The Dynamic Structures of the Type IV Pilus

CryoEM reveals the structure of an archaeal pilus involved in twitching motility

Amongst the major types of archaeal filaments, several have been shown to closely resemble bacterial homologues of the Type IV pili (T4P). Within Sulfolobales, member species encode for three types of T4P, namely the archaellum, the UV-inducible pilus system (Ups) and the archaeal adhesive pilus (Aap). Whereas the archaellum functions primarily in swimming motility, and the Ups in UV-induced cell aggregation and DNA-exchange, the Aap plays an important role in adhesion and twitching motility. Here, we present a cryoEM structure of the Aap of the archaeal model organism Sulfolobus acidocaldarius. We identify the component subunit as AapB and find that while its structure follows the canonical T4P blueprint, it adopts three distinct conformations within the pilus. The tri-conformer Aap structure that we describe challenges our current understanding of pilus structure and sheds new light on the principles of twitching motility.

Functional Analysis of the Major Pilin Proteins of Type IV Pili in Streptococcus sanguinis CGMH010

The pil gene cluster for Type IV pilus (Tfp) biosynthesis is commonly present and highly conserved in Streptococcus sanguinis. Nevertheless, Tfp-mediated twitching motility is less common among strains, and the factors determining twitching activity are not fully understood. Here, we analyzed the functions of three major pilin proteins (PilA1, PilA2, and PilA3) in the assembly and activity of Tfp in motile S. sanguinis CGMH010. Using various recombinant pilA deletion strains, we found that Tfp composed of different PilA proteins varied morphologically and functionally. Among the three PilA proteins, PilA1 was most critical in the assembly of twitching-active Tfp, and recombinant strains expressing motility generated more structured biofilms under constant shearing forces compared to the non-motile recombinant strains. Although PilA1 and PilA3 shared 94% identity, PilA3 could not compensate for the loss of PilA1, suggesting that the nature of PilA proteins plays an essential role in twitching activity. The single deletion of individual pilA genes had little effect on the invasion of host endothelia by S. sanguinis CGMH010. In contrast, the deletion of all three pilA genes or pilT, encoding the retraction ATPase, abolished Tfp-mediated invasion. Tfp- and PilT-dependent invasion were also detected in the non-motile S. sanguinis SK36, and thus, the retraction of Tfp, but not active twitching, was found to be essential for invasion.

The swimming defect caused by the absence of the transcriptional regulator LdtR in Sinorhizobium meliloti is restored by mutations in the motility genes motA and motS

The flagellar motor is a powerful macromolecular machine used to propel bacteria through various environments. We determined that flagellar motility of the alpha-proteobacterium Sinorhizobium meliloti is nearly abolished in the absence of the transcriptional regulator LdtR, known to influence peptidoglycan remodeling and stress response. LdtR does not regulate motility gene transcription. Remarkably, the motility defects of the ΔldtR mutant can be restored by secondary mutations in the motility gene motA or a previously uncharacterized gene in the flagellar regulon, which we named motS. MotS is not essential for S. meliloti motility and may serve an accessory role in flagellar motor function. Structural modeling predicts that MotS comprised an N-terminal transmembrane segment, a long-disordered region, and a conserved β-sandwich domain. The C terminus of MotS is localized in the periplasm. Genetics based substitution of MotA with MotAG12S also restored the ΔldtR motility defect. The MotAG12S variant protein features a local polarity shift at the periphery of the MotAB stator units. We propose that MotS may be required for optimal alignment of stators in wild-type flagellar motors but becomes detrimental in cells with altered peptidoglycan. Similarly, the polarity shift in stator units composed of MotB/MotAG12S might stabilize its interaction with altered peptidoglycan.

