Corepressor subnuclear organization is regulated by estrogen receptor via a mechanism that requires the DNA-binding domain

The restriction of transcription factors to certain domains within the cell nucleus must serve an important regulatory function. The silencing mediator of retinoic acid and thyroid hormone (SMRT) and other members of the corepressor complex are enriched in spherical intranuclear foci, and repress estrogen receptor alpha (ERα)-dependent transcriptional activity. When fluorescent protein (FP)-labeled SMRT and ERα were co-expressed, the proteins co-localized. The subnuclear organization and positioning of the complexes, however, depended on the ligand state of the receptor. Automated image analysis was used to quantify the ERα-dependent change in SMRT organization in randomly selected living cell populations. The results demonstrate that the subnuclear positioning of SMRT is influenced by the ligand-bound ERα, and this activity is dependent on the ratio of the co-expressed ERα and SMRT. A deletion mutant of ERα showed that the receptor DNA-binding domain was necessary for the ligand-dependent positioning of SMRT. These results define important organizational mechanisms that underlie nuclear receptor regulation of gene expression.