Acylation of the 47-kilodalton major membrane immunogen of Treponema pallidum determines its hydrophobicity

Evidence that immunization with TP0751, a bipartite Treponema pallidum lipoprotein with an intrinsically disordered region and lipocalin fold, fails to protect in the rabbit model of experimental syphilis

Deconvolution of syphilis pathogenesis and selection of candidate syphilis vaccinogens requires detailed knowledge of the molecular architecture of the Treponema pallidum outer membrane (OM). The T. pallidum OM contains a low density of integral OM proteins, while the spirochete's many lipoprotein immunogens are periplasmic. TP0751, a lipoprotein with a lipocalin fold, is reportedly a surface-exposed protease/adhesin and protective antigen. The rapid expansion of calycin/lipocalin structures in the RCSB PDB database prompted a comprehensive reassessment of TP0751. Small angle X-ray scattering analysis of full-length protein revealed a bipartite topology consisting of an N-terminal, intrinsically disordered region (IDR) and the previously characterized C-terminal lipocalin domain. A DALI server query using the lipocalin domain yielded 97 hits, 52 belonging to the calycin superfamily, including 15 bacterial lipocalins, but no Gram-negative surface proteins. Surprisingly, Tpp17 (TP0435) was identified as a structural ortholog of TP0751. In silico docking predicted that TP0751 can bind diverse ligands along the rim of its eight-stranded β-barrel; high affinity binding of one predicted ligand, heme, to the lipocalin domain was demonstrated. qRT-PCR and immunoblotting revealed very low expression of TP0751 compared to other T. pallidum lipoproteins. Immunoblot analysis of immune rabbit serum failed to detect TP0751 antibodies, while only one of five patients with secondary syphilis mounted a discernible TP0751-specific antibody response. In opsonophagocytosis assays, neither TP0751 nor Tpp17 antibodies promoted uptake of T. pallidum by rabbit peritoneal macrophages. Rabbits immunized with intact, full-length TP0751 showed no protection against local or disseminated infection following intradermal challenge with T. pallidum. Our data argue that, like other lipoprotein lipocalins in dual-membrane bacteria, TP0751 is periplasmic and binds small molecules, and we propose that its IDR facilitates ligand binding by and offloading from the lipocalin domain. The inability of TP0751 to elicit opsonic or protective antibodies is consistent with a subsurface location.

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.

Construction and evaluation of a plasmid vector for the expression of recombinant lipoproteins in Escherichia coli

Outer membrane lipoproteins are emerging as key targets for protective immunity to many bacterial pathogens. Heterologous expression of lipoproteins in Escherichia coli does not always result in high level expression of acylated recombinant protein. Thus, these proteins do not take up their correct membrane topology and are lacking the immunostimulatory properties endowed by the lipid. To this end, we have designed a lipoprotein expression vector (pDUMP) that results in the production of fusion proteins containing the E. coli major outer membrane lipoprotein (Lpp) signal sequence, lipoprotein signal peptidase recognition site, and the +2 outer membrane sorting signal at their N termini. To test the ability of pDUMP to express lipoproteins from heterologous hosts, the surface lipoprotein PsaA from the Gram-positive organism Streptococcus pneumoniae and the outer membrane lipoproteins MlpA from the Gram-negative Pasteurella multocida and BlpA from the spirochete Brachyspira hyodysenteriae were cloned into both hexahistidine fusion vectors and pDUMP. High level expression of antigenically active protein from both the hexahistidine fusion vectors and pDUMP resulted in abundant bands of the predicted molecular masses when analyzed by SDS-PAGE. When grown in the presence of 3[H]palmitic acid, proteins encoded by pDUMP were observed to incorporate palmitic acid whilst the hexahistidine fusion proteins did not. Using mass spectrometry and image analysis we determined the efficiency of lipidation between the three clones to vary from 31.7 to 100%. In addition, lipidated, but not hexahistidine, forms of the proteins were presented on the E. coli surface.

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.

