The unique C-termini of the thyroid hormone receptor variant, c-erbAα2, and thyroid hormone receptor α1 mediate different DNA-binding and heterodimerization properties

Abstract

Thyroid hormone receptors (TRs) mediate the regulation of gene transcription by thyroid hormone (T3) by binding to T3-responsive elements (TREs) in target genes. c-erbAα2 is a C-terminal TR variant which does not bind T3 and is a dominant inhibitor of T3 action. When synthesized in Escherichia Coli, α2 formed two TRE-binding complexes similar to the monomeric and homodimeric forms of TRα1 However, α2 did not bind nearly as well as TRα1 Furthermore, α2 failed to bind DNA with proteins that heterodimerized with TRα1 TRα1 and α2 also did not bind DNA as heterodimers with one another. The differences between TRα1 and α2 were further analyzed by studying a variety of C-terminal mutants synthesized in reticulocyte lysates. Deletion of the last 20 of the 122 unique amino acids (aa) of α2 increased its DNA binding to approximately the level of TRα1, indicating that the C-terminus of α2 is an inhibitory domain. This α2 mutant (α2ΔC) was still unable to heterodimerize with nuclear proteins, as were C-terminal deletion mutants of TRα1. We hypothesized that fusion of TRα1-specific sequences to the C-terminus of α2ΔC would transfer the property of heterodimerization. Indeed, although α2/α1 chimeras containing the last 40 and 70 aa of TRα1 failed to heterodimerize with nuclear proteins, addition of the last 100 or 150 aa of TRα1 did render α2ΔC heterodimerization competent. Thus, TRα1 contains a C-terminal structure which is necessary for heterodimerization and can confer this property on α2, which lacks this domain. The effects of the unique C-termini of TRα1 and α2 on their in vitro DNA binding have important implications for their mechanisms of action in vivo.