Abstract
Nuclear receptors transduce hormonal signals by binding directly to DNA target sites in promoters and modulating the transcription of linked genes. Receptor-mediated transactivation appears to be potentiated in response to ligand by a number of coactivators that may provide key interactions with components of the transcription preinitiation complex and/or alter chromatin structure. Here, we use the vitamin D3 receptor ligand-binding domain (VDR LBD) as an affinity matrix to identify components of a transcriptionally active nuclear extract that interact with VDR in response to ligand. We describe the purification of a complex of at least 10 VDR interacting proteins (DRIPs) ranging from 65 to 250 kD that associate with the receptor in a strictly 1,25-dihydroxyvitamin D3-dependent manner. These proteins also appear to interact with other, but not all, nuclear receptors, such as the thyroid hormone receptor. The DRIPs are distinct from known nuclear receptor coactivators, although like these coactivators, their interaction also requires the AF-2 transactivation motif of VDR. In addition, the DRIP complex contains histone acetyltransferase activity, indicating that at least one or more of the DRIPs may function at the level of nucleosomal modification. However, we show that the DRIPs selectively enhance the transcriptional activity of VDR on a naked DNA template utilizing a cell-free, ligand-dependent transcription assay. Moreover, this activity can be specifically depleted from the extract by liganded, but not unliganded, VDR-LBD. Overexpression of DRIP100 in vivo resulted in a strong squelching of VDR transactivation, suggesting the sequestration of other limiting factors, including components of the DRIP complex. These results demonstrate the existence of a new complex of novel functional nuclear receptor coactivators.