Helicobacter pylori cheV1 mutants recover semisolid agar migration due to loss of a previously uncharacterized Type IV filament membrane alignment complex homolog

The bacterial chemotaxis system is a well-understood signaling pathway that promotes bacterial success. Chemotaxis systems comprise chemoreceptors and the CheA kinase, linked by CheW or CheV scaffold proteins. Scaffold proteins provide connections between chemoreceptors and CheA and also between chemoreceptors to create macromolecular arrays. Chemotaxis is required for host colonization by many microbes, including the stomach pathogen Helicobacter pylori. This bacterium builds chemoreceptor-CheA contacts with two distinct scaffold proteins, CheW and CheV1. H. pylori cheW or cheV1 deletion mutants both lose chemoreceptor array formation, but show differing semisolid agar chemotaxis assay behaviors: ∆cheW mutants exhibit total migration failure, whereas ∆cheV1::cat mutants display a 50% reduction. On investigating these varied responses, we found that both mutants initially struggle with migration. However, over time, ∆cheV1::cat mutants develop a stable, enhanced migration capability, termed "migration-able" (Mig+). Whole-genome sequencing analysis of four distinct ∆cheV1::cat Mig+ strains identified single-nucleotide polymorphisms (SNPs) in hpg27_252 (hp0273) that were predicted to truncate the encoded protein. Computational analysis of the hpg27_252-encoded protein revealed it encoded a hypothetical protein that was a remote homolog of the PilO Type IV filament membrane alignment complex protein. Although H. pylori lacks Type IV filaments, our analysis showed it retains an operon of genes for homologs of PilO, PilN, and PilM. Deleting hpg27_252 in the ∆cheV1::cat or wild type strain resulted in enhanced migration in semisolid agar. Our study thus reveals that while cheV1 mutants initially have significant migration defects, they can recover the migration ability through genetic suppressors, highlighting a complex regulatory mechanism in bacterial migration.

IMPORTANCE

Chemotactic motility, present in over half of bacteria, depends on chemotaxis signaling systems comprising receptors, kinases, and scaffold proteins. In Helicobacter pylori, a stomach pathogen, chemotaxis is crucial for colonization, with CheV1 and CheW as key scaffold proteins. While both scaffolds are essential for building chemoreceptor complexes, their roles vary in other assays. Our research reexamines cheV1 mutants' behavior in semisolid agar, a standard chemotaxis test. Initially, cheV1 mutants exhibited defects similar to those of cheW mutants, but they evolved genetic suppressors that enhanced migration. These suppressors involve mutations in a previously uncharacterized gene, unknown in motility behavior. Our findings highlight the significant chemotaxis defects in cheV1 mutants and identify new elements influencing bacterial motility.

Understanding a Core Pilin of the Type IVa Pili of Acidithiobacillus thiooxidans, PilV

Pilins are protein subunits of pili. The pilins of type IV pili (T4P) in pathogenic bacteria are well characterized, but anything is known about the T4P proteins in acidophilic chemolithoautotrophic microorganisms such as the genus Acidithiobacillus. The interest in T4P of A. thiooxidans is because of their possible role in cell recruitment and bacterial aggregation on the surface of minerals during biooxidation of sulfide minerals. In this study we present a successful ad hoc methodology for the heterologous expression and purification of extracellular proteins such as the minor pilin PilV of the T4P of A. thiooxidans, a pilin exposed to extreme conditions of acidity and high oxidation-reduction potentials, and that interact with metal sulfides in an environment rich in dissolved minerals. Once obtained, the model structure of A. thiooxidans PilV revealed the core basic architecture of T4P pilins. Because of the acidophilic condition, we carried out in silico characterization of the protonation status of acidic and basic residues of PilV in order to calculate the ionization state at specific pH values and evaluated their pH stability. Further biophysical characterization was done using UV-visible and fluorescence spectroscopy and the results showed that PilV remains soluble and stable even after exposure to significant changes of pH. PilV has a unique amino acid composition that exhibits acid stability, with significant biotechnology implications such as biooxidation of sulfide minerals. The biophysics profiles of PilV open new paradigms about resilient proteins and stimulate the study of other pilins from extremophiles.

