Two-dimensional electrophoretic analysis of the protein family at 90 kDa of abortifacient Chlamydia psittaci

Analysis of Polymorphic Membrane Protein Expression in Cultured Cells Identifies PmpA and PmpH of Chlamydia psittaci as Candidate Factors in Pathogenesis and Immunity to Infection

The polymorphic membrane protein (Pmp) paralogous families of Chlamydia trachomatis, Chlamydia pneumoniae and Chlamydia abortus are putative targets for Chlamydia vaccine development. To determine whether this is also the case for Pmp family members of C. psittaci, we analyzed transcription levels, protein production and localization of several Pmps of C. psittaci. Pmp expression profiles were characterized using quantitative real-time PCR (RT-qPCR), immunofluorescence (IF) and immuno-electron microscopy (IEM) under normal and stress conditions. We found that PmpA was highly produced in all inclusions as early as 12 hpi in all biological replicates. In addition, PmpA and PmpH appeared to be unusually accessible to antibody as determined by both immunofluorescence and immuno-electron microscopy. Our results suggest an important role for these Pmps in the pathogenesis of C. psittaci, and make them promising candidates in vaccine development.

Identification of immunologically relevant proteins of Chlamydophila abortus using sera from experimentally infected pregnant ewes

Chlamydophila abortus is an intracellular pathogen and the etiological agent of enzootic abortion of ewes (EAE). C. abortus has a biphasic development cycle; extracellular infectious elementary bodies (EB) attach and penetrate host cells, where they give rise to intracellular, metabolically active reticulate bodies (RB). RB divide by binary fission and subsequently mature to EB, which, on rupture of infected cells, are released to infect new host cells. Pregnant ewes were challenged with 2 x 10(6) inclusion forming units (IFU) of C. abortus cultured in yolk sac (comprising both EB and RB). Serum samples were collected at 0, 7, 14, 21, 27, 30, 35, 40, and 43 days postinfection (dpi) and used to identify antigens of C. abortus expressed during disease. Additionally, sera from fetal lambs were collected at 30, 35, 40, and 43 dpi. All serum samples collected from experimentally infected pregnant ewes reacted specifically with several antigens of EB as determined by one-dimensional (1-D) and 2-D gel electrophoresis; reactive antigens identified by mass spectrometry included the major outer membrane protein (MOMP), polymorphic outer membrane protein (POMP), and macrophage infectivity potentiator (MIP) lipoprotein.

Transcriptional analysis of in vitro expression patterns of Chlamydophila abortus polymorphic outer membrane proteins during the chlamydial developmental cycle

Chlamydophila abortus is the aetiological agent of ovine enzootic abortion. Sequencing, annotation and comparative analysis of the genome of C. abortus strain S26/3 has revealed variation in the loci encoding the polymorphic membrane proteins (Pmps). These Pmps resemble autotransporter proteins of the type V secretion system, suggesting an important role in chlamydial pathogenesis. The purpose of this study was to characterise the transcriptional expression patterns of this family during the developmental cycle of C. abortus. McCoy cells were infected with C. abortus and analysed for pmp mRNA expression over a 72 h period. Few pmp transcripts were detected in the early stages of the developmental cycle. Peak expression occurred at 48 h post-infection (p.i.) other than for pmp5E, where it was observed at 24 h p.i. Overall, expression of pmps 5E, 18D and 10G were found to be 40 to 100-fold higher than the lowest expressing pmps (6H, 1 3G and 15G) at 24 h p.i., while pmps 18D and 17G were 14 to 16-fold higher than the lowest (11G, 14G and 15G) at 48 h. Levels of expression for all the other pmp genes were below one copy per genome at any time point. The expression of all the pmps reduced to near base-line levels by 60 h p.i. These results demonstrate that pmp expression in C. abortus is mid to late cycle, consistent with conversion of the reticulate body to the elementary body. The low level of pmp transcription may be indicative of heterogeneity in expression, suggesting a possible role for some of the Pmps in antigenic variation and chlamydial pathogenesis.

