Analysis of two forms of molybdate-stabilized estrogen receptor

We recently reported that the calf uterine estrogen receptor prepared in 10-mM molybdate elutes as two components (peaks I and II) from DEAE-Sephadex columns with a linear KCl gradient. We have extended these studies and report that unoccupied cytosol receptors also elute as two peaks (0.21 and 0.25 m KCl), as determined by postlabeling experiments with [3H]estradiol. Since molybdate-stabilized cytosol estrogen receptors incubated in the presence of 0.3 M KCl as well as estrogen receptors in peaks I and II did not demonstrate receptor binding to DNA-cellulose, we conclude that both receptor forms are nonactivated. Also, [3H]estradiol-receptor complexes from cytosol prepared in 10 mM molybdate sediment as 6.7S in 5-20% sucrose density gradients containing 0.3 M KCl, whereas receptor complexes from cytosol prepared without molybdate sediment as 4.5S. Both peaks I and II were eluted from DEAE-Sephadex columns prepared with either 10 mM molybdate or 10 mM tungstate, with both phosphatase inhibitors having effectively blocked salt activation of the receptor. However, receptor preparations in the presence of 10 mM arsenate were not eluted from DEAE-Sephadex, and arsenate was unable to inhibit receptor activation by KCl. Sucrose density gradient analysis of peaks I and II indicates that peak I sediments at approximately 4.8S, whereas peak II sediments at approximately 6.3S. If pooled fractions of the leading portion of peak I are rechromatographed on another DEAE-Sephadex column, both peaks I and II are recovered. Likewise, if pooled fractions of the descending portion of peak II are rechromatographed, both peaks I and II were measured. Therefore, we conclude that there may be an equilibrium between two forms of the molybdate-stabilized calf uterine estrogen receptor.