Regulation of bacterial cell polarity by small GTPases

Mechanism of GTPase activation of a prokaryotic small Ras-like GTPase MglA by an asymmetrically interacting MglB dimer

Cell polarity oscillations in Myxococcus xanthus motility are driven by a prokaryotic small Ras-like GTPase, mutual gliding protein A (MglA), which switches from one cell pole to the other in response to extracellular signals. MglA dynamics is regulated by MglB, which functions both as a GTPase activating protein (GAP) and a guanine nucleotide exchange factor (GEF) for MglA. With an aim to dissect the asymmetric role of the two MglB protomers in the dual GAP and GEF activities, we generated a functional MglAB complex by coexpressing MglB with a linked construct of MglA and MglB. This strategy enabled us to generate mutations of individual MglB protomers (MglB1 or MglB2 linked to MglA) and delineate their role in GEF and GAP activities. We establish that the C-terminal helix of MglB1, but not MglB2, stimulates nucleotide exchange through a site away from the nucleotide-binding pocket, confirming an allosteric mechanism. Interaction between the N-terminal β-strand of MglB1 and β0 of MglA is essential for the optimal GEF activity of MglB. Specific residues of MglB2, which interact with Switch-I of MglA, partially contribute to its GAP activity. Thus, the role of the MglB2 protomer in the GAP activity of MglB is limited to restricting the conformation of MglA active site loops. The direct demonstration of the allosteric mechanism of GEF action provides us new insights into the regulation of small Ras-like GTPases, a feature potentially present in many uncharacterized GEFs.

Unorthodox regulation of the MglA Ras-like GTPase controlling polarity in Myxococcus xanthus

Motile cells have developed a large array of molecular machineries to actively change their direction of movement in response to spatial cues from their environment. In this process, small GTPases act as molecular switches and work in tandem with regulators and sensors of their guanine nucleotide status (GAP, GEF, GDI and effectors) to dynamically polarize the cell and regulate its motility. In this review, we focus on Myxococcus xanthus as a model organism to elucidate the function of an atypical small Ras GTPase system in the control of directed cell motility. M. xanthus cells direct their motility by reversing their direction of movement through a mechanism involving the redirection of the motility apparatus to the opposite cell pole. The reversal frequency of moving M. xanthus cells is controlled by modular and interconnected protein networks linking the chemosensory-like frizzy (Frz) pathway - that transmits environmental signals - to the downstream Ras-like Mgl polarity control system - that comprises the Ras-like MglA GTPase protein and its regulators. Here, we discuss how variations in the GTPase interactome landscape underlie single-cell decisions and consequently, multicellular patterns.

Flagellar Motor Transformed: Biophysical Perspectives of the Myxococcus xanthus Gliding Mechanism

Many bacteria move on solid surfaces using gliding motility, without involvement of flagella or pili. Gliding of Myxococcus xanthus is powered by a proton channel homologous to the stators in the bacterial flagellar motor. Instead of being fixed in place and driving the rotation of a circular protein track like the flagellar basal body, the gliding machinery of M. xanthus travels the length of the cell along helical trajectories, while mechanically engaging with the substrate. Such movement entails a different molecular mechanism to generate propulsion on the cell. In this perspective, we will discuss the similarities and differences between the M. xanthus gliding machinery and bacterial flagellar motor, and use biophysical principles to generate hypotheses about the operating mechanism, efficiency, sensitivity to control, and mechanosensing of M. xanthus gliding.

