The human progesterone receptor (hPR) exists as two isoforms, hPR-A and hPR-B, which differ only in that hPR-A lacks 164 amino acids present at the amino-terminus of hPR-B. In this study we have separately expressed hPR-A and hPR-B and asked whether the progesterone-binding mechanisms are the same or different for the two forms of hPR and for their mixture. We investigated 1) the cooperativity of binding [H-3]progesterone to the receptor, as measured by the Hill coefficient (n(H)); and 2) the dissociation rate of [H-3]progesterone from the receptor. To compare the effects of dimerization, these ligand-binding properties were measured over a range of receptor concentrations. Binding of [H-3]progesterone to hPR-A was positively cooperative at all concentrations used; the limiting value for the Hill coefficient was 1.47 +/- 0.11 at high receptor concentrations (5-19 nM) and 1.31 +/- 0.06 at low receptor concentrations (1-4 nM). Similarly, little change was observed in the dissociation rate constant over the same concentration range; the values at high and low concentrations were 4.59 +/- 0.15 and 3.03 +/- 0.25 X 10(-3) min(-1), respectively. By contrast, the hPR-B concentration had a marked effect on positive cooperative binding and the dissociation rate of progesterone. At high hPR-B concentrations (3-5 nM), the limiting Hill coefficient was 1.49 +/- 0.11, which is indicative of moderately strong positive cooperativity, whereas at lower hPR-B concentrations (1-3 nM), the Hill coefficient was reduced to 1.1, which is essentially noncooperative. The [H-3]progesterone dissociation rate was 4.52 +/- 0.44 X 10(-3) min(-1) at the higher concentrations of hPR-B and was increased to 1.6 +/- 0.11 X 10(-3) min(-1) at the lower concentrations. Thus, over the same concentration range where hPR-A exhibited no significant change in positive cooperativity or the dissociation rate, these progesterone-binding properties were highly dependent on the concentration of hPR-B. When hPR-A and hPR-B were mixed, positive cooperative binding and the dissociation rate were more similar to hPR-B than to hPR-A, in that both binding parameters were dependent on the concentration of receptor. However, the hPR-AB mixture differed from hPR-B alone in that the mixture required a greater receptor concentration (7-10 vs. 3-5 nM) to exhibit positive cooperativity and the increased dissociation rate. These results show, first, that each hPR isoform displays different [H-3]progesterone-binding properties, which are most prominent at low concentrations of receptor, and second, that one isoform can influence the other. As the two receptor forms differ only at the N-terminus, yet positive cooperativity and changes in the dissociation rate constant are indicative of conformational changes affecting hormone binding, these results also strongly suggest that the N-terminus may directly or indirectly interact with the C-terminal ligand-binding domain.
