PHOSPHORYLATION OF THE HUMAN ESTROGEN-RECEPTOR BY MITOGEN-ACTIVATED PROTEIN-KINASE AND CASEIN KINASE-II - CONSEQUENCE ON DNA-BINDING

We determined the amino acid and radiolabel sequences of tryptic [P-32 ]phosphopeptides of the purified human estrogen receptor (hER) from MC F-7 cells and Sf9 cells. Serine 118 was identified as a site that was phosphorylated independently of estradiol-binding in MCF-7 cells. Prol ine is on the carboxy terminus of serine 118, which suggests that the serine-proline may be a consensus phosphorylation site motif for eithe r the mitogen-activated protein (MAP) kinase or p34(cdc2) kinase. MAP kinase selectively phosphorylated the recombinant hER in vitro on seri ne 118 independent of estradiol-binding, whereas p34(cdc2) did not pho sphorylate the hER. We demonstrated previously that serine 167 of the hER was phosphorylated in an estradiol-dependent manner. We therefore compared the consequence of hER phosphorylation at serine 118 by MAP k inase and phosphorylation at serine 167 by casein kinase II on the rec eptor's affinity for specific DNA binding. The binding of the hER to a n estrogen response element was not altered by phosphorylation with MA P kinase at serine 118 but was significantly increased when phosphoryl ated at serine 167 by casein kinase II. These data suggest that phosph orylation of the hER by MAP kinase(s) pathways may influence receptor action by a mechanism other than the estradiol-dependent phosphorylati on of hER by casein kinase II.