Structural characterization of Myxococcus xanthus MglC, a component of the polarity control system, and its interactions with its paralog MglB

The δ-proteobacteria Myxococcus xanthus displays social (S) and adventurous (A) motilities, which require pole-to-pole reversal of the motility regulator proteins. Mutual gliding motility protein C (MglC), a paralog of GTPase-activating protein Mutual gliding motility protein B (MglB), is a member of the polarity module involved in regulating motility. However, little is known about the structure and function of MglC. Here, we determined ∼1.85 Å resolution crystal structure of MglC using Selenomethionine Single-wavelength anomalous diffraction. The crystal structure revealed that, despite sharing <9% sequence identity, both MglB and MglC adopt a Regulatory Light Chain 7 family fold. However, MglC has a distinct ∼30° to 40° shift in the orientation of the functionally important α2 helix compared with other structural homologs. Using isothermal titration calorimetry and size-exclusion chromatography, we show that MglC binds MglB in 2:4 stoichiometry with submicromolar range dissociation constant. Using small-angle X-ray scattering and molecular docking studies, we show that the MglBC complex consists of a MglC homodimer sandwiched between two homodimers of MglB. A combination of size-exclusion chromatography and site-directed mutagenesis studies confirmed the MglBC interacting interface obtained by molecular docking studies. Finally, we show that the C-terminal region of MglB, crucial for binding its established partner MglA, is not required for binding MglC. These studies suggest that the MglB uses distinct interfaces to bind MglA and MglC. Based on these data, we propose a model suggesting a new role for MglC in polarity reversal in M. xanthus.

Dual specificity of a prokaryotic GTPase-activating protein (GAP) to two small Ras-like GTPases in Myxococcus xanthus

Two small Ras-like GTPases, MglA and SofG, work in synchrony to drive cell polarity and motility in the soil bacterium, Myxococcus xanthus. While MglA regulates two types of motility in Myxococcus and drives cell polarity reversals, SofG regulates social motility enabled by the type IV pili (T4P) machinery. In order to understand the molecular basis of how multiple GTPases act concertedly, we initiated biochemical studies on SofG. A construct of SofG (SofG^∆60 ) was purified as a homogenous monomer and could bind to GDP and GTP. Intrinsic GTP hydrolysis by SofG^∆60 was negligible. Earlier work from the laboratory revealed that MglB functions both as a GTPase-activating protein (GAP) and a guanine nucleotide exchange factor (GEF) for MglA. Biochemical assays of SofG^∆60 established that MglB interacts with GTP-bound SofG^∆60 and acts as a GAP for SofG^∆60 . Interaction of MglB with SofG^∆60 in the GDP-bound conformation was not observed, thereby suggesting that MglB might not act as a GEF for SofG^∆60 . The existence of a common GAP for both SofG and MglA could potentially contribute to concerted regulation of their GTPase activities, and mediate crosstalk between the two GTPases involved in motility of M. xanthus. Sequence analysis revealed the features for a SofG-like subclass of prokaryotic small Ras-like GTPases that enable MglB to act as a dual-specificity GAP.

Flow Induced Symmetry Breaking in a Conceptual Polarity Model

Important cellular processes, such as cell motility and cell division, are coordinated by cell polarity, which is determined by the non-uniform distribution of certain proteins. Such protein patterns form via an interplay of protein reactions and protein transport. Since Turing’s seminal work, the formation of protein patterns resulting from the interplay between reactions and diffusive transport has been widely studied. Over the last few years, increasing evidence shows that also advective transport, resulting from cytosolic and cortical flows, is present in many cells. However, it remains unclear how and whether these flows contribute to protein-pattern formation. To address this question, we use a minimal model that conserves the total protein mass to characterize the effects of cytosolic flow on pattern formation. Combining a linear stability analysis with numerical simulations, we find that membrane-bound protein patterns propagate against the direction of cytoplasmic flow with a speed that is maximal for intermediate flow speed. We show that the mechanism underlying this pattern propagation relies on a higher protein influx on the upstream side of the pattern compared to the downstream side. Furthermore, we find that cytosolic flow can change the membrane pattern qualitatively from a peak pattern to a mesa pattern. Finally, our study shows that a non-uniform flow profile can induce pattern formation by triggering a regional lateral instability.

