1.B.2 The Chlamydial Porin (CP) Family

The chlamydial major outer membrane porin, MomP, OmpA, Omp1L2, or Omp1, functions to permit the diffusion of solutes through the intrareticular body membrane. It has ~402 amino acyl residues and is believed to be disulfide-bonded to two other outer membrane constituents, OmcA, a lipid-anchored 9 kDa protein with ~ 14 cys residues, and OmcB, a 60 kDa protein with ~37 cys (Findlay et al. 2005). A homologue, PorB, which transports neutral solutes poorly, has been shown to transport dicarboxylates such as 2-ketoglutarate (Kubo and Stephens, 2001).

MOMP trimers are stable under reducing conditions, although disulfide bonds appear to be present between the monomers of a trimer and between trimers (Sun et al., 2007). Cross-linking of the Chlamydial outer membrane complex (COMC) demonstrated that the MOMP is probably disulfide-linked and in a close spatial relationship with the 60- and 12-kDa cysteine-rich proteins, OmcB and OmcA, respectively. The trimers consist mainly of β-pleated sheet structures. Using a liposomal swelling assay, the MOMP was found to have porin activity, approximately 2 nm in diameter (Sun et al., 2007). 

This family belongs to the Outer Membrane Pore-forming Protein I (OMPP-I) Superfamily .



Atanu, F.O., E. Oviedo-Orta, and K.A. Watson. (2013). A Novel Transport Mechanism for MOMP in Chlamydophila pneumoniae and Its Putative Role in Immune-Therapy. PLoS One 8: e61139.

Findlay, H.E., H. McClafferty, and R.H. Ashley. (2005). Surface expression, single-channel analysis and membrane topology of recombinant Chlamydia trachomatis Major Outer Membrane Protein. BMC Microbiol 5: 5.

Jeanteur, D., J.H. Lakey, and F. Pattus. (1991). The bacterial porin superfamily: sequence alignment and structure prediction. Mol. Microbiol. 5: 2153-2164.

Jeanteur, D., J.H. Lakey, and F. Pattus. (1994). The porin superfamily: diversity and common features. In: Bacterial Cell Wall. Edited by Ghuysen, J.M., Hakenbeck, R. Elsevier, Amsterdam, pp. 363-380.

Kubo, A. and R.S. Stephens. (2000). Characterization and functional analysis of PorB, a Chlamydia porin and neutralizing target. Mol. Microbiol. 38: 772-780.

Kubo, A. and R.S. Stephens. (2001). Substrate-specific diffusion of select dicarboxylates through Chlamydia trachomatis PorB. Microbiology 147: 3135-3140.

Longbottom, D., M. Livingstone, K.D. Aitchison, L. Imrie, E. Manson, N. Wheelhouse, and N.F. Inglis. (2019). Proteomic characterisation of the Chlamydia abortus outer membrane complex (COMC) using combined rapid monolithic column liquid chromatography and fast MS/MS scanning. PLoS One 14: e0224070.

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O'Neill, C.E., H.M. Seth-Smith, B. Van Der Pol, S.R. Harris, N.R. Thomson, L.T. Cutcliffe, and I.N. Clarke. (2013). Chlamydia trachomatis clinical isolates identified as tetracycline resistant do not exhibit resistance in vitro: whole-genome sequencing reveals a mutation in porB but no evidence for tetracycline resistance genes. Microbiology 159: 748-756.

Phillips, S., B.L. Quigley, and P. Timms. (2019). Seventy Years of Vaccine Research - Limitations of the Past and Directions for the Future. Front Microbiol 10: 70.

Pickett, M.A., M.E. Ward, and I.N. Clarke. (1988b). High-level expression and epitope localization of the major outer membrane protein of Chlamydia trachomatis serovar L1. Mol. Microbiol. 2: 681-685.

Pickett, M.A., S.J. Everson, and I.N. Clarke. (1988a). Chlamydia psittaci ewe abortion agent: complete nucleotide sequence of the major outer membrane protein gene. FEMS Microbiol. Lett. 55: 229-234.

Rodríguez-Marañón, M.J., R.M. Bush, E.M. Peterson, T. Schirmer, and L.M. de la Maza. (2002). Prediction of the membrane-spanning β-strands of the major outer membrane protein of Chlamydia. Protein. Sci. 11: 1854-1861.

Sadhasivam, A., H. Nagarajan, and V. Umashankar. (2019). Structure-based drug target prioritisation and rational drug design for targeting eye infections. J Biomol Struct Dyn 1-13. [Epub: Ahead of Print]

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Sun, G., S. Pal, A.K. Sarcon, S. Kim, E. Sugawara, H. Nikaido, M.J. Cocco, E.M. Peterson, and L.M. de la Maza. (2007). Structural and Functional Analyses of the Major Outer Membrane Protein of Chlamydia trachomatis. J. Bacteriol. 189:6222-6235.


TC#NameOrganismal TypeExample

MomP (Omp1) major general porin (transports many small molecules including sugars and amino acids).  A transport mechanism and antigenic properties have been studied for the closely related C. pneumoniae protein (Atanu et al. 2013). The ortholog of this protein in Clamydia abortus is likely to be part of the outer membrane complex (COMC) used as a potential vaccine candidate against ovine enzootic abortion (Longbottom et al. 2019).


Omp1 of Chlamydia psittaci


Major outer membrance protein MomP (OmpA, Omp1, Omp1L2) of 394 aas; 1 N-terminal TMS Rodríguez-Marañón et al., 2002). This protein is covalently linked to two other cys-rich proteins via disulfide bonds, OmcA (Omp2A; Omp3) with 88 aas and 1 N-terminal TMS, and OmcB (Omp2, Omp2B) with 547 aas and 1 N-terminal TMS. Together these proteins comprise the Chlamydial outer membrane complex (COMC) (Findlay et al. 2005). MOMP is the most suitable substitute for whole cell targets for vaccine production, and its delivery as a combined systemic and mucosal vaccine is most effective (Phillips et al. 2019). It is also a drug target (Sadhasivam et al. 2019).

Gram-negative bacteria

MomP of Chlamydia trachomatis with two auxiliary proteins, OmcA and OmcB


Outer membrane porin, OmpA-A of 259 aas.  This protein may be C-terminally truncated since it is substantially smaller than most of the other members of this family.


OmpA-A of Simkania negevensis


PorB dicarboxylate-specific porin (Kubo and Stephens 2001).  Mutations in porB can give rise to tetracycline resistance ().


PorB of Chlamydia trachomatis


Uncharacterized protein of 299 aas and 1 N-terminal TMS.  May be C-terminally truncated.

UP of Deltaproteobacteria bacterium


Uncharacterized protein of 271 aas and 1 N-terminal TMS.

UP of Thiohalophilus thiocyanatoxydans


Uncharacterized protein of 230 aas and 1 N-terminal TMS

UP of Candidatus Omnitrophica bacterium


Uncharacterized protein of 325 aas and 1 N-terminal TMS.

UP of unclassified Thioalkalivibrio


TC#NameOrganismal TypeExample