The 34-kilodalton membrane immunogen of Treponema pallidum is a lipoprotein

Insight into the invasion process and immune-protective evaluation of Tp0971, a membrane lipoprotein from Treponema pallidum

IMPORTANCE

The past two decades have seen a worldwide resurgence in infections caused by Treponema pallidum (T. pallidum) subsp. pallidum, the syphilis spirochete. The well-recognized capacity of the syphilis spirochete for early dissemination and immune evasion has earned it the designation "the stealth pathogen." There are many hurdles to studying syphilis pathogenesis, most notably the difficulty of culturing and genetically manipulating T. pallidum, as well as the absence of an effective vaccine for T. pallidum prevention. T. pallidum infection in humans is a complex and lengthy process. In this study, we investigated the invasion process and the function of the infection-dependent antigen Tp0971 as an immunogen to inhibit the dissemination of T. pallidum in an animal infection model. This enables a better understanding of the specific pathogenic mechanism of this pathogen, syphilis pathogenesis, and vaccine research.

Protein Secretion in Spirochetes

Spirochetes form a separate phylum of bacteria with two membranes but otherwise unusual morphologies and envelope structures. Distinctive common features of Borrelia, Leptospira, and Treponema include the sequestration of flagella to the periplasm and thin peptidoglycan cell walls that are more closely associated with the inner membrane. Outer membrane compositions differ significantly between the genera. Leptospira most closely track Gram-negative bacteria due to the incorporation of lipopolysaccharides. Treponema and Borrelia outer membranes lack lipopolysaccharide, with treponemes expressing only a few outer membrane proteins and Borrelia displaying a dizzying diversity of abundant surface lipoproteins instead. Phylogenetic and experimental evidence indicates that spirochetes have adapted various modules of bacterial export and secretion pathways to build and maintain their envelopes. Export and insertion pathways in the inner membrane appear conserved, while spirochetal experimentation with various envelope architectures over time has led to variations in secretion pathways in the periplasm and outer membrane. Classical type I to III secretion systems have been identified, with demonstrated roles in drug efflux and export of flagellar proteins only. Unique activities of periplasmic proteases, including a C-terminal protease, are involved in maturation of some periplasmic proteins. Proper lipoprotein sorting within the periplasm appears to be dependent on functional Lol pathways that lack the outer membrane lipoprotein insertase LolB. The abundance of surface lipoproteins in Borrelia and detailed protein sorting studies suggest a lipoprotein secretion pathway that either extends Lol through the outer membrane or bypasses it altogether. Proteins can be released from cells in outer membrane vesicles or, rarely, as soluble proteins.

The Treponema pallidum Outer Membrane

The outer membrane (OM) of Treponema pallidum, the uncultivatable agent of venereal syphilis, has long been the subject of misconceptions and controversy. Decades ago, researchers postulated that T. pallidum's poor surface antigenicity is the basis for its ability to cause persistent infection, but they mistakenly attributed this enigmatic property to the presence of a protective outer coat of serum proteins and mucopolysaccharides. Subsequent studies revealed that the OM is the barrier to antibody binding, that it contains a paucity of integral membrane proteins, and that the preponderance of the spirochete's immunogenic lipoproteins is periplasmic. Since the advent of recombinant DNA technology, the fragility of the OM, its low protein content, and the lack of sequence relatedness between T. pallidum and Gram-negative outer membrane proteins (OMPs) have complicated efforts to characterize molecules residing at the host-pathogen interface. We have overcome these hurdles using the genomic sequence in concert with computational tools to identify proteins predicted to form β-barrels, the hallmark conformation of OMPs in double-membrane organisms and evolutionarily related eukaryotic organelles. We also have employed diverse methodologies to confirm that some candidate OMPs do, in fact, form amphiphilic β-barrels and are surface-exposed in T. pallidum. These studies have led to a structural homology model for BamA and established the bipartite topology of the T. pallidum repeat (Tpr) family of proteins. Recent bioinformatics has identified several structural orthologs for well-characterized Gram-negative OMPs, suggesting that the T. pallidum OMP repertoire is more Gram-negative-like than previously supposed. Lipoprotein adhesins and proteases on the spirochete surface also may contribute to disease pathogenesis and protective immunity.

Bifunctional role of the Treponema pallidum extracellular matrix binding adhesin Tp0751

Treponema pallidum, the causative agent of syphilis, is a highly invasive pathogenic spirochete capable of attaching to host cells, invading the tissue barrier, and undergoing rapid widespread dissemination via the circulatory system. The T. pallidum adhesin Tp0751 was previously shown to bind laminin, the most abundant component of the basement membrane, suggesting a role for this adhesin in host tissue colonization and bacterial dissemination. We hypothesized that similar to that of other invasive pathogens, the interaction of T. pallidum with host coagulation proteins, such as fibrinogen, may also be crucial for dissemination via the circulatory system. To test this prediction, we used enzyme-linked immunosorbent assay (ELISA) methodology to demonstrate specific binding of soluble recombinant Tp0751 to human fibrinogen. Click-chemistry-based palmitoylation profiling of heterologously expressed Tp0751 confirmed the presence of a lipid attachment site within this adhesin. Analysis of the Tp0751 primary sequence revealed the presence of a C-terminal putative HEXXH metalloprotease motif, and in vitro degradation assays confirmed that recombinant Tp0751 purified from both insect and Escherichia coli expression systems degrades human fibrinogen and laminin. The proteolytic activity of Tp0751 was abolished by the presence of the metalloprotease inhibitor 1,10-phenanthroline. Further, inductively coupled plasma-mass spectrometry showed that Tp0751 binds zinc and calcium. Collectively, these results indicate that Tp0751 is a zinc-dependent, membrane-associated protease that exhibits metalloprotease-like characteristics. However, site-directed mutagenesis of the HEXXH motif to HQXXH did not abolish the proteolytic activity of Tp0751, indicating that further mutagenesis studies are required to elucidate the critical active site residues associated with this protein. This study represents the first published description of a T. pallidum protease capable of degrading host components and thus provides novel insight into the mechanism of T. pallidum dissemination.