FlhF regulates the number and configuration of periplasmic flagella in Borrelia burgdorferi

The Lyme disease bacterium Borrelia burgdorferi has 7-11 periplasmic flagella (PF) that arise from the cell poles and extend toward the midcell as a flat-ribbon, which is distinct from other bacteria. FlhF, a signal recognition particle (SRP)-like GTPase, has been found to regulate the flagellar number and polarity; however, its role in B. burgdorferi remains unknown. B. burgdorferi has an FlhF homolog (BB0270). Structural and biochemical analyses show that BB0270 has a similar structure and enzymatic activity as its counterparts from other bacteria. Genetics and cryo-electron tomography studies reveal that deletion of BB0270 leads to mutant cells that have less PF (4 ± 2 PF per cell tip) and fail to form a flat-ribbon, indicative of a role of BB0270 in the control of PF number and configuration. Mechanistically, we demonstrate that BB0270 localizes at the cell poles and controls the number and position of PF via regulating the flagellar protein stability and the polar localization of the MS-ring protein FliF. Our study not only provides the detailed characterizations of BB0270 and its profound impacts on flagellar assembly, morphology and motility in B. burgdorferi, but also unveils mechanistic insights into how spirochetes control their unique flagellar patterns.

Allosteric regulation of a prokaryotic small Ras-like GTPase contributes to cell polarity oscillations in bacterial motility

Mutual gliding motility A (MglA), a small Ras-like GTPase; Mutual gliding motility B (MglB), its GTPase activating protein (GAP); and Required for Motility Response Regulator (RomR), a protein that contains a response regulator receiver domain, are major components of a GTPase-dependent biochemical oscillator that drives cell polarity reversals in the bacterium Myxococcus xanthus. We report the crystal structure of a complex of M. xanthus MglA and MglB, which reveals that the C-terminal helix (Ct-helix) from one protomer of the dimeric MglB binds to a pocket distal to the active site of MglA. MglB increases the GTPase activity of MglA by reorientation of key catalytic residues of MglA (a GAP function) combined with allosteric regulation of nucleotide exchange by the Ct-helix (a guanine nucleotide exchange factor [GEF] function). The dual GAP-GEF activities of MglB accelerate the rate of GTP hydrolysis over multiple enzymatic cycles. Consistent with its GAP and GEF activities, MglB interacts with MglA bound to either GTP or GDP. The regulation is essential for cell polarity, because deletion of the Ct-helix causes bipolar localization of MglA, MglB, and RomR, thereby causing reversal defects in M. xanthus. A bioinformatics analysis reveals the presence of Ct-helix in homologues of MglB in other bacterial phyla, suggestive of the prevalence of the allosteric mechanism among other prokaryotic small Ras-like GTPases.

A study on the mechanism of cell polarity oscillations in Myxococcus xanthus reveals a novel allosteric regulatory mechanism for a small Ras-like GTPase. The motility protein MglB is the first example of both GTPase activating protein (GAP) and guanosine nucleotide exchange factor (GEF) activities being integrated into a single regulator of the small Ras-like GTPase MglA.

Interaction of the cyclic-di-GMP binding protein FimX and the Type 4 pilus assembly ATPase promotes pilus assembly

Type IVa pili (T4P) are bacterial surface structures that enable motility, adhesion, biofilm formation and virulence. T4P are assembled by nanomachines that span the bacterial cell envelope. Cycles of T4P assembly and retraction, powered by the ATPases PilB and PilT, allow bacteria to attach to and pull themselves along surfaces, so-called “twitching motility”. These opposing ATPase activities must be coordinated and T4P assembly limited to one pole for bacteria to show directional movement. How this occurs is still incompletely understood. Herein, we show that the c-di-GMP binding protein FimX, which is required for T4P assembly in Pseudomonas aeruginosa, localizes to the leading pole of twitching bacteria. Polar FimX localization requires both the presence of T4P assembly machine proteins and the assembly ATPase PilB. PilB itself loses its polar localization pattern when FimX is absent. We use two different approaches to confirm that FimX and PilB interact in vivo and in vitro, and further show that point mutant alleles of FimX that do not bind c-di-GMP also do not interact with PilB. Lastly, we demonstrate that FimX positively regulates T4P assembly and twitching motility by promoting the activity of the PilB ATPase, and not by stabilizing assembled pili or by preventing PilT-mediated retraction. Mutated alleles of FimX that no longer bind c-di-GMP do not allow rapid T4P assembly in these assays. We propose that by virtue of its high-affinity for c-di-GMP, FimX can promote T4P assembly when intracellular levels of this cyclic nucleotide are low. As P. aeruginosa PilB is not itself a high-affinity c-di-GMP receptor, unlike many other assembly ATPases, FimX may play a key role in coupling T4P mediated motility and adhesion to levels of this second messenger.

