PorZ, an Essential Component of the Type IX Secretion System of Porphyromonas gingivalis, Delivers Anionic Lipopolysaccharide to the PorU Sortase for Transpeptidase Processing of T9SS Cargo Proteins

Structural and functional insights into the C-terminal signal domain of the Bacteroidetes type-IX secretion system

Gram-negative bacteria from the Bacteroidota phylum possess a type-IX secretion system (T9SS) for protein secretion, which requires cargoes to have a C-terminal domain (CTD). Structurally analysed CTDs are from Porphyromonas gingivalis proteins RgpB, HBP35, PorU and PorZ, which share a compact immunoglobulin-like antiparallel 3+4 β-sandwich (β1-β7). This architecture is essential as a P. gingivalis strain with a single-point mutant of RgpB disrupting the interaction of the CTD with its preceding domain prevented secretion of the protein. Next, we identified the C-terminus ('motif C-t.') and the loop connecting strands β3 and β4 ('motif Lβ3β4') as conserved. We generated two strains with insertion and replacement mutants of PorU, as well as three strains with ablation and point mutants of RgpB, which revealed both motifs to be relevant for T9SS function. Furthermore, we determined the crystal structure of the CTD of mirolase, a cargo of the Tannerella forsythia T9SS, which shares the same general topology as in Porphyromonas CTDs. However, motif Lβ3β4 was not conserved. Consistently, P. gingivalis could not properly secrete a chimaeric protein with the CTD of peptidylarginine deiminase replaced with this foreign CTD. Thus, the incompatibility of the CTDs between these species prevents potential interference between their T9SSs.

Comparative analysis of Porphyromonas gingivalis A7436 and ATCC 33277 strains reveals differences in the expression of heme acquisition systems

Porphyromonas gingivalis strains exhibit different phenotypes in vitro, different virulence potential in animal models, and different associations with human diseases, with strains classified as virulent/more virulent (e.g., A7436 and W83) or as less virulent/avirulent (e.g., ATCC 33277). In this study, we comparatively analyzed the A7436 and ATCC 33277 strains to better understand their variability. Global gene expression analysis in response to heme and iron limitation revealed more pronounced differences in the A7436 than in the ATCC 33277 strain; however, in both strains, the largest changes were observed in genes encoding hypothetical proteins, genes whose products participate in energy metabolism, and in genes encoding proteins engaged in transport and binding proteins. Our results confirmed that variability between P. gingivalis strains is due to differences in the arrangement of their genomes. Analysis of gene expression of heme acquisition systems demonstrated that not only the availability of iron and heme in the external environment but also the ability to store iron intracellularly can influence the P. gingivalis phenotype. Therefore, we assume that differences in virulence potential may also be due to differences in the production of systems involved in iron and heme acquisition, mainly the Hmu system. In addition, our study showed that hemoglobin, in a concentration-dependent manner, differentially influences the virulence potential of P. gingivalis strains. We conclude that iron and heme homeostasis may add to the variability observed between P. gingivalis strains.

IMPORTANCE

Periodontitis belongs to a group of multifactorial diseases, characterized by inflammation and destruction of tooth-supporting tissues. P. gingivalis is one of the most important microbial factors involved in the initiation and progression of periodontitis. To survive in the host, the bacterium must acquire heme as a source of iron and protoporphyrin IX. P. gingivalis strains respond differently to changing iron and heme concentrations, which may be due to differences in the expression of systems involved in iron and heme acquisition. The ability to accumulate iron intracellularly, being different in more and less virulent P. gingivalis strains, may influence their phenotypes, production of virulence factors (including proteins engaged in heme acquisition), and virulence potential of this bacterium.

