Factors That Enhance Escherichia coli -Expressed TRβ Binding to T 3 and DNA

Thyroid hormone receptors (TRs) recently have been produced in E. coli by several laboratories. We produced E. coli-expressed human TRβ using the histidine/fusion protein system. Surprisingly, we observed that reticulocyte lysate, nonspecific proteins, and 1% Triton X dramatically increased both the T₃- and DNA-binding activities of human TRβ. These studies demonstrate that there are a number of factors that will enhance ligand and DNA binding of E. coli-expressed TRβ. Addition of these factors to reaction samples containing E. coli-expressed TRs will help to optimize measurement conditions. These findings also suggest that experiments in which cellular proteins are added to highly purified TR preparations may require controls to eliminate contributions by nonspecific proteins.