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Envelope proteins

Cysteine rich proteins (CRP) of the chlamydial outer membrane complex

Apart from MOMP, the major proteins in the chlamydial outer membrane complex (COMC) are the two cysteine rich proteins, OmcA and OmcB. The tandem organisation of these genes on the C. trachomatis [with the CT number in the Los Alamos STD database entry shown in brackets] is shown in Fig 1. 

Fig 1.

CT442 encodes a hypothetical 15 KDa cysteine rich outer membrane complex protein.

OmcB (or omp2, or EnvB; CT443) is the large 60 KDa cysteine rich protein (CRP), forming a prominent doublet on SDS-PAGE gels of chlamydial EBs or COMC, (probably as a result of post-translational processing) and easily recognisable on 2-dimensional proteomics gels [Sanchez-Campillo et al., 1999]. The large CRP is highly immunogenic [Sanchez-Campillo et al., 1999] carries genus specific epitopes [Watson et al., 1994] but is thought not to be surface exposed [Watson et al., 1994; Everett & Hatch; 1995]. Its sequence, particularly the position of the cysteines, is strongly conserved, presumably because it is functionally important [de la Maza et al., 1991; Watson et al., 1989]. This would explain why OmcB is much less likely to be involved in recombination events than MOMP  [Millman et al., 2001]  [See: recombination in MOMP].  However there is sequence variability  in the N-terminal fifth [Hsia et al., 1996]. The large CRP includes an MAxxxST amino acid motif [single letter code for amino acids, where x is any amino acid] that is associated elsewhere with autoimmune myocarditis directed against myosin. Not surprisingly injection of a  peptide encompassing this sequence can cause experimental myocarditis in laboratory rodents [Bachmaier et al., 1999].  [MEW Comment: This may be a fortuitous rather than real association, as myocarditis following systemic chlamydial infection is rare and there is little evidence that when it occurs it is autoimmune. C. pneumoniae infection, although systemic, is associated with coronary artery disease, not myocarditis. Views please to the discussion forum]. 

OmcA (or omp3, or EnvA; CT444), first identified and sequenced by Lambden et al., 1990, is a cysteine rich lipoprotein with a molecular weight of 9 KDa [Lay reader: 1 Dalton is the weight of a hydrogen atom]. Residues 1 - 88 of the C. trachomatis gene are between 55 - 59% homologous with the equivalent region of the corresponding C. psittaci and C. pneumoniae genes. In C. psittaci 6BC the gene encodes 87 amino acids, of which 15 are cysteine. There is a putative signal peptide for insertion in or across membrane together with a potential signal peptidase II-lipid modification site. It is thought the lipoprotein [Everett & Hatch, 1991].

The CRPs of the COMC are present in greatest amounts in the elementary body [Newhall, 1987; Sardinia et al., 1988]. Transcription and translation of the CRP genes must be developmentally regulated, the proteins being re-synthesized late in the growth cycle at the differentiation of reticulate bodies to elementary bodies. Transcription of the CRP genes results in a polycistronic mRNA. Analysis of the upstream sequences around the transcriptional start point for these mRNAs revealed the presence of three inverted repeat structures that might act as regulatory binding domains [Lambden et al., 1990].  In the C. psittaci 6BC elementary bodies COMC it is calculated there is about one large-CRP molecule to two small-CRP molecules to five MOMP molecules [Everett & Hatch, 1991].

A model for the structure of the COMC of C. psittaci 6BC was proposed by Everett & Hatch 1995 on the basis of experiment. They proposed that OmcA is anchored to the outer membrane by its lipid moiety, with a hydrophilic peptide portion extending into the periplasm. It was suggested OmcB is located exclusively within the periplasm and is not surface exposed. It was also suggested that disulphide cross-linked polymers of OmcB were the functional equivalent of peptidoglycan in other gram negative bacteria, forming a disulphide cross-linked network with the periplasmic domains of OmcA and other membrane proteins. This might account for the considerable structural stability of elementary bodies [Everett & Hatch, 1995].

