Production of recombinant androgen receptor in a heterologous expression system

To facilitate detailed studies of androgen receptor, we have produced a full-length receptor protein and some of its deletion mutants in Spodoptera frugiperda (Sf9) insect cells, using the baculovirus expression system. Recombinant baculovirus DNA-infected Sf9 cells expressed these proteins in very high quantities, which represented as much as 30-40% of total insect cell protein at 72 h after infection. Only < 1% of the recombinant protein was soluble in low-salt buffers; the majority formed electron-dense cytoplasmic aggregates 30-40 nm in diameter. These aggregates could be solubilized in 6 mol/L guanidine HCl, and biologically active receptor was generated by diluting the guanidine HCl preparation 20- to 50-fold. The full-length receptor, expressed either in a soluble or aggregated form, had characteristics typical of a native receptor: it bound steroids with high affinity and specificity, interacted with DNA in a sequence-specific fashion, and was recognized by domain-specific receptor antibodies. Androgen-receptor protein purified to homogeneity in guanidine HCl required the presence of Zn2+ ions during the refolding to reconstitute its DNA-binding form; ZnCl2 was not, however, needed to restore the receptor's steroid-binding activity.