Characterization of ligand-dependent phosphorylation of the estrogen receptor.

The mouse estrogen receptor is phosphorylated upon estrogen binding at multiple serine residues located mainly between residues 121 and 599. Phosphorylation is progressively reduced in mutant receptors that are defective in estrogen- and DNA-binding activities, suggesting that it occurs in stages, initially as a consequence of hormone binding and subsequently after DNA binding. Phosphopeptide maps of the receptor expressed in the presence of estrogen or 4-hydroxytamoxifen are similar, suggesting that the effects of this antiestrogen on transcriptional activity are not mediated by differences in phosphorylation. Although it is unclear whether phosphorylation is a prerequisite for transcriptional activity, the similarity in the phosphopeptide maps of the wild-type receptor and the transcriptionally defective mutant confirm that phosphorylation does not occur simply as a consequence of estrogen-dependent transcriptional activation.