Structural and topographical studies of the type IV bundle-forming pilus assembly complex of enteropathogenic Escherichia coli

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.

Type IV pilus secretins have extracellular C termini

Type IV pili (T4Ps) are surface appendages used by Gram-negative and Gram-positive pathogens for motility and attachment to epithelial surfaces. In Gram-negative bacteria, such as the important pediatric pathogen enteropathogenic Escherichia coli (EPEC), during extension and retraction, the pilus passes through an outer membrane (OM) pore formed by the multimeric secretin complex. The secretin is common to Gram-negative assemblies, including the related type 2 secretion (T2S) system and the type 3 secretion (T3S) system. The N termini of the secretin monomers are periplasmic and in some systems have been shown to mediate substrate specificity. In this study, we mapped the topology of BfpB, the T4P secretin from EPEC, using a combination of biochemical and biophysical techniques that allowed selective identification of periplasmic and extracellular residues. We applied rules based on solved atomic structures of outer membrane proteins (OMPs) to generate our topology model, combining the experimental results with secondary structure prediction algorithms and direct inspection of the primary sequence. Surprisingly, the C terminus of BfpB is extracellular, a result confirmed by flow cytometry for BfpB and a distantly related T4P secretin, PilQ, from Pseudomonas aeruginosa. Keeping with prior evidence, the C termini of two T2S secretins and one T3S secretin were not detected on the extracellular surface. On the basis of our data and structural constraints, we propose that BfpB forms a beta barrel with 16 transmembrane beta strands. We propose that the T4P secretins have a C-terminal segment that passes through the center of each monomer.

IMPORTANCE

Secretins are multimeric proteins that allow the passage of secreted toxins and surface structures through the outer membranes (OMs) of Gram-negative bacteria. To date, there have been no atomic structures of the C-terminal region of a secretin, although electron microscopy (EM) structures of the complex are available. This work provides a detailed topology prediction of the membrane-spanning domain of a type IV pilus (T4P) secretin. Our study used innovative techniques to provide new and comprehensive information on secretin topology, highlighting similarities and differences among secretin subfamilies. Additionally, the techniques used in this study may prove useful for the study of other OM proteins.

Function-related positioning of the type II secretion ATPase of Xanthomonas campestris pv. campestris

Gram-negative bacteria use the type II secretion (T2S) system to secrete exoproteins for attacking animal or plant cells or to obtain nutrients from the environment. The system is unique in helping folded proteins traverse the outer membrane. The secretion machine comprises multiple proteins spanning the cell envelope and a cytoplasmic ATPase. Activity of the ATPase, when copurified with the cytoplasmic domain of an interactive ATPase partner, is stimulated by an acidic phospholipid, suggesting the membrane-associated ATPase is actively engaged in secretion. How the stimulated ATPase activity is terminated when secretion is complete is unclear. We fused the T2S ATPase of Xanthomonas campestris pv. campestris, the causal agent of black rot in the crucifers, with fluorescent protein and found that the ATPase in secretion-proficient cells was mainly diffused in cytoplasm. Focal spots at the cell periphery were detectable only in a few cells. The discrete foci were augmented in abundance and intensity when the secretion channel was depleted and the exoprotein overproduced. The foci abundance was inversely related to secretion efficiency of the secretion channel. Restored function of the secretion channel paralleled reduced ATPase foci abundance. The ATPase foci colocalized with the secretion channel. The ATPase may be transiently associated with the T2S machine by alternating between a cytoplasmic and a machine-associated state in a secretion-dependent manner. This provides a logical means for terminating the ATPase activity when secretion is completed. Function-related dynamic assembly may be the essence of the T2S machine.

PilMNOPQ from the Pseudomonas aeruginosa type IV pilus system form a transenvelope protein interaction network that interacts with PilA