Bacterial secretion system functions: evidence of interactions and downstream implications

Unprecedented insights into the biology and functions of bacteria have been and continue to be gained through studying bacterial secretion systems in isolation. This method, however, results in our understanding of the systems being primarily based on the idea that they operate independently, ignoring the subtleties of downstream interconnections. Gram-negative bacteria are naturally able to adapt to and navigate their frequently varied and dynamic surroundings, mostly because of the covert connections between secretion systems. Therefore, to comprehend some of the linked downstream repercussions for organisms that follow this discourse, it is vital to have mechanistic insights into how the intersecretion system functions in bacterial rivalry, virulence, and survival, among other things. To that purpose, this paper discusses a few key instances of molecular antagonistic and interdependent relationships between bacterial secretion systems and their produced functional products.

Structural Model of a Porphyromonas gingivalis type IX Secretion System Shuttle Complex

Porphyromonas gingivalis is a gram-negative oral anaerobic pathogen and is one of the key causative agents of periodontitis. P. gingivalis utilises a range of virulence factors, including the cysteine protease RgpB, to drive pathogenesis and these are exported and attached to the cell surface via the type IX secretion system (T9SS). All cargo proteins possess a conserved C-terminal signal domain (CTD) which is recognised by the T9SS, and the outer membrane β-barrel protein PorV (PG0027/LptO) can interact with cargo proteins as they are exported to the bacterial surface. Using a combination of solution nuclear magnetic resonance (NMR) spectroscopy, biochemical analyses, machine-learning-based modelling and molecular dynamics (MD) simulations, we present a structural model of a PorV:RgpB-CTD complex from P. gingivalis. This is the first structural insight into CTD recognition by the T9SS and shows how the conserved motifs in the CTD are the primary sites that mediate binding. In PorV, interactions with extracellular surface loops are important for binding the CTD, and together these appear to cradle and lock RgpB-CTD in place. This work provides insight into cargo recognition by PorV but may also have important implications for understanding other aspects of type-IX dependent secretion.

Differential affinity chromatography reveals a link between Porphyromonas gingivalis-induced changes in vascular smooth muscle cell differentiation and the type 9 secretion system

Porphyromonas gingivalis is implicated in adverse pregnancy outcome. We previously demonstrated that intrauterine infection with various strains of P. gingivalis impairs the physiologic remodeling of the uterine spiral arteries (IRSA) during pregnancy, which underlies the major obstetrical syndromes. Women diagnosed with IRSA also have a greater risk for premature cardiovascular disease in later life. The dysregulated plasticity of vascular smooth muscle cells (VSMCs) is present in both IRSA and premature cardiovascular events. We hypothesized that VSMCs could serve as a bait to identify P. gingivalis proteins associated with dysregulated VSMC plasticity as seen in IRSA. We first confirmed that dams with P. gingivalis A7UF-induced IRSA also show perturbed aortic smooth muscle cell (AoSMC) plasticity along with the P. gingivalis colonization of the tissue. The in vitro infection of AoSMCs with IRSA-inducing strain A7UF also perturbed AoSMC plasticity that did not occur with infection by non-IRSA-inducing strain W83. Far-Western blotting with strain W83 and strain A7UF showed a differential binding pattern to the rat aorta and primary rat AoSMCs. The affinity chromatography/pull-down assay combined with mass spectrometry was used to identify P. gingivalis/AoSMC protein interactions specific to IRSA. Membrane proteins with a high binding affinity to AoSMCs were identified in the A7UF pull-down but not in the W83 pull-down, most of which were the outer membrane components of the Type 9 secretion system (T9SS) and T9SS cargo proteins. Additional T9SS cargo proteins were detected in greater abundance in the A7UF pull-down eluate compared to W83. None of the proteins enriched in the W83 eluate were T9SS components nor T9SS cargo proteins despite their presence in the prey preparations used in the pull-down assay. In summary, differential affinity chromatography established that the components of IRSA-inducing P. gingivalis T9SS as well as its cargo directly interact with AoSMCs, which may play a role in the infection-induced dysregulation of VSMC plasticity. The possibility that the T9SS is involved in the microbial manipulation of host cell events important for cell differentiation and tissue remodeling would constitute a new virulence function for this system.

