An estrogen receptor from Xenopus laevis liver possibly connected with vitellogenin synthesis

This paper describes an estrogen receptor which is found in both the nucleus and cytoplasm of liver cells from male Xenopus laevis, and which seems to be involved in the induction of vitellogenin synthesis. It has a high affinity for estradiol (Kd = 0.5 x 10(-9) M), and the affinities of various steroids for the receptor correlate well with their ability to induce vitellogenin synthesis. It sediments at 3.5S at 0 degrees C in 0.5 M KCI. The rate of sedimentation is unaffected by incubation at 20 degrees C prior to centrifugation, but increases if the salt concentration is lowered to 0.1 M KCI or to zero. It has a Stokes radius of 2.6 nm and a molecular weight of approximately 40,000. The receptor is present at very low levels compared to other steroid target tissues (50--100 fold less than chick oviduct). The cytoplasm of a single hepatocyte contains 92 +/- 18 binding sites for estradiol, while each nucleus contains 99 +/- 19 sites.