Rat liver immuno-ultrastructural localization of the specific (4-5S) 3-methylcholanthrene-binding protein; evidence for its involvement as a receptor protein in cytochrome P-4501A1 induction.
Two major specific carcinogen-binding proteins are thought to be involved in the regulation of hepatic cytochrome P-4501A1 induction in response to polycyclic aromatic hydrocarbons. We have raised both mono- and polyclonal antibodies which specifically interact with one of these proteins, the 4-5S-specific binding protein. Antibody binding was found to both the specific (4-5S) 3-methylcholanthrene (3MC) binding activity present in rat hepatic cytosol (as quantified on sucrose gradients or by Sephacryl S-300 gel filtration chromatography) and to the purified 39,000-dalton protein previously reported as the specific 3MC-binding protein (determined by Western blot analyses). No immunoreactivity was observed to cytosolic proteins in the region of 70,000 or 95,000 daltons (i.e. corresponding to the Ah receptor). We have used Western blot analyses and immunostaining of cryostat sections to demonstrate that the 4-5S-specific binding protein is present predominantly in the cytoplasm but also (at lower concentration) in the nuclei of untreated Wistar rat hepatocytes. Electron micrographs of immunostained sections indicate that, following exposure to 3MC, the concentration of specific binding proteins in the nucleus increases and this is assumed to be due to translocation of specific binding protein-3MC complexes from the cytoplasm.