Crystal structure of the Tp34 (TP0971) lipoprotein of treponema pallidum: implications of its metal-bound state and affinity for human lactoferrin

The Tp34 (TP0971) membrane lipoprotein of Treponema pallidum, an obligate human pathogen and the agent of syphilis, was previously reported to have lactoferrin binding properties. Given the non-cultivatable nature of T. pallidum, a structure-to-function approach was pursued to clarify further potential relationships between the Tp34 structural and biochemical properties and its propensity to bind human lactoferrin. The crystal structure of a nonacylated, recombinant form of Tp34 (rTp34), solved to a resolution of 1.9A(,) revealed two metaloccupied binding sites within a dimer; the identity of the ion most likely was zinc. Residues from both of the monomers contributed to the interfacial metal-binding sites; a novel feature was that the delta-sulfur of methionine coordinated the zinc ion. Analytical ultracentrifugation showed that, in solution, rTp34 formed a metal-stabilized dimer and that rTp34 bound human lactoferrin with a stoichiometry of 2:1. Isothermal titration calorimetry further revealed that rTp34 bound human lactoferrin at high (submicromolar) affinity. Finally, membrane topology studies revealed that native Tp34 is not located on the outer surface (outer membrane) of T. pallidum but, rather, is periplasmic. How propensity of Tp34 to bind zinc and the iron-sequestering lactoferrin may relate overall to the biology of T. pallidum infection in humans is discussed.

The leptospiral major outer membrane protein LipL32 is a lipoprotein expressed during mammalian infection

We report the cloning of the gene encoding the 32-kDa lipoprotein, designated LipL32, the most prominent protein in the leptospiral protein profile. We obtained the N-terminal amino acid sequence of a staphylococcal V8 proteolytic-digest fragment to design an oligonucleotide probe. A Lambda-Zap II library containing EcoRI fragments of Leptospira kirschneri DNA was screened, and a 5.0-kb DNA fragment which contained the entire structural lipL32 gene was identified. Several lines of evidence indicate that LipL32 is lipid modified in a manner similar to that of other procaryotic lipoproteins. The deduced amino acid sequence of LipL32 would encode a 272-amino-acid polypeptide with a 19-amino-acid signal peptide, followed by a lipoprotein signal peptidase cleavage site. LipL32 is intrinsically labeled during incubation of L. kirschneri in media containing [(3)H]palmitate. The linkage of palmitate and the amino-terminal cysteine of LipL32 is acid labile. LipL32 is completely solubilized by Triton X-114 extraction of L. kirschneri; phase separation results in partitioning of LipL32 exclusively into the hydrophobic, detergent phase, indicating that it is a component of the leptospiral outer membrane. CaCl(2) (20 mM) must be present during phase separation for recovery of LipL32. LipL32 is expressed not only during cultivation but also during mammalian infection. Immunohistochemistry demonstrated intense LipL32 reactivity with L. kirschneri infecting proximal tubules of hamster kidneys. LipL32 is also a prominent immunogen during human leptospirosis. The sequence and expression of LipL32 is highly conserved among pathogenic Leptospira species. These findings indicate that LipL32 may be important in the pathogenesis, diagnosis, and prevention of leptospirosis.

Borrelia burgdorferi supercoiled plasmids encode multicopy tandem open reading frames and a lipoprotein gene family

DNA sequencing and Southern blot analyses of a Borrelia burgdorferi DNA fragment encoding a signal sequence led to the discovery of a genetic locus, designated 2.9, which appears to be present in at least seven copies in virulent B. burgdorferi 297. DNA sequence analysis of these regions revealed that each 2.9 locus contained an operon of four genes (ABCD) and open reading frames designated rep+ (positive strand) and rep- (negative strand) which encoded multiple repeat motifs. Downstream of the rep+ gene(s) in six of the completely cloned and sequenced 2.9 loci also were lipoprotein (LP) genes possessing highly similar signal sequences but encoding variable mature polypeptides. The lipoproteins could he separated into two classes on the basis of hydrophilicity profiles, sequence similarities, and reactivity with specific antibodies. The 2.9 loci were localized to two (20- and 30-kb) supercoiled plasmids in B. burgdorferi 297. Northern (RNA) blot analysis established that the 2.9 ABCD operon was only minimally expressed, whereas the rep- gene(s) and at least three of the seven LP genes were expressed by B. burgdorferi in vitro. A single putative promoter element was identified by RNA primer extension analysis upstream of the ABCD operon, whereas a number of potential promoter regions existed upstream of the LP genes. The combined data indicate that the ABCD operon, rep+ and rep- genes, and LP genes are separately transcribed during in vitro growth. The 2.9 loci possess a repetitiveness, diversity, and complexity not previously described for B. burgdorferi; differential expression of these genes may facilitate the spirochete's ability to survive in diverse host environments.

Molecular characterization, genomic arrangement, and expression of bmpD, a new member of the bmp class of genes encoding membrane proteins of Borrelia burgdorferi

An expression library made with Borrelia burgdorferi DNA in the vector lambda ZapII was screened with serum from a monkey infected with the Lyme disease agent. This serum killed B. burgdorferi in vitro by an antibody-dependent, complement-mediated mechanism and contained antibodies to at least seven spirochetal antigens, none of which were the major outer surface proteins OspA or OspB. Among several positive clones, a clone containing the B. burgdorferi bmpA gene encoding the immunodominant antigen P39 was obtained. Chromosome walking and DNA sequence analysis permitted the identification of two additional upstream genes homologous to the bmpA gene and its related companion, bmpB. The first of these was the recently characterized bmpC gene, and adjacent to it was the fourth and new member of this class, which has been designated bmpD. The gene product encoded by bmpD is 34l residues long, contains a signal sequence with a potential signal peptidase II cleavage site, and has 26% identity with TmpC of Treponema pallidum. Southern blotting confirmed the tandem arrangement of all four bmp genes in the chromosome of B. burgdorferi JD1. However, Northern (RNA) blotting revealed that bmpD is expressed as a monocistronic transcript, which indicates that it is not part of an operon at the bmp locus. The bmpD gene was found to be conserved in representative members of the three species of the B. burgdorferi sensu lato complex, suggesting that it serves an important biological function in the spirochete.

