Binding of liganded vitamin D receptor to the vitamin D receptor interacting protein coactivator complex induces interaction with RNA polymerase II holoenzyme

Because the vitamin D receptor interacting protein (DRIP) coactivator compl ex shares components with the RNA polymerase II (Pol II) holoenzyme complex , we tested whether the two protein complexes associate in cellular extract s. On initial purification steps, the DRIP complex copurified with the Pol II holoenzyme. Pol II was found to bind to the vitamin D receptor in a liga nd-dependent fashion when either nuclear extracts or partially purified pre parations were used as sources of DRIP and Pol II holoenzyme. A subpopulati on of holoenzyme complexes bound to the receptor because BRCA1, which assoc iates with the Pol II holoenzyme, did not associate with the liganded recep tor, and only in certain of the holoenzyme- and DRIP-containing fractions d id Pol II bind to the liganded receptor. Immunoprecipitation experiments re vealed that the DRIP complex was not pre-associated with the Pol II holoenz yme, hut the interaction between these two complexes was induced only in th e presence of receptor and ligand. These data support a model in which the activation of transcription by hormone-bound receptor requires binding to t he DRIP coactivator, and this induced ternary complex can then bind to the Pol II holoenzyme to activate transcription.