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).