Synergism between ERalpha transactivation function 1 (AF-1) and AF-2 mediated by steroid receptor coactivator protein-1: requirement for the AF-1 alpha-helical core and for a direct interaction between the N- and C-terminal domains.

The transcriptional activity of ERalpha (or NR3A1) after binding of ligand is mediated through synergistic action between activation functions (AFs) AF-1 and AF-2 and the transcriptional machinery. This is functionally achieved by bridging coactivators such as CEBP binding protein/p300 and members of the p160 subfamily such as steroid receptor coactivator protein-1 (SRC-1). We previously identified a conserved potential alpha-helical structure within the AF-1 functional core, and by evaluating point mutants of human ERalpha (hERalpha) within this region, we show that in transfection experiments this structure is required for synergism between SRC-1 and hERalpha. We report that the transcriptional synergism between AF-1 mutants and SRC-1 was abolished in AF-1-sensitive cells such as HepG2, whereas it was reduced by 50% in CHO-K1 cells, which have a mixed context that is sensitive to both the AF-1 and AF-2 regions of hERalpha. Glutathione-S-transferase pulldown assays demonstrate that the AF-1 core is able and sufficient for the hERalpha N-terminal region to interact with SRC-1. Interestingly, an enhancement of this recruitment in the presence of the hERalpha ligand-binding domain was observed, which was found to be dependent on a direct interaction between the N-terminal B domain and the ligand-binding domain. Another functional consequence of this physical interaction, which is promoted by both partial and full agonists of hERalpha, was an increase in the phosphorylation state of the N-terminal domain. Binding of 4-hydroxytamoxifen (OHT) to the hERalpha C-terminal region induced a functional AF-1 conformation in vitro through this N- and C-terminal interaction. The involvement of an SRC-1-mediated pathway in transactivation mediated by hERalpha AF-1 was further substantiated by transfection experiments using the OHTresponsive human C3 promoter, which showed that OHT-induced hERalpha AF-1 activity was enhanced by SRC-1 and required the AF-1 alpha-helical structure. In conclusion, we demonstrate that the synergism between AF-1 and AF-2 is mediated in part by a cooperative recruitment of SRC-1 by both the AF-1 alpha-helical core and AF-2 regions and that it is stabilized by a direct interaction between the B and C-terminal domains. This interaction of SRC-1 with the AF-1 alpha-helical core is essential for both E2- and OHT-induced ERalpha activity.