Rebecca D. Prokipcak

Equivalent molecular mass of cytosolic and nuclear forms of Ah receptor from Hepa-1 cells determined by photoaffinity labeling with 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin

The structure of the Ah receptor previously has been extensively characterized by reversible binding of the high affinity ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin. We report the use of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin as a photoaffinity ligand for Ah receptor from the mouse hepatoma cell line Hepa-1c1c9. Both cytosolic and nuclear forms of Ah receptor could be specifically photoaffinity-labeled, which allowed determination of molecular mass for the two forms under denaturing conditions. After analysis by fluorography of polyacrylamide gels run in the presence of sodium dodecyl sulfate, molecular mass for the cytosolic form of Ah receptor was estimated at 92,000 +/- 4,300 and that for the nuclear form was estimated at 93,500 +/- 3,400. Receptor in mixture of cytosol and nuclear extract (each labeled separately with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin) migrated as a single band. These results are consistent with the presence of a common ligand-binding subunit of identical molecular mass in both cytosolic and nuclear complexes.

Nuclear Ah receptor from mouse hepatoma cells: Effect of partial proteolysis on relative molecular mass and DNA-binding properties☆

The nuclear Ah receptor from mouse hepatoma (Hepa-1c1c9) cells is a 176-kDa multimeric protein which is stable under conditions of up to 1 M KCl. Under denaturing conditions, the Hepa-1 nuclear receptor can be dissociated into a ligand-binding subunit of Mr approximately 91,000. The identity of subunits that compose the nuclear Ah receptor is currently unknown. We used partial proteolysis under nondenaturing conditions as an approach to study the domain organization of the nuclear form of Ah receptor from Hepa-1c1c9 cells treated with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in culture. Low concentrations of trypsin (0.5 microgram/mg nuclear protein) generated heterogeneous fragments with the main fragment having a Stokes radius (Rs) approximately 6 nm. More discrete ligand-binding fragments of Mr approximately 84,000 (Rs approximately 4 nm/approximately 5 S) and Mr approximately 16,000 (Rs approximately 2 nm/approximately 2 S) could be generated using higher concentrations of trypsin (5 micrograms/mg nuclear protein). The relative concentration of the 84 and 16-kDa fragment was dependent on duration of protease treatment; formation of the 16-kDa fragment was accompanied by some loss in [3H]TCDD binding. Treatment of nuclear Ah receptor with alpha-chymotrypsin (1 microgram/mg nuclear protein) generated a single, apparently homogeneous ligand-binding fragment of Mr approximately 101,000 (Rs approximately 5 nm/approximately 5 S). When analyzed by DNA-cellulose chromatography, the chymotryptic fragment eluted at a significantly higher KCl concentration (462 mM) compared to native untreated nuclear Ah receptor (385 mM). Despite this increased affinity for DNA-cellulose columns, the ligand-binding fragment generated by chymotrypsin treatment was unable to interact with a dioxin responsive element in a gel retardation assay. DNA-cellulose binding ability, therefore, does not appear to be a reliable indicator of specific DNA interactions for these protease-modified fragments.

7-Substituted-2,3-dichlorodibenzo-p-dioxins as competitive ligands for the Ah receptor: Quantitative structure-activity relationships (QSARs) and a comparison of human receptor with Ah receptor from rodents

Abstract The competitive binding affinities of thirteen 7-substituted-2,3,-dichlorodibenzo-p-dioxins to the human Ah placental cytosolic Ah receptor were determined versus [ 3 H ]-2,3,7,8- tetrachlorodibenzo -p- dioxin (TCDD) as the radioligand. Multiple parameter linear regression analysis of the competitive binding EC50 values for these compounds gave the following equation: pEC 50 (M) =6.246+1.632 π − 1.764σ m° + 1.282 HB were π is the substituent lipophilicity, σm° the meta-directing electronegativity and HB the hydrogen binding capacity. The equation obtained using human placental receptor was different than correlations previously derived for the binding of the same series of compounds to the rat, mouse, guinea pig or hamster cytosolic Ah receptor, providing further evidence for interspecies differences in the properties of the Ah receptor protein.

Regulation of c-myc mRNA Half-Life by an RNA-Binding Protein

c-myc mRNA contains at least two discrete sequence elements that account for its short half-life, one in the 3′-untranslated region, the other in the carboxy terminal coding region (coding region determinant). A combination of experiments with cell-free extracts and intact cells suggests that the coding region determinant interacts with an abundant RNA-binding protein that, when bound to the mRNA, protects it from endonucleolytic attack. The mRNA-protein complex might serve as a fail-safe mechanism to permit rapid c-myc mRNA destruction at certain times.