title: Thermodynamic analysis of progesterone receptor-promoter interactions reveals a molecular model for isoform-specific function.
abstract: Human progesterone receptors (PR) exist as two functionally distinct isoforms, PR-A and PR-B. The proteins are identical except for an additional 164 residues located at the N terminus of PR-B. To determine the mechanisms responsible for isoform-specific functional differences, we present here a thermodynamic dissection of PR-A-promoter interactions and compare the results to our previous work on PR-B. This analysis has generated a number of results inconsistent with the traditional, biochemically based model of receptor function. Specifically, statistical models invoking preformed PR-A dimers as the active binding species demonstrate that intrinsic binding energetics are over an order of magnitude greater than is apparent. High-affinity binding is opposed, however, by a large energetic penalty. The consequences of this penalty are 2-fold: Successive monomer binding to a palindromic response element is thermodynamically favored over preformed dimer binding, and DNA-induced dimerization of the monomers is largely abolished. Furthermore, PR-A binding to multiple PREs is only weakly cooperative, as judged by a 5-fold increase in overall stability. Comparison of these results to our work on PR-B demonstrates that whereas both isoforms appear to have similar DNA binding affinities, PR-B in fact has a greatly increased intrinsic binding affinity and cooperative binding ability relative to PR-A. These differences thus suggest that residues unique to PR-B allosterically regulate the energetics of cooperative promoter assembly. From a functional perspective, the differences in microscopic affinities predict receptor-promoter occupancies that accurately correlate with the transcriptional activation profiles seen for each isoform.
authors.affiliation: Department of Pharmaceutical Sciences, University of Colorado Health Sciences Center, Denver, CO 80262, USA.
authors.forename: Keith D
authors.initials: KD
authors.lastname: Connaghan-Jones
authors.forename: Aaron F
authors.initials: AF
authors.lastname: Heneghan
authors.forename: Michael T
authors.initials: MT
authors.lastname: Miura
authors.forename: David L
authors.initials: DL
authors.lastname: Bain
mesh_terms: D000494:Allosteric Regulation; D016172:DNA Fingerprinting; D019281:Dimerization; D006801:Humans; D008958:Models, Molecular; D015233:Models, Statistical; D011401:Promoter Regions, Genetic / Q000235:genetics*; D011485:Protein Binding; D020033:Protein Isoforms / Q000502:physiology; D011980:Receptors, Progesterone / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology*; D020218:Response Elements; D013816:Thermodynamics; D015533:Transcriptional Activation
chemical_list: D020033:Protein Isoforms; D011980:Receptors, Progesterone; C101118:progesterone receptor A; C097314:progesterone receptor B