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

The chlamydial major outer membrane porin, MomP 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 other outer membrane constituents. 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 outer membrane complex demonstrated that the MOMP is most likely not in a close spatial relationship with the 60- and 12-kDa cysteine-rich proteins. 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 (OMPP) Superfamily I.



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.

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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.

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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.

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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.

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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) 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).


Omp1 of Chlamydia psittaci

1.B.2.1.2Major outer membrance protein MomP (310aas; Rodríguez-Marañón et al., 2002)

Gram-negative bacteria

MomP of Chlamydia trachomatis (B7U3X1)


Outer membrane porin, OmpA-A of 259 aas


OmpA-A of Simkania negevensis


TC#NameOrganismal TypeExample

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


PorB of Chlamydia trachomatis