Membrane platform protein PulF of the Klebsiella type II secretion system forms a trimeric ion channel essential for endopilus assembly and protein secretion

IMPORTANCE

Type IV pili and type II secretion systems are members of the widespread type IV filament (T4F) superfamily of nanomachines that assemble dynamic and versatile surface fibers in archaea and bacteria. The assembly and retraction of T4 filaments with diverse surface properties and functions require the plasma membrane platform proteins of the GspF/PilC superfamily. Generally considered dimeric, platform proteins are thought to function as passive transmitters of the mechanical energy generated by the ATPase motor, to somehow promote insertion of pilin subunits into the nascent pilus fibers. Here, we generate and experimentally validate structural predictions that support the trimeric state of a platform protein PulF from a type II secretion system. The PulF trimers form selective proton or sodium channels which might energize pilus assembly using the membrane potential. The conservation of the channel sequence and structural features implies a common mechanism for all T4F assembly systems. We propose a model of the oligomeric PulF-PulE ATPase complex that provides an essential framework to investigate and understand the pilus assembly mechanism.

Tad and toxin-coregulated pilus structures reveal unexpected diversity in bacterial type IV pili

Type IV pili (T4P) are ubiquitous in both bacteria and archaea. They are polymers of the major pilin protein, which has an extended and protruding N-terminal helix, α1, and a globular C-terminal domain. Cryo-EM structures have revealed key differences between the bacterial and archaeal T4P in their C-terminal domain structure and in the packing and continuity of α1. This segment forms a continuous α-helix in archaeal T4P but is partially melted in all published bacterial T4P structures due to a conserved helix breaking proline at position 22. The tad (tight adhesion) T4P are found in both bacteria and archaea and are thought to have been acquired by bacteria through horizontal transfer from archaea. Tad pilins are unique among the T4 pilins, being only 40 to 60 residues in length and entirely lacking a C-terminal domain. They also lack the Pro22 found in all high-resolution bacterial T4P structures. We show using cryo-EM that the bacterial tad pilus from Caulobacter crescentus is composed of continuous helical subunits that, like the archaeal pilins, lack the melted portion seen in other bacterial T4P and share the packing arrangement of the archaeal T4P. We further show that a bacterial T4P, the Vibrio cholerae toxin coregulated pilus, which lacks Pro22 but is not in the tad family, has a continuous N-terminal α-helix, yet its α1 s are arranged similar to those in other bacterial T4P. Our results highlight the role of Pro22 in helix melting and support an evolutionary relationship between tad and archaeal T4P.

Disorder and amino acid composition in proteins: their potential role in the adaptation of extracellular pilins to the acidic media, where Acidithiobacillus thiooxidans grows

There are few biophysical studies or structural characterizations of the type IV pilin system of extremophile bacteria, such as the acidophilic Acidithiobacillus thiooxidans. We set out to analyze their pili-comprising proteins, pilins, because these extracellular proteins are in constant interaction with protons of the acidic medium in which At. thiooxidans grows. We used the web server Operon Mapper to analyze and identify the cluster codified by the minor pilin of At. thiooxidans. In addition, we carried an in-silico characterization of such pilins using the VL-XT algorithm of PONDR® server. Our results showed that structural disorder prevails more in pilins of At. thiooxidans than in non-acidophilic bacteria. Further computational characterization showed that the pilins of At. thiooxidans are significantly enriched in hydroxy (serine and threonine) and amide (glutamine and asparagine) residues, and significantly reduced in charged residues (aspartic acid, glutamic acid, arginine and lysine). Similar results were obtained when comparing pilins from other Acidithiobacillus and other acidophilic bacteria from another genus versus neutrophilic bacteria, suggesting that these properties are intrinsic to pilins from acidic environments, most likely by maintaining solubility and stability in harsh conditions. These results give guidelines for the application of extracellular proteins of acidophiles in protein engineering.

Discovery of Type IV filament membrane alignment complex homologs in H. pylori that promote soft-agar migration