Glycosyltransferase-Related Protein GtrA Is Essential for Localization of Type IX Secretion System Cargo Protein Cellulase Cel9A and Affects Cellulose Degradation in Cytophaga hutchinsonii

The Gram-negative bacterium Cytophaga hutchinsonii digests cellulose through a novel cellulose degradation mechanism. It possesses the lately characterized type IX secretion system (T9SS). We recently discovered that N-glycosylation of the C-terminal domain (CTD) of a hypothetical T9SS substrate protein in the periplasmic space of C. hutchinsonii affects protein secretion and localization. In this study, green fluorescent protein (GFP)-CTD_Cel9A recombinant protein was found with increased molecular weight in the periplasm of C. hutchinsonii. Site-directed mutagenesis studies on the CTD of cellulase Cel9A demonstrated that asparagine residue 900 in the D-X-N-X-S motif is important for the processing of the recombinant protein. We found that the glycosyltransferase-related protein GtrA (CHU_0012) located in the cytoplasm of C. hutchinsonii is essential for outer membrane localization of the recombinant protein. The deletion of gtrA decreased the abundance of the outer membrane proteins and affected cellulose degradation by C. hutchinsonii. This study provided a link between the glycosylation system and cellulose degradation in C. hutchinsonii. IMPORTANCE N-Glycosylation systems are generally limited to some pathogenic bacteria in prokaryotes. The disruption of the N-glycosylation pathway is related to adherence, invasion, colonization, and other phenotypic characteristics. We recently found that the cellulolytic bacterium Cytophaga hutchinsonii also has an N-glycosylation system. The cellulose degradation mechanism of C. hutchinsonii is novel and mysterious; cellulases and other proteins on the cell surface are involved in utilizing cellulose. In this study, we identified an asparagine residue in the C-terminal domain of cellulase Cel9A that is necessary for the processing of the T9SS cargo protein. Moreover, the glycosyltransferase-related protein GtrA is essential for the localization of the GFP-CTD_Cel9A recombinant protein. Deletion of gtrA affected cellulose degradation and the abundance of outer membrane proteins. This study enriched the understanding of the N-glycosylation system in C. hutchinsonii and provided a link between N-glycosylation and cellulose degradation, which also expanded the role of the N-glycosylation system in bacteria.

Prevotella: An insight into its characteristics and associated virulence factors

Prevotella species, a gram-negative obligate anaerobe, is commonly associated with human infections such as dental caries and periodontitis, as well as other conditions such as chronic osteomyelitis, bite-related infections, rheumatoid arthritis and intestinal diseases like ulcerative colitis. This generally harmless commensal possesses virulence factors such as adhesins, hemolysins, secretion systems exopolysaccharide, LPS, proteases, quorum sensing molecules and antibiotic resistance to evolve into a well-adapted pathogen capable of causing successful infection and proliferation in the host tissue. This review describes several of these virulence factors and their advantage to Prevotella spp. in causing inflammatory diseases like periodontitis. In addition, using genome analysis of Prevotella reference strains, we examined other putative virulence determinants which can provide insights as biomarkers and be the targets for effective interventions in Prevotella related diseases like periodontitis.

Design Principles of the Rotary Type 9 Secretion System

The Fo ATP synthase, the bacterial flagellar motor, and the bacterial type 9 secretion system (T9SS) are the three known proton motive force driven biological rotary motors. In this review, we summarize the current information on the nuts and bolts of T9SS. Torque generation by T9SS, its role in gliding motility of bacteria, and the mechanism via which a T9SS-driven swarm shapes the microbiota are discussed. The knowledge gaps in our current understanding of the T9SS machinery are outlined.

