The structural organization of the oxysterol receptor, postulated to be involved in the regulation of 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol biosynthesis in mammalian cells, has been explored by limited proteolysis with trypsin, alpha-chymotrypsin, and endoproteinase GluC. Treatment with each of these proteases converts the receptor from a homodimer of approximately 95 kDa subunits to a 44-kDa form, based on hydrodynamic measurements by sucrose density gradient centrifugation and gel filtration chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of photoaffinity-labeled preparations indicates that the oxysterol binding site is on a 28-kDa fragment within the 44-kDa limit form of the receptor. The limit proteolytc form exhibits the high affinity and structural specificity for oxysterols of the native dimeric receptor with an increase in the rate constant of association for 25-hydroxycholesterol. The proteolytic form also shows an increased binding affinity for nonspecific DNA, but no sequence specificity for the oxysterol regulatory element from the reductase gene was detected
