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).