Nuclear receptor (NR) transcriptional activity is controlled by agonist binding and concomitant exchange of receptor-associating corepressor proteins for NR box-containing, receptor AF-2-targeting coactivator proteins. We report here that TNIP1 is an atypical NR coregulator. Requirements for TNIP1-RAR interaction-its NR boxes, ligand, and the receptor's AF-2 domain-are characteristic of coactivators. However, TNIP1 reduces RAR activity. Repression is partially relieved by SRC1, Suggesting interference with coactivator recruitment as a mechanism of TNIP1 repression. TNIP1 does not bind RXR alpha and RAR alpha AF-2 domain, necessary for that receptor's association with TNIP1, is insufficient to confer upon RXR alpha interaction with TNIP1. Preferential interaction of RAR alpha over RAR gamma with TNIP1 can be mapped to RAR alpha ligand binding domain helices 5-9 and suggests regions outside the receptor helix 12 modulate interaction of NRs and NR box-containing corepressors. TNIP1 repression of RARs in the presence of RA places it in a small category of corepressors of agonist-bound NRs. (C) 2009 Elsevier Inc. All rights reserved.