The stomach pathogen Helicobacter pylori utilizes two scaffold proteins, CheW and CheV1, to build critical chemotaxis arrays. Chemotaxis helps bacteria establish and maintain infection. Mutants lacking either of these chemotaxis proteins have different soft agar phenotypes: deletion of cheW creates non-chemotactic strains, while deletion of cheV1 results in 50% loss of chemotaxis. In this work, we characterized the cheV1 deletion mutant phenotype in detail. cheV1 deletion mutants had poor soft-agar migration initially, but regained migration ability over time. This improved bacterial migration was stable, suggesting a genetic suppressor phenotype, termed Che+. Whole-genome sequencing analysis of four distinct cheV1 Che+ strains revealed single nucleotide polymorphisms (SNPs) in a common gene, HPG27_252 (HP0273). These SNPs were predicted to truncate the encoded protein. To confirm the role of HPG27_252 in the cheV1 phenotype, we created a targeted deletion of HPG27_252 and found that loss of HPG27_252 enhanced soft-agar migration. HPG27_252 and CheV1 appear to interact directly, based on bacterial two-hybrid analysis. HPG27_252 is predicted to encode a 179 amino acid, 21 kDa protein annotated as a hypothetical protein. Computational analysis revealed this protein to be a remote homolog of the PilO Type IV filament membrane alignment complex protein. Although H. pylori is not known to possess Type IV filaments, our analysis showed it retains an operon of genes for homologs of PilO, PilN, and PilM, but does not possess other Type IV pili genes. Our data suggest the PilO homolog plays a role in regulating H. pylori chemotaxis and motility, suggesting new ideas about evolutionary steps for controlling migration through semi-solid media.

Bacterial growth in multicellular aggregates leads to the emergence of complex life cycles

Facultative multicellular behaviors expand the metabolic capacity and physiological resilience of bacteria. Despite their ubiquity in nature, we lack an understanding of how these behaviors emerge from cellular-scale phenomena. Here, we show how the coupling between growth and resource gradient formation leads to the emergence of multicellular lifecycles in a marine bacterium. Under otherwise carbon-limited growth conditions, Vibrio splendidus 12B01 forms clonal multicellular groups to collectively harvest carbon from soluble polymers of the brown-algal polysaccharide alginate. As they grow, groups phenotypically differentiate into two spatially distinct sub-populations: a static "shell" surrounding a motile, carbon-storing "core." Differentiation of these two sub-populations coincides with the formation of a gradient in nitrogen-source availability within clusters. Additionally, we find that populations of cells containing a high proportion of carbon-storing individuals propagate and form new clusters more readily on alginate than do populations with few carbon-storing cells. Together, these results suggest that local metabolic activity and differential partitioning of resources leads to the emergence of reproductive cycles in a facultatively multicellular bacterium.

Bactericidal Effect of Pseudomonas oryziphila sp. nov., a Novel Pseudomonas Species Against Xanthomonas oryzae Reduces Disease Severity of Bacterial Leaf Streak of Rice

Pseudomonas is a diverse genus of Gammaproteobacteria with increasing novel species exhibiting versatile trains including antimicrobial and insecticidal activity, as well as plant growth-promoting, which make them well suited as biocontrol agents of some pathogens. Here we isolated strain 1257 that exhibited strong antagonistic activity against two pathovars of Xanthomonas oryzae, especially X. oryzae pv. oryzicola (Xoc) responsible for the bacterial leaf streak (BLS) in rice. The phylogenetic, genomic, physiological, and biochemical characteristics support that strain 1257 is a representative of a novel Pseudomonas species that is most closely related to the entomopathogenic bacterium Pseudomonas entomophila. We propose to name it Pseudomonas oryziphila sp. nov. Comparative genomics analyses showed that P. oryziphila 1257 possesses most of the central metabolic genes of two closely related strains P. entomophila L48 and Pseudomonas mosselii CFML 90-83, as well as a set of genes encoding the type IV pilus system, suggesting its versatile metabolism and motility properties. Some features, such as insecticidal toxins, phosphate solubilization, indole-3-acetic acid, and phenylacetic acid degradation, were disclosed. Genome-wide random mutagenesis revealed that the non-ribosomal peptide catalyzed by LgrD may be a major active compound of P. oryziphila 1257 against Xoc RS105, as well as the critical role of the carbamoyl phosphate and the pentose phosphate pathway that control the biosynthesis of this target compound. Our findings demonstrate that 1257 could effectively inhibit the growth and migration of Xoc in rice tissue to prevent the BLS disease. To our knowledge, this is the first report of a novel Pseudomonas species that displays a strong antibacterial activity against Xoc. The results suggest that the P. oryziphila strain could be a promising biological control agent for BLS.

