The mechanism by which the v-rel oncogene of the avian Rev-T retrovirus transforms chicken spleen cells is not known. We have created v-rel mutants that show conditional properties by fusing sequences encoding the ligand-binding domain of the human estrogen receptor (ER) in-frame at the 3' end of the v-rel oncogene. Two vRel-ER fusion proteins showed estrogen-dependent subcellular localization in chicken embryo fibroblasts (CEF): vRel-ER proteins were located in the cytoplasm of CEF in the absence of estrogen and were located in the nucleus of CEF in the presence of estrogen. Wild-type vRel was located in the nucleus of CEF in the presence or absence of estrogen. Mobility shift assays using extracts from infected CEF showed that the ability of vRel-ER to bind DNA was also dependent on estrogen. However, the ability of vRel-ER to repress transcription from κB site-containing promoters was not dependent on estrogen. Finally, we were able to isolate a vRel-ER-transformed avian spleen cell line whose growth is dependent on estrogen; this indicates that a vRel function is needed for both the initiation and the maintenance of the transformed state. The vRel-ER protein may be useful for determining genes controlled by vRel.
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