Steroid-receptor fusion of the human immunodeficiency virus type 1 Rev transactivator: mapping cryptic functions of the arginine-rich motif.

The human immunodeficiency virus type 1 (HIV-1) transactivator Rev is a nuclear protein that regulates expression of certain HIV-1 transcripts by binding to an RNA target element (the RRE) present in these transcripts. A short arginine-rich sequence in Rev contains the signals required to direct this protein into nuclei, where it associates preferentially with nucleoli. We created a steroid-inducible transactivator by fusing Rev with the steroid-binding domain of the glucocorticoid receptor (GR). This Rev/GR protein remains inactive in the cytoplasm when steroids are absent, but it enters the nucleus and initiates transactivation within minutes after exposure to dexamethasone. Although the GR moiety is sufficient to transport Rev/GR into nuclei, mutation of certain residues in the arginine-rich region blocks nucleolar localization and also inhibits transactivation. We find that other mutations in this region, however, can abolish the function of Rev/GR without affecting its localization; the latter phenotype may reflect a specific defect in binding of the RRE.