Limited proteolysis of the mouse liver glucocorticoid receptor

Abstract

Activation of the mouse liver glucocorticoid receptor resulted in the generation of a protein of very different characteristics from that found previously in the mouse AtT-20 pituitary tumor cell line. Ion-exchange and adsorption chromatography showed that the activated liver receptor was a more basic protein, it eluted earlier from a DEAE-cellulose column, while a later elution was observed on phosphocellulose and DNA-cellulose chromatography. Further experiments showed that this was due to proteolysis of the liver receptor to a smaller form (3.2 S; Rs = 3.9 nm; MW = 53,000; f/f0 = 1.45) after activation. Mero-receptor (2.4 S; Rs = 2.4 nm; MW = 24,000; f/f0 = 1.15) was detectable when cytosol was chromatographed on hydroxylapatite or was treated with trypsin. These proteolytic fragments are similar to those obtained for various other steroid hormone receptors. Mixing experiments with liver cytosol showed that the AtT-20 glucocorticoid receptor could also be cleaved to these fragments. Endogenous proteases are apparently in low concentration in this cell line. Finally, activation appears to result in the exposure of the protease-sensitive regions, since sodium molybdate, which prevents receptor activation, also renders the receptor resistant to proteolysis.