FXXLF and WXXLF Sequences Mediate the NH2-terminal Interaction with the Ligand Binding Domain of the Androgen Receptor

The nuclear receptor superfamily members of eukaryotic transcriptional regulators contain a highly conserved activation function 2 (AF2) in the hormone binding carboxyl-terminal domain and, for some, an additional activation function 1 in the NH2-terminal region which is not conserved. Recent biochemical and crystallographic studies revealed the molecular basis of AF2 is hormone-dependent recruitment of LXXLL motif-containing coactivators, including the p160 family, to a hydrophobic cleft in the ligand binding domain. Our previous studies demonstrated that AF2 in the androgen receptor (AR) binds only weakly to LXXLL motif-containing coactivators and instead mediates an androgen-dependent interaction with the AR NH2-terminal domain required for its physiological function. Here we demonstrate in a mammalian two-hybrid assay, glutathione S-transferase fusion protein binding studies, and functional assays that two predicted α-helical regions that are similar, but functionally distinct from the p160 coactivator interaction sequence, mediate the androgen-dependent, NH2- and carboxyl-terminal interaction. FXXLF in the AR NH2-terminal domain with the sequence23FQNLF27 mediates interaction with AF2 and is the predominant androgen-dependent interaction site. This FXXLF sequence and a second NH2-terminal WXXLF sequence 433WHTLF437 interact with different regions of the ligand binding domain to stabilize the hormone-receptor complex and may compete with AF2 recruitment of LXXLL motif-containing coactivators. The results suggest a unique mechanism for AR-mediated transcriptional activation.