The differential expression of PilY1 proteins by the HsfBA phosphorelay allows twitching motility in the absence of exopolysaccharides

Tad pili with adaptable tips mediate contact-dependent killing during bacterial predation

The predatory bacterium, Myxococcus xanthus, kills its prey by contact, using a putative Tight Adherence pilus, known as the Kil system, along with a protein complex resembling the basal body a type-III secretion system, named the "needleless" T3SS*. In this work, we provide direct evidence that Myxococcus polymerizes a Kil pilus at the prey contact site, which is constituted by the major pilin KilP. We also genetically demonstrate that the predation function of this pilus is linked to four different minor pilin complexes, which work in specific combinations to detect and kill phylogenetically diverse bacterial species. Structural models of the Kil pilus suggest that these minor pilin complexes form interchangeable "Tips", exposing variable domains at the extremity of the pilus to interact with prey cells. Remarkably, the activity of these Tips also depends on the T3SS*, revealing a tight functional connection between the Kil system and the T3SS*. While these Tips are mostly restricted to predatory bacteria, genomic and structural analyses suggest that in other bacteria, including pathogens, Tad pili are also customized and functionalized by similar minor pilin complexes exposing variable domains.

Functional investigation of Zur in metal ion homeostasis, motility and multiple stresses resistance in cyanobacteria Synechocystis sp. PCC 6803

Zur (zinc uptake regulator), a member of the Fur (ferric uptake regulator) family of transcriptional regulators, plays multifaceted roles by regulating the gene expressions, such as modulating zinc ion uptake by regulating the znuABC gene cluster and influencing bacterial motility by modulating genes associated with flagella or pili. The photosynthetic autotroph Synechocystis sp. PCC 6803 is frequently used as an indicator organism for water pollution and a cell factory for high-value biochemical production in synthetic biology. During its growth, this organism often encounters various abiotic stresses, including oxidative, salt, and antibiotic stress. In this study, we conducted transcriptomic analysis on both Δzur mutant and wild-type (WT) strains to identify potential Zur-regulated genes in Synechocystis sp. PCC 6803. These genes primarily participate in multiple pathways such as inorganic ion transport, carbohydrate transport, energy production and conversion, and cell motility. Zur not only controls zinc ion homeostasis within the cell but also influences the iron balance by directly regulating the expression of the fur gene. In terms of motility, Zur regulates the expression of bacterial pili gene cluster and other motility-related genes, thereby affecting the twitching motility of Synechocystis sp. PCC 6803. Furthermore, Zur plays a crucial role in promoting biofilm formation and enhancing resistance to salt, oxidative, and antibiotic stresses by modulating relative gene expression. In conclusion, as a global transcriptional regulator, Zur plays pivotal roles in metal ion homeostasis, motility, and resistance to multiple stresses in Synechocystis sp. PCC 6803. This study illustrates the Zur regulons in Synechocystis sp. PCC 6803, and underscores the importance of Zur in enhancing the environmental adaptability of cyanobacteria.

Gut commensal Lachnospiraceae bacteria contribute to anti-colitis effects of Lactiplantibacillus plantarum exopolysaccharides

The probiotic Lactiplantibacillus plantarum (L. plantarum) could ameliorate colitis. Alterations in the composition of gut microbiota (GM) have been proved in cases of colitis. The exopolysaccharides from L. plantarum HMPM2111 (LPE) could be effective in colitis through altering the composition of the GM. These effects were linked to inhibiting intestinal inflammation, regulating the TXNIP/NLRP3 inflammasome axis, and attenuating colonic barrier dysfunction. The combination of fecal microbiota transplantation (FMT) and antibiotic inducement showed that gut bacteria susceptible to vancomycin were inversely associated with colitis features and were necessary for the anti-inflammatory effects of LPE. The elevated abundances of gut commensal Lachnospiraceae bacteria were associated with the restoration of colitis treated by LPE. Metabolomics analysis showed that colitis mice treated with LPE had higher levels of propionate and tryptophan metabolites generated from gut bacteria. The administration of these metabolites protected colitis and resulted in a reduction in inflammatory responses. The outcomes of our investigation emerge the significance of the GM in controlling the protective implications of LPE against colitis. Lachnospiraceae bacteria, together with downstream metabolites, contribute substantially to protection. This work elucidates the essential function of the GM-metabolite axis in producing comprehensive protection versus colitis and identifies prospective treatment targets.