Molecular cloning and sequence analysis of the gene encoding LipL41, a surface-exposed lipoprotein of pathogenic Leptospira species

We report the cloning of the gene encoding a surface-exposed leptospiral lipoprotein, designated LipL41. In a previous study, a 41-kDa protein antigen was identified on the surface of Leptospira kirschneri (D. A. Haake, E. M. Walker, D. R. Blanco, C. A. Bolin, J. N. Miller, and M. A. Lovett, Infect. Immun. 59:1131-1140, 1991). We obtained the N-terminal amino acid sequence of a staphylococcal V8 proteolytic-digest fragment in order to design an oligonucleotide probe.A Lambda ZAP II library containing EcoRI fragments of L. kirschneri DNA was screened, and a 2.3-kb DNA fragment which contained the entire structural lipL41 gene was identified. The deduced amino acid sequence of LipL41 would encode a 355-amino-acid polypeptide with a 19-amino-acid signal peptide, followed by an L-X-Y-C lipoprotein signal peptidase cleavage site. A recombinant His6-LipL41 fusion protein was expressed in Escherichia coli in order to generate specific rabbit antiserum. LipL41 is solubilized by Triton X-114 extraction of L. kirschneri; phase separation results in partitioning of LipL41 exclusively into the detergent phase. At least eight proteins, including LipL41 and the other major Triton X-114 detergent phase proteins, are intrinsically labeled during incubation of L. kirschneri in media containing [3H] palmitate. Processing of LipL41 is inhibited by globomycin, a selective inhibitor of lipoprotein signal peptidase. Triton X-100 extracts of L. kirschneri contain immunoprecipitable OmpL1 (porin), LipL41, and another lipoprotein, LipL36. However, in contrast to LipL36, only LipL41 and OmpL1 were exposed on the surface of intact organisms. Immunoblot analysis of a panel of Leptospira species reveals that LipL41 expression is highly conserved among leptospiral pathogens.

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.

Isolation of the outer membranes from Treponema pallidum and Treponema vincentii

The outer membranes from Treponema pallidum subsp. pallidum and Treponema vincentii were isolated by a novel method. Purified outer membranes from T. pallidum and T. vincentii following sucrose gradient centrifugation banded at 7 and 31% (wt/wt) sucrose, respectively. Freeze fracture electron microscopy of purified membrane vesicles from T. pallidum and T. vincentii revealed an extremely low density of protein particles; the particle density of T. pallidum was approximately six times less than that of T. vincentii. The great majority of T. vincentii lipopolysaccharide was found in the outer membrane preparation. The T. vincentii outer membrane also contained proteins of 55 and 65 kDa. 125I-penicillin V labeling demonstrated that t. pallidum penicillin-binding proteins were found exclusively with the protoplasmic cylinders and were not detectable with purified outer membrane material, indicating the absence of inner membrane contamination. Isolated T. pallidum outer membrane was devoid of the 19-kDa 4D protein and the normally abundant 47-kDa lipoprotein known to be associated with the cytoplasmic membrane; only trace amounts of the periplasmic endoflagella were detected. Proteins associated with the T. pallidum outer membrane were identified by one- and two-dimensional electrophoretic analysis using gold staining and immunoblotting. Small amounts of strongly antigenic 17- and 45-kDa proteins were detected and shown to correspond to previously identified lipoproteins which are found principally with the cytoplasmic membrane. Less antigenic proteins of 65, 31 (acidic pI), 31 (basic pI), and 28 kDa were identified. Compared with whole-organism preparations, the 65- and the more basic 31-kDa proteins were found to be highly enriched in the outer membrane preparation, indicating that they may represent the T. pallidum rare outer membrane proteins. Reconstitution of solubilized T. pallidum outer membrane into lipid bilayer membranes revealed porin activity with two estimated channel diameters of 0.35 and 0.68 nm based on the measured single-channel conductances in 1 M KCl of 0.40 and 0.76 nS, respectively.

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.

