Progesterone is a key reproductive hormone that7 binds to nuclear progesterone receptors (PR) to mediate their effects. There are two isoforms of PR, 85 kDa A-receptors (PR-A) and 103 kDa B-receptors (PR-B). They are identical except for a 164 amino acid extension at the N-terminus of PR-B. Although the binding of PR-A and PR-B to hormone and DNA are similar, in vitro transcriptional studies and cDNA array analyses of endogenous genes have demonstrated differences in the biological activities of these two isoforms. Of particular note is the fact that human breast cancers express widely differing ratios of PR-A vs. PR-B, unlike the normal breast in which the two receptors are equimolar. The goal of this work is to define the structural elements in the N-termini of PR-A and PR-B that that give rise to their functional differences. To do so, an analysis of the conformational differences between PR-A, PR-B, and functionally-altered mutants of PR will be performed, using techniques such as circular dichroism and fluorescence spectroscopy. Additionally, co-immunoprecipitation assays will be used to identify proteins that bind differentially to the N-terminus of PR-B and assess how coregulator binding affects the N-terminal structure. Successful completion of these studies will explain how structural differences between the two PR, and differences among the proteins that interact with the receptors, account for the function differences between them.