Dynamic proton-dependent motors power type IX secretion and gliding motility in Flavobacterium

Motile bacteria usually rely on external apparatus like flagella for swimming or pili for twitching. By contrast, gliding bacteria do not rely on obvious surface appendages to move on solid surfaces. Flavobacterium johnsoniae and other bacteria in the Bacteroidetes phylum use adhesins whose movement on the cell surface supports motility. In F. johnsoniae, secretion and helicoidal motion of the main adhesin SprB are intimately linked and depend on the type IX secretion system (T9SS). Both processes necessitate the proton motive force (PMF), which is thought to fuel a molecular motor that comprises the GldL and GldM cytoplasmic membrane proteins. Here, we show that F. johnsoniae gliding motility is powered by the pH gradient component of the PMF. We further delineate the interaction network between the GldLM transmembrane helices (TMHs) and show that conserved glutamate residues in GldL TMH2 are essential for gliding motility, although having distinct roles in SprB secretion and motion. We then demonstrate that the PMF and GldL trigger conformational changes in the GldM periplasmic domain. We finally show that multiple GldLM complexes are distributed in the membrane, suggesting that a network of motors may be present to move SprB along a helical path on the cell surface. Altogether, our results provide evidence that GldL and GldM assemble dynamic membrane channels that use the proton gradient to power both T9SS-dependent secretion of SprB and its motion at the cell surface.

Motile bacteria usually rely on external apparatus like flagella or pili, but gliding bacteria do not rely on obvious surface appendages for their movement. This study shows that bacteria in the phylum Bacteroidetes use proton-dependent motors to power protein secretion and gliding motility.

Protein Interactome Analysis of the Type IX Secretion System Identifies PorW as the Missing Link between the PorK/N Ring Complex and the Sov Translocon

The type IX secretion system (T9SS) transports cargo proteins through the outer membrane of Bacteroidetes and attaches them to the cell surface for functions including pathogenesis, gliding motility, and degradation of carbon sources. The T9SS comprises at least 20 different proteins and includes several modules: the trans-envelope core module comprising the PorL/M motor and the PorK/N ring, the outer membrane Sov translocon, and the cell attachment complex. However, the spatial organization of these modules is unknown. We have characterized the protein interactome of the Sov translocon in Porphyromonas gingivalis and identified Sov-PorV-PorA as well as Sov-PorW-PorN-PorK to be novel networks. PorW also interacted with PGN_1783 (PorD), which was required for maximum secretion efficiency. The identification of PorW as the missing link completes a continuous interaction network from the PorL/M motor to the Sov translocon, providing a pathway for cargo delivery and energy transduction from the inner membrane to the secretion pore.

IMPORTANCE The T9SS is a newly identified protein secretion system of the Fibrobacteres-Chlorobi-Bacteroidetes superphylum used by pathogens associated with diseases of humans, fish, and poultry for the secretion and cell surface attachment of virulence factors. The T9SS comprises three known modules: (i) the trans-envelope core module comprising the PorL/M motor and the PorK/N ring, (ii) the outer membrane Sov translocon, and (iii) the cell surface attachment complex. The spatial organization and interaction of these modules have been a mystery. Here, we describe the protein interactome of the Sov translocon in the human pathogen Porphyromonas gingivalis and have identified PorW as the missing link which bridges PorN with Sov and so completes a continuous interaction network from the PorL/M motor to the Sov translocon, providing, for the first time, a pathway for cargo delivery and energy transduction from the inner membrane to the secretion pore.

Structural and functional analyses of the Porphyromonas gingivalis type IX secretion system PorN protein