The minor pilin PilV provides a conserved adhesion site throughout the antigenically variable meningococcal type IV pilus

Neisseria meningitidis utilizes type IV pili (T4P) to adhere to and colonize host endothelial cells, a process at the heart of meningococcal invasive diseases leading to meningitis and sepsis. T4P are polymers of an antigenically variable major pilin building block, PilE, plus several core minor pilins that initiate pilus assembly and are thought to be located at the pilus tip. Adhesion of N. meningitidis to human endothelial cells requires both PilE and a conserved noncore minor pilin PilV, but the localization of PilV and its precise role in this process remains to be clarified. Here, we show that both PilE and PilV promote adhesion to endothelial vessels in vivo. The substantial adhesion defect observed for pilV mutants suggests it is the main adhesin. Consistent with this observation, superresolution microscopy showed the abundant distribution of PilV throughout the pilus. We determined the crystal structure of PilV and modeled it within the pilus filament. The small size of PilV causes it to be recessed relative to adjacent PilE subunits, which are dominated by a prominent hypervariable loop. Nonetheless, we identified a conserved surface-exposed adhesive loop on PilV by alanine scanning mutagenesis. Critically, antibodies directed against PilV inhibit N. meningitidis colonization of human skin grafts. These findings explain how N. meningitidis T4P undergo antigenic variation to evade the humoral immune response while maintaining their adhesive function and establish the potential of this highly conserved minor pilin as a vaccine and therapeutic target for the prevention and treatment of N. meningitidis infections.

Bacterial motility: machinery and mechanisms

Bacteria have developed a large array of motility mechanisms to exploit available resources and environments. These mechanisms can be broadly classified into swimming in aqueous media and movement over solid surfaces. Swimming motility involves either the rotation of rigid helical filaments through the external medium or gyration of the cell body in response to the rotation of internal filaments. On surfaces, bacteria swarm collectively in a thin layer of fluid powered by the rotation of rigid helical filaments, they twitch by assembling and disassembling type IV pili, they glide by driving adhesins along tracks fixed to the cell surface and, finally, non-motile cells slide over surfaces in response to outward forces due to colony growth. Recent technological advances, especially in cryo-electron microscopy, have greatly improved our knowledge of the molecular machinery that powers the various forms of bacterial motility. In this Review, we describe the current understanding of the physical and molecular mechanisms that allow bacteria to move around.

The activation and limitation of the bacterial natural transformation system: The function in genome evolution and stability

Bacteria can take up exogenous naked DNA and integrate it into their genomes, which has been regarded as a main contributor to bacterial evolution. The competent status of bacteria is influenced by environmental cues and by the immune systems of bacteria. Here, we review recent advances in understanding the working mechanisms underlying activation of the natural transformation system and limitations thereof. Environmental stresses including the presence of antimicrobials can activate the natural transformation system. However, bacterial enzymes (nucleases), non-coding RNAs, specific DNA sequences, the restriction-modification (R-M) systems, CRISPR-Cas systems and prokaryotic Argonaute proteins (Agos) are have been found to be involved in the limitation of the natural transformation system. Together, this review represents an opportunity to gain insight into bacterial genome stability and evolution.

Computational prediction of secreted proteins in gram-negative bacteria

Gram-negative bacteria harness multiple protein secretion systems and secrete a large proportion of the proteome. Proteins can be exported to periplasmic space, integrated into membrane, transported into extracellular milieu, or translocated into cytoplasm of contacting cells. It is important for accurate, genome-wide annotation of the secreted proteins and their secretion pathways. In this review, we systematically classified the secreted proteins according to the types of secretion systems in Gram-negative bacteria, summarized the known features of these proteins, and reviewed the algorithms and tools for their prediction.

How bacteria block their own biofilms

Bacterial biofilms are surface-associated multicellular communities that are highly resistant to removal. Scheffler et al. discovered that Pseudomonas aeruginosa secretes a small molecule that inhibits other P. aeruginosa cells from adsorbing to surfaces by interfering with type IV pili dynamics. The inhibition of cell adsorption could present a method to prevent biofilm formation on sensitive surfaces in hospitals and industry.

