Effects of calcium on the chick oviduct progesterone receptor

Abstract

The chick oviduct cytosol progesterone receptor can be transformed to a small form (R s = 21 A ̊ , S 20, w: 2.9 ) denoted “mero-receptor” by incubation in the presence of Ca 2+ [8]. In the molybdate-free cytosol all the progestin binding components could be completely transformed to mero-form by 1 h treatment with 100 mM Ca 2+ at 0°C. If EDTA was secondarily added, the ligand was rapidly released. If molybdate (20 mM) containing cytosol was incubated with Ca 2+, no radioactivity was found in the meroposition on the Agarose A 0.5 m column, but the bound steroid sedimented at 2.9 S in sucrose gradients containing Ca 2+ (and no molybdate). When 20 nM molybdate was added to cytosol containing receptor activated by 0.3 M KCl, complete mero-transformation by Ca 2+ was obtained also by the gel filtration criterion, indicating that molybdate does not inhibit the mero-transforming factor. Ligand-free progesterone receptor could also be completely converted to mero-form by endogenous cytosolic transforming factor and calcium. The transforming factor was completely inactivated, when cytosol was run through Agarose A 0.5 m gel. Mero-transformation was found to be irreversible. The purified progesterone receptor subunit 110 K (B) was partially converted to smaller forms by calcium alone (100 mM, 0°C, 1 h) whereas addition of a small amount of cytosol allowed complete conversion to mero-form.