The chlamydial life cycle involves the intimate interaction of components of the infectious elementary body (EB) surface with receptors on the susceptible eukaryotic cell plasma membrane. We have developed an in vitro
ligand binding assay system for the identification and characterization of
detergent-extracted EB envelope
proteins capable of binding to
glutaraldehyde-fixed HeLa cell surfaces. With this assay, the developmentally regulated
cysteine-rich envelope
protein Omp2 of Chlamydia psittaci strain guinea pig
inclusion conjunctivitis was shown to bind specifically to HeLa cells. HeLa cells bound Omp2 selectively over other cell wall-associated
proteins, including the major outer
membrane protein, and the binding of Omp2 was abolished under conditions which alter its conformation. Furthermore,
trypsin treatment, which reduces EB adherence, resulted in the proteolytic removal of a small terminal
peptide of Omp2 at the EB surface and inactivated Omp2 in the
ligand binding assay, while having a negligible effect on the major outer
membrane protein. Collectively, our results suggest that Omp2 possesses the capacity to engage in a specific interaction with the host eukaryotic cell. We speculate that, since Omp2 is present only in the infectious EB form, the observed in vitro interaction may be representative of a determining step of the chlamydial pathogenic process.