The ability of steroid receptor RNA activator (SRA), an AF-1 coactivator, to contribute to differences in estrogen receptor (ER)-alpha and ERP transcriptional activity was tested. In transient transfections, SRA expression increased ERalpha- and ERbeta-dependent gene expression. However, when the receptors' amino-terminal A/B regions were examined as GAL4 DNA binding domain fusions, SRA enhanced the activity of GAL-ABalpha but not GAL-ABbeta. Exogenous SRA also enhanced AF-2 activity for both receptors, indicating that SRA effects are not limited to AF-1. Simultaneously mutating three phosphorylation sites within GAL-ABalpha domain only modestly reduced SRA coactivation of GAL-ABalpha, suggesting that phosphorylation does not play a major role in SRA function relative to this domain. SRA enhanced ERalpha activity stimulated by 4-hydroxytamoxifen, but was unable to convert this mixed anticstrogen to an ERP agonist. Thus, SRA is an ERalpha AF-1-specific coactivator that enhances the agonist activity of tamoxifen-bound ERalpha and may contribute to tamoxifen resistance. (C) 2004 Elsevier Inc. All rights reserved.