Biophysical Studies of the Binding of ERα Nuclear Receptor to DNA

Estrogen receptor α (ERα) is a member of a family of ligand-modulated transcription factors that have come to be known as nuclear receptors. ERα mediates the action of estrogens and plays an integral role in a wide range of physiological processes ranging from embryonic development and morphogenesis to reproduction to cardiovascular health. Not surprisingly, malfunction of the estrogen system is associated with a host of pathological conditions such as osteoporosis, heart disease and most notably breast cancer. Essential to its functioning as a transcription factor are specific protein-DNA interactions which are mediated by the binding of the DNA-binding (DB) domain of ERα to particular DNA sequences located within target gene promoters called estrogen response elements (EREs). Here, using a diverse array of biophysical techniques, including in particular isothermal titration calorimetry coupled with molecular modeling and semi-empirical analysis, I provide new insights into the ERα-DNA interaction in thermodynamic and structural terms. My data show that the binding of the DB domain of ERα to DNA is coupled to protonation at two specific amino acids, H196 and E203. Protonation of these residues is non-trivial and is required for high affinity binding. Amino acid sequence alignment of the DB domains of the NR family suggests that this may be a hallmark feature common to the functioning of all nuclear receptors. Furthermore, I demonstrate that the DB domain can tolerate all single nucleotide substitutions within the ERE and bind in the physiologically relevant nanomolar to micromolar range. Comparative thermodynamic analysis reveals that the DB domain binds to these ERE sequences utilizing a considerable range of energetic signatures such that any one thermodynamic component of binding is not predictive of associated affinity. In addition, it is shown that nucleotide substitution results in significant changes in secondary and three-dimensional features of the oligonucleotides and may impact binding affinity. Finally, I demonstrate that the zinc-finger of the DB domain of ERα is relatively promiscuous and can accommodate several heavy-metal divalent cations. Other than zinc, only DB domains reconstituted with cobalt, cadmium and mercury were capable of binding DNA. Incorporation of the metals resulted in a wide range of CD spectroscopic features which were found not to be predictive of DNA binding capacity. Thus, isostructure does not equate to isofunction in the case of metal reconstituted DB domain of ERα. This analysis suggests that metal coordination is not likely to be required for domain folding, but rather is required to bind DNA. Taken together, this thesis provides novel insights into the physicochemical basis of a key protein-DNA interaction essential to human health and disease. My studies bear the potential to impact the development of novel therapies harboring greater efficacy coupled with lower toxicity for the treatment of disease