TIF1β/KAP-1 is a coactivator of the orphan nuclear receptor NGFI-B/ Nur77

Abstract

In efforts to define mechanisms of transcriptional activation by the orphan nuclear receptor NGFI-B (Nur77), we identified TIF1β by mass spectrometry within a nuclear protein complex containing NGFI-B. TIF1β, also known as KAP-1 (KRAB domain-associated protein) or KRIP-1, acts as a transcriptional corepressor for many transcription factors, in particular for the Krüppel-associated box domain-containing zinc finger transcription factors. TIF1β is also an intrinsic component of two chromatin remodeling and histone deacetylase complexes, the N-CoR1 and nucleosome remodeling and deacetylation complexes. In contrast to these activities, we report that TIF1β is a coactivator of NGFI-B and that it is as potent as the SRC coactivators in this context. Using pull-down assays and immunoprecipitation, we showed that TIF1β interacts directly with NGFI-B and with other Nur family members. NGFI-B is an important mediator of hypothalamic corticotropin-releasing hormone (CRH) activation of proopiomelanocortin (POMC) transcription, and TIF1β enhances transcription mediated through the NGFI-B target, the Nur response element (NurRE). The NurRE binds Nur factor dimers and is responsive to signaling pathways. In keeping with the role of NGFI-B as mediator of CRH signaling, we found that TIF1β is recruited to the POMC promoter following CRH stimulation and that TIF1β potentiates CRH and protein kinase A signaling through the NurRE; it acts synergistically with the SRC2 coactivator. However, the actions of TIF1β and SRC2 were mapped to different NGFI-B AF-1 subdomains. Taken together, these results indicate that TIF1β is an important coactivator of NGFI-B-dependent transcription. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.