Expression and purification of the mouse mammary tumor virus gag-pro transframe protein p30 and characterization of its dUTPase activity

The mouse mammary tumor virus gag-pro transframe protein (p30) contains the nucleocapsid protein domain derived from the 3' end of gag, fused to 154 residues encoded by the 5' region of the pro open reading frame. The DNA coding for p30 was cloned into the plasmid pALTER-1, and an additional nucleotide was inserted by site-directed mutagenesis to allow the read-through from the gag into the pro open reading frame. The obtained insert was then cloned into pGEX-2T, a plasmid containing the glutathione S-transferase gene of Schistosoma japonicum and a nucleotide sequence encoding for a thrombin cleavage site. The chimeric protein (GST-p30) was isolated by affinity chromatography on a glutathione-Sepharose 4B column, and after thrombin treatment, the excised p30 was further purified on a single-stranded DNA-agarose column. This protein showed dUTPase activity, with only negligible cleavage of dATP, dGTP, dCTP, dTTP, or UTP. Its apparent Km for dUTP was 28 microM. The enzyme was inhibited by EDTA, but its effect could be reversed by Mg2+ and other divalent cations. dUTPase activity was also detected in purified mouse mammary tumor virus, and p30 was the only protein recognized by antibodies directed towards the carboxyl-terminal sequence of the dUTPase coding region.

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.

Delayed hypersensitivity, immune deviation, antigen processing and T-cell subset selection in syphilis pathogenesis and vaccine design

T-cell mediated delayed-type hypersensitivity (DTH) is the predominant immune mechanism for clearing tissues of infecting organisms in the primary lesion of syphilis. Here, Stewart Sell and Pei-Ling Hsu propose a strategy for vaccination against syphilis in which selective induction of DTH may be accomplished by using BCG vectored vaccines containing selected DNA sequences for T. pallidum antigens.

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.

Genome structure of spirochetes

The genome structures of several pathogenic spirochetes have recently been determined. The genomes of Borrelia species consist of a linear chromosome of approximately one million base pairs (Mb) and various linear and circular plasmids. Analysis of restriction fragment length polymorphisms and 16S ribosomal RNA sequence data indicate the division of Borrelia burgdorferi into at least three distinct genetic groups. Leptospira interrogans has a circular chromosome 5 Mb in size and a 0.35 Mb extrachromosomal element. Repetitive sequence elements similar to insertion sequences have been identified in the Leptospira interrogans genome. The chromosome of Treponema pallidum subsp. pallidum is circular and has a size of approximately one Mb. Genetic studies conducted to date indicate that B. burgdorferi and L. interrogans have a high degree of genetic diversity, whereas remarkably few genetic differences have been observed among the pathogenic Treponema. Knowledge of the genomic structure of these organisms will serve as a basis for future genetic studies.

Purification of immature cores of mouse mammary tumor virus and immunolocalization of protein domains

The immature capsids of the mouse mammary tumor virus (MMTV), known as intracytoplasmic A particles, have been isolated from murine L1210 leukemia cells. The diameter of the isolated particles was 80 nm as determined by negative staining. Two polypeptides of 77 and 110 kDa were found to be their major polypeptide components, in agreement with the expected sizes of the Gag and Gag-Pro precursor polypeptides of the mature MMTV proteins. Both polypeptides were recognized by antibodies directed toward the matrix (p10) and capsid (p27) proteins of MMTV. Immunogold labeling of p10 on isolated A particles, visualized by negative staining, showed that this protein is located at the surface of the immature capsids, whereas p27 can be detected only in broken or disrupted particles, suggesting that it has an internal location. These observations were confirmed by immunolabeling of both proteins on thin sections of A particle-producing cells. In addition, the viral protease had a more internal position than p27. Since the sequential order of the viral proteins in the Gag precursor is p10-pp21-p27-p14 and that in Gag-Pro is p10-pp21-p27-p30-protease, our results demonstrate the radial organization of the polypeptide precursors forming the intracytoplasmic A particles.

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.

