Prime time for minor subunits of the type II secretion and type IV pilus systems

A review on pilus assembly mechanisms in Gram-positive and Gram-negative bacteria

The surface of Gram-positive and Gram-negative bacteria contains long hair-like proteinaceous protrusion known as pili or fimbriae. Historically, pilin proteins were considered to play a major role in the transfer of genetic material during bacterial conjugation. Recent findings however elucidate their importance in virulence, biofilm formation, phage transduction, and motility. Therefore, it is crucial to gain mechanistic insights on the subcellular assembly of pili and the localization patterns of their subunit proteins (major and minor pilins) that aid the macromolecular pilus assembly at the bacterial surface. In this article, we review the current knowledge of pilus assembly mechanisms in a wide range of Gram-positive and Gram-negative bacteria, including subcellular localization patterns of a few pilin subunit proteins and their role in virulence and pathogenesis.

Discovering the Molecular Determinants of Phaeobacter inhibens Susceptibility to Phaeobacter Phage MD18

Bacteriophages have immense potential as antibiotic therapies and in genetic engineering. Understanding the mechanisms that bacteriophages implement to infect their hosts will allow researchers to manipulate these systems and adapt them to specific bacterial targets. In this study, we isolated a bacteriophage capable of infecting the marine alphaproteobacterium Phaeobacter inhibens and determined its mechanism of infection. Phaeobacter virus MD18, a novel species of bacteriophage isolated in Woods Hole, MA, exhibits potent lytic ability against P. inhibens and appears to be of the Siphoviridae morphotype. The genomic sequence of MD18 displayed significant similarity to another siphophage, the recently discovered Roseobacter phage DSS3P8, but genomic and phylogenetic analyses, assessing host range and a search of available metagenomes are all consistent with the conclusion that Phaeobacter phage MD18 is a novel lytic phage. We incubated MD18 with a library of barcoded P. inhibens transposon insertion mutants and identified 22 genes that appear to be required for phage predation of this host. Network analysis of these genes using genomic position, Gene Ontology (GO) term enrichment, and protein associations revealed that these genes are enriched for roles in assembly of a type IV pilus (T4P) and regulators of cellular morphology. Our results suggest that T4P serve as receptors for a novel marine virus that targets P. inhibens. IMPORTANCE Bacteriophages are useful nonantibiotic therapeutics for bacterial infections as well as threats to industries utilizing bacterial agents. This study identified Phaeobacter virus MD18, a phage antagonist of Phaeobacter inhibens, a bacterium with promising use as a probiotic for aquatic farming industries. Genomic analysis suggested that Phaeobacter phage MD18 has evolved to enhance its replication in P. inhibens by adopting favorable tRNA genes as well as through genomic sequence adaptation to resemble host codon usage. Lastly, a high-throughput analysis of P. inhibens transposon insertion mutants identified genes that modulate host susceptibility to phage MD18 and implicated the type IV pilus as the likely receptor recognized for adsorption. This study marks the first characterization of the relationship between P. inhibens and an environmentally sampled phage, which informs our understanding of natural threats to the bacterium and may promote the development of novel phage technologies for genetic manipulation of this host.

Regulation of Protein Secretion Systems Mediated by Cyclic Diguanylate in Plant-Interacting Bacteria

The ubiquitous second messenger cyclic diguanylate (c-di-GMP) is involved in the regulation of different processes in bacteria. In phytopathogens, intracellular fluctuations in the concentration of this molecule contribute to the lifestyle switching from a motile and virulent stage to a sessile and biofilm-forming phase. Among the virulence mechanisms used by bacterial pathogens, different specific type secretion systems (TSSs) and the effector proteins that they translocate are included. Some of these TSS are conceived to suppress host immune responses during bacterial colonization. The modulation of the expression of secretion systems components and/or effector proteins can be influenced by c-di-GMP levels at transcriptional, translational, or post-translational levels and can take place directly by binding to specific or global regulators, or via transducer proteins. Different genera of plant-interacting bacteria have been analyzed to shed some light in the implications of c-di-GMP in the regulation of host plant colonization through protein secretion systems. Expression of (1) adhesins secreted by Type 1 secretion systems to bind the host plant in Pectobacterium (formerly Erwinia) and some beneficial Pseudomonas strains; (2) catalytic exoproteins delivered by Type 2 secretion systems to break plant cell wall in Dickeya; (3) effectors secreted by Type 3 secretion systems to suppress plant immunity in Xanthomonas; or (4) the activity of Type 6 secretion systems to export an ATPase in Pseudomonas, are finely tuned by c-di-GMP levels. In this minireview, we summarize the knowledge available about the implications of c-di-GMP in the regulation of protein secretion in different plant-interacting bacteria. Topic: Secretion systems and effector proteins of phytopathogenic and beneficial bacteria regulated by NSM.