Antigenic organization of the N-terminal part of the polymorphic outer membrane proteins 90, 91A, and 91B of Chlamydophila abortus

A series of overlapping recombinant antigens, 61 to 74 residues in length, representing polymorphic outer membrane protein 90 (POMP90) of Chlamydophila abortus and two recombinant peptides spanning gene fragment p91Bf99 of POMP91B were assessed by immunoblotting to determine the antigen-binding sites of 20 monoclonal antibodies to POMP90, -91A, and -91B. The epitopes were further restricted by scanning 52 overlapping synthetic 12-mer peptides representing the N-terminal part of POMP90, and the 12-mer epitopes were then analyzed by using hexapeptides to the resolution of a single amino acid. Ten epitopes were defined: 1, TSEEFQVKETSSGT; 2, SGAIYTCEGNVCISYAGKDSPL; 3, SLVFHKNCSTAE; 4, AIYADKLTIVSGGPTLFS; 5, SPKGGAISIKDS; 6, ITFDGNKIIKTS; 7, LRAKDGFGIFFY; 7a, DGFGIF; 7b, GIFFYD; 8, IFFYDPITGGGS; 8a, FFYDPIT; 9, GKIVFSGE; and 10, DLGTTL. The 20-mer peptide LRAKDGFGIFFYDPITGGGS was a major epitope that was recognized by seven antibodies. Epitopes 7 to 10 were conserved in reference strains of the former species C. psittaci, whereas the strong antigenic peptides FYDPIT and IVFSGE were conserved among members of the genus CHLAMYDOPHILA: Epitopes 3 to 8 were located within the best-scoring beta-helical wrap (residues 148 to 293) predicted for POMP91B by the program BETAWRAP. Other studies have suggested an association of the POMPs with type V secretory autotransporter proteins. The results presented in this study provide some evidence for a passenger domain that is folded as a beta-helix pyramid with compact antigenic organization.

Polymorphic membrane protein H has evolved in parallel with the three disease-causing groups of Chlamydia trachomatis

Chlamydia trachomatis is a human pathogen causing trachoma, urogenital disease, and lymphogranuloma venereum (LGV). A family of nine polymorphic membrane protein genes (pmpA to pmpI), resembling autotransporter proteins, has recently been discovered in C. trachomatis. pmp genes are large and predicted to be outer membrane proteins. We hypothesized that they would contain useful nucleotide sequence variability for epidemiologic studies. Since sequence information is available only for serovars D and L2, we sought to determine the amount of diversity within an individual pmp gene among serovars. We used restriction fragment length polymorphism (RFLP) analysis as a primary screen to assess the amount of sequence divergence among the pmp genes for serovars A to L3 of C. trachomatis. RFLP analysis showed little variation for some of the genes, such as pmpA, but substantial variation in others, such as pmpI. pmpH and pmpE yielded RFLP patterns that clustered the 15 serovars into ocular, urogenital, and LGV groups, and both proteins have been localized to the outer membrane. Therefore, we chose to sequence pmpE, pmpH, and pmpI from each of the 15 serovars. Evolutionary analysis showed three distinct divergence patterns. PmpI was least variable, resulting in an ambiguous evolutionary pattern. PmpE showed a high degree of diversity in the ocular strains compared to the other strains. Finally, the evolution of PmpH shows three groups that reflect disease groups, suggesting this protein may play a role in pathogenesis.

The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029

BACKGROUND

Chlamydiae are obligate intracellular bacteria, which are important human pathogens. Genome sequences of C. trachomatis and C. pneumoniae have revealed the presence of a Chlamydia specific gene family encoding polymorphic outer membrane proteins, Pmps. In C. pneumoniae the family comprises twenty-one members, which are all transcribed. In the present study, the expression, processing and localisation of the sixteen full-length Pmps in C. pneumoniae strain CWL029 have been further investigated by two-dimensional gel electrophoresis and immunofluorescence microscopy.

