M. Puzianowskakuznicka et al., FUNCTIONAL-CHARACTERIZATION OF A MUTANT THYROID-HORMONE RECEPTOR IN XENOPUS-LAEVIS, The Journal of biological chemistry, 271(52), 1996, pp. 33394-33403
Thyroid hormone plays a causative role during frog metamorphosis, and
its effect is mediated by thyroid hormone receptors (TRs), To investig
ate the function of Xenopus TRs, we have recently developed a thyroid
hormone dependent in vivo transcription system by introducing TRs and
RXRs (9-cis-retinoic acid receptors) into Xenopus oocytes. Interesting
ly, using this system, we have found that the TR alpha B cloned previo
usly is defective in transcriptional activation compared with TR alpha
A, In vitro DNA binding experiments show that TR alpha B . RXR hetero
dimers have drastically reduced affinity for a thyroid hormone respons
e element, Site-directed mutagenesis shows that two of the seven amino
acid residues that differ between TR alpha A and TR alpha B are respo
nsible for the defect in TR alpha B function. These two residues affec
t the DNA binding by both TR . RXR heterodimers and TR homodimers, In
contrast, heterodimer formation with RXRs is not affected as demonstra
ted by coimmunoprecipitation and dominant-transcriptional inhibition e
xperiments, By cDNA and genomic DNA sequence analysis, we have demonst
rated that the residues, which affect TR alpha B function when mutated
, are identical between the wild type TR alpha B and TR alpha A, Thus,
our experiments have discovered the first amphibian TR mutant, The DN
A binding and transcription activation functions of the mutant are dis
cussed in relation to the recently published TR crystal structure.