Estrogen Receptor Transcription Factor Assay Kit (Colorimetric) (ab207203)

ER belongs to the superfamily of ligand-inducible transcription factors. Human ERa is comprised of 595 amino acids and displays an approximate molecular weight of 66-70 kDa. Six functional regions have been identified. A hypervariable domain (aa 1-184) contains activation function 1 (AF1). The DNA binding domain (DBD, aa 185-263) contains two zinc finger motifs and is highly conserved across the nuclear receptor superfamily. It is responsible for the binding of the receptor to estrogen response elements (EREs) and contributes to dimerization and activation. Typically, EREs consist of two inverted half-sites separated by 3 bp (5´-GGTCAnnnTGACC-3´). The region which separates the ligand binding domain (LBD) and the DBD is called the hinge region (aa 264-302).

The LBD (aa 303-553) consists of 12 a-helices, which form a hydrophobic pocket responsible for ligand binding. The function of the final domain (aa 554-595) is not clear but is thought to play a role in distinguishing between agonist and antagonist binding. Human ERb is expressed as multiple isoforms. Structure and function studies have shown that the DBD of ERa and ERb are highly homologous, approaching 96%, whereas the LBD showed only 59% homology. The general mechanism of action of ERb is thought to be similar to that of ERa. ERa and ERb have the ability to interact with target promoters in three different complexes: ERa homodimers, ERb homodimers and ERa/ERb heterodimers.

The transcriptional effects of ER can be mediated through several mechanisms other than E₂-ER complexes binding to EREs. E₂-ER complexes can also trans activate genes through protein-protein interactions with transcription factors such as AP-1 or Sp1 that bind DNA, with coaccessory proteins (Src, ACTR), some of which have histone acetylase activity, and with RNA Polymerase II complex proteins. In addition, ERs serve to repress genes, which also plays an important role in E₂ action.