Physical map of the genome of Treponema pallidum subsp. pallidum (Nichols)

A physical map of the chromosome of Treponema pallidum subsp. pallidum (Nichols), the causative agent of syphilis, was constructed from restriction fragments produced by NotI, SfiI, and SrfI. These rare-cutting restriction endonucleases cleaved the T. pallidum genome into 16, 8, and 15 fragments, respectively. Summation of the physical lengths of the fragments indicates that the chromosome of T. pallidum subsp. pallidum is approximately 1,030 to 1,080 kbp in size. The physical map was constructed by hybridizing a variety of probes to Southern blots of single and double digests of T. pallidum genomic DNA separated by contour-clamped homogeneous electric field electrophoresis. Probes included cosmid clones constructed from T. pallidum subsp. pallidum genomic DNA, restriction fragments excised from gels, and selected genes. Physical mapping confirmed that the chromosome of T. pallidum subsp. pallidum is circular, as the SfiI and SrfI maps formed complete circles. A total of 13 genes, including those encoding five membrane lipoproteins (tpn47, tpn41, tpn29-35, tpn17, and tpn15), a putative outer membrane porin (tpn50), the flagellar sheath and hook proteins (flaA and flgE), the cytoplasmic filament protein (cfpA), 16S rRNA (rrnA), a major sigma factor (rpoD), and a homolog of cysteinyl-tRNA synthetase (cysS), have been localized in the physical map as a first step toward studying the genetic organization of this noncultivable pathogen.

Treponema pallidum in gel microdroplets: a novel strategy for investigation of treponemal molecular architecture

Controversy exists regarding the constituents and antigenic properties of the Treponema pallidum outer membrane; a major point of contention concerns the cellular location(s) of the spirochaete's lipoprotein immunogens. To address these issues and circumvent problems associated with prior efforts to localize treponemal surface antigens, we developed a novel strategy for investigating T. pallidum molecular architecture. Virulent treponemes were encapsulated in porous agarose beads (gel microdroplets) and then probed in the presence or absence of Triton X-100. Intact, encapsulated treponemes were not labelled by monospecific antisera directed against four major T. pallidum lipoproteins or a candidate T. pallidum outer membrane protein (TpN50) with C-terminal sequence homology to Escherichia coli OmpA or by human or rabbit syphilitic serum. Each of these immunologic reagents, however, labelled encapsulated treponemes co-incubated with detergent. In contrast, antibodies generated against isolated T. pallidum outer membranes labelled intact organisms and the pattern of fluorescence was consistent with the distribution of rare outer membrane proteins visualized by freeze-fracture electron microscopy. In addition to providing strong evidence that the protein portions of treponemal lipoproteins are located within the periplasmic space, these studies have extended our understanding of the topographical relationships among T. pallidum cell envelope constituents. They also demonstrate the feasibility of generating antibodies against rare outer membrane proteins and detecting them on the surfaces of virulent treponemes.

The 47-kDa major lipoprotein immunogen of Treponema pallidum is a penicillin-binding protein with carboxypeptidase activity

The recent model of Treponema pallidum molecular architecture proposes that the vast majority of the bacterium's integral membrane proteins are lipoprotein immunogens anchored in the cytoplasmic membrane while the outer membrane contains only a limited number of surface-exposed transmembrane proteins. This unique model explains, in part, the organism's remarkable ability to evade host immune defenses and establish persistent infection. Our strategy for refining this model involves demonstrating that the physiological functions of treponemal membrane proteins are consistent with their proposed cellular locations. In this study, we used an ampicillin-digoxigenin conjugate to demonstrate by chemiluminescence that the 47-kDa lipoprotein immunogen of T. pallidum (Tpp47) is a penicillin-binding protein. Reexamination of the Tpp47 primary sequence revealed the three amino acid motifs characteristic of penicillin-binding proteins. A recombinant, nonlipidated, soluble form of Tpp47 was used to demonstrate that Tpp47 is a zinc-dependent carboxypeptidase. Escherichia coli expressing Tpp47 was characterized by cell wall abnormalities consistent with altered peptidoglycan biosynthesis. Though the inability to cultivate T. pallidum in vitro and the lack of genetic exchange systems continue to impede treponemal research, this study advances strategies for utilizing E. coli molecular genetics as a means of elucidating the complex relationships between syphilis pathogenesis and T. pallidum membrane biology.

Nucleotide sequence and analysis of the gene in Borrelia burgdorferi encoding the immunogenic P39 antigen

The P39 antigen is a specific, highly conserved, and immunogenic protein of Lyme disease spirochetes, Borrelia burgdorferi sensu lato. The nucleotide sequence of the gene encoding this protein was determined and found to be the first of two tandemly arranged open reading frames located on the spirochete's chromosome. These two open reading frames were designated bmpA for the gene encoding P39 and bmpB for the gene encoding the putative protein ORF2 encoded by the second open reading frame. The nucleic acid sequence identity for the two open reading frames was 62% while their deduced amino acid sequences were 52% identical. Comparison to sequence data bases demonstrated that the deduced amino acid sequences of both P39 and ORF2 were homologous to TmpC, a putative outer or cytoplasmic membrane lipoprotein of the syphilis spirochete, Treponema pallidum.