Viral Hijack of Filamentous Surface Structures in Archaea and Bacteria

The bacterial and archaeal cell surface is decorated with filamentous surface structures that are used for different functions, such as motility, DNA exchange and biofilm formation. Viruses hijack these structures and use them to ride to the cell surface for successful entry. In this review, we describe currently known mechanisms for viral attachment, translocation, and entry via filamentous surface structures. We describe the different mechanisms used to exploit various surface structures bacterial and archaeal viruses. This overview highlights the importance of filamentous structures at the cell surface for entry of prokaryotic viruses.

Discovery of a New Neisseria gonorrhoeae Type IV Pilus Assembly Factor, TfpC

Most bacterial species express one or more extracellular organelles called pili/fimbriae that are required for many properties of each bacterial cell. The Neisseria gonorrhoeae type IV pilus is a major virulence and colonization factor for the sexually transmitted infection gonorrhea. We have discovered a new protein of Neisseria gonorrhoeae called TfpC that is required to maintain type IV pili on the bacterial cell surface. There are similar proteins found in other members of the Neisseria genus and many other bacterial species important for human health.

Neisseria gonorrhoeae relies on type IV pili (T4p) to promote colonization of their human host and to cause the sexually transmitted infection gonorrhea. This organelle cycles through a process of extension and retraction back into the bacterial cell. Through a genetic screen, we identified the NGO0783 locus of N. gonorrhoeae strain FA1090 as containing a gene encoding a protein required to stabilize the type IV pilus in its extended, nonretracted conformation. We have named the gene tfpC and the protein TfpC. Deletion of tfpC produces a nonpiliated colony morphology, and immuno-transmission electron microscopy confirms that the pili are lost in the ΔtfpC mutant, although there is some pilin detected near the bacterial cell surface. A copy of the tfpC gene expressed from a lac promoter restores pilus expression and related phenotypes. A ΔtfpC mutant shows reduced levels of pilin protein, but complementation with a tfpC gene restored pilin to normal levels. Bioinformatic searches show that there are orthologues in numerous bacterial species, but not all type IV pilin-expressing bacteria contain orthologous genes. Coevolution and nuclear magnetic resonance (NMR) analysis indicates that TfpC contains an N-terminal transmembrane helix, a substantial extended/unstructured region, and a highly charged C-terminal coiled-coil domain.

The Clustered Regularly Interspaced Short Palindromic Repeat System and Argonaute: An Emerging Bacterial Immunity System for Defense Against Natural Transformation?

Clustered regularly interspaced short palindromic repeat (CRISPR) systems and prokaryotic Argonaute proteins (Agos) have been shown to defend bacterial and archaeal cells against invading nucleic acids. Indeed, they are important elements for inhibiting horizontal gene transfer between bacterial and archaeal cells. The CRISPR system employs an RNA-guide complex to target invading DNA or RNA, while Agos target DNA using single stranded DNA or RNA as guides. Thus, the CRISPR and Agos systems defend against exogenous nucleic acids by different mechanisms. It is not fully understood how antagonization of these systems occurs during natural transformation, wherein exogenous DNA enters a host cell as single stranded DNA and is then integrated into the host genome. In this review, we discuss the functions and mechanisms of the CRISPR system and Agos in cellular defense against natural transformation.

Protein Dynamics in F-like Bacterial Conjugation

Efficient in silico development of novel antibiotics requires high-resolution, dynamic models of drug targets. As conjugation is considered the prominent contributor to the spread of antibiotic resistance genes, targeted drug design to disrupt vital components of conjugative systems has been proposed to lessen the proliferation of bacterial antibiotic resistance. Advancements in structural imaging techniques of large macromolecular complexes has accelerated the discovery of novel protein-protein interactions in bacterial type IV secretion systems (T4SS). The known structural information regarding the F-like T4SS components and complexes has been summarized in the following review, revealing a complex network of protein-protein interactions involving domains with varying degrees of disorder. Structural predictions were performed to provide insight on the dynamicity of proteins within the F plasmid conjugative system that lack structural information.