Type IV pili (T4P) are assembled on the surfaces of many bacterial pathogens and commensals through the action of specialized assembly machines whose components and structures are the subject of intense study. Repeated cycles of T4P assembly, attachment and retraction allow bacteria to move or “twitch” along surfaces, efficiently colonize and intoxicate host tissues, and elaborate multicellular structures such as biofilms. Assembly and retraction are powered by specific ATPases, PilB and PilT respectively, but the manner in which their activity is coordinated is still poorly understood. In this work, we provide evidence that a high-affinity c-di-GMP binding protein of Pseudomonas aeruginosa, FimX, interacts with the ATPase PilB and promotes PilB-dependent assembly of T4P. Live cell imaging of twitching bacteria shows that FimX localizes to the leading pole of motile P. aeruginosa and that its recruitment requires both components of the T4P assembly machine and the PilB ATPase. Our work highlights a novel regulatory strategy employed by P. aeruginosa to control assembly of this broadly conserved virulence factor.

Regulation of Cell Polarity in Motility and Cell Division in Myxococcus xanthus

Rod-shaped Myxococcus xanthus cells are polarized with proteins asymmetrically localizing to specific positions. This spatial organization is important for regulation of motility and cell division and changes over time. Dedicated protein modules regulate motility independent of the cell cycle, and cell division dependent on the cell cycle. For motility, a leading-lagging cell polarity is established that is inverted during cellular reversals. Establishment and inversion of this polarity are regulated hierarchically by interfacing protein modules that sort polarized motility proteins to the correct cell poles or cause their relocation between cell poles during reversals akin to a spatial toggle switch. For division, a novel self-organizing protein module that incorporates a ParA ATPase positions the FtsZ-ring at midcell. This review covers recent findings concerning the spatiotemporal regulation of motility and cell division in M. xanthus and illustrates how the study of diverse bacteria may uncover novel mechanisms involved in regulating bacterial cell polarity.

Identification and Characterization of Differentially-Regulated Type IVb Pilin Genes Necessary for Predation in Obligate Bacterial Predators

Bdellovibrio bacteriovorus is an obligate predator of bacteria that grows and divides within the periplasm of its prey. Functions involved in the early steps of predation have been identified and characterized, but mediators of prey invasion are still poorly detailed. By combining omics data available for Bdellovibrio and like organisms (BALO’s), we identified 43 genes expressed in B. bacteriovorus during the early interaction with prey. These included genes in a tight adherence (TAD) operon encoding for two type IVb fimbriae-like pilin proteins (flp1 and flp2), and their processing and export machinery. Two additional flp genes (flp3 and flp4) were computationally identified at other locations along the chromosome, defining the largest and most diverse type IVb complement known in bacteria to date. Only flp1, flp2 and flp4 were expressed; their respective gene knock-outs resulted in a complete loss of the predatory ability without losing the ability to adhere to prey cells. Additionally, we further demonstrate differential regulation of the flp genes as the TAD operon of BALOs with different predatory strategies is controlled by a flagellar sigma factor FliA, while flp4 is not. Finally, we show that FliA, a known flagellar transcriptional regulator in other bacteria, is an essential Bdellovibrio gene.

