Two Distinct Isoforms of cDNA Encoding Rainbow Trout Androgen Receptors

Androgens play an important role in male sexual differentiation and development. The activity of androgens is mediated by an androgen receptor (AR), which binds to specific DNA recognition sites and regulates transcription. We describe here the isolation of two distinct rainbow trout cDNA clones, designated rtAR-α and rtAR-β, which contain the entire androgen receptor coding region. Comparison of the predicted amino acid sequence of rtAR-α to that of rtAR-β revealed 85% identity. Interestingly, despite this high homology, rtAR-α activated transcription of an androgen-responsive reporter gene in co-transfection assays, but rtAR-β did not. These results suggest that rainbow trout contains two distinct isoforms of androgen receptors whose functions differ. The region of rtAR-β responsible for its inactivity was mapped to its ligand binding domain by analyzing chimeras of the rtAR-α, rtAR-β, and rtGR-I (glucocorticoid) receptors. Alteration of any one of three out of four segments within this domain restored activity. Extracts made from COS-1 cells transfected with an rtAR-α expression plasmid produced a high level of [ 3 H]mibolerone binding, whereas no binding was observed by extracts of cells transfected with an rtAR-β expression plasmid. These data demonstrate that the lack of transactivation activity of rtAR-β is due to its inability to bind hormone.