Pseudomonas aeruginosa type IV pili (T4P) are virulence factors that promote infection of cystic fibrosis and immunosuppressed patients. As the absence of T4P impairs colonization, they are attractive targets for the development of novel therapeutics. Genes in the pilMNOPQ operon are important for both T4P assembly and a form of bacterial movement, called twitching motility, that is required for pathogenicity. The type II membrane proteins, PilN and PilO, dimerize via their periplasmic domains and anchor this complex in the inner membrane. Our earlier work showed that PilNO binds PilP, a periplasmic lipoprotein (S. Tammam, L. M. Sampaleanu, J. Koo, P. Sundaram, M. Ayers, P. A. Chong, J. D. Forman-Kay, L. L. Burrows, and P. L. Howell, Mol. Microbiol. 82:1496-1514, 2011). Here, we show that PilP interacts with the N0 segment of the outer membrane secretin PilQ via its C-terminal domain, and that the N-terminal cytoplasmic tail of PilN binds to the actin-like protein PilM, thereby connecting all cellular compartments via the PilMNOPQ protein interaction network. We show that PilA, the major pilin subunit, interacts with PilNOPQ. The results allow us to propose a model whereby PilA makes extensive contacts with the transenvelope complex, possibly to increase local concentrations of PilA monomers for polymerization. The PilNOP complex could provide a stable anchor in the inner membrane, while the PilMNOPQ transenvelope complex facilitates transit of the pilus through the periplasm and clamps the pilus in the cell envelope. The PilMN interaction is proposed to be responsible for communicating signals from the cytoplasmic to periplasmic components of this complex macromolecular machine.

Structure of an essential type IV pilus biogenesis protein provides insights into pilus and type II secretion systems

Type IV pili (T4Ps) are long cell surface filaments, essential for microcolony formation, tissue adherence, motility, transformation, and virulence by human pathogens. The enteropathogenic Escherichia coli bundle-forming pilus is a prototypic T4P assembled and powered by BfpD, a conserved GspE secretion superfamily ATPase held by inner-membrane proteins BfpC and BfpE, a GspF-family membrane protein. Although the T4P assembly machinery shares similarity with type II secretion (T2S) systems, the structural biochemistry of the T4P machine has been obscure. Here, we report the crystal structure of the two-domain BfpC cytoplasmic region (N-BfpC), responsible for binding to ATPase BfpD and membrane protein BfpE. The N-BfpC structure reveals a prominent central cleft between two α/β-domains. Despite negligible sequence similarity, N-BfpC resembles PilM, a cytoplasmic T4P biogenesis protein. Yet surprisingly, N-BfpC has far greater structural similarity to T2S component EpsL, with which it also shares virtually no sequence identity. The C-terminus of the cytoplasmic domain, which leads to the transmembrane segment not present in the crystal structure, exits N-BfpC at a positively charged surface that most likely interacts with the inner membrane, positioning its central cleft for interactions with other Bfp components. Point mutations in surface-exposed N-BfpC residues predicted to be critical for interactions among BfpC, BfpE, and BfpD disrupt pilus biogenesis without precluding interactions with BfpE and BfpD and without affecting BfpD ATPase activity. These results illuminate the relationships between T4P biogenesis and T2S systems, imply that subtle changes in component residue interactions can have profound effects on function and pathogenesis, and suggest that T4P systems may be disrupted by inhibitors that do not preclude component assembly.

Outer membrane targeting, ultrastructure, and single molecule localization of the enteropathogenic Escherichia coli type IV pilus secretin BfpB

Type IV pili (T4P) are filamentous surface appendages required for tissue adherence, motility, aggregation, and transformation in a wide array of bacteria and archaea. The bundle-forming pilus (BFP) of enteropathogenic Escherichia coli (EPEC) is a prototypical T4P and confirmed virulence factor. T4P fibers are assembled by a complex biogenesis machine that extrudes pili through an outer membrane (OM) pore formed by the secretin protein. Secretins constitute a superfamily of proteins that assemble into multimers and support the transport of macromolecules by four evolutionarily ancient secretion systems: T4P, type II secretion, type III secretion, and phage assembly. Here, we determine that the lipoprotein transport pathway is not required for targeting the BfpB secretin protein of the EPEC T4P to the OM and describe the ultrastructure of the single particle averaged structures of the assembled complex by transmission electron microscopy. Furthermore, we use photoactivated localization microscopy to determine the distribution of single BfpB molecules fused to photoactivated mCherry. In contrast to findings in other T4P systems, we found that BFP components predominantly have an uneven distribution through the cell envelope and are only found at one or both poles in a minority of cells. In addition, we report that concurrent mutation of both the T4bP secretin and the retraction ATPase can result in viable cells and found that these cells display paradoxically low levels of cell envelope stress response activity. These results imply that secretins can direct their own targeting, have complex distributions and provide feedback information on the state of pilus biogenesis.