Processing and surface presentation of the Mycoplasma hyorhinis variant lipoprotein VlpC

The variant surface lipoprotein VlpC of Mycoplasma hyorhinis was shown to be processed by cleavage of a characteristic prokaryotic prolipoprotein signal peptide. In addition, a vlpC::phoA fusion protein expressed and translocated in Escherichia coli was recognized by surface-binding monoclonal antibodies, which identified the characteristic region II of Vlps, containing divergent external sequences proximal to the membrane, as an exposed portion of these surface proteins subject to immune recognition and selection.

Similarity between the 38-kilodalton lipoprotein of Treponema pallidum and the glucose/galactose-binding (MglB) protein of Escherichia coli

The recent discovery that abundant and immunogenic lipoproteins constitute the integral membrane proteins of Treponema pallidum has prompted efforts to investigate their importance in the physiology and ultrastructure of the organism and in immune responses during infection. Earlier studies identified a 38-kDa lipoprotein of T. pallidum believed to be specific to the pathogen. In the present study, monoclonal antibodies generated against the 38-kDa lipoprotein of T. pallidum reacted with cognate 37-kDa molecules in the nonpathogens Treponema phagedenis, Treponema denticola, and Treponema refringens. Cloning and expression of the 38-kDa-lipoprotein gene of T. pallidum in Escherichia coli revealed that the recombinant product displayed a slightly larger (39-kDa) apparent molecular mass but remained reactive with anti-38-kDa-protein monoclonal antibodies. The recombinant product was processed and acylated in E. coli. DNA and amino acid sequence analyses indicated an open reading frame encoding 403 amino acids, with the first 25 amino acids corresponding to a leader peptide terminated by a signal peptidase II processing site of Val-Val-Gly-Cys. The predicted mature protein is 378 amino acids in length with a deduced molecular weight of 40,422 (excluding acylation). Southern blotting failed to demonstrate in nonpathogenic treponemes genomic sequences homologous with the 38-kDa-lipoprotein gene of T. pallidum. Computer analysis revealed that the 38-kDa lipoprotein of T. pallidum had 34.2% identity and 58.9% similarity with the glucose/galactose-binding protein (MglB) of E. coli and Salmonella typhimurium. Furthermore, of the 19 amino acids of MglB involved in carbohydrate binding, the 38-kDa lipoprotein had identity with 11. These studies have allowed the first putative functional assignment (carbohydrate binding) to a T. pallidum integral membrane protein. Recognition of this potential physiological role for the 38-kDa lipoprotein underscores the possibility that the membrane biology of T. pallidum may more closely resemble that of gram-positive organisms, which also utilize lipoproteins as anchored transporters, than that of gram-negative bacteria to which T. pallidum often is analogized.

Fatty acids of Treponema pallidum and Borrelia burgdorferi lipoproteins

A fundamental ultrastructural feature shared by the spirochetal pathogens Treponema pallidum subsp. pallidum (T. pallidum) and Borrelia burgdorferi, the etiological agents of venereal syphilis and Lyme disease, respectively, is that their most abundant membrane proteins contain covalently attached fatty acids. In this study, we identified the fatty acids covalently bound to lipoproteins of B. burgdorferi and T. pallidum and examined potential acyl donors to these molecules. Palmitate was the predominant fatty acid of both B. burgdorferi and T. pallidum lipoproteins. T. pallidum lipoproteins also contained substantial amounts of stearate, a fatty acid not typically prevalent in prokaryotic lipoproteins. In both spirochetes, the fatty acids of cellular lipids differed from those of their respective lipoproteins. To characterize phospholipids in these organisms, spirochetes were metabolically labeled with [3H]palmitate or [3H]oleate; B. burgdorferi contained only phosphatidylglycerol and phosphatidylcholine, while T. pallidum contained phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin. Although palmitate predominated in the lipoproteins, there were no apparent differences in the incorporation of these two fatty acids into phospholipids (putative acyl donors). Phospholipase A1 and A2 digestion of phosphatidylcholine from B. burgdorferi and T. pallidum labeled with either [3H]palmitate or [3H]oleate also revealed that neither fatty acid was incorporated preferentially into the 1 and 2 positions (potential acyl donor sites) of the glycerol backbone. The combined findings suggest that fatty acid utilization during lipoprotein synthesis is determined largely by the fatty acid specificities of the lipoprotein acyl transferases. These findings also provide the basis for ongoing efforts to elucidate the relationship between lipoprotein acylation and the physiological functions and inflammatory activities of these molecules.

Treponema pallidum rare outer membrane proteins: analysis of mobility by freeze-fracture electron microscopy

Freeze-fracture and deep-etch electron microscopy were used to investigate the molecular architecture of the Treponema pallidum outer membrane (OM). Freeze-fracture electron microscopy of treponemes freshly harvested from rabbit testes revealed that the intramembranous particles (IMPs) in both the concave and convex OM leaflets were distributed into alternating areas of relatively high and low particle density; in many OM fractures, IMPs formed rows that ran either parallel to or obliquely across the fracture faces. Statistical analysis (runs test) confirmed that the IMPs were nonrandomly distributed in both OM leaflets. Examination of deep-etched specimens revealed that the particles observed in freeze-fractured OMs also were surface exposed. Combined analysis of deep-etched and cross-fractured treponemes revealed that the OM particles were located in regions of the OM away from the endoflagella and closely apposed to the cytoplasmic membrane-peptidoglycan complex. When treponemes were incubated for extended periods with heat-inactivated immune rabbit syphilitic serum, no alteration in the distribution of OM IMPs was detected. In further experiments, approximately 1:1 mixtures of T. pallidum and Escherichia coli or separate suspensions of the nonpathogenic Treponema phagedenis biotype Reiter were fixed at 34 degrees C or after cooling to 0 degree C (to induce lateral phase separations that would aggregate IMPs). Only particles in the T. pallidum OM failed to aggregate in cells fixed at the lower temperature. The combined data suggest that the mobility of T. pallidum rare OM proteins is limited, perhaps as a result of interactions between their periplasmic domains and components of the peptidoglycan-cytoplasmic membrane complex.

