Formation and characteristics of hepatic dexamethasone-receptor complexes of different molecular weight

The dexamethasone-binding receptor protein in rat liver cytosol has a Stokes radius of 61 A and a sedimentation coefficient of 4.0 S. In contrast, cell nuclei labelled with [3H]dexamethasone in vivo or in vitro (reconstitution experiments with [3H]dexamethasone-labelled cytosol and isolated unlabelled nuclei) contain a high-salt-extractable dexamethasone-receptor complex with a Stokes radius of 30-36 A and a sedimentation coefficient of 3.2 S. Exposure of liver homogenate or 1000 X g homogenate supernatant to low ionic strength during preparation of cytosol resulted in conversion of the 61 A to a 36 A complex very similar to the intranuclear form of dexamethasone receptor. 61 leads to 36 A complex-converting activity was present in both the 100 X g-10 000 X g sediment of liver homogenate, from which it could be extracted by hypotonic media, and in the liver cell nuclei, from which it could be extracted by hypertonic media. Mild digestion of the 61 A dexamethasone-receptor complex with trypsin also gave rise to a complex with a Stokes radius of 36 A. Reconstitution experiments with isolated liver cell nuclei indicated that both the 61 A and 36 A dexamethasone-receptor complexes were taken up by the nuclei; reextraction of the nuclei incubated with the 61 A complex revealed that this form had been converted to the 30-36 A complex. Further digestion of the 61 and 36 A [3H]dexamethasone-receptor complexes with hypotonic extract of the 1000 X g-10 000 X g sediment of liver homogenate or with trypsin resulted in formation of a third complex with Stokes radius of 19 A and a sedimentation coefficient of 2.5 S. The approximate molecular weights of the 61, 36 and 19 A dexamethasone-receptor complexes were calculated as 102 000, 46 000 and 19 000, respectively, and the frictional ratios of the molecules as 1.84, 1.38 and 1.00, respectively. It is concluded that the nuclear 30-36 A dexamethasone-receptor complex is formed from the cytosol 61 A complex by proteolytic digestion and that this latter protein contains at least two sites with a relatively high sensitivity to protelytic cleavage.