Sticky situation: localized adherence of enteropathogenic Escherichia coli to the small intestine epithelium

Enteropathogenic Escherichia coli (EPEC) primarily cause gastrointestinal illness in neonates. They accomplish this by a complex coordinated multistage strategy, whereby the organisms colonize the epithelial lining of the small intestine. This process can be divided into four stages: first, localized, nonintimate adherence; second, type III secretion-mediated injection of effector proteins, third effacement of microvilli and, finally, intimate adherence. In this article, we review the history and current state of knowledge, as well as present potential future directions for further investigating the fascinating processes by which EPEC and related organisms colonize the human intestine and cause disease.

PilM/N/O/P proteins form an inner membrane complex that affects the stability of the Pseudomonas aeruginosa type IV pilus secretin

The highly conserved pilM/N/O/P/Q gene cluster is among the core set of genes required for cell surface expression of type IV pili and associated twitching motility. With the exception of the outer membrane secretin, a multimer of PilQ subunits, the specific functions of the products encoded by this gene cluster are poorly characterized. Orthologous proteins in the related bacterial type II secretion system have been shown to interact to form an inner membrane complex required for protein secretion. In this study, we provide evidence that the PilM/N/O/P proteins form a functionally equivalent type IVa pilus complex. Using Pseudomonas aeruginosa as model organism, we found that all four proteins, including the nominally cytoplasmic PilM, colocalized to the inner membrane. Stability studies via Western blot analyses revealed that loss of one component has a negative impact on the levels of other members of the putative complex. Furthermore, complementation studies revealed that the stoichiometry of the components is important for the correct formation of a stable complex in vivo. We provide evidence that an intact inner membrane complex is required for optimal formation of the outer membrane complex of the type IVa pilus system in P. aeruginosa, as PilQ stability is negatively affected in its absence. Finally, we show that, in the absence of the pilin subunit, the levels of membrane-bound components of the inner membrane complex are negatively regulated by the PilR/S two-component system, suggesting a role for PilR/S in sensing the piliation status of the cell.

Regulation of the type IV pili molecular machine by dynamic localization of two motor proteins

Type IV pili (T4P) are surface structures that undergo extension/retraction oscillations to generate cell motility. In Myxococcus xanthus, T4P are unipolarly localized and undergo pole-to-pole oscillations synchronously with cellular reversals. We investigated the mechanisms underlying these oscillations. We show that several T4P proteins localize symmetrically in clusters at both cell poles between reversals, and these clusters remain stationary during reversals. Conversely, the PilB and PilT motor ATPases that energize extension and retraction, respectively, localize to opposite poles with PilB predominantly at the piliated and PilT predominantly at the non-piliated pole, and these proteins oscillate between the poles during reversals. Therefore, T4P pole-to-pole oscillations involve the disassembly of T4P machinery at one pole and reassembly of this machinery at the opposite pole. Fluorescence recovery after photobleaching experiments showed rapid turnover of YFP–PilT in the polar clusters between reversals. Moreover, PilT displays bursts of accumulation at the piliated pole between reversals. These observations suggest that the spatial separation of PilB and PilT in combination with the noisy PilT accumulation at the piliated pole allow the temporal separation of extension and retraction. This is the first demonstration that the function of a molecular machine depends on disassembly and reassembly of its individual parts.

High-force generation is a conserved property of type IV pilus systems

The type IV pilus (T4P) system of Neisseria gonorrhoeae is the strongest linear molecular motor reported to date, but it is unclear whether high-force generation is conserved between bacterial species. Using laser tweezers, we found that the average stalling force of single-pilus retraction in Myxococcus xanthus of 149 +/- 14 pN exceeds the force generated by N. gonorrhoeae. Retraction velocities including a bimodal distribution were similar between M. xanthus and N. gonorrhoeae, but force-dependent directional switching was not. Force generation by pilus retraction is energized by the ATPase PilT. Surprisingly, an M. xanthus mutant lacking PilT apparently still retracted T4P, although at a reduced frequency. The retraction velocity was comparable to the high-velocity mode in the wild type at low forces but decreased drastically when the force increased, with an average stalling force of 70 +/- 10 pN. Thus, M. xanthus harbors at least two different retraction motors. Our results demonstrate that the major physical properties are conserved between bacteria that are phylogenetically distant and pursue very different lifestyles.