Identification and characterization of the Treponema pallidum tpn50 gene, an ompA homolog

Treponema pallidum is a pathogenic spirochete that has no known genetic exchange mechanisms. In order to identify treponemal genes encoding surface and secreted proteins, we carried out TnphoA mutagenesis of a T. pallidum genomic DNA library in Escherichia coli. Several of the resulting clones expressed enzymatically active T. pallidum-alkaline phosphatase fusion proteins. The DNA sequence of the 5' portion of a number of the treponemal genes was obtained and analyzed. A recombinant clone harboring plasmid p4A2 that encoded a treponemal protein with an approximate molecular mass of 50,000 Da was identified. Plasmid p4A2 contained an open reading frame of 1,251 nucleotides that resulted in a predicted protein of 417 amino acids with a calculated molecular mass of 47,582 Da. We have named this gene tpn50 in accordance with the current nomenclature for T. pallidum genes. A 1.9-kb HincII-ClaI fragment from p4A2 that contained the tpn50 gene was subcloned to produce p4A2HC2. Comparison of the predicted amino acid sequence of TpN50 with protein sequences in the National Center for Biotechnology Information data base indicated statistically significant homology to the Pseudomonas sp. OprF, E. coli OmpA, Bordetella avium OmpA, Neisseria meningitidis RmpM, Neisseria gonorrhoeae PIII, Haemophilus influenzae P6, E. coli PAL, and Legionella pneumophila PAL proteins. These proteins are all members of a family of outer membrane proteins that are present in gram-negative bacteria. The tpn50 gene complemented E. coli ompA mutations on the basis of two separate criteria. First, morphometry and electron microscopy data showed that E. coli C386 (ompA lpp) cells harboring plasmid vector pEBH21 were rounded while cells of the same strain harboring p4A2HC2 (TpN50+), pWW2200 (OprF+), or pRD87 (OmpA+) were rod shaped. Second, E. coli BRE51 (MC4100 delta sulA-ompA) cells harboring pEBH21 grew poorly at 42 degrees C in minimal medium, while the growth of BRE51 cells harboring p4A2HC2 was similar to that of the parental MC4100 cells. These results demonstrate that the TpN50 protein is functionally equivalent to the E. coli OmpA protein. If TpN50 functions in a similar fashion in T. pallidum, then it may be localized to the treponemal outer membrane.

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

Common and specific antigens of several treponemes detected by polyclonal antisera against major cellular proteins

Thirteen polypeptide antigens with molecular weights ranging from 34 kDa to 83 kDa were selected and their antigenic behaviors and distribution were examined in 12 strains of microorganisms including Treponema, Borrelia, Leptospira and Leptonema. Immunoblot analysis demonstrated that 45 kDa and 83 kDa polypeptides of Treponema socranskii subsp. buccale ATCC 35534, 53 kDa antigen of Treponema denticola ATCC 33520 and 44 kDa polypeptide of the strain G7201 were strain-specific. The 34, 62, 66 and 84 kDa polypeptide antigens were detected in all 8 treponemal strains examined. T. denticola ATCC 33520 and ATCC 35404 possessed 38 kDa, 48 kDa, 52 kDa and 72 kDa common polypeptide antigens. All 12 strains possessed the 84 kDa polypeptide antigen. Immunoelectron microscopy demonstrated that the 34 kDa and 38 kDa polypeptide antigens were located on the axial flagella and that other polypeptide antigens were located on the outer envelopes or wall-membrane complexes.

Polypeptides of Treponema pallidum: progress toward understanding their structural, functional, and immunologic roles. Treponema Pallidum Polypeptide Research Group

Treponema pallidum subsp. pallidum, the spirochete that causes syphilis, is unusual in a number of respects, including its small genome size, inability to grow under standard in vitro culture conditions, microaerophilism, apparent paucity of outer membrane proteins, structurally complex periplasmic flagella, and ability to evade the host immune responses and cause disease over a period of years to decades. Many of these attributes are related ultimately to its protein content. Our knowledge of the activities, structure, and immunogenicity of its proteins has been expanded by the application of recombinant DNA, hybridoma, and structural fractionation techniques. The purpose of this monograph is to summarize and correlate this new information by using two-dimensional gel electrophoresis, monoclonal antibody reactivity, sequence data, and other properties as the bases of polypeptide identification. The protein profiles of the T. pallidum subspecies causing syphilis, yaws, and endemic syphilis are virtually indistinguishable but differ considerably from those of other treponemal species. Among the most abundant polypeptides are a group of lipoproteins of unknown function that appear to be important in the immune response during syphilitic infection. The periplasmic flagella of T. pallidum and other spirochetes are unique with regard to their protein content and ultrastructure, as well as their periplasmic location. They are composed of three core proteins (homologous to the other members of the eubacterial flagellin family) and a single, unrelated sheath protein; the functional significance of this arrangement is not understood at present. Although the bacterium contains the chaperonins GroEL and DnaK, these proteins are not under the control of the heat shock regulon as they are in most organisms. Studies of the immunogenicity of T. pallidum proteins indicate that many may be useful for immunodiagnosis and immunoprotection. Future goals in T. pallidum polypeptide research include continued elucidation of their structural locations and functional activities, identification and characterization of the low-abundance outer membrane proteins, further study of the immunoprotective and immunodiagnostic potential of T. pallidum proteins, and clarification of the roles of treponemal proteins in pathogenesis.