RESULTS

Ten Pmps were identified in elementary bodies (EBs). Eight of these were investigated with respect to time dependent expression and all were found to be up-regulated between 36 and 48 hours post infection. Antibodies against Pmp6, 8, 10, 11 and 21 reacted with chlamydiae when infected cells were formalin fixed. Pmp6, Pmp20 and Pmp21 were found in cleaved forms, and the cleavage sites of Pmp6 and Pmp21 were identified.

CONCLUSIONS

The Pmps are heavily up-regulated at the time of conversion of RB to EB, and at least ten Pmps are present in EBs. Due to their reaction in formalin fixation it is likely that Pmp6, 8, 10, 11 and 21 are surface exposed. The identified cleavage sites of Pmp6 and Pmp21 are in agreement with the theory that the Pmps are autotransporters.

Polymorphic outer-membrane proteins of Chlamydophila abortus are glycosylated

Antigenic profiles of mono-, bi- and poly-specific monoclonal antibodies against 90 kDa polymorphic outer-membrane proteins (POMPs) and a 105 kDa POMP-related protein of Chlamydophila abortus ATCC VR 656(T), after one- and two-dimensional electrophoretic analysis, helped identify each one of the triplets POMP 90, 91A and 91B, and a POMP-related protein at 85 kDa. The lectin concanavalin A bound to the four POMPs and the POMP-related protein in a specific manner and the binding was sensitive to treatment with the amidase N-endoglycosidase F, suggesting the presence of small asparagine-linked oligosaccharide chains. The exposure of the five proteins on the chlamydial surface and the orientation of the attached oligosaccharide chains was examined by protease and endoglycosidase treatments of intact bacteria. The results were consistent with the concept that some of the oligosaccharides in the POMPs face outwards, possibly protecting the polypeptides from proteolytic enzymes, whereas the oligosaccharides in the 105 kDa POMP-related protein are oriented inwards, thereby rendering the polypeptide chain accessible to proteases. A possible role for the N-linked oligosaccharides in the POMPs might be the promotion of the proper folding and processing of these proteins.

Polymorphic proteins of Chlamydia spp.--autotransporters beyond the Proteobacteria

Gram-negative bacteria secrete a variety of proteins to the cell surface and beyond, a process with many inherent difficulties. An exceptionally widespread answer to these problems is the type V (or autotransporter) secretion pathway. By exploiting the data made available by bacterial genome sequencing, we have discovered that the previously described polymorphic proteins of Chlamydia spp. resemble members of the autotransporter family, and we suggest that they follow the same secretion pathway.

Identification of polymorphic outer membrane proteins of Chlamydia psittaci 6BC

The genomes of Chlamydia spp. encode a family of putative outer membrane proteins, referred to as polymorphic outer membrane proteins (POMPs), which may play a role in the avoidance of host immune defenses. We analyzed avian strain 6BC of Chlamydia psittaci by polyacrylamide gel electrophoresis for the expression of POMPs. At least six putative POMPs were identified on the basis of their size (90 to 110 kDa) and labeling with an outer membrane-specific probe, 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine. Three of the putative POMPs reacted with antiserum raised against a recombinant ovine C. psittaci strain POMP, and two possessed surface-exposed, trypsin-sensitive sites. The POMPs were dependent on disulfide bonds for their maintenance in sodium lauryl sarcosine- and sodium dodecyl sulfate-insoluble complexes but did not appear to be interpeptide disulfide bond cross-linked. The putative POMPs were found to be synthesized during the late phase of the chlamydial developmental cycle, cotemporally with the cysteine-rich doublet periplasmic proteins.

Characterization of outer membrane proteins in Chlamydia trachomatis LGV serovar L2

We used a photoactivatable, lipophilic reagent, 3'-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine, to label proteins in the outer membrane of elementary bodies of Chlamydia trachomatis LGV serovar L2 and mass spectrometry to identify the labeled proteins. The identified proteins were polymorphic outer membrane proteins E, G, and H, which were made late in the developmental cycle, the major outer membrane protein, and a mixture of 46-kDa proteins consisting of the open reading frame 623 protein and possibly a modified form of the major outer membrane protein.