Spatial clustering of the curlin secretion lipoprotein requires curli fiber assembly

Gram-negative bacteria assemble functional amyloid surface fibers called curli. CsgB nucleates the major curli subunit protein, CsgA, into a self-propagating amyloid fiber on the cell surface. The CsgG lipoprotein is sufficient for curlin transport across the outer membrane and is hypothesized to be the central molecule of the curli fiber secretion and assembly complex. We tested the hypothesis that the curli secretion protein, CsgG, was restricted to certain areas of the cell to promote the interaction of CsgA and CsgB during curli assembly. Here, electron microscopic analysis of curli-producing strains showed that relatively few cells in the population contacted curli fibers and that curli emanated from spatially discrete points on the cell surface. Microscopic analysis revealed that CsgG was surface exposed and spatially clustered around curli fibers. CsgG localization to the outer membrane and exposure of the surface domain were not dependent on any other csg-encoded protein, but the clustering of CsgG required the csg-encoded proteins CsgE, CsgF, CsgA, and CsgB. CsgG formed stable oligomers in all the csg mutant strains, but these oligomers were distinct from the CsgG complexes assembled in wild-type cells. Finally, we found that efficient fiber assembly was required for the spatial clustering of CsgG. These results suggest a new model where curli fiber formation is spatially coordinated with the CsgG assembly apparatus.

Type IV pili: e pluribus unum?

The widespread role of pili as colonization factors in pathogens has long been recognized in Gram-negative bacteria and more recently in Gram-positive bacteria, making the study of these hair-like filaments a perennial hot topic for research. No other pili are found in as many or as diverse bacteria as type IV pili. This is likely a consequence of their ancient origin and unique ability to promote multiple and strikingly different phenotypes such as attachment to surfaces, aggregation, uptake of DNA during transformation, motility, etc. Two decades of investigations in several model species have shed some light on the structure of these filaments and the molecular basis of some of the properties they confer. Moreover, recent discoveries have led to a better knowledge of the genetic basis and molecular mechanisms of type IV pili biogenesis. This brings us a few steps closer to understanding how these filaments are produced, but leaves us wondering whether (as in the famous motto that inspired the title) out of the many models studied will emerge one unifying mechanism.

Type IV pili: paradoxes in form and function

Type IV pili are filaments on the surfaces of many Gram-negative bacteria that mediate an extraordinary array of functions, including adhesion, motility, microcolony formation and secretion of proteases and colonization factors. Their prominent display on the surfaces of many bacterial pathogens, their vital role in virulence, and their ability to elicit an immune response make Type IV pilus structures particularly relevant for study as targets for component vaccines and therapies. Structural studies of the pili and components of the pilus assembly apparatus have proven extremely challenging, but new approaches and methods have produced important breakthroughs that are advancing our understanding of pilus functions and their complex assembly mechanism. These structures provide insights into the biology of Type IV pili as well as that of the related bacterial secretion and archaeal flagellar systems. This review will summarize the most recent structural advances on Type IV pili and their assembly components and highlight their significance.

Outer membrane components of the Tad (tight adherence) secreton of Aggregatibacter actinomycetemcomitans

Prokaryotic secretion relies on proteins that are widely conserved, including NTPases and secretins, and on proteins that are system specific. The Tad secretion system in Aggregatibacter actinomycetemcomitans is dedicated to the assembly and export of Flp pili, which are needed for tight adherence. Consistent with predictions that RcpA forms the multimeric outer membrane secretion channel (secretin) of the Flp pilus biogenesis apparatus, we observed the RcpA protein in multimers that were stable in the presence of detergent and found that rcpA and its closely related homologs form a novel and distinct subfamily within a well-supported gene phylogeny of the entire secretin gene superfamily. We also found that rcpA-like genes were always linked to Aggregatibacter rcpB- or Caulobacter cpaD-like genes. Using antisera, we determined the localization and gross abundances of conserved (RcpA and TadC) and unique (RcpB, RcpC, and TadD) Tad proteins. The three Rcp proteins (RcpA, RcpB, and RcpC) and TadD, a putative lipoprotein, localized to the bacterial outer membrane. RcpA, RcpC, and TadD were also found in the inner membrane, while TadC localized exclusively to the inner membrane. The RcpA secretin was necessary for wild-type abundances of RcpB and RcpC, and TadC was required for normal levels of all three Rcp proteins. TadC abundance defects were observed in rcpA and rcpC mutants. TadD production was essential for wild-type RcpA and RcpB abundances, and RcpA did not multimerize or localize to the outer membrane without the expression of TadD. These data indicate that membrane proteins TadC and TadD may influence the assembly, transport, and/or function of individual outer membrane Rcp proteins.