Highly Signal-Responsive Gene Regulatory Network Governing Myxococcus Development

The bacterium Myxococcus xanthus undergoes multicellular development when starved. Thousands of cells build mounds in which some differentiate into spores. This remarkable feat and the genetic tractability of Myxococcus provide a unique opportunity to understand the evolution of gene regulatory networks (GRNs). Recent work has revealed a GRN involving interconnected cascades of signal-responsive transcriptional activators. Initially, starvation-induced intracellular signals direct changes in gene expression. Subsequently, self-generated extracellular signals provide morphological cues that regulate certain transcriptional activators. However, signals for many of the activators remain to be discovered. A key insight is that activators often work combinatorially, allowing signal integration. The Myxococcus GRN differs strikingly from those governing sporulation of Bacillus and Streptomyces, suggesting that Myxococcus evolved a highly signal-responsive GRN to enable complex multicellular development.

Where are things inside a bacterial cell?

Bacterial cells are intricately organized, despite the lack of membrane-bounded organelles. The extremely crowded cytoplasm promotes macromolecular self-assembly and formation of distinct subcellular structures, which perform specialized functions. For example, the cell poles act as hubs for signal transduction complexes, thus providing a platform for the coordination of optimal cellular responses to environmental cues. Distribution of macromolecules is mostly mediated via specialized transport machineries, including the MreB cytoskeleton. Recent evidence shows that RNAs also specifically localize within bacterial cells, raising the possibility that gene expression is spatially organized. Here we review the current understanding of where things are in bacterial cells and discuss emerging questions that need to be addressed in the future.

Myxobacteria: Moving, Killing, Feeding, and Surviving Together

Myxococcus xanthus, like other myxobacteria, is a social bacterium that moves and feeds cooperatively in predatory groups. On surfaces, rod-shaped vegetative cells move in search of the prey in a coordinated manner, forming dynamic multicellular groups referred to as swarms. Within the swarms, cells interact with one another and use two separate locomotion systems. Adventurous motility, which drives the movement of individual cells, is associated with the secretion of slime that forms trails at the leading edge of the swarms. It has been proposed that cellular traffic along these trails contributes to M. xanthus social behavior via stigmergic regulation. However, most of the cells travel in groups by using social motility, which is cell contact-dependent and requires a large number of individuals. Exopolysaccharides and the retraction of type IV pili at alternate poles of the cells are the engines associated with social motility. When the swarms encounter prey, the population of M. xanthus lyses and takes up nutrients from nearby cells. This cooperative and highly density-dependent feeding behavior has the advantage that the pool of hydrolytic enzymes and other secondary metabolites secreted by the entire group is shared by the community to optimize the use of the degradation products. This multicellular behavior is especially observed in the absence of nutrients. In this condition, M. xanthus swarms have the ability to organize the gliding movements of 1000s of rods, synchronizing rippling waves of oscillating cells, to form macroscopic fruiting bodies, with three subpopulations of cells showing division of labor. A small fraction of cells either develop into resistant myxospores or remain as peripheral rods, while the majority of cells die, probably to provide nutrients to allow aggregation and spore differentiation. Sporulation within multicellular fruiting bodies has the benefit of enabling survival in hostile environments, and increases germination and growth rates when cells encounter favorable conditions. Herein, we review how these social bacteria cooperate and review the main cell–cell signaling systems used for communication to maintain multicellularity.

