Thermal activation of the purified rat hepatic glucocorticoid receptor. Evidence for a two-step mechanism.

Thermal "activation" or "transformation" of rat hepatic [6,7-3H]triamcinolone acetonide (TA)-receptor complexes purified in the unactivated state to near homogeneity (Grandics, P., Miller, A., Schmidt, T. J., Mittman, D., and Litwack, G. (1984) J. Biol. Chem. 259, 3173-3180) has been further investigated. The data generated in reconstitution experiments demonstrate that warming (25 degrees C for 30 min) of the purified unactivated complexes promotes their activation as judged by an increase in DNA-cellulose binding, but to a lower extent than that observed after warming of glucocorticoid-receptor complexes in crude cytosols. However, maximal DNA-cellulose binding capacity can be detected in reconstituted systems (also heated at 25 degrees C for 30 min) consisting of purified unactivated [3H]TA-receptor complexes and a cytoplasmic "stimulator(s)." This cytoplasmic factor(s), which does not copurify with the receptor, is heat-stable (90 degrees C for 30 min), excluded from Sephadex G-25, and trypsin-sensitive and stimulates DNA-cellulose binding in a dose-dependent manner. The ability of Na2MoO4 to block thermal activation of the highly purified receptor complexes suggests that this transition metal anion interacts directly with the receptor protein itself. The fact that the cytoplasmic stimulator(s) enhances DNA-cellulose binding of the [3H]TA-receptor complexes without increasing the proportion of those complexes eluted in the activated (low salt) position from DEAE-cellulose is consistent with a proposed two-step model of in vitro activation. During the Na2MoO4-sensitive Step 1, elevated temperature (25 degrees C for 30 min) may directly alter the conformation of the purified receptor complexes (i.e. subunit dissociation or disaggregation), resulting in the appropriate shift in the elution profile of the [3H]TA-receptor complexes on DEAE-cellulose but only in a minimal (approximately 2-3-fold) increase in the binding of these complexes to DNA-cellulose. During the Na2MoO4-insensitive and temperature-independent Step 2, a heat-stable cytoplasmic protein(s) may interact with these thermally activated [3H]TA-receptor complexes and enhance their ability to bind to DNA-cellulose without further increasing the percentage of those complexes which elute from DEAE-cellulose in the activated position. In crude cytosols these two steps would presumably occur simultaneously, and addition of Na2MoO4 prior to warming would block Step 1 and hence Step 2 would not occur.