A systematic genetic analysis in Neisseria meningitidis defines the Pil proteins required for assembly, functionality, stabilization and export of type IV pili

Although type IV pili (Tfp) are among the commonest virulence factors in bacteria, their biogenesis by complex machineries of 12-15 proteins, and thereby their function remains poorly understood. Interestingly, some of these proteins were reported to merely antagonize the retraction of the fibres powered by PilT, because piliation could be restored in their absence by a mutation in the pilT gene. The recent identification of the 15 Pil proteins dedicated to Tfp biogenesis in Neisseria meningitidis offered us the unprecedented possibility to define their exact contribution in this process. We therefore systematically introduced a pilT mutation into the corresponding non-piliated mutants and characterized them for the rescue of Tfp and Tfp-mediated virulence phenotypes. We found that in addition to the pilin, the main constituent of Tfp, only six Pil proteins were required for the actual assembly of the fibres, because apparently normal fibres were restored in the remaining mutants. Restored fibres were surface-exposed, except in the pilQ/T mutant in which they were trapped in the periplasm, suggesting that the PilQ secretin was the sole Pil component necessary for their emergence on the surface. Importantly, although in most mutants the restored Tfp were not functional, the pilG/T and pilH/T mutants could form bacterial aggregates and adhere to human cells efficiently, suggesting that Tfp stabilization and functional maturation are two discrete steps. These findings have numerous implications for understanding Tfp biogenesis/function and provide a useful groundwork for the characterization of the precise function of each Pil protein in this process.

Archaeal flagella, bacterial flagella and type IV pili: a comparison of genes and posttranslational modifications

The archaeal flagellum is a unique motility organelle. While superficially similar to the bacterial flagellum, several similarities have been reported between the archaeal flagellum and the bacterial type IV pilus system. These include the multiflagellin nature of the flagellar filament, N-terminal sequence similarities between archaeal flagellins and bacterial type IV pilins, as well as the presence of homologous proteins in the two systems. Recent advances in archaeal flagella research add to the growing list of similarities. First, the preflagellin peptidase that is responsible for processing the N-terminal signal peptide in preflagellins has been identified. The preflagellin peptidase is a membrane-bound enzyme topologically similar to its counterpart in the type IV pilus system (prepilin peptidase); the two enzymes are demonstrated to utilize the same catalytic mechanism. Second, it has been suggested that the archaeal flagellum and the bacterial type IV pilus share a similar mode of assembly. While bacterial flagellins and type IV pilins can be modified with O-linked glycans, N-linked glycans have recently been reported on archaeal flagellins. This mode of glycosylation, as well as the observation that the archaeal flagellum lacks a central channel, are both consistent with the proposed assembly model. On the other hand, the failure to identify other genes involved in archaeal flagellation by homology searches likely implies a novel aspect of the archaeal flagellar system. These interesting features remain to be deciphered through continued research. Such knowledge would be invaluable to motility and protein export studies in the Archaea.

Interaction and localization studies of enteropathogenic Escherichia coli type IV bundle-forming pilus outer membrane components

Typical enteropathogenicEscherichia colistrains express an established virulence factor belonging to the type IV pili family, called the bundle-forming pilus (BFP). BFP are present on the cell surface as bundled filamentous appendages, and are assembled and retracted by proteins encoded by thebfpoperon. These proteins assemble to form a molecular machine. The BFP machine may be conceptually divided into three components: the cytoplasmic membrane (CM) subassembly, which is composed of CM proteins and cytoplasmic nucleotide-binding proteins; the outer membrane (OM) subassembly and the pilus itself. The authors have previously characterized the CM subassembly and the pilus. In this study, a more complete characterization of the OM subassembly was carried out using a combination of biochemical, biophysical and genetic approaches. It is reported that targeting of BfpG to the OM was influenced by the secretin BfpB. BfpG and BfpU interacted with the amino terminus of BfpB. BfpU had a complex cellular distribution pattern and, along with BfpB and BfpG, was part of the OM subassembly.

