Structural characterization of the native NH2-terminal transactivation domain of the human androgen receptor: a collapsed disordered conformation underlies structural plasticity and protein-induced folding

The androgen receptor (AR) mediates the action of the steroid hormones testosterone and dihydrotestosterone. The protein contains two globular alpha-helical domains responsible for binding hormone and DNA. In contrast, the N-terminal domain is less well structurally defined and contains the main determinants for receptor-dependent transactivation, termed AF1. Previously, we have shown this region has the propensity to form alpha-helix structure. Significantly, the binding of specific protein targets or a natural osmolyte resulted in a more protease resistant conformation for the AF1 domain, consistent with an increase in conformational stability. Computational and experimental analyses were used to investigate the conformational properties of the native AF1 domain. This region of the receptor is predicted to contain significant regions of natural disordered structure, when analyzed by amino acid composition, PONDR (Predictor of Natural Disordered Regions), RONN (Regional Order Neural Network), and GlobPlot, but is grouped with ordered proteins on a charge-hydropathy plot. The binding of a hydrophobic fluorescence probe, 8-anilinonaphthalene-1-sulfonic acid (ANS), together with size-exclusion chromatography suggests that native AR-AF1 exists in a collapsed disordered conformation, distinct from extended disordered (random coil) and a stable globular fold. This state has also been described as premolten or molten globule-like. These findings are discussed in terms of the functional importance of the intrinsic plasticity of the AF1 domain.