Molecular characterization of a chemotaxis operon in the oral spirochete, Treponema denticola

Atypical chemoreceptor arrays accommodate high membrane curvature

The prokaryotic chemotaxis system is arguably the best-understood signaling pathway in biology. In all previously described species, chemoreceptors organize into a hexagonal (P6 symmetry) extended array. Here, we report an alternative symmetry (P2) of the chemotaxis apparatus that emerges from a strict linear organization of the histidine kinase CheA in Treponema denticola cells, which possesses arrays with the highest native curvature investigated thus far. Using cryo-ET, we reveal that Td chemoreceptor arrays assume an unusual arrangement of the supra-molecular protein assembly that has likely evolved to accommodate the high membrane curvature. The arrays have several atypical features, such as an extended dimerization domain of CheA and a variant CheW-CheR-like fusion protein that is critical for maintaining an ordered chemosensory apparatus. Furthermore, the previously characterized Td oxygen sensor ODP influences CheA ordering. These results suggest a greater diversity of the chemotaxis signaling system than previously thought.

Regulation of the chemotaxis histidine kinase CheA: A structural perspective

Bacteria sense and respond to their environment through a highly conserved assembly of transmembrane chemoreceptors (MCPs), the histidine kinase CheA, and the coupling protein CheW, hereafter termed "the chemosensory array". In recent years, great strides have been made in understanding the architecture of the chemosensory array and how this assembly engenders sensitive and cooperative responses. Nonetheless, a central outstanding question surrounds how receptors modulate the activity of the CheA kinase, the enzymatic output of the sensory system. With a focus on recent advances, we summarize the current understanding of array structure and function to comment on the molecular mechanism by which CheA, receptors and CheW generate the high sensitivity, gain and dynamic range emblematic of bacterial chemotaxis. The complexity of the chemosensory arrays has motivated investigation with many different approaches. In particular, structural methods, genetics, cellular activity assays, nanodisc technology and cryo-electron tomography have provided advances that bridge length scales and connect molecular mechanism to cellular function. Given the high degree of component integration in the chemosensory arrays, we ultimately aim to understand how such networked molecular interactions generate a whole that is truly greater than the sum of its parts. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

Major proteins and antigens of Treponema denticola

Treponema denticola is a Gram-negative, motile, asaccharolytic, anaerobic spirochaete which along with Porphyromonas gingivalis and Tannerella forsythia has been shown to form a bacterial consortium called the Red Complex that is strongly associated with the clinical progression of chronic periodontitis. T. denticola was grown in continuous culture in a complex medium with a mean generation time of 15.75 h. Samples from two different membrane-enriched preparations and a cytoplasm-enriched preparation were separated by two-dimensional gel electrophoresis and the proteins identified by MALDI-TOF/TOF mass spectrometry. In total, 219 non-redundant proteins were identified including numerous virulence factors, lipoproteins, ABC transporter proteins and enzymes involved in the metabolism of nine different amino acids of which glycine seems to be of particular importance. Novel findings include the identification of several abundant peptide uptake systems, and the identification of three flagellar filament outer layer proteins. Two-dimensional Western blot analysis using sera from mice immunized with formalin-killed T. denticola cells suggested that Msp, PrcA, OppA, OppA10, MglB, TmpC and several flagellar filament proteins are antigenic.

The diverse CheC-type phosphatases: chemotaxis and beyond

A new class of protein phosphatases has emerged in the study of bacterial/archaeal chemotaxis, the CheC-type phosphatases. These proteins are distinct and unrelated to the well-known CheY-P phosphatase CheZ, though they have convergently evolved to dephosphorylate the same target. The family contains a common consensus sequence D/S-X(3)-E-X(2)-N-X(22)-P that defines the phosphatase active site, of which there are often two per protein. Three distinct subgroups make up the family: CheC, FliY and CheX. Further, the CheC subgroup can be divided into three classes. Bacillus subtilis CheC typifies the first class and might function as a regulator of CheD. Class II CheCs likely function as phosphatases in systems other than chemotaxis. Class III CheCs are found in the archaeal class Halobacteria and might function as class I CheCs. FliY is the main phosphatase in the B. subtilis chemotaxis system. CheX is quite divergent from the rest of the family, forms a dimer and some may function outside chemotaxis. A model for the evolution of the family is discussed.

