Ligand Binding Domain Mutations of the Glucocorticoid Receptor Selectively Modify the Effects with, but Not Binding of, Cofactors

We previously reported that several point mutations in the ligand binding domain (LBD) of glucocorticoid receptors (GRs) marginally affect the binding affinity of the synthetic glucocorticoids dexamethasone (Dex) and deacylcortivazol (DAC). However, these mutations dramatically alter the efficacy (A(max)) and potency (EC50) of agonists, along with the partial agonist activity (PAA) of the antisteroid Dex-mesylate (DM), for gene induction and repression in a steroid-dependent manner. This was proposed to result, in part, from altered protein protein interactions in the complex of GR with the coactivator TIF2 despite normal TIF2 binding. To explore the generality of this phenomenon, we now ask whether these mutations also affect the transactivation properties, but not binding, of other GR-bound factors. We find that an elevated concentration of GR, to probe unidentified cofactors, or of the comodulator Ubc9 does not reverse the effects of GR LBD mutations that increase the EC50 and lower the PAA with the GREtkLUC reporter in both CV-1 and U2OS cells. This behavior is more dramatic with Ubc9 and the isolated GR LBD fused to the GAL4 DNA binding domain, despite normal binding of Ubc9 to the mutant GRs. Similar effects, albeit gene, steroid, and transcriptional property-specific, are seen with full-length GRs and three endogenous genes in U2OS cells. Thus, changes in simple steady-state binding capacities of mutant receptors for factors cannot account for the modified transcriptional properties. In all cases, the nuclear translocation of Dex- and DAC-bound wild-type and mutant receptors is the same. These results are consistent with the earlier results with TIF2 and support the hypothesis that small changes in the GR LBD can alter the activities of the bound cofactor without modifying cofactor binding. We propose that this separation of binding and the modulation of transactivation parameters occurs for a wide variety of GR-associated cofactors.