass are found on a diverse group of human pathogens, whereas Type IVb pili are found almost exclusively on enteric bacteria. The Type IVa and IVb subclasses are distinguished by differences in the pilin subunits, including the fold of the globular domain. To understand the implications of the distinct pilin folds, we compared the stabilities of pilin subunits and pilus filaments for the Type IVa GC pilus from Neisseria gonorrhoeae and the Type IVb toxin-coregulated pilus (TCP) from Vibrio cholerae. We show that while recombinant TCP pilin is more stable than GC pilin, the GC pili are more resistant to proteolysis, heat and chemical denaturation than TCP, remaining intact in 8 M urea. To understand these differences, we determined the TCP structure by electron microscopy and three-dimensional image reconstruction. TCP have an architecture similar to that of GC pili, with subunits arranged in a right-handed 1-start helix and related by an 8.4-Å axial rise and a 96.8° azimuthal rotation. However, the TCP subunits are not as tightly packed as GC pilins, and the distinct Type IVb pilin fold exposes a segment of the α-helical core of TCP. Hydrophobic interactions dominate for both pilus subtypes, but base stacking by aromatic residues conserved among the Type IVa pilins may contribute to GC pilus stability. The extraordinary stability of GC pili may represent an adaptation of the Type IVa pili to harsh environments and the need to retract against external forces.

Focusing homologous recombination: pilin antigenic variation in the pathogenic Neisseria

Some pathogenic microbes utilize homologous recombination to generate antigenic variability in targets of immune surveillance. These specialized systems rely on the cellular recombination machinery to catalyse dedicated, high-frequency reactions that provide extensive diversity in the genes encoding surface antigens. A description of the specific mechanisms that allow unusually high rates of recombination without deleterious effects on the genome in the well-characterized pilin antigenic variation systems of Neisseria gonorrhoeae and Neisseria meningitidis is presented. We will also draw parallels to selected bacterial and eukaryotic antigenic variation systems, and suggest the most pressing unanswered questions related to understanding these important processes.

Pseudomonas aeruginosa interacts with epithelial cells rapidly forming aggregates that are internalized by a Lyn-dependent mechanism

Growing evidence is pointing to the importance of multicellular bacterial structures in the interaction of pathogenic bacteria with their host. Transition from planktonic to host cell-associated multicellular structures is an essential infection step that has not been described for the opportunistic human pathogen Pseudomonas aeruginosa. In this study we show that P. aeruginosa interacts with the surface of epithelial cells mainly forming aggregates. Dynamics of aggregate formation typically follow a sigmoidal curve. First, a single bacterium attaches at cell-cell junctions. This is followed by rapid recruitment of free-swimming bacteria and association of bacterial cells resulting in the formation of an aggregate on the order of minutes. Aggregates are associated with phosphatidylinositol 3,4,5-trisphosphate (PIP3)-enriched host cell membrane protrusions. We further show that aggregates can be rapidly internalized into epithelial cells. Lyn, a member of the Src family tyrosine kinases previously implicated in P. aeruginosa infection, mediates both PIP3-enriched protrusion formation and aggregate internalization. Our results establish the first framework of principles that define P. aeruginosa transition to multicellular structures during interaction with host cells.

Vibrio cholerae El Tor TcpA crystal structure and mechanism for pilus-mediated microcolony formation

Type IV pili (T4P) are critical to virulence for Vibrio cholerae and other bacterial pathogens. Among their diverse functions, T4P mediate microcolony formation, which protects the bacteria from host defences and concentrates secreted toxins. The T4P of the two V. cholerae O1 disease biotypes, classical and El Tor, share 81% identity in their TcpA subunits, yet these filaments differ in their interaction patterns as assessed by electron microscopy. To understand the molecular basis for pilus-mediated microcolony formation, we solved a 1.5 A resolution crystal structure of N-terminally truncated El Tor TcpA and compared it with that of classical TcpA. Residues that differ between the two pilins are located on surface-exposed regions of the TcpA subunits. By iteratively changing these non-conserved amino acids in classical TcpA to their respective residues in El Tor TcpA, we identified residues that profoundly affect pilus:pilus interaction patterns and bacterial aggregation. These residues lie on either the protruding d-region of the TcpA subunit or in a cavity between pilin subunits in the pilus filament. Our results support a model whereby pili interact via intercalation of surface protrusions on one filament into depressions between subunits on adjacent filaments as a means to hold V. cholerae cells together in microcolonies.

