Transformation of the glucocorticoid receptor in the cell-free cytosol of the neural retina of the chick embryo: changes in the size and charge of the receptor complex during transformation suggest a multistage process.

The physicochemical properties of the glucocorticoid receptors (GR), and the molecular changes induced during their transformation in the cell-free cytosol of the neural retina of the chick embryo, were investigated. The surface charge of the various size forms of the GR complex was determined on gel filtration and/or glycerol density gradient-isolated GR, by electrofocusing under nondenaturing conditions. The nontransformed molybdate-stabilized GR in hypotonic buffer (containing PMSF) appears as a 350 kilodalton (kDa) complex (Rs = 8.6 nm, S = 9.5), with an apparent pI value (pI') of 4.4 +/- 0.1. The GRs in heat or salt-activated cytosols appear as a 90 kDa hormone-receptor complex (Rs = 5.6 +/- 0.2, S = 3.9 +/- 0.1), which is resolved as a major peak with a pI' value of 6.2 +/- 0.1 and a minor peak with a pI' value of 5.4. The transformation of the 350 kDa oligomer to the 90 kDa monomer occurs in three stages. Two distinct dissociation steps were induced by 0.4 M KCl: (a) the dissociation of the 350 kDa complex to a 170 kDa complex (Rs = 7.8 +/- 0.2, S = 5.1 +/- 0.2), exhibiting a pI' value of 5.6 +/- 0.2, induced by salt and not inhibited by molybdate; and (b) the dissociation of the 170 kDa complex to the 102 kDa complex (Rs = 5.6 +/- 0.2, S = 4.4), also exhibiting a pI' value of 5.6 +/- 0.2, which is blocked by molybdate. The third step, the transition of the 102 kDa complex to the activated (nuclear-like), 90 kDa form, is dependent on cytosolic factors. It is induced in the isotonic milieu by physiological temperatures, and in the cold by exposing the crude cytosol to 0.4 M KCl. The nature of this cytosolic processing step is unknown. It occurs in the presence of PMSF, which presumably inhibits proteolytic GR degradation in the cytosol of the neural retina. Activated GR complexes tend to aggregate. Molybdate inhibits activation-induced GR-aggregation.