The nuclear receptor corepressor (N-CoR) contains three isoleucine motifs (I/LXXII) that serve as receptor interaction domains (IDs)

Unliganded thyroid hormone receptors (TRs) repress transcription through re cruitment of corepressors, including nuclear receptor corepressor (N-CoR). We find that N-CoR contains three interaction domains (IDs) that bind to TR , rather than the previously reported two. The hitherto unrecognized ID (ID 3) serves as a fully functional TR binding site, both in vivo and in vitro, and may be the most important for TR binding. Each ID motif contains a con served hydrophobic core (I/LXXII) that resembles the hydrophobic core of nu clear receptor boxes (LXXLL), which mediates p160 coactivator binding to li ganded nuclear receptors. Although the integrity of the I/LXXII motif is re quired for ID function, substitution of ID isoleucines with leucines did no t allow ID peptides to bind to liganded TR, and substitution of NR box leuc ines with isoleucines did not allow NR box peptides to bind unliganded TR. This indicates that the binding preferences of N-CoR for unliganded TR and p160s for liganded TR are not dictated solely by the identity of conserved hydrophobic residues within their TR binding motifs. Examination of sequenc e conservation between IDs, and mutational analysis of individual IDs, sugg ests that they are comprised of the central hydrophobic core and distinct a djacent sequences that may make unique contacts with the TR surface. Accord ingly, a hybrid peptide that contains distinct adjacent sequences from ID3 and ID1 shows enhanced binding to TR.