The Longus type IV pilus of enterotoxigenic Escherichia coli (ETEC) mediates bacterial self-aggregation and protection from antimicrobial agents

Enterotoxigenic Escherichia coli (ETEC) strains are leading causes of childhood diarrhea in developing countries. ETEC pili and non-pili adherence factors designated colonization surface antigens (CSA) are believed to be important in the pathogenesis of diarrhea. Longus, a type IV pilus identified as the CSA(21), is expressed in up to one-third of ETEC strains, and share similarities to the toxin-coregulated pilus of Vibrio cholerae, and the bundle-forming pilus of enteropathogenic E. coli. To identify longus phenotype and possible function, a site-directed mutation of the lngA major subunit gene in the E9034A wild type ETEC strain was constructed. Lack of longus expression from the lngA mutant was demonstrated by immunoblot analysis and electron microscopy using specific anti-LngA antibody. Formation of self-aggregates by ETEC was shown to be dependent on longus expression as the lngA mutant or wild type grown under poor longus expression conditions was unable to express this phenotype. Longus-expressing ETEC were also associated with improved survival when exposed to antibacterial factors including lysozyme and antibiotics. This suggests that longus-mediated bacterial self-aggregates protect bacteria against antimicrobial environmental agents and may promote gut colonization.

Pseudomonas aeruginosa Type IV pilus expression in Neisseria gonorrhoeae: effects of pilin subunit composition on function and organelle dynamics

Type IV pili (TFP) play central roles in the expression of many phenotypes including motility, multicellular behavior, sensitivity to bacteriophages, natural genetic transformation, and adherence. In Neisseria gonorrhoeae, these properties require ancillary proteins that act in conjunction with TFP expression and influence organelle dynamics. Here, the intrinsic contributions of the pilin protein itself to TFP dynamics and associated phenotypes were examined by expressing the Pseudomonas aeruginosa PilA(PAK) pilin subunit in N. gonorrhoeae. We show here that, although PilA(PAK) pilin can be readily assembled into TFP in this background, steady-state levels of purifiable fibers are dramatically reduced relative those of endogenous pili. This defect is due to aberrant TFP dynamics as it is suppressed in the absence of the PilT pilus retraction ATPase. Functionally, PilA(PAK) pilin complements gonococcal adherence for human epithelial cells but only in a pilT background, and this property remains dependent on the coexpression of both the PilC adhesin and the PilV pilin-like protein. Since P. aeruginosa pilin only moderately supports neisserial sequence-specific transformation despite its assembly proficiency, these results together suggest that PilA(PAK) pilin functions suboptimally in this environment. This appears to be due to diminished compatibility with resident proteins essential for TFP function and dynamics. Despite this, PilA(PAK) pili support retractile force generation in this background equivalent to that reported for endogenous pili. Furthermore, PilA(PAK) pili are both necessary and sufficient for bacteriophage PO4 binding, although the strain remains phage resistant. Together, these findings have significant implications for TFP biology in both N. gonorrhoeae and P. aeruginosa.

Conserved regions from Neisseria gonorrhoeae pilin are immunosilent and not immunosuppressive

PilE is the primary subunit of type IV pili from Neisseria gonorrhoeae and contains a surface-exposed hypervariable region thought to be one feature of pili that has prevented development of a pilin-based vaccine. We have created a three-dimensional structure-based antigen by replacing the hypervariable region of PilE with an aspartate-glutamine linker chosen from the sequence of Pseudomonas aeruginosa PilA. We then characterized murine immune responses to this novel protein to determine if conserved PilE regions could serve as a vaccine candidate. The control PilE protein elicited strong T-cell-dependent B-cell responses that are specific to epitopes in both the hypervariable deletion and control proteins. In contrast, the hypervariable deletion protein was unable to elicit an immune response in mice, suggesting that in the absence of the hypervariable region, the conserved regions of PilE alone are not sufficient for antibody production. Further analysis of these PilE proteins with suppressor cell assays showed that neither suppresses T- or B-cell responses, and flow cytometry experiments suggested that they do not exert suppressor effects by activating T regulatory cells. Our results show that in the murine model, the hypervariable region of PilE is required to activate immune responses to pilin, whereas the conserved regions are unusually nonimmunogenic. In addition, we show that both hypervariable and conserved regions of pilin are not suppressive, suggesting that PilE does not cause the decrease in T-cell populations observed during gonococcal cervicitis.

