Isolation of a 48.5-kDa membrane protein from Pseudomonas maltophilia which exhibits immunologic cross-reaction to the beta-subunit of human chorionic gonadotropin

In separate studies we have shown that Pseudomonas maltophilia (American Type Culture Collection 13637) possesses an immunoglobulin Fc-binding protein. We have found that this protein prevents the application of immunoassays using monoclonal antibodies to study possible production of a CG-like material by this bacteria. Employing an immunoglobulin saturation technique to block this protein as well as a zwitterion detergent membrane solubilization technique, we now report the isolation of a membrane protein from Pseudomonas maltophilia which shows immunological relationships to the beta-subunit and carboxyl tail of human pregnancy CG. This pseudomonas immunoreactive material produced dose-response curves in the following CG immunoassays: 1) a polyclonal rabbit anti-CG equilibrium assay, 2) carboxyl tail CG equilibrium assay, and 3) two CG equilibrium assays using monoclonal antibodies. The pseudomonas CG-like protein did not react in equilibrium assays for human TSH, human LH, or free alpha-subunit of CG. The pseudomonas CG-like protein was purified by affinity chromatography. The purified protein showed only 0.25% cross-reaction with pregnancy CG in the monoclonal antibody equilibrium assays. Furthermore, the purified protein showed no binding to rat testicular CG/LH receptors, but showed avid binding to the pseudomonas CG-binding protein previously described by Richert and Ryan. The pseudomonas protein showed no binding to Concanavalin-A, which avidly binds pregnancy CG. When assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting, this protein had a mol wt of 48,500 daltons, which is larger than the mol wt of the unglycosylated beta-subunit of pregnancy CG. We conclude that pseudomonas contain a protein that has partial homology to the beta-subunit of pregnancy CG. This material does not bind to mammalian CG receptors, but does bind to a pseudomonas CG-binding site. The latter suggests it has a function, as yet unknown, in pseudomonas.