Porphyromonas gingivalis, the major human pathogen bacterium associated with periodontal diseases, secretes virulence factors through the Bacteroidetes-specific type IX secretion system (T9SS). Effector proteins of the T9SS are recognized by the complex via their conserved C-terminal domains (CTDs). Among the 18 proteins essential for T9SS function in P. gingivalis, PorN is a periplasmic protein that forms large ring-shaped structures in association with the PorK outer membrane lipoprotein. PorN also mediates contacts with the PorM subunit of the PorLM energetic module, and with the effector’s CTD. However, no information is available on the PorN structure and on the implication of PorN domains for T9SS assembly and effector recognition. Here we present the crystal structure of PorN at 2.0-Å resolution, which represents a novel fold with no significant similarity to any known structure. In agreement with in silico analyses, we also found that the N- and C-terminal regions of PorN are intrinsically disordered. Our functional studies showed that the N-terminal disordered region is involved in PorN dimerization while the C-terminal disordered region is involved in the interaction with PorK. Finally, we determined that the folded PorN central domain is involved in the interaction with PorM, as well as with the effector’s CTD. Altogether, these results lay the foundations for a more comprehensive model of T9SS architecture and effector transport.

The Type IX Secretion System and Its Role in Bacterial Function and Pathogenesis

Porphyromonas, Tannerella, and Prevotella species found in severe periodontitis use the Type IX Secretion System (T9SS) to load their outer membrane surface with an array of virulence factors. These virulence factors are then released on outer membrane vesicles (OMVs), which penetrate the host to dysregulate the immune response to establish a positive feedback loop of chronic, inflammatory destruction of the tooth's supporting tissues. In this review, we present the latest information on the molecular architecture of the T9SS and provide mechanistic insight into its role in secretion and attachment of cargo proteins to produce a virulence coat on cells and OMVs. The recent molecular structures of the T9SS motor comprising PorL and PorM as well as the secretion pore Sov, together with advances in the overall interactome, have provided insight into the possible mechanisms of secretion. We propose the presence of PorL/M motors arranged in a circle at the inner membrane with bent periplasmic rotors interacting with the PorN protein. At the outer membrane, we envisage a slide carousel model where the PorN protein is driven around a circular track composed of PorK. Cargo proteins are transported by PorN to PorW and the Sov translocon just as slides are rotated to the projection window. Secreted proteins are proposed to then be shuttled along highways consisting of the PorV shuttle protein to an array of attachment complexes distributed around the cell. The cell surface attachment of cargo is a hallmark of the T9SS, and in Porphyromonas gingivalis and Tannerella forsythia, this attachment is achieved via covalent bonding to a linking sugar synthesized by the Wbp/Vim pathway. The cell-surface attached cargo are enriched on OMVs, which are then released from the cell.

Intermolecular latency regulates the essential C-terminal signal peptidase and sortase of the Porphyromonas gingivalis type-IX secretion system

Porphyromonas gingivalis is a keystone pathogen of the human dysbiotic oral microbiome that causes severe periodontitis. It employs a type-IX secretion system (T9SS) to shuttle proteins across the outer membrane (OM) for virulence. Uniquely, T9SS cargoes carry a C-terminal domain (CTD) as a secretion signal, which is cleaved and replaced with anionic lipopolysaccharide by transpeptidation for extracellular anchorage to the OM. Both reactions are carried out by PorU, the only known dual-function, C-terminal signal peptidase and sortase. PorU is itself secreted by the T9SS, but its CTD is not removed; instead, intact PorU combines with PorQ, PorV, and PorZ in the OM-inserted "attachment complex." Herein, we revealed that PorU transits between active monomers and latent dimers and solved the crystal structure of the ∼260-kDa dimer. PorU has an elongated shape ∼130 Å in length and consists of seven domains. The first three form an intertwined N-terminal cluster likely engaged in substrate binding. They are followed by a gingipain-type catalytic domain (CD), two immunoglobulin-like domains (IGL), and the CTD. In the first IGL, a long "latency β-hairpin" protrudes ∼30 Å from the surface to form an intermolecular β-barrel with β-strands from the symmetric CD, which is in a latent conformation. Homology modeling of the competent CD followed by in vivo validation through a cohort of mutant strains revealed that PorU is transported and functions as a monomer through a C⁶⁹⁰/H⁶⁵⁷ catalytic dyad. Thus, dimerization is an intermolecular mechanism for PorU regulation to prevent untimely activity until joining the attachment complex.