A novel homeobox protein which recognizes a TGT core and functionally interferes with a retinoid-responsive motif

Abstract

We describe here a novel homeobox gene, denoted TGIF (5'TG3' interacting factor), which belongs to an expanding TALE (three amino acid loop extension) superclass of atypical homeodomains. The TGIF homeodomain binds to a previously characterized retinoid X receptor (RXR) responsive element from the cellular retinol-binding protein II promoter (CRBPII-RXRE), which contains an unusual DNA target for a homeobox. The interactions of both the homeoprotein TGIF and receptor RXRα with the CRHPII-RXRE DNA motif occur on overlapping areas and generate a mutually exclusive binding in vitro. Transient cellular transfections demonstrate that TGIF inhibits the 9-cis- retinoic acid-dependent RXRα transcription activation of the retinoic acid responsive clement. TGIF transcripts were detected in a restricted number of tissues. The canonical binding site of TGIF is conserved and is an integral part of several responsive elements which are organized like the CRHPII- RXRE. Hence, a novel auxiliary factor to the steroid receptor superfamily may participate in the transmission of nuclear signals during development and in the adult, as illustrated by the down-modulation of the RXRα activities.