MglC, a Paralog of Myxococcus xanthus GTPase-Activating Protein MglB, Plays a Divergent Role in Motility Regulation

In order to optimize interactions with their environment and one another, bacteria regulate their motility. In the case of the rod-shaped cells of Myxococcus xanthus, regulated motility is essential for social behaviors. M. xanthus moves over surfaces using type IV pilus-dependent motility and gliding motility. These two motility systems are coordinated by a protein module that controls cell polarity and consists of three polarly localized proteins, the small G protein MglA, the cognate MglA GTPase-activating protein MglB, and the response regulator RomR. Cellular reversals are induced by the Frz chemosensory system, and the output response regulator of this system, FrzZ, interfaces with the MglA/MglB/RomR module to invert cell polarity. Using a computational approach, we identify a paralog of MglB, MXAN_5770 (MglC). Genetic epistasis experiments demonstrate that MglC functions in the same pathway as MglA, MglB, RomR, and FrzZ and is important for regulating cellular reversals. Like MglB, MglC localizes to the cell poles asymmetrically and with a large cluster at the lagging pole. Correct polar localization of MglC depends on RomR and MglB. Consistently, MglC interacts directly with MglB and the C-terminal output domain of RomR, and we identified a surface of MglC that is necessary for the interaction with MglB and for MglC function. Together, our findings identify an additional member of the M. xanthus polarity module involved in regulating motility and demonstrate how gene duplication followed by functional divergence can add a layer of control to the complex cellular processes of motility and motility regulation.

IMPORTANCE Gene duplication and the subsequent divergence of the duplicated genes are important evolutionary mechanisms for increasing both biological complexity and regulation of biological processes. The bacterium Myxococcus xanthus is a soil bacterium with an unusually large genome that carries out several social processes, including predation of other bacterial species and formation of multicellular, spore-filled fruiting bodies. One feature of the large M. xanthus genome is that it contains many gene duplications. Here, we compare the products of one example of gene duplication and divergence, in which a paralog of the cognate MglA GTPase-activating protein MglB has acquired a different and opposing role in the regulation of cellular polarity and motility, processes critical to the bacterium's social behaviors.

How Bacteria Use Type IV Pili Machinery on Surfaces

The bacterial type IV pilus (T4P) is a versatile molecular machine with a broad range of functions. Recent advances revealed that the molecular components and the biophysical properties of the machine are well conserved among phylogenetically distant bacterial species. However, its functions are diverse, and include adhesion, motility, and horizontal gene transfer. This review focusses on the role of T4P in surface motility and bacterial interactions. Different species have evolved distinct mechanisms for intracellular coordination of multiple pili and of pili with other motility machines, ranging from physical coordination to biochemical clocks. Coordinated behavior between multiple bacteria on a surface is achieved by active manipulation of surfaces and modulation of pilus-pilus interactions. An emerging picture is that the T4P actively senses and responds to environmental conditions.

Investigation into FlhFG reveals distinct features of FlhF in regulating flagellum polarity in Shewanella oneidensis

Rod-shaped bacterial cells are polarized, with many organelles confined to a polar cellular site. In polar flagellates, FlhF and FlhG, a multiple-domain (B-N-G) GTPase and a MinD-like ATPase respectively, function as a cognate pair to regulate flagellar localization and number as revealed in Vibrio and Pseudomonas species. In this study, we show that FlhFG of Shewanella oneidensis (SoFlhFG), a monotrichous γ-proteobacterium renowned for respiratory diversity, also play an important role in the flagellar polar placement and number control. Despite this, SoFlhFG exhibit distinct features that are not observed in the characterized counterparts. Most strikingly, the G domain of SoFlhF determines the polar placement, contrasting the N domain of the Vibrio cholerae FlhF. The SoFlhF N domain in fact counteracts the function of the G domain with respect to the terminal targeting in the absence of the B domain. We further show that GTPase activity of SoFlhF is essential for motility but not positioning. Overall, our results suggest that mechanisms underlying the polar placement of organelles appear to be diverse, even for evolutionally relatively conserved flagellum.

