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The  Chlamydiae  constitute an evolutionary well separated group of intracellular bacteria comprising important pathogens of humans as well as symbionts of protozoa. The amoeba symbiont  Protochlamydia amoebophila  lacks a homologue of the most abundant outer membrane protein of the  Chlamydiaceae , the major outer membrane protein MOMP, highlighting a major difference between environmental chlamydiae and their pathogenic counterparts. We recently identified a novel family of putative porins encoded in the genome of  P. amoebophila  by  in silico  analysis. Two of these   P rotochlamydia   o uter  m embrane proteins, PomS (pc1489) and PomT (pc1077), are highly abundant in outer membrane preparations of this organism. Here we show that all four members of this putative porin family are toxic when expressed in the heterologous host  Escherichia coli . Immunofluorescence analysis using antibodies against heterologously expressed PomT and PomS purified directly from elementary bodies, respectively, demonstrated the location of both proteins in the outer membrane of  P. amoebophila . The location of the most abundant protein PomS was further confirmed by immuno-transmission electron microscopy. We could show that  pomS  is transcribed, and the corresponding protein is present in the outer membrane throughout the complete developmental cycle, suggesting an essential role for  P. amoebophila . Lipid bilayer measurements demonstrated that PomS functions as a porin with anion-selectivity and a pore size similar to the  Chlamydiaceae  MOMP. Taken together, our results suggest that PomS, possibly in concert with PomT and other members of this porin family, is the functional equivalent of MOMP in  P. amoebophila . This work contributes to our understanding of the adaptations of symbiotic and pathogenic chlamydiae to their different eukaryotic hosts.

Identification and Characterization of a Novel Porin Family Highlights a Major Difference in the Outer Membrane of Chlamydial Symbionts and Pathogens