Lipid modification of the 17-kilodalton membrane immunogen of Treponema pallidum determines macrophage activation as well as amphiphilicity

A murine monoclonal antibody specific for a 17-kDa major membrane immunogen of Treponema pallidum was used to select recombinant Escherichia coli clones expressing the molecule from a T. pallidum genomic library. Sequence analysis of the structural gene for the immunogen (designated tpp17) revealed a 468-bp open reading frame encoding a polypeptide of 156 amino acids with a calculated molecular mass of 16,441 Da. The deduced amino acid sequence included a putative leader peptide terminated by a consensus tetrapeptide for the modification and processing of prokaryotic lipoproteins. Immunoprecipitation of the cloned immunogen radiolabeled with [3H]palmitate confirmed that it was a lipoprotein. The amino acid sequence also predicted that the mature protein contains four cysteine residues in addition to the lipid-modified cysteine of the N terminus. The existence of disulfide-bonded multimeric forms of the native immunogen was demonstrated by immunoblotting T. pallidum solubilized in the presence and absence of 2-mercaptoethanol. Triton X-114 phase partitioning of a nonlipidated form of the 17-kDa immunogen cleaved from a glutathione S-transferase fusion protein demonstrated that lipid modification is responsible for the immunogen's hydrophobic character. The same nonlipidated form of the immunogen also was used to demonstrate that lipid modification is essential for the molecule's ability to stimulate production of tumor necrosis factor alpha by murine macrophages. We conclude that covalently attached fatty acids not only anchor T. pallidum lipoproteins to spirochetal membranes but also confer upon these molecules the ability to activate immune effector cells.

Molecular cloning, expression, and DNA sequence analysis of the gene that encodes the 16-kilodalton outer membrane lipoprotein of Serpulina hyodysenteriae

The gene (smpA) that encodes the 16-kDa outer membrane lipoprotein of Serpulina hyodysenteriae was cloned in Escherichia coli, and its primary structure was determined by nucleotide sequencing. The putative open reading frame encodes a prolipoprotein of 16.8 kDa which in its fully acylated and cleaved form is 15.1 kDa. Analysis of the N-terminal amino acid sequence derived from the DNA sequence revealed the presence of a signal sequence and a putative acylation and signal peptidase II cleavage site (Phe-Ala-Val-Ser-Cys). In E. coli, processing of the prolipoprotein was less efficient than that observed in S. hyodysenteriae, and globomycin, an inhibitor of signal peptidase II, inhibited cleavage of the lipoprotein expressed in E. coli but did not inhibit cleavage in S. hyodysenteriae.

A 16-kilodalton lipoprotein of the outer membrane of Serpulina (Treponema) hyodysenteriae

Serpulina (Treponema) hyodysenteriae P18A and VS1 were extracted by using the detergent Triton X-114 and separated into detergent and aqueous phases. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western immunoblot analysis confirmed that a membrane-associated 16-kDa antigen was hydrophobic, since it was found in the detergent phase. A 45-kDa antigen partitioned into the aqueous phase, suggesting that it was hydrophilic and may be of periplasmic origin. When spirochetes were grown in the presence of [3H]palmitic acid, a predominant 16-kDa antigen was labeled; from the results of immunoprecipitation experiments, this antigen appeared to be the same as that recognized by both polyclonal and monoclonal antisera to a previously described 16-kDa antigen. This antigen was proteinase K sensitive and was not a component of the lipopolysaccharide, which, although [3H]palmitate labeled, was resistant to proteinase K digestion. The most probable explanation is that the 16-kDa antigen is a membrane-associated, surface-exposed, immunodominant lipoprotein.

The outer membrane, not a coat of host proteins, limits antigenicity of virulent Treponema pallidum

Virulent Treponema pallidum reacts poorly with the specific antibodies present in human and rabbit syphilitic sera, a phenomenon often attributed to an outer coat of host serum proteins. Here we present additional evidence that the limited antigenicity of virulent organisms actually is due to a paucity of proteins in the outer membrane. Initially, we used electron microscopy to demonstrate that the outer membrane is highly susceptible to damage from physical manipulation (i.e., centrifugation and resuspension) and nonionic detergents. Organisms with disrupted outer membranes were markedly more antigenic than intact treponemes as determined by immunoelectron microscopy (IEM) with rabbit syphilitic and antiendoflagellar antisera. Data obtained with a new radioimmunoassay, designated the T. pallidum surface-specific radioimmunoassay, corroborated these IEM findings by demonstrating that the major T. pallidum immunogens are not surface exposed; the assay also was unable to detect serum proteins, including fibronectin, on the surfaces of intact organisms. Furthermore, IEM of T. pallidum on ultrathin cryosections with monospecific anti-47-kDa-immunogen antiserum confirmed the intracellular location of the 47-kDa immunogen. On the basis of these and previous findings, we proposed a new model for T. pallidum ultrastructure in which the outer membrane contains a small number of transmembrane proteins and the major membrane immunogens are anchored by lipids to the periplasmic leaflet of the cytoplasmic membrane. This unique ultrastructure explains the remarkable ability of virulent organisms to evade the humoral immune response of the T. pallidum-infected host.

Localization of outer surface proteins A and B in both the outer membrane and intracellular compartments of Borrelia burgdorferi

Borrelia burgdorferi B31 with and without outer membranes contained nearly identical amounts of outer surface proteins A and B. The majority of each immunogen also was localized intracellularly by immunocryoultramicrotomy. These results are inconsistent with the widely held belief that outer surface proteins A and B are exclusively outer membrane proteins.