The small G-protein MglA connects to the MreB actin cytoskeleton at bacterial focal adhesions

In Myxococcus xanthus the gliding motility machinery is assembled at the leading cell pole to form focal adhesions, translocated rearward to propel the cell, and disassembled at the lagging pole. We show that MglA, a Ras-like small G-protein, is an integral part of this machinery. In this function, MglA stimulates the assembly of the motility complex by directly connecting it to the MreB actin cytoskeleton. Because the nucleotide state of MglA is regulated spatially and MglA only binds MreB in the guanosine triphosphate-bound form, the motility complexes are assembled at the leading pole and dispersed at the lagging pole where the guanosine triphosphatase activating protein MglB disrupts the MglA-MreB interaction. Thus, MglA acts as a nucleotide-dependent molecular switch to regulate the motility machinery spatially. The function of MreB in motility is independent of its function in peptidoglycan synthesis, representing a coopted function. Our findings highlight a new function for the MreB cytoskeleton and suggest that G-protein-cytoskeleton interactions are a universally conserved feature.

Pilin Processing Follows a Different Temporal Route than That of Archaellins in Methanococcus maripaludis

Methanococcus maripaludis has two different surface appendages: type IV-like pili and archaella. Both structures are believed to be assembled using a bacterial type IV pilus mechanism. Each structure is composed of multiple subunits, either pilins or archaellins. Both pilins and archaellins are made initially as preproteins with type IV pilin-like signal peptides, which must be removed by a prepilin peptidase-like enzyme. This enzyme is FlaK for archaellins and EppA for pilins. In addition, both pilins and archaellins are modified with N-linked glycans. The archaellins possess an N-linked tetrasaccharide while the pilins have a pentasaccharide which consists of the archaellin tetrasaccharide but with an additional sugar, an unidentified hexose, attached to the linking sugar. In this report, we show that archaellins can be processed by FlaK in the absence of N-glycosylation and N-glycosylation can occur on archaellins that still retain their signal peptides. In contrast, pilins are not glycosylated unless they have been acted on by EppA to have the signal peptide removed. However, EppA can still remove signal peptides from non-glycosylated pilins. These findings indicate that there is a difference in the order of the posttranslational modifications of pilins and archaellins even though both are type IV pilin-like proteins.

Different effects of MglA and MglB on pilus-mediated functions and natural competence in Thermus thermophilus

The thermophilic bacterium Thermus thermophilus is known for its high natural competence. Uptake of DNA is mediated by a DNA translocator that shares components with type IV pili. Localization and function of type IV pili in other bacteria depend on the cellular localization at the poles of the bacterium, a process that involves MglA and MglB. T. thermophilus contains homologs of MglA and MglB. The genes encoding MglA and MglB were deleted and the physiology of the mutants was studied. Deletion of the genes individually or in tandem had no effect on pili formation but pili lost their localization at the poles. The mutants abolished pilus-mediated functions such as twitching motility and adherence but had no effect on uptake of DNA by natural competence. These data demonstrate that MglA and MglB are dispensable for natural transformation and are consistent with the hypothesis that uptake of DNA does not depend on type IV pili or their cellular localization.

Zinc and ATP binding of the hexameric AAA-ATPase PilF from Thermus thermophilus: role in complex stability, piliation, adhesion, twitching motility, and natural transformation

The traffic AAA-ATPase PilF is essential for pilus biogenesis and natural transformation of Thermus thermophilus HB27. Recently, we showed that PilF forms hexameric complexes containing six zinc atoms coordinated by conserved tetracysteine motifs. Here we report that zinc binding is essential for complex stability. However, zinc binding is neither required for pilus biogenesis nor natural transformation. A number of the mutants did not exhibit any pili during growth at 64 °C but still were transformable. This leads to the conclusion that type 4 pili and the DNA translocator are distinct systems. At lower growth temperatures (55 °C) the zinc-depleted multiple cysteine mutants were hyperpiliated but defective in pilus-mediated twitching motility. This provides evidence that zinc binding is essential for the role of PilF in pilus dynamics. Moreover, we found that zinc binding is essential for complex stability but dispensable for ATPase activity. In contrast to many polymerization ATPases from mesophilic bacteria, ATP binding is not required for PilF complex formation; however, it significantly increases complex stability. These data suggest that zinc and ATP binding increase complex stability that is important for functionality of PilF under extreme environmental conditions.