The molecular mechanisms of coactivator utilization in ligand-dependent transactivation by the androgen receptor.

Androgens drive sex differentiation, bone and muscle development, and promote growth of hormone-dependent cancers by binding the nuclear androgen receptor (AR), which recruits coactivators to responsive genes. Most nuclear receptors recruit steroid receptor coactivators (SRCs) to their ligand binding domain (LBD) using a leucine-rich motif (LXXLL). AR is believed to recruit unique coactivators to its LBD using an aromatic-rich motif (FXXLF) while recruiting SRCs to its N-terminal domain (NTD) through an alternate mechanism. Here, we report that the AR-LBD interacts with both FXXLF motifs and a subset of LXXLL motifs and that contacts with these LXXLL motifs are both necessary and sufficient for SRC-mediated AR regulation of transcription. Crystal structures of the activated AR in complex with both recruitment motifs reveal that side chains unique to the AR-LBD rearrange to bind either the bulky FXXLF motifs or the more compact LXXLL motifs and that AR utilizes subsidiary contacts with LXXLL flanking sequences to discriminate between LXXLL motifs.