Identification of Treponema pallidum subspecies pallidum genes encoding signal peptides and membrane-spanning sequences using a novel alkaline phosphatase expression vector

Treponema pallidum subspecies pallidum is a pathogenic spirochaete for which there are no systems of genetic exchange. In order to provide a system for the identification of T. pallidum surface proteins and potential virulence factors, we have developed a novel expression vector which confers the utility of TnphoA transposition. The relevant features of this plasmid vector, termed pMG, include an inducible tac promoter, a polylinker with multiple cloning sites in three reading frames, and an alkaline phosphatase (AP) gene lacking the signal sequence-encoding region. Library construction with Sau3A-digested T. pallidum genomic DNA resulted in the creation of functional T. pallidum-AP fusion proteins. Analysis of fusion proteins and their corresponding DNA and deduced amino acid sequences demonstrated that they could be grouped into three categories: (i) those with signal peptides containing leader peptidase I cleavage sites, (ii) those with signal peptides containing leader peptidase II cleavage sites, and (iii) those with non-cleavable hydrophobic membrane-spanning sequences. Triton X-114 detergent phase partitioning of individual T. pallidum-AP fusions revealed several clones whose AP activity partitioned preferentially into the hydrophobic detergent phase. Several of these fusion proteins were subsequently shown to be acylated by Escherichia coli following [3H]-palmitate labelling, indicating their lipoproteinaceous nature. DNA and amino acid sequence analysis of one acylated fusion protein, Tp75, confirmed the presence of a hydrophobic N-terminal signal sequence containing a consensus leader peptidase II recognition site. The DNA sequence of Tp75 also indicates that this is a previously unreported T. pallidum lipoprotein. T. pallidum-AP fusion proteins which partitioned into the hydrophobic detergent phase but did not incorporate palmitate were also identified. DNA and amino acid analysis of one such clone, Tp70, showed no cleavable signal but had a significant hydrophobic region of approximately 20 residues, consistent with a membrane-spanning domain. Immunoblot analysis of T. pallidum-AP fusions detected with a monoclonal antibody specific for AP identified several fusion proteins which migrated as doublets separated in apparent electrophoretic mobility by no more than 3 kDa. [35S]-methionine pulse-chase incorporation showed that the doublet AP fusions represented precursor and processed forms of the same protein. DNA and amino acid sequence analysis of clones expressing processed fusion proteins demonstrated hydrophobic N-terminal signal sequences containing consensus leader peptidase I recognition sites.

Treponema phagedenis encodes and expresses homologs of the Treponema pallidum TmpA and TmpB proteins

We cloned and sequenced the genes from Treponema phagedenis Kazan 5 encoding proteins homologous to the TmpA and TmpB proteins of Treponema pallidum subsp. pallidum Nichols (hereafter referred to as T. pallidum). Although previous reports suggested that the TmpA and TmpB proteins were specific for T. pallidum, we found that homologs for both were expressed in T. phagedenis Kazan 5 and Reiter. The TmpA protein from T. phagedenis contained the consensus sequence that bacterial lipoproteins require for posttranslational modification and subsequent proteolytic cleavage by signal peptidase II and showed 42% amino acid sequence identity with the TmpA protein from T. pallidum. The TmpB proteins of T. phagedenis and T. pallidum had similar amino acid sequences at their amino- and carboxy-terminal ends. The central portions of both of these proteins contained four repeats of the amino acid sequence EAARKAAE. The TmpB protein from T. phagedenis had an additional amino acid sequence repeat (consensus sequence KAAKE/D) that was not found in the TmpB protein from T. pallidum; this repeat was most remarkable, as it occurred 17 times in succession. These repeated amino acid sequences probably created an extensive alpha-helix region within the TmpB proteins. As with T. pallidum, the stop codon of the T. phagedenis tmpA gene overlapped the start codon of its tmpB gene. Northern blot analysis showed that the T. phagedenis tmpA and tmpB genes were probably transcribed into a single 2.5-kb mRNA molecule. Western blot (immunoblot) analysis demonstrated that both proteins were expressed by T. phagedenis. The high degree of amino acid sequence conservation seen with the TmpA and TmpB proteins from two different Treponema species suggests that they may play crucial roles in the biology of these organisms.

Characterization of the 35-kilodalton Treponema pallidum subsp. pallidum recombinant lipoprotein TmpC and antibody response to lipidated and nonlipidated T. pallidum antigens

The gene encoding the 35-kDa immunogenic Treponema pallidium subsp. pallidum (T. pallidum) membrane protein C, TmpC, was cloned, sequenced, and expressed in Escherichia coli. The deduced amino acid sequence carries on N-terminal signal sequence with a four-amino-acid motif, which is characteristic for bacterial lipoproteins. Metabolic labeling with radioactive palmitic acid of E. coli expressing TmpC revealed incorporation of the fatty acid into the antigen. The antigen was overproduced, purified to near homogeneity and used in an enzyme-linked immunosorbent assay (ELISA) to evaluate its potential for the serodiagnosis of syphilis. Although all sera from untreated secondary syphilis patients were reactive in this TmpC ELISA, only a minority of the serum samples from untreated patients in the primary or early latent stage of the disease contained significant anti-TmpC antibodies. To study the influence of the lipid moiety on the antigenic properties of the TmpC, TmpA, and TpD lipoproteins, plasmids encoding nonlipidated forms of these antigens were constructed. In addition, a plasmid expressing a lipidated form of the otherwise non-lipid-modified antigen TmpB was constructed. Immunization and absorption experiments with these lipidated and nonlipidated antigens showed that antibodies against the lipid moiety of lipoproteins could not be detected on immunoblots, neither in sera from infected rabbits nor in sera from animals immunized with the lipoproteins. In addition, we were unable to demonstrate cross-reactivity between antibodies against the T. pallidum lipoproteins and those reactive to the Venereal Diseases Research Laboratories test, suggesting that antibodies reactive to the Venereal Diseases Research Laboratories test are unrelated to antilipoprotein antibodies.