Analysis of the N-terminal region of the 47-kilodalton integral membrane lipoprotein of Treponema pallidum

The 47-kDa lipoprotein is an abundant integral membrane protein and dominant immunogen of Treponema pallidum subsp. pallidum. Previous DNA sequencing of the 47-kDa-lipoprotein gene did not reveal consensus features representative of other bacterial lipoprotein genes; this prompted further analyses with emphasis on elucidation of the N terminus of the molecule. To assist in localizing start signals for the protein, the transcription initiation site for the 47-kDa-antigen gene was determined. RNA isolated from both T. pallidum and recombinant Escherichia coli expressing the 47-kDa antigen was used as a template in reverse transcriptase primer extension. Upon analysis of cDNA products, transcription initiation was localized to one nucleotide in T. pallidum and to two adjacent nucleotides in E. coli. When various primers were used in DNA sequencing reactions for these analyses, a previously undetected nucleotide (G) was found in the purported 5' untranslated region; this altered the upstream reading frame to reveal plausible sites for ribosome binding (GGAGG), translation initiation (GTG start codon), and signal peptidase II processing (Val-Val-Gly-Cys). Discounting acylation, the molecular weight of the mature polypeptide is 45,756 (approximately 46,600 with acylation). To derive nonacylated 47-kDa antigen for further structure-function studies, the 47-kDa-antigen gene was subcloned without acylation signals as a genetic construct encoding a glutathione S-transferase fusion protein; following cleavage with thrombin, the nonacylated 47-kDa protein was hydrophilic rather than amphiphilic but retained its antigenicity when tested against 116 human serum samples from patients with various stages of syphilis.

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.

Chemical and biological activities of a 64-kilodalton outer sheath protein from Treponema denticola strains

This study examined the distribution of the major outer sheath proteins (MOSP) in several Treponema denticola strains and reports the isolation of a 64-kDa protein from the outer sheath of human clinical isolate T. denticola GM-1. The outer sheath was isolated by freeze-thaw procedures, and the distribution of outer sheath proteins was examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). T. denticola GM-1, MS25, SR-5, and three low-passage clinical isolates possessed an MOSP with a relative molecular mass of 60 to 64 kDa. This MOSP was absent in T. denticola ATCC 35404 (TD-4) and clinical isolate SR-4. The latter possessed an MOSP of 58 kDa. 125I labeling revealed both MOSP to be dissociated forms of higher-molecular-mass oligomeric units between 116 and 162 kDa. Two-dimensional SDS-PAGE confirmed the modifiability of these MOSP. Isoelectric focusing of the 64-kDa MOSP indicated a pI of 6.7. Immunoblots with antiserum to GM-1 whole cells revealed the 64-kDa protein to be immunogenic and not cross-reactive with the MOSP of TD-4 or SR-4, and monospecific antibody to the 64-kDa protein recognized common epitopes on the high-molecular-weight oligomeric protein. These antibodies did not react with any component of TD-4 whole cells in immunoblots or in immunogold electron microscopy. Fab fragments inhibited the adherence of T. denticola GM-1 to human gingival fibroblasts by 78% (1:1,600; 0.72 micrograms of protein per ml), while TD-4 adherence was not inhibited. Amino acid analysis revealed a slightly acidic protein, devoid of cysteine, with 36% hydrophobic residues. Cyanogen bromide fragmentation of the 64-kDa protein revealed that a 42-kDa fragment contained a T-L-D-L-A-L-D segment which was 100% homologous with an integrin alpha subunit of a human leukocyte adhesion glycoprotein p 150,95.

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.

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.

Immunogenic integral membrane proteins of Borrelia burgdorferi are lipoproteins

The pathogenic spirochete Borrelia burgdorferi contains a set of integral membrane proteins which were selectively extracted into the detergent phase upon solubilization of intact B. burgdorferi with the nonionic detergent Triton X-114. Virtually all of these hydrophobic proteins were recognized by antibodies in pooled sera from patients with chronic Lyme arthritis, demonstrating that proteins partitioning into the detergent phase of Triton X-114 encompass the major B. burgdorferi immunogens. Furthermore, most of these immunogenic proteins, including the previously characterized OspA and OspB membrane antigens, could be biosynthetically labeled when B. burgdorferi was incubated in vitro with [3H]palmitate. The OspA and OspB antigens were radioimmunoprecipitated from [3H]palmitate-labeled detergent-phase proteins with monoclonal antibodies, and [3H]palmitate was recovered unaltered from these proteins after sequential alkaline and acid hydrolyses. The combined results provide formal confirmation that the major B. burgdorferi immunogens extracted by Triton X-114 are lipoproteins. The demonstration that B. burgdorferi integral membrane antigens are lipoproteins may explain the basis of their immunogenicity and may help to improve our understanding of the surface topology of B. burgdorferi.

