THE NONACTIVATED ESTROGEN-RECEPTOR (NAER) OF THE GOAT UTERUS IS A TYROSINE KINASE

The nonactivated estrogen receptor (naER) has been isolated and purifi ed to absolute homogeneity from the goat uterine cytosol. It is a 66-k Da protein, sedimenting at 4.2 S on linear sucrose density gradients a nd having a Stokes radius of 36 Angstrom. It displays high affinity an d specificity for estradiol and diethyl stilbestrol with a K-d of 1 X 10(-10) M. CNBr peptide analysis reveals that it has a primary structu re distinctly different from that of the regular estrogen receptor eve n though anti-ER antibody cross-reacts with the nonactivated ER. The p rotein gains access to the DNA only upon dimerization with the estroge n receptor activation factor (E-RAF), a DNA-binding protein having no capacity to bind estradiol. Analysis reveals that both naER and E-RAF are protein kinases. While the E-RAF is a serine kinase, naER function s as a tyrosine kinase. No protein kinase activity is displayed by the regular estrogen receptor. The protein kinase activity of the naER is inhibited in the presence of estradiol. Similarly, the protein kinase activities associated with the proteins disappear when the naER and E -RAF are brought together. (C) 1994 Academic Press, Inc.