The Rich Tapestry of Bacterial Protein Translocation Systems

The translocation of proteins across membranes is a fundamental cellular function. Bacteria have evolved a striking array of pathways for delivering proteins into or across cytoplasmic membranes and, when present, outer membranes. Translocated proteins can form part of the membrane landscape, reside in the periplasmic space situated between the inner and outer membranes of Gram-negative bacteria, deposit on the cell surface, or be released to the extracellular milieu or injected directly into target cells. One protein translocation system, the general secretory pathway, is conserved in all domains of life. A second, the twin-arginine translocation pathway, is also phylogenetically distributed among most bacteria and plant chloroplasts. While all cell types have evolved additional systems dedicated to the translocation of protein cargoes, the number of such systems in bacteria is now known to exceed nine. These dedicated protein translocation systems, which include the types 1 through 9 secretion systems (T1SSs-T9SSs), the chaperone-usher pathway, and type IV pilus system, are the subject of this review. Most of these systems were originally identified and have been extensively characterized in Gram-negative or diderm (two-membrane) species. It is now known that several of these systems also have been adapted to function in Gram-positive or monoderm (single-membrane) species, and at least one pathway is found only in monoderms. This review briefly summarizes the distinctive mechanistic and structural features of each dedicated pathway, as well as the shared properties, that together account for the broad biological diversity of protein translocation in bacteria.

Bridging Bacteria and the Gut: Functional Aspects of Type IV Pili

Cell-surface-located proteinaceous appendages, such as flagella and fimbriae or pili, are ubiquitous in bacterial communities. Here, we focus on conserved type IV pili (T4P) produced by bacteria in the intestinal tract, one of the most densely populated human ecosystems. Computational analysis revealed that approximately 30% of known intestinal bacteria are predicted to produce T4P. To rationalize how T4P allow intestinal bacteria to interact with their environment, other microbiota members, and host cells, we review their established role in gut commensals and pathogens with respect to adherence, motility, and biofilm formation, as well as protein secretion and DNA uptake. This work indicates that T4P are widely spread among the known members of the intestinal microbiota and that their contribution to human health might be underestimated.

Virulence determinants of the emerging pathogen Kingella kingae

Kingella kingae is a gram-negative coccobacillus that is a fastidious commensal organism in the oropharynx and is being recognized increasingly as a common cause of osteoarticular infections and other invasive diseases in young children. The pathogenesis of K. kingae disease begins with bacterial adherence to respiratory epithelium, followed by translocation across the epithelial barrier, survival in the bloodstream, and dissemination to distant sites, including bones, joints, and the endocardium, among others. Characterization of the determinants of K. kingae pathogenicity has revealed a novel model of adherence that involves the interplay of type IV pili, a non-pilus adhesin, and a polysaccharide capsule and a novel model of resistance to serum killing and neutrophil killing that involves complementary functions of a polysaccharide capsule and an exopolysaccharide. These models likely apply to other bacterial pathogens as well.

Multiple conformations facilitate PilT function in the type IV pilus

Type IV pilus-like systems are protein complexes that polymerize pilin fibres. They are critical for virulence in many bacterial pathogens. Pilin polymerization and depolymerization are powered by motor ATPases of the PilT/VirB11-like family. This family is thought to operate with C2 symmetry; however, most of these ATPases crystallize with either C3 or C6 symmetric conformations. The relevance of these conformations is unclear. Here, we determine the X-ray structures of PilT in four unique conformations and use these structures to classify the conformation of available PilT/VirB11-like family member structures. Single particle electron cryomicroscopy (cryoEM) structures of PilT reveal condition-dependent preferences for C2, C3, and C6 conformations. The physiologic importance of these conformations is validated by coevolution analysis and functional studies of point mutants, identifying a rare gain-of-function mutation that favours the C2 conformation. With these data, we propose a comprehensive model of PilT function with broad implications for PilT/VirB11-like family members.

Monoderm bacteria: the new frontier for type IV pilus biology

In the diverse world of bacterial pili, type IV pili (Tfp) are unique for two reasons: their multifunctionality and ubiquity. This latter feature offers an extraordinary possibility, that is, to perform comparative studies in evolutionarily distant species in order to improve our fragmentary understanding of Tfp biology. Regrettably, such potential has remained largely untapped, because, for 20 years, Tfp have only been characterised in diderm bacteria. However, recent studies of Tfp in monoderms have started closing the gap, revealing many interesting commonalities and a few significant differences, extending the frontiers of knowledge of Tfp biology. Here, I review the current state of the art of the Tfp field in monoderm bacteria and discuss resulting implications for our general understanding of the assembly and function of these widespread filamentous nanomachines.

