Purification, subunit structure, and DNA binding properties of the mouse oxysterol receptor

A cytosolic receptor protein for oxygenated sterols, postulated to be involved in the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and cholesterol biosynthesis, has been purified from mouse L cell cytosol greater than 3,600-fold in its undenatured form and to apparent homogeneity upon further electrophoresis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified 7.5 S receptor appears to be a dimer with similar or identical subunits of Mr 95,000. Proteolytic cleavage by an endogenous factor(s) gives rise to a 4.2 S form of the receptor which is resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into a heterogeneous mixture of ligand binding fragments of Mr 30,000-60,000. This 4.2 S form of the receptor retains high affinity for the oxysterol ligand and exhibits a more rapid oxysterol binding rate than the 7.5 S form. The 7.5 S form of the receptor binds to DNA-cellulose at low salt concentrations at neutral pH, and its affinity increases at low pH or in the presence of Zn2+. Receptor preparations from mouse liver were purified approximately 900-fold by the same purification procedure, but this was accompanied by conversion of the 7.5 S liver receptor to a approximately 4 S form, Mr approximately 55,000.