Combinatorial control of type IVa pili formation by the four polarized regulators MglA, SgmX, FrzS, and SopA

Type IVa pili (T4aP) are widespread and enable bacteria to translocate across surfaces. T4aP engage in cycles of extension, surface adhesion, and retraction, thereby pulling cells forward. Accordingly, the number and localization of T4aP are critical to efficient translocation. Here, we address how T4aP formation is regulated in Myxococcus xanthus, which translocates with a well-defined leading and lagging cell pole using T4aP at the leading pole. This localization is orchestrated by the small GTPase MglA and its downstream effector SgmX that both localize at the leading pole and recruit the PilB extension ATPase to the T4aP machinery at this pole. Here, we identify the previously uncharacterized protein SopA and show that it interacts directly with SgmX, localizes at the leading pole, stimulates polar localization of PilB, and is important for T4aP formation. We corroborate that MglA also recruits FrzS to the leading pole, and FrzS stimulates SgmX recruitment. In addition, FrzS and SgmX separately recruit SopA. Precise quantification of T4aP-formation and T4aP-dependent motility in various mutants supports a model whereby the main pathway for stimulating T4aP formation is the MglA/SgmX pathway. FrzS stimulates this pathway by recruiting SgmX and SopA. SopA stimulates the MglA/SgmX pathway by stimulating the function of SgmX, likely by promoting the SgmX-dependent recruitment of PilB to the T4aP machinery. The architecture of the MglA/SgmX/FrzS/SopA protein interaction network for orchestrating T4aP formation allows for combinatorial regulation of T4aP levels at the leading cell pole resulting in discrete levels of T4aP-dependent motility.

IMPORTANCE

Type IVa pili (T4aP) are widespread bacterial cell surface structures with important functions in translocation across surfaces, surface adhesion, biofilm formation, and virulence. T4aP-dependent translocation crucially depends on the number of pili. To address how the number of T4aP is regulated, we focused on M. xanthus, which assembles T4aP at the leading cell pole and is a model organism for T4aP biology. Our results support a model whereby the four proteins MglA, SgmX, FrzS, and the newly identified SopA protein establish a highly intricate interaction network for orchestrating T4aP formation at the leading cell pole. This network allows for combinatorial regulation of the number of T4aP resulting in discrete levels of T4aP-dependent motility.

Deciphering the enigmatic PilY1 of Acidithiobacillus thiooxidans: An in silico analysis

Thirty years since the first report on the PilY1 protein in bacteria, only the C-terminal domain has been crystallized; there is no study in which the N-terminal domain, let alone the complete protein, has been crystallized. In our laboratory, we are interested in characterizing the Type IV Pili (T4P) of Acidithiobacillus thiooxidans. We performed an in silico characterization of PilY1 and other pilins of the T4P of this acidophilic bacterium. In silico characterization is crucial for understanding how proteins adapt and function under extreme conditions. By analyzing the primary and secondary structures of proteins through computational methods, researchers can gain valuable insights into protein stability, key structural features, and unique amino acid compositions that contribute to resilience in harsh environments. Here, it is presented a description of the particularities of At. thiooxidans PilY1 through predictor software and homology data. Our results suggest that PilY1 from At. thiooxidans may have the same role as has been described for other PilY1 associated with T4P in neutrophilic bacteria; also, its C-terminal interacts (interface interaction) with the minor pilins PilX, PilW and PilV. The N-terminal region comprises domains such as the vWA and the MIDAS, involved in signaling, ligand-binding, and protein-protein interaction. In fact, the vWA domain has intrinsically disordered regions that enable it to maintain its structure over a wide pH range, not only at extreme acidity to which At. thiooxidans is adapted. The results obtained helped us design the correct methodology for its heterologous expression. This allowed us partially experimentally characterize it by obtaining the N-terminal domain recombinantly and evaluating its acid stability through fluorescence spectroscopy. The data suggest that it remains stable across pH changes. This work thus provides guidance for the characterization of extracellular proteins from extremophilic organisms.

The genetic basis of predation by myxobacteria

Myxobacteria (phylum Myxococcota) are abundant and virtually ubiquitous microbial predators. Facultatively multicellular organisms, they are able to form multicellular fruiting bodies and swarm across surfaces, cooperatively hunting for prey. Myxobacterial communities are able to kill a wide range of prey microbes, assimilating their biomass to fuel population growth. Their mechanism of predation is exobiotic - hydrolytic enzymes and toxic metabolites are secreted into the extracellular environment, killing and digesting prey cells from without. However, recent observations of single-cell predation and contact-dependent prey killing challenge the dogma of myxobacterial predation being obligately cooperative. Regardless of their predatory mechanisms, myxobacteria have a broad prey range, which includes Gram-negative bacteria, Gram-positive bacteria and fungi. Pangenome analyses have shown that their extremely large genomes are mainly composed of accessory genes, which are not shared by all members of their species. It seems that the diversity of accessory genes in different strains provides the breadth of activity required to prey upon such a smorgasbord of microbes, and also explains the considerable strain-to-strain variation in predatory efficiency against specific prey. After providing a short introduction to general features of myxobacterial biology which are relevant to predation, this review brings together a rapidly growing body of work into the molecular mechanisms and genetic basis of predation, presenting a summary of current knowledge, highlighting trends in research and suggesting strategies by which we can potentially exploit myxobacterial predation in the future.