Effect of glucose on Treponema denticola cell behavior

INTRODUCTION

Treponema denticola inhabits the oral subgingival environment and is part of a proteolytic benzoyl-dl-arginine-naphthylamide-positive 'red complex' associated with active periodontal disease. Spirochetes have a unique form of chemotactic motility that may contribute to their virulence. Chemotaxis is essential for efficient nutrient-directed translocation.

METHODS

We examined the effect of glucose on T. denticola cell velocity, expression of periplasmic flagella proteins, and chemotaxis, e.g. translocation into capillary tubes.

RESULTS

The presence of glucose did not significantly effect T. denticola cell velocity in high viscosity conditions nor did it alter periplasmic flagella protein expression. The addition of glucose to capillary tubes resulted in greater numbers of T. denticola cells in tubes containing glucose. A non-motile mutant did not migrate into capillary tubes containing glucose.

CONCLUSION

These results are consistent with a chemotactic response to glucose that is motility dependent.

Protein-protein interactions in the chemotaxis signalling pathway of Treponema denticola

Motile bacteria employ sophisticated chemotaxis signal transduction systems to transform environmental cues into corresponding behavioural responses. The proteins involved in this signalling pathway have been extensively studied on a molecular level in various model organisms, including enterobacteria and Bacillus subtilis, and specific protein-protein interactions have been identified. The chemotaxis operon of spirochaetes encodes a novel chemotaxis protein, CheX, in addition to homologues to the central components of established chemotaxis systems. Interestingly, the closest functionally characterized homologue of CheX is CheC of the complex B. subtilis chemotaxis pathway. In this study, the yeast two-hybrid system was applied to investigate protein-protein interactions within the chemotaxis signalling pathway of Treponema denticola, with special focus on CheX. CheX was found to interact with CheA and with itself. The other chemotaxis proteins exhibited interactions comparable to their homologues in known chemotaxis systems. Based on these findings, a model integrating CheX in the chemotaxis signal transduction pathway of T. denticola is proposed.

Diversity in chemotaxis mechanisms among the bacteria and archaea

The study of chemotaxis describes the cellular processes that control the movement of organisms toward favorable environments. In bacteria and archaea, motility is controlled by a two-component system involving a histidine kinase that senses the environment and a response regulator, a very common type of signal transduction in prokaryotes. Most insights into the processes involved have come from studies of Escherichia coli over the last three decades. However, in the last 10 years, with the sequencing of many prokaryotic genomes, it has become clear that E. coli represents a streamlined example of bacterial chemotaxis. While general features of excitation remain conserved among bacteria and archaea, specific features, such as adaptational processes and hydrolysis of the intracellular signal CheY-P, are quite diverse. The Bacillus subtilis chemotaxis system is considerably more complex and appears to be similar to the one that existed when the bacteria and archaea separated during evolution, so that understanding this mechanism should provide insight into the variety of mechanisms used today by the broad sweep of chemotactic bacteria and archaea. However, processes even beyond those used in E. coli and B. subtilis have been discovered in other organisms. This review emphasizes those used by B. subtilis and these other organisms but also gives an account of the mechanism in E. coli.

Chemotaxis-guided movements in bacteria

Motile bacteria often use sophisticated chemotaxis signaling systems to direct their movements. In general, bacterial chemotactic signal transduction pathways have three basic elements: (1) signal reception by bacterial chemoreceptors located on the membrane; (2) signal transduction to relay the signals from membrane receptors to the motor; and (3) signal adaptation to desensitize the initial signal input. The chemotaxis proteins involved in these signal transduction pathways have been identified and extensively studied, especially in the enterobacteria Escherichia coli and Salmonella enterica serovar typhimurium. Chemotaxis-guided bacterial movements enable bacteria to adapt better to their natural habitats via moving toward favorable conditions and away from hostile surroundings. A variety of oral microbes exhibits motility and chemotaxis, behaviors that may play important roles in bacterial survival and pathogenesis in the oral cavity.

