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 .
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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).
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