Identification of diverse archaeal proteins with class III signal peptides cleaved by distinct archaeal prepilin peptidases

Most secreted archaeal proteins are targeted to the membrane via a tripartite signal composed of a charged N terminus and a hydrophobic domain, followed by a signal peptidase-processing site. Signal peptides of archaeal flagellins, similar to class III signal peptides of bacterial type IV pilins, are distinct in that their processing sites precede the hydrophobic domain, which is crucial for assembly of these extracytoplasmic structures. To identify the complement of archaeal proteins with class III signal sequences, a PERL program (FlaFind) was written. A diverse set of proteins was identified, and many of these FlaFind positives were encoded by genes that were cotranscribed with homologs of pilus assembly genes. Moreover, structural conservation of primary sequences between many FlaFind positives and subunits of bacterial pilus-like structures, which have been shown to be critical for pilin assembly, have been observed. A subset of pilin-like FlaFind positives contained a conserved domain of unknown function (DUF361) within the signal peptide. Many of the genes encoding these proteins were in operons that contained a gene encoding a novel euryarchaeal prepilin-peptidase, EppA, homolog. Heterologous analysis revealed that Methanococcus maripaludis DUF361-containing proteins were specifically processed by the EppA homolog of this archaeon. Conversely, M. maripaludis preflagellins were cleaved only by the archaeal preflagellin peptidase FlaK. Together, the results reveal a diverse set of archaeal proteins with class III signal peptides that might be subunits of as-yet-undescribed cell surface structures, such as archaeal pili.

Substitutions in the N-terminal alpha helical spine of Neisseria gonorrhoeae pilin affect Type IV pilus assembly, dynamics and associated functions

Type IV pili (Tfp) are multifunctional surface appendages expressed by many Gram negative species of medical, environmental and industrial importance. The N-terminally localized, so called alpha-helical spine is the most conserved structural feature of pilin subunits in these organelles. Prevailing models of pilus assembly and structure invariably implicate its importance to membrane trafficking, organelle structure and related functions. Nonetheless, relatively few studies have examined the effects of missense substitutions within this domain. Using Neisseria gonorrhoeae as a model system, we constructed mutants with single and multiple amino acid substitutions localized to this region of the pilin subunit PilE and characterized them with regard to pilin stability, organelle expression and associated phenotypes. The consequences of simultaneous expression of the mutant and wild-type PilE forms were also examined. The findings document for the first time in a defined genetic background the phenomenon of pilin intermolecular complementation in which assembly defective pilin can be rescued into purifiable Tfp by coexpression of wild-type PilE. The results further demonstrate that pilin subunit composition can impact on organelle dynamics mediated by the PilT retraction protein via a process that appears to monitor the efficacy of subunit-subunit interactions. In addition to confirming and extending the evidence for PilE multimerization as an essential component for competence for natural genetic transformation, this work paves the way for detailed studies of Tfp subunit-subunit interactions including self-recognition within the membrane and packing within the pilus polymer.

Structure of the Bundle-forming Pilus from Enteropathogenic Escherichia coli

Bundle-forming pili (BFP) are essential for the full virulence of enteropathogenic Escherichia coli (EPEC) because they are required for localized adherence to epithelial cells and auto-aggregation. We report the high resolution structure of bundlin, the monomer of BFP, solved by NMR. The structure reveals a new variation in the topology of type IVb pilins with significant differences in the composition and relative orientation of elements of secondary structure. In addition, the structural parameters of native BFP filaments were determined by electron microscopy after negative staining. The solution structure of bundlin was assembled according to these helical parameters to provide a plausible atomic resolution model for the BFP filament. We show that EPEC and Vibriocholerae type IVb pili display distinct differences in their monomer subunits consistent with data showing that bundlin and TcpA cannot complement each other, but assemble into filaments with similar helical organization.

PilX, a pilus-associated protein essential for bacterial aggregation, is a key to pilus-facilitated attachment of Neisseria meningitidis to human cells

The attachment of pathogenic Neisseria species to human cells, in which type IV pili (Tfp) play a key but incompletely defined role, depends on the ability of these bacteria to establish contacts with the target cells but also interbacterial interactions. In an effort to improve our understanding of the molecular mechanisms of N. meningitidis adherence to human cells, we screened a collection of defined mutants for those presenting reduced attachment to a human cell line. Besides underscoring the central role of Tfp in this process, this analysis led to the identification of mutants interrupted in a novel gene termed pilX, that displayed an adherence as impaired as that of a non-piliated mutant but quantitatively and qualitatively unaltered fibres. Moreover, the pilX gene, which encodes a pilin-like protein that copurifies with Tfp fibres, was also found to be essential for bacterial aggregation. We provide here several piece of evidence suggesting that PilX has intrinsic aggregative but no adhesive properties and that the reduced numbers of adherent bacteria seen with a pilX mutant result from the absence of interbacterial interactions. These data extend the current model for Tfp-facilitated adherence of N. meningitidis to human cells by suggesting that the pili lead to an increase in net initial adherence primarily by mediating a cooperation between the bacteria, which is supported by the finding that a major effect on initial adherence could be observed in a wild-type (WT) genetic background after a mechanical removal of the bacterial aggregates.

