Two transcription activation functions in the amino terminus of the mouse estrogen receptor that are affected by the carboxy terminus

To determine the characteristics of the N-terminal transactivation domain (AF-I) of the mouse estrogen receptor (ER), we constructed a number of deletion mutants. Wild-type and mutant receptors were expressed in yeast cells and assayed for their ability to transactivate an estrogen-responsive reporter plasmid (ERE-CYC1-LacZ) that contained a single estrogen response element of the vitellogenin A2 gene promoter. Deletion of the N-terminal 121 amino acids from the mouse ER resulted in a 50% reduction in transactivation activity compared with the full-length wild-type ER. Deletion of the first 150 amino acids resulted in loss of 90% transactivation activity. An ER deletion mutant lacking residues 121–154 retained full transcriptional activity, suggesting that this region plays a significant transacting role only when the first portion is deleted. A point mutation was introduced in the C-terminal region at Met-521 in order to study the possible interaction between the C-terminal ligand-binding domain and the N-terminal AF-1 region. This mutant ER, M521G, exhibited 150% of the transcriptional activity of the wild-type ER. An M521G mutant lacking the N-terminal 121 amino acids retained full transactivation activity, whereas, M521G lacking 150 amino acids resulted in only 10% of wild-type activity. These results suggest that residues 121–154 might interact with the C terminus to affect transcription. In summary, multiple N-terminal regions in the ER were identified that function in transactivation. Furthermore, a point mutation in the C-terminal portion of the ER may change the conformation of the ER ligand-binding domain, producing a more stable receptor/ligand complex that increases transcriptional activity. These data suggest that the N-and C-terminal portions of the ER interact in a cooperative manner to activate transcription from target genes.