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-alpha and rtAR-beta, which contain the entire androgen receptor coding region. Comparison of the predicted amino acid sequence of rtAR-alpha to that of rtAR-beta revealed 85% identity. Interestingly, despite this high homology, rtAR-alpha activated transcription of an androgen-responsive reporter gene in co-transfection assays, but rtAR-beta did not. These results suggest that rainbow trout contains two distinct isoforms of androgen receptors whose functions differ. The region of rtAR-beta responsible for its inactivity was mapped to its ligand binding domain by analyzing chimeras of the rtAR-alpha, rtAR-beta, 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-alpha expression plasmid produced a high level of [3H]mibolerone binding, whereas no binding was observed by extracts of cells transfected with an rtAR-beta expression plasmid. These data demonstrate that the lack of transactivation activity of rtAR-beta is due to its inability to bind hormone.