Differential recruitment of coactivator RIP140 by Ah and estrogen receptors - Absence of a role for LXXL motifs

The Ah receptor (AhR), a soluble cytosolic protein, mediates most of the to xic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related envir onmental contaminants. The mechanism of ligand-mediated AhR activation has been, in part, elucidated. The sequence of events following the binding of the AhR/AhR nuclear translocator protein (ARNT) heterodimer to dioxin respo nse elements has yet to be completely understood. The role of coactivator, RIP140, in the modulation of transcriptional activity of AhR/ARNT heterodim er was examined. RIP140 enhanced TCDD-mediated, dioxin response element-dri ven reporter gene activity in three cell lines. Co-immunoprecipitation and co-localization assays revealed that RIP140 interacted with AhR, but not wi th ARNT, both in vitro and in cells. Mapping of the interaction sites revea led that RIP140 was recruited by the AhR transactivation domain via the Q-r ich subdomain. The RIP140 domain that interacts with the AhR was mapped to a location between amino acid residues 154 and 350, which is distinct from those involved in estrogen receptor binding. The signature motif, LXXLL, wh ich is responsible for binding of several coactivators to nuclear receptors , is not required for RIP140 binding to AhR. These results demonstrate that the AhR recruits coactivators that are capable of enhancing transcription and, thus, the AhR may compete with steroid receptors for a common coactiva tor pool. In addition, the data suggest that there are distinct motif(s) fo r the recruitment of RIP140 to AhR and possibly other non-steroid receptors /transcription factors.