Tight-packing of large pilin subunits provides distinct structural and mechanical properties for the Myxococcus xanthus type IVa pilus

Type IVa pili (T4aP) are ubiquitous cell surface filaments important for surface motility, adhesion to surfaces, DNA uptake, biofilm formation, and virulence. T4aP are built from thousands of copies of the major pilin subunit and tipped by a complex composed of minor pilins and in some systems also the PilY1 adhesin. While major pilins of structurally characterized T4aP have lengths of <165 residues, the major pilin PilA of Myxococcus xanthus is unusually large with 208 residues. All major pilins have a conserved N-terminal domain and a variable C-terminal domain, and the additional residues of PilA are due to a larger C-terminal domain. We solved the structure of the M. xanthus T4aP (T4aPMx) at a resolution of 3.0 Å using cryo-EM. The T4aPMx follows the structural blueprint of other T4aP with the pilus core comprised of the interacting N-terminal α1-helices, while the globular domains decorate the T4aP surface. The atomic model of PilA built into this map shows that the large C-terminal domain has more extensive intersubunit contacts than major pilins in other T4aP. As expected from these greater contacts, the bending and axial stiffness of the T4aPMx is significantly higher than that of other T4aP and supports T4aP-dependent motility on surfaces of different stiffnesses. Notably, T4aPMx variants with interrupted intersubunit interfaces had decreased bending stiffness, pilus length, and strongly reduced motility. These observations support an evolutionary scenario whereby the large major pilin enables the formation of a rigid T4aP that expands the environmental conditions in which the T4aP system functions.

Large pilin subunits provide distinct structural and mechanical properties for the Myxococcus xanthus type IV pilus

Type IV pili (T4P) are ubiquitous bacterial cell surface filaments important for surface motility, adhesion to biotic and abiotic surfaces, DNA uptake, biofilm formation, and virulence. T4P are built from thousands of copies of the major pilin subunit and tipped by a complex composed of minor pilins and in some systems also the PilY1 adhesin. While the major pilins of structurally characterized T4P have lengths of up to 161 residues, the major pilin PilA of Myxococcus xanthus is unusually large with 208 residues. All major pilins have a highly conserved N-terminal domain and a highly variable C-terminal domain, and the additional residues in the M. xanthus PilA are due to a larger C-terminal domain. We solved the structure of the M. xanthus T4P (T4P Mx ) at a resolution of 3.0 Å using cryo-electron microscopy (cryo-EM). The T4P Mx follows the structural blueprint observed in other T4P with the pilus core comprised of the extensively interacting N-terminal α1-helices while the globular domains decorate the T4P surface. The atomic model of PilA built into this map shows that the large C-terminal domain has much more extensive intersubunit contacts than major pilins in other T4P. As expected from these greater contacts, the bending and axial stiffness of the T4P Mx is significantly higher than that of other T4P and supports T4P-dependent motility on surfaces of different stiffnesses. Notably, T4P Mx variants with interrupted intersubunit interfaces had decreased bending stiffness and strongly reduced motility on all surfaces. These observations support an evolutionary scenario whereby the large major pilin enables the formation of a rigid T4P that expands the environmental conditions in which the T4P system functions.

Mechanism of assembly of type 4 filaments: everything you always wanted to know (but were afraid to ask)

Type 4 filaments (T4F) are a superfamily of filamentous nanomachines - virtually ubiquitous in prokaryotes and functionally versatile - of which type 4 pili (T4P) are the defining member. T4F are polymers of type 4 pilins, assembled by conserved multi-protein machineries. They have long been an important topic for research because they are key virulence factors in numerous bacterial pathogens. Our poor understanding of the molecular mechanisms of T4F assembly is a serious hindrance to the design of anti-T4F therapeutics. This review attempts to shed light on the fundamental mechanistic principles at play in T4F assembly by focusing on similarities rather than differences between several (mostly bacterial) T4F. This holistic approach, complemented by the revolutionary ability of artificial intelligence to predict protein structures, led to an intriguing mechanistic model of T4F assembly.