Identification and properties of chlamydial polypeptides that bind eucaryotic cell surface components.

An electroblotting technique was used to identify proteins of Chlamydia that bound surface-radioiodinated and Triton X-100-solubilized HeLa cell extracts. Two proteins, with apparent molecular masses of 18 and 32 kilodaltons (kDa), that bound HeLa cell surface components were identified on Chlamydia trachomatis L2 elementary bodies (EBs). Radioiodinated heparin, which disrupts chlamydial association with cultured cells, was also bound by these proteins. These two proteins were found on EBs but were absent or were present in reduced amounts on the noninfectious reticulate bodies. All C. trachomatis strains tested displayed two such proteins, although the apparent molecular weight of the larger protein varied with serotype in correlation with biotype and the disease that it caused. Two Chlamydia psittaci strains examined displayed only a single binding protein in the range of 17 to 19 kDa. All of the binding proteins stained intensely and distinctively on silver-stained sodium dodecyl sulfate-polyacrylamide gels and displayed an unusual sensitivity to reducing agents. The 32-kDa protein was not seen and did not bind 125I-labeled HeLa cell components if the EBs were solubilized in the presence of 2-mercaptoethanol. The 32-kDa protein was not affected by dithiothreitol, however. Similar to the effect of 2-mercaptoethanol, the 32-kDa protein was not visualized after treatment of EBs with the protease inhibitors tosyl-phenylalanine chloromethyl ketone (TPCK) or tosyl-lysine chloromethyl ketone (TLCK). TPCK and TLCK also abolished infectivity as did the alkylating agents N-ethylmaleimide and iodoacetamide, yet the latter two agents did not affect the appearance of the 32-kDa protein. These proteins were not detected in immunoblots with either rabbit antisera to C. trachomatis L2 EBs or by serum from a patient with lymphogranuloma venereum. The role of these proteins in the interaction of chlamydiae with host cells is not clear, but the binding of eucaryotic cell surface components and heparin, presence only during the infectious stage of the life cycle, variation between serotypes in correlation with disease, and sensitivity to reducing agents or protease inhibitors, collectively, suggest a role for these proteins in parasite-host interactions.