Gram-Positive Type IV Pili and Competence

Type IV pili (T4P) are remarkable bacterial surface appendages that carry out a range of functions. Various types of T4P have been identified in bacteria and archaea, making them almost universal structures in prokaryotes. T4P are best characterized in Gram-negative bacteria, in which pilus biogenesis and T4P-mediated functions have been studied for decades. Recent advances in microbial whole-genome sequencing have provided ample evidence for the existence of T4P also in many Gram-positive species. However, comparatively little is known, and T4P in Gram-positive bacteria are just beginning to be dissected. So far, they have mainly been studied in Clostridium and Streptococcus spp. and are involved in diverse cellular processes such as adhesion, motility, and horizontal gene transfer. Here we summarize the current understanding of T4P in Gram-positive species and their functions, with particular focus on the type IV competence pilus produced by the human pathogen Streptococcus pneumoniae and its role in natural transformation.

Pili of Mycobacterium tuberculosis: current knowledge and future prospects

Many pathogenic bacteria express filamentous appendages, termed pili, on their surface. These organelles function in several important bacterial processes, including mediating bacterial interaction with, and colonization of the host, signalling events, locomotion, DNA uptake, electric conductance, and biofilm formation. In the last decade, it has been established that the tuberculosis-causing bacterium, Mycobacterium tuberculosis, produces two pili types: curli and type IV pili. In this paper, we review studies on M. tuberculosis pili, highlighting their structure and biological significance to M. tuberculosis pathogenesis, and discuss their potential as targets for therapeutic intervention and diagnostic test development.

Archaeal type IV pili and their involvement in biofilm formation

Type IV pili are ancient proteinaceous structures present on the cell surface of species in nearly all bacterial and archaeal phyla. These filaments, which are required for a diverse array of important cellular processes, are assembled employing a conserved set of core components. While type IV pilins, the structural subunits of pili, share little sequence homology, their signal peptides are structurally conserved allowing for in silico prediction. Recently, in vivo studies in model archaea representing the euryarchaeal and crenarchaeal kingdoms confirmed that several of these pilins are incorporated into type IV adhesion pili. In addition to facilitating surface adhesion, these in vivo studies also showed that several predicted pilins are required for additional functions that are critical to biofilm formation. Examples include the subunits of Sulfolobus acidocaldarius Ups pili, which are induced by exposure to UV light and promote cell aggregation and conjugation, and a subset of the Haloferax volcanii adhesion pilins, which play a critical role in microcolony formation while other pilins inhibit this process. The recent discovery of novel pilin functions such as the ability of haloarchaeal adhesion pilins to regulate swimming motility may point to novel regulatory pathways conserved across prokaryotic domains. In this review, we will discuss recent advances in our understanding of the functional roles played by archaeal type IV adhesion pili and their subunits, with particular emphasis on their involvement in biofilm formation.

Pseudomonas aeruginosa minor pilins prime type IVa pilus assembly and promote surface display of the PilY1 adhesin

Type IV pili (T4P) contain hundreds of major subunits, but minor subunits are also required for assembly and function. Here we show that Pseudomonas aeruginosa minor pilins prime pilus assembly and traffic the pilus-associated adhesin and anti-retraction protein, PilY1, to the cell surface. PilV, PilW, and PilX require PilY1 for inclusion in surface pili and vice versa, suggestive of complex formation. PilE requires PilVWXY1 for inclusion, suggesting that it binds a novel interface created by two or more components. FimU is incorporated independently of the others and is proposed to couple the putative minor pilin-PilY1 complex to the major subunit. The production of small amounts of T4P by a mutant lacking the minor pilin operon was traced to expression of minor pseudopilins from the P. aeruginosa type II secretion (T2S) system, showing that under retraction-deficient conditions, T2S minor subunits can prime T4P assembly. Deletion of all minor subunits abrogated pilus assembly. In a strain lacking the minor pseudopilins, PilVWXY1 and either FimU or PilE comprised the minimal set of components required for pilus assembly. Supporting functional conservation of T2S and T4P minor components, our 1.4 Å crystal structure of FimU revealed striking architectural similarity to its T2S ortholog GspH, despite minimal sequence identity. We propose that PilVWXY1 form a priming complex for assembly and that PilE and FimU together stably couple the complex to the major subunit. Trafficking of the anti-retraction factor PilY1 to the cell surface allows for production of pili of sufficient length to support adherence and motility.

