Association and dissociation of estrogen receptor with estrogen receptor-binding factors is regulated by Mg2+

It was recently shown that uterine estrogen receptor (ER) is translocated from the cytoplasm into the nucleus in the form of vero-ER X E (basic ER molecule bound with estradiol), and the translocation is inhibited by the specific binding of vero-ER X E with the cytoplasmic protein factors designated as ER-binding factors (ERBFs) ["5S" ER-forming factor ("5S" ER-FF), "6S" ER-FF, "8S" ER-FF] [Murayama, A. & Fukai, F. (1983) J. Biochem. 94, 511]. It was found that the specific interaction of vero-ER X E with the ERBFs is regulated by Mg2+ at physiological concentrations. The apparent Kd values of vero-ER X E for the ERBFs ["5S" ER-FF, 2.7 X 10(-9) M; "6S" ER-FF, 6.0 X 10(-8) M; "8S" ER-FF, 3.5 X 10(-8) M] observed in the presence of 1 mM Mg2+ were all increased over 10 times as compared with in the absence of Mg2+. The inhibitory effects of the ERBFs on the nuclear translocation of vero-ER X E were reduced by Mg2+.