Bacillus subtilis CheC and FliY are members of a novel class of CheY-P-hydrolyzing proteins in the chemotactic signal transduction cascade

Rapid restoration of prestimulus levels of the chemotactic response regulator, CheY-P, is important for preparing bacteria and archaea to respond sensitively to new stimuli. In an extension of previous work (Szurmant, H., Bunn, M. W., Cannistraro, V. J., and Ordal, G. W. (2003) J. Biol. Chem. 278, 48611-48616), we describe a new family of CheY-P phosphatases, the CYX family, that is widespread among the bacteria and archaea. These proteins provide another pathway, in addition to the ones involving CheZ of the gamma- and beta-proteobacteria (e.g. Escherichia coli) or the alternative CheY that serves as a "phosphate sink" among the alpha-proteobacteria (e.g. Sinorhizobium meliloti), for dephosphorylating CheY-P. In particular, we identify CheC, known previously to be involved in adaptation to stimuli in Bacillus subtilis, as a CheY-P phosphatase. Using an in vitro assay used previously to demonstrate that the switch protein FliY is a CheY-P phosphatase, we have shown that increasing amounts of CheC accelerate the hydrolysis of CheY-P. In vivo, a double mutant lacking cheC and the region of fliY that encodes the CheY-P binding domain is almost completely smooth swimming, implying that these cells contain very high levels of CheY-P. CheC appears to be primarily involved in restoring normal CheY-P levels following the addition of attractant, whereas FliY seems to act on CheY-P constitutively. The activity of CheC is relatively low compared to that of FliY, but we have shown that the chemotaxis protein CheD enhances the activity of CheC 5-fold. We suggest a model for how FliY, CheC, and CheD work together to regulate CheY-P levels in the bacterium.

Construction and characterization of a cheA mutant of Treponema denticola

The Treponema denticola cheA gene, encoding the central kinase of the general chemotaxis pathway, was analyzed for its role in chemotaxis and tissue penetration. The cheA gene was interrupted by insertion of an ermF-ermAM gene cassette. Reverse transcription-PCR confirmed that the other downstream chemotaxis genes within the same operon (cheW, cheX, and cheY) were still expressed in the cheA mutant strain. Lack of cheA resulted in decreased swarming on soft-agar swarm plates and failure to respond chemotactically to a mixture of nutrients. Behavioral analyses using video microscopy revealed that the cheA mutant exhibited coordinated cell movement. The cellular reversal frequency, however, was severely reduced, indicating that CheA in T. denticola mainly controls cellular reversal and that active chemotaxis signaling input is not required for coordination of flagellar rotation at both cell poles.

Role of Treponema denticola in periodontal diseases

Among periodontal anaerobic pathogens, the oral spirochetes, and especially Treponema denticola, have been associated with periodontal diseases such as early-onset periodontitis, necrotizing ulcerative gingivitis, and acute pericoronitis. Basic research as well as clinical evidence suggest that the prevalence of T denticola, together with other proteolytic gram-negative bacteria in high numbers in periodontal pockets, may play an important role in the progression of periodontal disease. The accumulation of these bacteria and their products in the pocket may render the surface lining periodontal cells highly susceptible to lysis and damage. T. denticola has been shown to adhere to fibroblasts and epithelial cells, as well as to extracellular matrix components present in periodontal tissues, and to produce several deleterious factors that may contribute to the virulence of the bacteria. These bacterial components include outer-sheath-associated peptidases, chymotrypsin-like and trypsin-like proteinases, hemolytic and hemagglutinating activities, adhesins that bind to matrix proteins and cells, and an outer-sheath protein with pore-forming properties. The effects of T. denticola whole cells and their products on a variety of host mucosal and immunological cells has been studied extensively (Fig. 1). The clinical data regarding the presence of T. denticola in periodontal health and disease, together with the basic research results involving the role of T. denticola factors and products in relation to periodontal diseases, are reviewed and discussed in this article.