[Comment: The macromolecular net of peptidoglycan is the main structural component of the  cell wall for most gram negative bacteria. Most (but not all) Chlamydiales are gram negative. At the time of Everett & Hatch 1995, it was believed that chlamydiae were unusual in not having peptidoglycan. Genomic sequencing showed that chlamydiae do have the genes necessary to synthesize peptidoglycan, although peptidoglycan, if present, is probably in there insufficient amount to play a major structural role. Hence the continuing belief that the -S-S bridges of the CRPs may fulfil a structural role].

[MEW] April 2002

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References

Bachmaier, K., Neu, N., de la Maza L. M., Pal, S., Hessel, A. & Penninger, J. M.  (1999). Chlamydia Infections and Heart Disease Linked Through Antigenic Mimicry. Science 283, 1335 - 1339. [Personally I remain unconvinced]

Clarke, I. N., Ward, M. E. & Lambden, P. R. (1988). Molecular cloning and sequence analysis of a developmentally regulated cysteine-rich outer membrane protein from Chlamydia trachomatis. Gene 71, 307 - 314. [First CRP sequences]

de la Maza, L. M., Fielder, T. J., Carlson, E. J., Markoff, B. A.& Peterson, E. M. (1991). Sequence diversity of the 60-kilodalton protein and of a putative 15-kilodalton protein between the trachoma and lymphogranuloma venereum biovars of Chlamydia trachomatis. Infection and Immunity 59, 1196 - 1201.

Everett, K. D., Desiderio, D. M. & Hatch, T. P. (1994). Characterization of lipoprotein EnvA in Chlamydia psittaci 6BC. Journal of Bacteriology 176, 6082 - 6087. 

Everett, K. D. & Hatch, T. P. (1991). Sequence analysis and lipid modification of the cysteine-rich envelope proteins of Chlamydia psittaci 6BC. Journal of Bacteriology 173, 3821 - 3830.

Everett, K. D. & Hatch, T. P. (1995). Architecture of the cell envelope of Chlamydia psittaci 6BC. Journal of Bacteriology 177, 877 - 882. Full article  [Elegant work proposing a model that still stands]

Hsia, R. C. and Bavoil, P. M. (1996). Sequence analysis of the omp2 region of Chlamydia psittaci strain GPIC: structural and functional implications. Gene 176, 155 - 162.

Lambden, P. R., Everson, J. S., Ward, M. E. & Clarke, I. N. (1990). Sulfur-rich proteins of Chlamydia trachomatis: developmentally regulated transcription of polycistronic mRNA from tandem promoters. Gene 87, 105 - 112.

Millman, K. L., Tavare, S. & Dean, D. (2001). Recombination in the ompA gene but not the omcB gene of Chlamydia contributes to serovar-specific differences in tissue tropism, immune surveillance, and persistence of the organism. Journal of Bacteriology 183, 5997 - 6008. Full article [Excellent and thoughtful study. Good literature review].

Newhall, W. J. 1987. Biosynthesis and disulfide cross-linking of outer membrane components during the growth cycle of Chlamydia trachomatis. Infection and Immunity 55, 162 - 168

Sanchez-Campillo, M., Bini, L., Comanducci, M., Raggiaschi, R., Marzocchi, B., Pallini, V. & Ratti, G. (1999). Identification of immunoreactive proteins of Chlamydia trachomatis by Western blot analysis of a two-dimensional electrophoresis map with patient sera. Electrophoresis 20, 2269 - 2279.

Sardinia, L. M., Segal, E. and Ganem, D. (1988). Developmental regulation of the cysteine-rich outer-membrane proteins of murine Chlamydia trachomatis. Journal of General Microbiology 134, 997 - 1004.

Watson, M. W., Lambden, P. R., Everson, J. S. & Clarke, I. N. (1994). Immunoreactivity of the 60-kDa cysteine-rich proteins of Chlamydia trachomatis, Chlamydia psittaci and Chlamydia pneumoniae expressed in Escherichia coli. Microbiology 140, 2003 - 2011.

Watson, M. W., Lambden, P. R., Ward, M. E. & Clarke, I. N. (1989). Chlamydia trachomatis 60 kDa cysteine rich outer membrane protein: sequence homology between trachoma and LGV biovars. FEMS Microbiology Letters 53, 293 - 297.

NEXT: The polymorphic outer membrane proteins