Subunit structure of the nonactivated human estrogen receptor

Area of Science:

• Molecular Biology
• Cellular Biology
• Endocrinology

Background:

• The estrogen receptor (ER) plays a crucial role in human mammary carcinoma.
• Understanding the nonactivated ER's subunit stoichiometry is vital for comprehending its function.
• Previous studies have suggested varying compositions for nuclear hormone receptor complexes.

Purpose of the Study:

• To determine the protein subunit stoichiometry of the nonactivated estrogen receptor.
• To investigate the molecular architecture of the ER complex in human MCF-7 cells.
• To compare the ER's structure with other nuclear hormone receptors.

Main Methods:

• Chemical cross-linking of the ER complex in cell extracts and intact cells.
• Immunoprecipitation using a monoclonal antibody against the ER.
• Denaturing gel electrophoresis and purification of the cross-linked receptor complex.
• Mass spectrometry and protein component analysis.

Main Results:

• The native, molybdate-stabilized ER complex has a molecular mass of approximately 300 kDa.
• Chemical cross-linking confirmed the 300 kDa complex and revealed a heteromeric structure.
• Purified complex contained the ER polypeptide (~65 kDa), hsp90, and p59, but not hsp70 or cyclophilin.
• One antibody molecule binding indicated a single ER polypeptide per complex.

Conclusions:

• The nonactivated estrogen receptor exhibits a heterotetrameric structure: one ER polypeptide, two hsp90 molecules, and one p59 subunit.
• This molecular architecture is homologous to glucocorticoid and progesterone receptors.
• Despite phylogenetic distinctions, the ER shares structural similarities with other nuclear hormone receptors.