Molecular characterization of the Treponema denticola fliQ region

A Treponema denticola 4.2 kb DNA region containing four complete genes (orfl, fliQ, fliR, and flhB) and a truncated gene (flhA') was sequenced and analyzed. The deduced amino acid sequences of FliQ, FliR, FlhB and FlhA' have significant homology with bacterial proteins associated with the flagellar export apparatus, whereas the deduced amino acid sequence of Orf1 has homology with an E. coli alcohol dehydrogenase. A putative sigma70-like promoter was identified upstream of fliQ. RT-PCR analysis indicated that fliQ, fliR, flhB and flhA' are co-transcribed independently of orfl, suggesting that the motility-associated genes are components of an operon. The location of the T. denticola fliQ-flhA' genes differs from that of the corresponding T. pallidum and Borrelia burgdorferi genes which are present in the large fla or flgB flagellar operons, respectively.

Motility and chemotaxis in tissue penetration of oral epithelial cell layers by Treponema denticola

The ability to penetrate tissue is an important virulence factor for pathogenic spirochetes. Previous studies have recognized the role of motility in allowing pathogenic spirochetes to invade tissues and migrate to sites favorable for bacterial proliferation. However, the nature of the movements, whether they are random or controlled by chemotaxis systems, has yet to be established. In this study, we addressed the role of motility and chemotaxis in tissue penetration by the periodontal disease-associated oral spirochete Treponema denticola using an oral epithelial cell line-based experimental approach. Wild-type T. denticola ATCC 35405 was found to penetrate the tissue layers effectively, whereas a nonmotile mutant was unable to overcome the tissue barrier. Interestingly, the chemotaxis mutants also showed impaired tissue penetration. A cheA mutant that is motile but lacks the central kinase of the chemotaxis pathway showed only about 2 to 3% of the wild-type penetration rate. The two known chemoreceptors of T. denticola, DmcA and DmcB, also appear to be involved in the invasion process. The dmc mutants were actively motile but exhibited reduced tissue penetration of about 30 and 10% of the wild-type behavior, respectively. These data suggest that not only motility but also chemotaxis is involved in the tissue penetration by T. denticola.

Cloning and expression of two novel hemin binding protein genes from Treponema denticola

Treponema denticola does not appear to produce siderophores, so it must acquire iron by other pathways. Indeed, T. denticola has been shown to have an iron-regulated 44-kDa outer membrane protein (HbpA) with hemin binding ability. To characterize the HbpA protein, its gene was cloned from genomic DNA libraries of T. denticola. Sequence analysis of the hbpA open reading frame indicated that it encoded a 42.8-kDa protein with a 23-amino-acid signal peptide. HbpA has no significant homology to any proteins in the databases. Southern blot analysis demonstrated that hbpA is present in several T. denticola ATCC strains and clinical isolates, but not in Treponema pectinovorum, Treponema socranskii, or Escherichia coli. HbpA, expressed as a recombinant protein in E. coli and purified by antibody affinity chromatography, has hemin binding activity as determined by lithium dodecyl sulfate-polyacrylamide gel electrophoresis with tetramethylbenzidine staining. Northern blot analysis showed that there were two hbpA-containing transcripts, of approximately 1.3 and 2.6 kb, and that the RNA levels were low-iron induced. Interestingly, the 2.6-kb mRNA also encoded a second protein with significant homology to hbpA. This downstream gene, called hbpB, was cloned and sequenced and its product was expressed as a fusion protein in E. coli. The hbpB gene product is 49% identical to HbpA and binds hemin. Thus, T. denticola has two novel hemin binding proteins which may be part of a previously unrecognized iron acquisition pathway.