The type IV pilus protein PilU functions as a PilT-dependent retraction ATPase

Type IV pili are dynamic cell surface appendages found throughout the bacteria. The ability of these structures to undergo repetitive cycles of extension and retraction underpins their crucial roles in adhesion, motility and natural competence for transformation. In the best-studied systems a dedicated retraction ATPase PilT powers pilus retraction. Curiously, a second presumed retraction ATPase PilU is often encoded immediately downstream of pilT. However, despite the presence of two potential retraction ATPases, pilT deletions lead to a total loss of pilus function, raising the question of why PilU fails to take over. Here, using the DNA-uptake pilus and mannose-sensitive haemagglutinin (MSHA) pilus of Vibrio cholerae as model systems, we show that inactivated PilT variants, defective for either ATP-binding or hydrolysis, have unexpected intermediate phenotypes that are PilU-dependent. In addition to demonstrating that PilU can function as a bona fide retraction ATPase, we go on to make the surprising discovery that PilU functions exclusively in a PilT-dependent manner and identify a naturally occurring pandemic V. cholerae PilT variant that renders PilU essential for pilus function. Finally, we show that Pseudomonas aeruginosa PilU also functions as a PilT-dependent retraction ATPase, providing evidence that the functional coupling between PilT and PilU could be a widespread mechanism for optimal pilus retraction.

The Vibrio cholerae minor pilin TcpB mediates uptake of the cholera toxin phage CTXφ

Virulent strains of the bacterial pathogen Vibrio cholerae cause the diarrheal disease cholera by releasing cholera toxin into the small intestine. V. cholerae acquired its cholera toxin genes by lysogenic infection with the filamentous bacteriophage CTXφ. CTXφ uses its minor coat protein pIII, located in multiple copies at the phage tip, to bind to the V. cholerae toxin-coregulated pilus (TCP). However, the molecular details of this interaction and the mechanism of phage internalization are not well-understood. The TCP filament is a polymer of major pilins, TcpA, and one or more minor pilin, TcpB. TCP are retractile, with both retraction and assembly initiated by TcpB. Consistent with these roles in pilus dynamics, we hypothesized that TcpB controls both binding and internalization of CTXφ. To test this hypothesis, we determined the crystal structure of the C-terminal half of TcpB and characterized its interactions with CTXφ pIII. We show that TcpB is a homotrimer in its crystallographic form as well as in solution and is present in multiple copies at the pilus tip, which likely facilitates polyvalent binding to pIII proteins at the phage tip. We further show that recombinant forms of TcpB and pIII interact in vitro, and both TcpB and anti-TcpB antibodies block CTXφ infection of V. cholerae Finally, we show that CTXφ uptake requires TcpB-mediated retraction. Our data support a model whereby CTXφ and TCP bind in a tip-to-tip orientation, allowing the phage to be drawn into the V. cholerae periplasm as an extension of the pilus filament.

Architecture, Function, and Substrates of the Type II Secretion System

The type II secretion system (T2SS) delivers toxins and a range of hydrolytic enzymes, including proteases, lipases, and carbohydrate-active enzymes, to the cell surface or extracellular space of Gram-negative bacteria. Its contribution to survival of both extracellular and intracellular pathogens as well as environmental species of proteobacteria is evident. This dynamic, multicomponent machinery spans the entire cell envelope and consists of a cytoplasmic ATPase, several inner membrane proteins, a periplasmic pseudopilus, and a secretin pore embedded in the outer membrane. Despite the trans-envelope configuration of the T2S nanomachine, proteins to be secreted engage with the system first once they enter the periplasmic compartment via the Sec or TAT export system. Thus, the T2SS is specifically dedicated to their outer membrane translocation. The many sequence and structural similarities between the T2SS and type IV pili suggest a common origin and argue for a pilus-mediated mechanism of secretion. This minireview describes the structures, functions, and interactions of the individual T2SS components and the general architecture of the assembled T2SS machinery and briefly summarizes the transport and function of a growing list of T2SS exoproteins. Recent advances in cryo-electron microscopy, which have led to an increased understanding of the structure-function relationship of the secretin channel and the pseudopilus, are emphasized.