Estrogen-mediated ubiquitylation and subsequent degradation of the estrogen receptor alpha (ER alpha) appears to be involved in the transcriptional activity of ER alpha. We show that the estrogen-responsive finger protein (EFP) interacts with and ubiquitylates ER alpha. EFP promoted the ubiquitylation of ER alpha in vitro and in vivo and consequently promoted the degradation of ER alpha. The interaction between EFP and ER alpha was greatly enhanced in the presence of estrogen. The action of EFP on ER alpha in the presence of estrogen resulted in a robust interaction between ER alpha and Tip60, one of the transcriptional coactivators, leading to activation of ER alpha transcriptional activity. However, a dominant negative mutant of EFP lacking the RING domain prolonged the half-life of ER alpha and inhibited the transcription by ER alpha. Our results indicate that EFP functions as a cofactor for ER alpha-mediated transcription, thus suggesting that ER alpha-mediated transcription is closely linked to he ubiquitylation of ER alpha. (c) 2007 Elsevier Inc. All rights reserved.