Transcriptional activation by the estrogen receptor is mediated through its
interaction with coactivator proteins upon ligand binding. By systematic m
utagenesis, we have identified a group of conserved hydrophobic residues in
the ligand binding domain that are required for binding the p160 family of
coactivators. Together with helix 12 and lysine 366 at the C-terminal end
of helix 3, they form a hydrophobic groove that accommodates an LXXLL motif
, which is essential for mediating coactivator binding to the receptor. Fur
thermore, we demonstrated that the high-affinity binding of motif 2, conser
ved in the p160 family, is due to the presence of three basic residues N te
rminal to the core LXXLL motif. The recruitment of p160 coactivators to the
estrogen receptor is therefore likely to depend not only on the LXXLL moti
f making hydrophobic interactions with the docking surface on the receptor,
but also on adjacent basic residues, which may be involved in the recognit
ion of charged residues on the receptor to allow the initial docking of the
motif.