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

Treponema pallidum subsp. pallidum and Escherichia coli incorporated exogenous [3H]palmitate into the 34-kilodalton (kDa) pathogen-specific antigen of T. pallidum. Radiolabeled fatty acid remained associated with the protein upon immunoprecipitation and after boiling in sodium dodecyl sulfate, acetone precipitation, and extensive extractions in organic solvents, suggesting that the fatty acid was covalently bound to the protein. Detection of [3H]palmitate after alkaline and acid hydrolyses confirmed the identity of the incorporated label. Globomycin inhibited maturation of the recombinant 34-kDa antigen, suggesting that E. coli uses the lipoprotein-specific signal peptidase II to process the treponemal antigen. Globomycin also inhibited processing of the 34-kDa antigen, as well as the 44.5- and 15-kDa antigens, in T. pallidum, implying that T. pallidum also possesses the lipoprotein export pathway common to both gram-negative and gram-positive bacteria. Ethanol inhibited processing of the 34-kDa antigen in minicells, suggesting that the 34-kDa antigen normally is translocated through the cytoplasmic membrane. Comparison of the Triton X-114 phase partitioning behavior of the 34-kDa antigen produced either by minicells or by a cell-free translation system indicated that the covalent attachment of fatty acid conferred hydrophobic biochemical properties to the 34-kDa antigen, consistent with the hypothesis that the attached lipid anchors the 34-kDa antigen into the membrane.

Molecular cloning and characterization of the 15-kilodalton major immunogen of Treponema pallidum

Pathogen-specific membrane immunogens of Treponema pallidum subsp. pallidum (T. pallidum) have been identified previously by phase partitioning with the nonionic detergent Triton X-114. One of these antigens, a 15-kilodalton (kDa) polypeptide, is expressed in relatively small quantities in T. pallidum but is highly immunogenic in both human and experimental syphilis. The native T. pallidum antigen was purified to homogeneity from the mixture of Triton X-114 detergent-phase proteins by chromatofocusing. Recombinant Escherichia coli clones were selected from a T. pallidum genomic DNA library by using monoclonal antibodies specific to the 15-kDa antigen; immunoblotting and minicell analyses confirmed expression of the 15-kDa protein in the transformants. Southern hybridization with a 1.1-kilobase fragment of DNA encoding the 15-kDa-antigen gene indicated that the gene is probably present in a single copy within the genomes of both T. pallidum and T. pallidum subsp. pertenue (the agent of yaws), while it is absent from the genome of the nonpathogenic Treponema phagedenis biotype Reiter. Cell fractionation studies with Triton X-114 demonstrated that the recombinant polypeptide possesses hydrophobic properties similar to those of the native antigen and localized the cloned 15-kDa antigen to the inner membrane of E. coli. Protein processing experiments in minicells revealed that a precursor appears to be processed to the mature 15-kDa polypeptide.

Major integral membrane protein immunogens of Treponema pallidum are proteolipids

A number of the major pathogen-specific immunogens of Treponema pallidum were characterized recently as amphiphilic, integral membrane proteins by phase partitioning with Triton X-114 (J. D. Radolf, N. R. Chamberlain, A. Clausell, and M. V. Norgard. Infect. Immun. 56:490-498, 1988). In the present study, we demonstrated that the same membrane immunogens (designated as detergent phase proteins [DPPs]) become radiolabeled upon in vitro incubation of T. pallidum with various 3H-labeled fatty acids. Radioimmunoprecipitation with a monoclonal antibody confirmed that the 3H-labeled 47-kilodalton protein corresponded to the well-characterized treponemal antigen with the identical apparent molecular mass. Failure to detect 3H-labeled DPPs following incubation with erythromycin confirmed that protein acylation required de novo protein synthesis by the bacteria. When treponemes were incubated with [3H]myristate, [3H]palmitate, or [3H]oleate, radiolabeled proteins corresponding to the DPPs were detected upon autoradiography. Demonstration that a number of the abundant membrane immunogens of T. pallidum are proteolipids provides information to help clarify their membrane association(s) and may serve to explain their extraordinary immunogenicity.