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.

Identification of a TcpC-TcpQ outer membrane complex involved in the biogenesis of the toxin-coregulated pilus of Vibrio cholerae

The toxin-coregulated pilus (TCP) of Vibrio cholerae and the soluble TcpF protein that is secreted via the TCP biogenesis apparatus are essential for intestinal colonization. The TCP biogenesis apparatus is composed of at least nine proteins but is largely uncharacterized. TcpC is an outer membrane lipoprotein required for TCP biogenesis that is a member of the secretin protein superfamily. In the present study, analysis of TcpC in a series of strains deficient in each of the TCP biogenesis proteins revealed that TcpC was absent specifically in a tcpQ mutant. TcpQ is a predicted periplasmic protein required for TCP biogenesis. Fractionation studies revealed that the protein is not localized to the periplasm but is associated predominantly with the outer membrane fraction. An analysis of the amount of TcpQ present in the series of tcp mutants demonstrated the inverse of the TcpC result (absence of TcpQ in a tcpC deletion strain). Complementation of the tcpQ deletion restored TcpC levels and TCP formation, and similarly, complementation of tcpC restored TcpQ. Metal affinity pull-down experiments performed using His-tagged TcpC or TcpQ demonstrated a direct interaction between TcpC and TcpQ. In the presence of TcpQ, TcpC was found to form a high-molecular-weight complex that is stable in 2% sodium dodecyl sulfate and at temperatures below 65 degrees C, a characteristic of secretin complexes. Fractionation studies in which TcpC was overexpressed in the absence of TcpQ showed that TcpQ is also required for proper localization of TcpC to the outer membrane.

Interaction with type IV pili induces structural changes in the bacterial outer membrane secretin PilQ

Type IV pili are cell surface organelles found on many Gram-negative bacteria. They mediate a variety of functions, including adhesion, twitching motility, and competence for DNA uptake. The type IV pilus is a helical polymer of pilin protein subunits and is capable of rapid polymerization or depolymerization, generating large motor forces in the process. Here we show that a specific interaction between the outer membrane secretin PilQ and the type IV pilus fiber can be detected by far-Western analysis and sucrose density gradient centrifugation. Transmission electron microscopy of preparations of purified pili, to which the purified PilQ oligomer had been added, showed that PilQ was uniquely located at one end of the pilus fiber, effectively forming a "mallet-type" structure. Determination of the three-dimensional structure of the PilQ-type IV pilus complex at 26-angstroms resolution showed that the cavity within the protein complex was filled. Comparison with a previously determined structure of PilQ at 12-angstroms resolution indicated that binding of the pilus fiber induced a dissociation of the "cap" feature and lateral movement of the "arms" of the PilQ oligomer. The results demonstrate that the PilQ structure exhibits a dynamic response to the binding of its transported substrate and suggest that the secretin could play an active role in type IV pilus assembly as well as secretion.

Disparate subcellular localization patterns of Pseudomonas aeruginosa Type IV pilus ATPases involved in twitching motility

The opportunistic pathogen Pseudomonas aeruginosa expresses polar type IV pili (TFP), which are responsible for adhesion to various materials and twitching motility on surfaces. Twitching occurs by alternate extension and retraction of TFP, which arise from assembly and disassembly of pilin subunits at the base of the pilus. The ATPase PilB promotes pilin assembly, while the ATPase PilT or PilU or both promote pilin dissociation. Fluorescent fusions to two of the three ATPases (PilT and PilU) were functional, as shown by complementation of the corresponding mutants. PilB and PilT fusions localized to both poles, while PilU fusions localized only to the piliated pole. To identify the portion of the ATPases required for localization, sequential C-terminal deletions of PilT and PilU were generated. The conserved His and Walker B boxes were dispensable for polar localization but were required for twitching motility, showing that localization and function could be uncoupled. Truncated fusions that retained polar localization maintained their distinctive distribution patterns. To dissect the cellular factors involved in establishing polarity, fusion protein localization was monitored with a panel of TFP mutants. The localization of yellow fluorescent protein (YFP)-PilT and YFP-PilU was independent of the subunit PilA, other TFP ATPases, and TFP-associated proteins previously shown to be associated with the membrane or exhibiting polar localization. In contrast, YFP-PilB exhibited diffuse cytoplasmic localization in a pilC mutant, suggesting that PilC is required for polar localization of PilB. Finally, localization studies performed with fluorescent ATPase chimeras of PilT and PilU demonstrated that information responsible for the characteristic localization patterns of the ATPases likely resides in their N termini.