Characterization of a Legionella pneumophila gene encoding a lipoprotein antigen

A prominent 19 kDa surface antigen of Legionella pneumophila, cloned in Escherichia coli, was found to be intimately associated with peptidoglycan. The DNA region encoding this antigen was mapped on an 11.9 kb plasmid by means of deletion analysis and transposon mutagenesis. PhoA+ gene fusions, gene-rated by TnphoA insertions into this region, confirmed the presence of a gene encoding a secreted protein. PhoA+ transposon insertions were also associated with loss of the 19 kDa antigen in immunoassays using a monoclonal antibody (mAb1E9) and the replacement of the 19 kDa antigen with larger fusion proteins in immunoblots using Legionella immune serum. A 1540bp PstI fragment carrying the gene was sequenced, and the open reading frame encoding the antigen was identified. The gene encodes a polypeptide 176 amino acid residues long and 18913Da in size. The presence of a signal sequence of 22 amino acids with a consensus sequence for cleavage by signal peptidase II indicates that the antigen is a lipoprotein, and striking similarity with peptidoglycan-associated lipoproteins (PALs) from E. coli (51% amino acid homology) and Haemophilus influenzae (55% homology) is noted. We conclude that the 19kDa antigen of L. pneumophila is the structural equivalent of the PAL found in other Gram-negative species and suggest that its post-translational acylation may explain its potency as an immunogen.

Molecular cloning and sequence analysis of antigen gene tdpA of Treponema denticola

We isolated and characterized an immunogenic protein of an oral spirochete, Treponema denticola Johnson. A genomic DNA library constructed with bacteriophage lambda EMBL3 as a vector was immunologically screened with a rabbit antiserum against the whole cells. Using Western immunoblot analysis, we found a particular clone encoding an antigen with a molecular weight of 53,000; we designated the antigen as T. denticola protein A (TdpA). Complete sequence determination revealed an open reading frame of 1,419 bp and a signal peptide sequence that was homologous to that of bacterial lipoprotein. Southern hybridization analysis revealed that the tdpA gene is highly conserved in six tested strains of T. denticola species. Furthermore, we found that sera from some periodontitis patients contained antibody against the TdpA protein, although the reactivities of the antibodies varied among individuals.

Molecular cloning and characterization of a 35.5-kilodalton lipoprotein of Treponema pallidum

A clone expressing a 35.5-kDa recombinant treponemal protein was isolated from a genomic DNA library constructed from Treponema pallidum street strain 14. Polyclonal antiserum raised against the recombinant protein reacted with a corresponding native protein of comparable size in T. pallidum that is specific to the pathogenic treponemes. Radiolabeling of the recombinant protein with [3H]palmitate demonstrated that it is lipid modified. Like other recently characterized T. pallidum lipoproteins, the 35.5-kDa lipoprotein partitioned into the detergent phase from T. pallidum cells fractionated with Triton X-114, suggesting that it is an integral membrane protein. Processing of the recombinant 35.5-kDa lipoprotein from a precursor form to a smaller mature form was not evident in pulse-chase experiments. However, pretreatment of Escherichia coli cells expressing the 35.5-kDa lipoprotein with inhibitors of protein processing or translocation revealed the existence of a higher-molecular-mass precursor. Gene fusion studies with the transposon TnphoA demonstrated the presence of an export signal in the 35.5-kDa lipoprotein that promotes the extracytoplasmic localization of a 35.5-kDa lipoprotein-PhoA hybrid.

Lipid modification of the 15 kiloDalton major membrane immunogen of Treponema pallidum

The 15 kiloDalton major membrane immunogen was included among the Treponema pallidum polypeptides selectively labelled with [3H]-palmitate. The cloned gene for this immunogen, tpp15, encoded a signal peptide of 17 amino acids, a consensus signal peptidase II cleavage site, and a mature protein of 124 amino acids (13,967 Daltons). As predicted by the DNA sequence, the recombinant 15 kiloDalton immunogen labelled selectively with [3H]-palmitate, and globomycin inhibited processing of the precursor to the mature polypeptide. While the native and recombinant immunogens are amphiphilic, the 15 kiloDalton immunogen synthesized in a cell-free system was hydrophilic. The covalent attachment of fatty acids appears to be responsible for the amphiphilicity of the immunogen and its membrane attachment.

Identification and mapping of an immunogenic region of Mycoplasma hyopneumoniae p65 surface lipoprotein expressed in Escherichia coli from a cloned genomic fragment

A previously characterized lipid-modified amphiphilic surface protein of Mycoplasma hyopneumoniae, p65, has been defined by its reaction with a surface-binding monoclonal antibody (MAb) and by its exclusive partitioning into the detergent phase during Triton X-114 phase fractionation (K. S. Wise and M. F. Kim, J. Bacteriol. 169:5546-5555, 1987). In the current study, polyclonal mouse antibody (PAb) to gel-purified p65 was used to identify recombinant phage plaques expressing p65-related epitopes. Several characteristic partial tryptic fragments of p65 were recognized by both PAb and p65 and MAb to p65, but the PAb population specifically eluted from recombinant phage plaques bound only epitopes restricted to the largest of these fragments. Graded carboxypeptidase-Y digestion of intact M. hyopneumoniae generated C terminally truncated peptides that were recognized by PAb to p65 and MAb to p65, indicating that the C terminus and much of the adjoining region of p65 were present and accessible on the external face of the membrane. However, antibody eluted from recombinant phage plaques bound only to the largest truncated polypeptide, suggesting that a recombinant product corresponding to the C-terminal region of p65 was expressed in Escherichia coli. A 19-kilodalton recombinant protein (p19), which was recognized by PAb to p65 but not by MAb to p65, was detected in recombinant phage lysates. Serum antibodies from swine taken after, but not before, experimentally induced M. hyopneumoniae pneumonia preferentially recognized the native, amphiphilic p65 lipoprotein and also bound specifically to the p19 recombinant product. This confirmed that the p65 lipoprotein is a major immunogen of M. hyopneumoniae recognized during disease and identified its C-terminal region as an immunogenic domain.