A series of human androgen receptor (AR) deletion mutants was constructed to study the relationship between the structural domains and their different functions in the AR protein. Human AR mutants were expressed in COS-1 and HeLa cells to investigate hormone binding, transcriptional activation, and subcellular localization. The wild-type human AR (AR 1–910) was expressed as a 110- to 112-kDa doublet, as revealed on immunoblots. All mutant AR proteins also migrated as doublets, except for one. This AR has a deletion from amino acid residues 51–211 and migrated as a single protein band, possibly due to altered posttranslational modification.

The AR steroid-binding domain is encoded by approximately 250 amino acid residues in the Cterminal end. Deletions in this domain as well as truncation of the last 12 C-terminal amino acid residues abolished hormone binding.

Cotransfection studies in HeLa cells showed that transcriptional activation of an androgen-regulated reporter gene construct was induced by the wildtype human AR. Mutational analysis revealed two regions in the N-terminal part, encoded by amino acid residues 51–211 and 244–360, to be essential for this transcriptional activation. Deletion of the hormone-binding domain yielded a constitutively active AR protein, indicating that in the absence of hormone this domain displays an inhibitory function.

In the presence of its ligand, the wild-type AR was located in the cell nucleus. In the absence of androgens the receptor was mainly nuclear, but cytoplasmic localization was observed as well. In contrast, an AR deletion mutant lacking part of the DNAbinding domain and part of the hinge region was exclusively cytoplasmic in the absence of hormone. This mutant AR lacks a region that is highly conserved among steroid receptors and homologous to the simian virus-40 large T-antigen- and nucleoplas-min-nuclear localization signals. Transformation by hormone directed this mutant to the nucleus, indicating the presence of a second, hormone-dependent nuclear targeting mechanism.