Presence of pili on the surface of Francisella tularensis

Francisella tularensis is a highly infectious gram-negative bacterium with potential for use as a bioweapon. Analysis of the F. tularensis live vaccine strain (LVS) ultrastructure by electron microscopy revealed the presence of long, thin fibers, similar in appearance to type 4 pili. The highly virulent F. tularensis Schu S4 strain was found to contain type 4 pilus genes, and we confirmed that these genes are present and expressed in the LVS.

Purification of post-translationally modified proteins from bacteria: homologous expression and purification of histidine-tagged pilin from Neisseria meningitidis

Until recently, glycosylation of proteins in prokaryotes was regarded as uncommon and thought to be limited to special cases such as S-layer proteins and some archeal outer membrane proteins. Now, there are an increasing number of reports of bacterial proteins that are glycosylated. Pilin of pathogenic Neisseria is one of the best characterised post-translationally modified bacterial proteins, with four different types of modifications reported, including a novel glycosylation. Pilin monomers assemble to form pilus fibres, which are long protein filaments that protrude from the surface of bacterial cells and are key virulence factors. To aid in the investigation of these modifications, pure pilin is required. A number of pilin purification methods have been published, but none are appropriate for the routine purification of pilin from many different isolates. This study describes a novel, rapid, and simple method of pilin purification from Neisseria meningitidis C311#3, which facilitates the production of consistent quantities of pure, native pilin. A 6x histidine tag was fused to the C-terminus of the pilin subunit structural gene, pilE, via homologous recombination placing the 6x histidine-tagged allele in the chromosome of N. meningitidis C311#3. Pilin was purified under non-denaturing conditions via a two-step process using immobilised metal affinity chromatography (IMAC), followed by dye affinity chromatography. Analysis of the purified pilin confirmed that it retained both of the post-translational modifications examined. This novel approach may prove to be a generally applicable method for purification and analysis of post-translationally modified proteins in bacteria.

Type IV pilin structure and assembly: X-ray and EM analyses of Vibrio cholerae toxin-coregulated pilus and Pseudomonas aeruginosa PAK pilin

Pilin assembly into type IV pili is required for virulence by bacterial pathogens that cause diseases such as cholera, pneumonia, gonorrhea, and meningitis. Crystal structures of soluble, N-terminally truncated pilin from Vibrio cholera toxin-coregulated pilus (TCP) and full-length PAK pilin from Pseudomonas aeruginosa reveal a novel TCP fold, yet a shared architecture for the type IV pilins. In each pilin subunit a conserved, extended, N-terminal alpha helix wrapped by beta strands anchors the structurally variable globular head. Inside the assembled pilus, characterized by cryo-electron microscopy and crystallography, the extended hydrophobic alpha helices make multisubunit contacts to provide mechanical strength and flexibility. Outside, distinct interactions of adaptable heads contribute surface variation for specificity of pilus function in antigenicity, motility, adhesion, and colony formation.

Microarray analyses of Xylella fastidiosa provide evidence of coordinated transcription control of laterally transferred elements

Genetically distinct strains of the plant bacterium Xylella fastidiosa (Xf) are responsible for a variety of plant diseases, accounting for severe economic damage throughout the world. Using as a reference the genome of Xf 9a5c strain, associated with citrus variegated chlorosis (CVC), we developed a microarray-based comparison involving 12 Xf isolates, providing a thorough assessment of the variation in genomic composition across the group. Our results demonstrate that Xf displays one of the largest flexible gene pools characterized to date, with several horizontally acquired elements, such as prophages, plasmids, and genomic islands (GIs), which contribute up to 18% of the final genome. Transcriptome analysis of bacteria grown under different conditions shows that most of these elements are transcriptionally active, and their expression can be influenced in a coordinated manner by environmental stimuli. Finally, evaluation of the genetic composition of these laterally transferred elements identified differences that may help to explain the adaptability of Xf strains to infect such a wide range of plant species.