Potential relevance of Chlamydia pneumoniae surface proteins to an effective vaccine

The surface of Chlamydia pneumoniae is covered with proteins but their exact identification is not known probably because of the presence of conformational epitopes. A family of 21 pmp genes has been found by DNA sequencing. In common, these genes have the capacity to encode the amino acid motif GGAI. Several of the genes have the capacity to encode outer membrane proteins of about 100 kDa. Thus, they are candidate genes to encode the protein(s) present in the 98-kDa protein band of the C. pneumoniae outer membrane complex. The production of recombinant GGAI proteins is described as is the use of polyclonal antibodies raised against the recombinant GGAI proteins to determine their expression in C. pneumoniae elementary bodies. At least three of the proteins, Omp4, 5, and 11, are expressed.

Molecular characterization of a bacteriophage (Chp2) from Chlamydia psittaci

Comparisons of the proteome of abortifacient Chlamydia psittaci isolates from sheep by two-dimensional gel electrophoresis identified a novel abundant protein with a molecular mass of 61.4 kDa and an isoelectric point of 6.41. C-terminal sequence analysis of this protein yielded a short peptide sequence that had an identical match to the viral coat protein (VP1) of the avian chlamydiaphage Chp1. Electron microscope studies revealed the presence of a 25-nm-diameter bacteriophage (Chp2) with no apparent spike structures. Thin sections of chlamydia-infected cells showed that Chp2 particles were located to membranous structures surrounding reticulate bodies (RBs), suggesting that Chp2 is cytopathic for ovine C. psittaci RBs. Chp2 double-stranded circular replicative-form DNA was purified and used as a template for DNA sequence analysis. The Chp2 genome is 4,567 bp and encodes up to eight open reading frames (ORFs); it is similar in overall organization to the Chp1 genome. Seven of the ORFs (1 to 5, 7, and 8) have sequence homologies with Chp1. However, ORF 6 has a different spatial location and no cognate partner within the Chp1 genome. Chlamydiaphages have three viral structural proteins, VP1, VP2, and VP3, encoded by ORFs 1 to 3, respectively. Amino acid residues in the phiX174 procapsid known to mediate interactions between the viral coat protein and internal scaffolding proteins are conserved in the Chp2 VP1 and VP3 proteins. We suggest that VP3 performs a scaffolding-like function but has evolved into a structural protein.

Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae

Whole sequence genome analysis is invaluable in providing complete profiles of related proteins and gene families. The genome sequences of the obligate intracellular bacteria Chlamydia trachomatis and Chlamydia pneumoniae both encode proteins with similarity to several 90-kDa Chlamydia psittaci proteins. These proteins are members of a large superfamily, C. trachomatis with 9 members and C. pneumoniae with 21 members. All polymorphic membrane protein (Pmp) are heterogeneous, both in amino acid sequence and in predicted size. Most proteins have apparent signal peptide leader sequences and hence are predicted to be localized to the outer membrane. The unifying features of all proteins are the conserved amino acid motifs GGAI and FXXN repeated in the N-terminal half of each protein. In both genomes, the pmp genes are clustered at various locations on the chromosome. Phylogenetic analysis suggests six related families, each with at least one C. trachomatis and one C. pneumoniae orthologue. One of these families has seen prolific expansion in C. pneumoniae, resulting in 13 protein paralogues. The maintenance of orthologues from each species suggests specific functions for the proteins in chlamydial biology.

Immunoelectron microscopic localisation of the OMP90 family on the outer membrane surface of Chlamydia psittaci

The putative outer membrane location of the OMP90 (formerly POMP) family from the ovine abortion strain of Chlamydia psittaci was investigated by immunoelectron microscopy. Using a non-embedding technique, antigens were shown to be localised on the outer membrane surface of both elementary and reticulate bodies, the infectious and non-infectious forms of Chlamydiae respectively. Antibodies affinity-purified against the expressed amino- and carboxy-terminal halves of one of the family members. OMP90A, demonstrated that the amino half is surface-exposed while the carboxyl half is most probably localised internally. Surface localisation on elementary bodies indicates the importance of these proteins as protective antigen candidates.