The Cpx envelope stress response affects expression of the type IV bundle-forming pili of enteropathogenic Escherichia coli

The Cpx envelope stress response mediates adaptation to potentially lethal envelope stresses in Escherichia coli. The two-component regulatory system consisting of the sensor kinase CpxA and the response regulator CpxR senses and mediates adaptation to envelope insults believed to result in protein misfolding in this compartment. Recently, a role was demonstrated for the Cpx response in the biogenesis of P pili, attachment organelles expressed by uropathogenic E. coli. CpxA senses misfolded P pilus assembly intermediates and initiates increased expression of both assembly and regulatory factors required for P pilus elaboration. In this report, we demonstrate that the Cpx response is also involved in the expression of the type IV bundle-forming pili of enteropathogenic E. coli (EPEC). Bundle-forming pili were not elaborated from an exogenous promoter in E. coli laboratory strain MC4100 unless the Cpx pathway was constitutively activated. Further, an EPEC cpxR mutant synthesized diminished levels of bundle-forming pili and was significantly affected in adherence to epithelial cells. Since type IV bundle-forming pili are very different from chaperone-usher-type P pili in both form and biogenesis, our results suggest that the Cpx envelope stress response plays a general role in the expression of envelope-localized organelles with diverse structures and assembly pathways.

Structure of the Neisseria meningitidis Outer Membrane PilQ Secretin Complex at 12 Å Resolution

The bacterial pathogen Neisseria meningitidis expresses long, thin, retractile fibers (called type IV pili) from its cell surface and uses these adhesive structures to mediate primary attachment to epithelial cells during host colonization and invasion. PilQ is an outer membrane protein complex that is essential for the translocation of these pili across the outer membrane. Here, we present the structure of the PilQ complex determined by cryoelectron microscopy to 12 A resolution. The dominant feature of the structure is a large central cavity, formed by four arm features that spiral upwards from a squared ring base and meet to form a prominent cap region. The cavity, running through the center of the complex, is continuous and is effectively sealed at both the top and bottom. Analysis of the complex using self-orientation and by examination of two-dimensional crystals indicates a strong C4 rotational symmetry, with a much weaker C12 rotational symmetry, consistent with PilQ possessing true C4 symmetry with C12 quasi-symmetry. We therefore suggest that the complex is a homododecamer, formed by association of 12 PilQ polypeptide chains into a tetramer of trimers. The structure of the PilQ complex, with its large and well defined central chamber, suggests that it may not function solely as a passive portal in the outer membrane, but could be actively involved in mediating pilus assembly or modification.

The inner membrane subassembly of the enteropathogenic Escherichia coli bundle-forming pilus machine

Type IV pili (Tfps) are filamentous surface appendages expressed by Gram-negative microorganisms and play numerous roles in bacterial cell biology. Tfp biogenesis machineries are highly conserved and resemble protein secretion and DNA uptake systems. Although components of Tfp biogenesis systems have been identified, it is not known how they interact to form these machineries. Using the bundle-forming pilus (BFP) of enteropathogenic Escherichia coli as a model Tfp system, we provide evidence of a cytoplasmic membrane subassembly of the Tfp assembly machine composed of putative cytoplasmic nucleotide-binding and cytoplasmic membrane proteins. A combination of genetic, biochemical and biophysical approaches revealed interactions among putative cytoplasmic nucleotide-binding proteins BfpD and BfpF and cytoplasmic membrane proteins BfpC and BfpE of the BFP biogenesis machine. The polytopic membrane protein BfpE appears to be a central component of this subassembly as it interacts with BfpC, BfpD and BfpF. We report that BFP biogenesis probably requires interactions among BfpC, BfpD and BfpE, whereas BFP retraction requires interaction of the PilT-like putative ATPase BfpF with a conserved domain of BfpE. BfpE is the first protein that is not a member of the PilT family to be implicated in Tfp retraction. Furthermore, we found that the putative ATPases BfpD and BfpF play antagonistic roles in BFP biogenesis and retraction, respectively, by interacting with distinct domains of the BFP biogenesis machine.