The
androgen receptor (AR) is transcriptionally activated by high affinity binding of
testosterone (T) or its 5alpha-reduced metabolite,
dihydrotestosterone (DHT), a more potent
androgen required for male reproductive tract development. The molecular basis for the weaker activity of T was investigated by determining T-bound
ligand binding domain crystal structures of wild-type AR and a
prostate cancer somatic mutant complexed with the AR FXXLF or coactivator LXXLL
peptide. Nearly identical interactions of T and DHT in the AR
ligand binding pocket correlate with similar rates of dissociation from an AR fragment containing the
ligand binding domain. However, T induces weaker AR FXXLF and coactivator LXXLL motif interactions at activation function 2 (
AF2). Less effective FXXLF motif binding to
AF2 accounts for faster T dissociation from full-length AR. T can nevertheless acquire DHT-like activity through an AR helix-10 H874Y
prostate cancer mutation. The Tyr-874 mutant side chain mediates a new hydrogen bonding scheme from exterior helix-10 to backbone
protein core helix-4 residue Tyr-739 to rescue T-induced AR activity by improving
AF2 binding of FXXLF and LXXLL motifs. Greater AR
AF2 activity by improved core helix interactions is supported by the effects of
melanoma antigen gene protein-11, an AR coregulator that binds the AR FXXLF motif and targets
AF2 for activation. We conclude that T is a weaker
androgen than DHT because of less favorable T-dependent AR FXXLF and coactivator LXXLL motif interactions at
AF2.