Type IV pili in Gram-positive bacteria

Type IV pili (T4P) are surface-exposed fibers that mediate many functions in bacteria, including locomotion, adherence to host cells, DNA uptake (competence), and protein secretion and that can act as nanowires carrying electric current. T4P are composed of a polymerized protein, pilin, and their assembly apparatuses share protein homologs with type II secretion systems in eubacteria and the flagella of archaea. T4P are found throughout Gram-negative bacterial families and have been studied most extensively in certain model Gram-negative species. Recently, it was discovered that T4P systems are also widespread among Gram-positive species, in particular the clostridia. Since Gram-positive and Gram-negative bacteria have many differences in cell wall architecture and other features, it is remarkable how similar the T4P core proteins are between these organisms, yet there are many key and interesting differences to be found as well. In this review, we compare the two T4P systems and identify and discuss the features they have in common and where they differ to provide a very broad-based view of T4P systems across all eubacterial species.

Identification of an additional minor pilin essential for piliation in the archaeon Methanococcus maripaludis

Methanococcus maripaludis is an archaeon with two studied surface appendages, archaella and type IV-like pili. Previously, the major structural pilin was identified as MMP1685 and three additional proteins were designated as minor pilins (EpdA, EpdB and EpdC). All of the proteins are likely processed by the pilin-specific prepilin peptidase EppA. Six other genes were identified earlier as likely encoding pilin proteins processed also by EppA. In this study, each of the six genes (mmp0528, mmp0600, mmp0601, mmp0709, mmp0903 and mmp1283) was deleted and the mutants examined by electron microscopy to determine their essentiality for pili formation. While mRNA transcripts of all genes were detected by RT-PCR, only the deletion of mmp1283 led to nonpiliated cells. This strain could be complemented back to a piliated state by supplying a wildtype copy of the mmp1283 gene in trans. This study adds to the complexity of the type IV pili system in M. maripaludis and raises questions about the functions of the remaining five pilin-like genes and whether M. maripaludis under other growth conditions may be able to assemble additional pili-like structures.

Complex formation and processing of the minor transformation pilins of Bacillus subtilis

Transformation in most bacteria is dependent on orthologues of Type 2 secretion and Type 4 pilus system proteins. In each system, pilin proteins (major and minor) are required to make the pilus structure and are essential to the process, although the precise roles of the minor pilins remain unclear. We have explored protein-protein interactions among the competence minor pilins of Bacillus subtilis through in vitro binding studies, immunopurification and mass spectrometry. We demonstrate that the minor pilins directly interact, and the minor pilin ComGG interacts with most of the known proteins required for transformation. We find that ComGG requires other ComG proteins for its stabilization and for processing by the pre-pilin peptidase. These observations, C-terminal mutations in ComGG that prevent processing and the inaccessibility of pre-ComGG to externally added protease suggest a model in which pre-ComGG must be associated with other minor pilins for processing to take place. We propose that ComGG does not become a transmembrane protein until after processing. These behaviours contrast with that of pre-ComGC, the major pilin, which is accessible to externally added protease and requires only the peptidase to be processed. The roles of the pilins and of the pilus in transformation are discussed.

Activity of Bdellovibrio hit locus proteins, Bd0108 and Bd0109, links Type IVa pilus extrusion/retraction status to prey-independent growth signalling

Bdellovibrio bacteriovorus are facultatively predatory bacteria that grow within gram-negative prey, using pili to invade their periplasmic niche. They also grow prey-independently on organic nutrients after undergoing a reversible switch. The nature of the growth switching mechanism has been elusive, but several independent reports suggested mutations in the hit (host-interaction) locus on the Bdellovibrio genome were associated with the transition to prey-independent growth. Pili are essential for prey entry by Bdellovibrio and sequence analysis of the hit locus predicted that it was part of a cluster of Type IVb pilus-associated genes, containing bd0108 and bd0109. In this study we have deleted the whole bd0108 gene, which is unique to Bdellovibrio, and compared its phenotype to strains containing spontaneous mutations in bd0108 and the common natural 42 bp deletion variant of bd0108. We find that deletion of the whole bd0108 gene greatly reduced the extrusion of pili, whereas the 42 bp deletion caused greater pilus extrusion than wild-type. The pili isolated from these strains were comprised of the Type IVa pilin protein; PilA. Attempts to similarly delete gene bd0109, which like bd0108 encodes a periplasmic/secreted protein, were not successful, suggesting that it is likely to be essential for Bdellovibrio viability in any growth mode. Bd0109 has a sugar binding YD- repeat motif and an N-terminus with a putative pilin-like fold and was found to interact directly with Bd0108. These results lead us to propose that the Bd0109/Bd0108 interaction regulates pilus production in Bdellovibrio (possibly by interaction with the pilus fibre at the cell wall), and that the presence (and possibly retraction state) of the pilus feeds back to alter the growth state of the Bdellovibrio cell. We further identify a novel small RNA encoded by the hit locus, the transcription of which is altered in different bd0108 mutation backgrounds.