Self-Association Energetics of an Intact, Full-Length Nuclear Receptor:  The B-Isoform of Human Progesterone Receptor Dimerizes in the Micromolar Range

Abstract

We are focused on understanding the mechanisms underlying eukaryotic gene regulation, using the human progesterone receptor (PR) and its interactions with its DNA response elements as a model system. An understanding of PR function is complicated by the presence of two transcriptionally distinct isoforms, an 83 kDa A-receptor (PR-A) and a 99 kDa B-receptor (PR-B). The two isoforms are identical except the B-receptor contains an additional 164 residues at its N-terminus. As a first step toward understanding the principles by which the two isoforms assemble at complex promoters, we examined the energetics of PR-B self-association using sedimentation velocity and sedimentation equilibrium methods. Full-length human PR-B was purified to 95% homogeneity from baculovirus-infected insect cells. Using a fluorescence hormone binding assay, we determined the purified protein to be highly active in its ability to bind ligand. Sedimentation velocity studies of hormone-bound PR-B at pH 8.0, 4 degrees C, and 50 mM NaCl demonstrate that it undergoes a concentration-dependent change in its sedimentation coefficient, existing as a 4.0S species at submicromolar concentrations but forming a 5.7S species at higher concentrations. These results strongly suggest that PR-B undergoes self-association in the micromolar range. This hypothesis was examined rigorously using sedimentation equilibrium. Global analysis of the sedimentation equilibrium data demonstrated that PR-B self-association was well described by a monomer-dimer model with a dimerization free energy of -7.2 +/- 0.7 kcal/mol. The role of NaCl in regulating PR-B dimerization was examined by carrying out sedimentation velocity and equilibrium studies under high salt conditions. At 300 mM NaCl, PR-B is exclusively monomeric in the micromolar range, thus revealing a significant ionic contribution to the assembly energetics. Further, the monomer sediments as a structurally homogeneous, but highly asymmetric, 4.0S species. Limited proteolysis of PR-B demonstrated that the hydrodynamic asymmetry is due in part to an extended, nonglobular conformation localized to the N-terminal region of PR-B. In contrast, the DNA binding domain (DBD) and hormone binding domain (HBD) exist as independent structural units, and the activation function N-terminal to the DBD (AF-1) shows moderate structure. These results represent the first rigorous analysis of the self-assembly energetics of an intact nuclear receptor and suggest that PR function is more complex than envisioned by traditional models.