Oxysterol binding protein.

A binding protein is described for certain oxygenated derivatives of cholesterol which suppress 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol synthesis in cultured mammalian cells. This protein is found in the cytosolic fraction of many cell types and is distinct from cytosolic proteins which bind cholesterol. The relative binding affinity of a wide variety of oxysterols correlates with their ability to suppress reductase and it is proposed that the binding protein functions as a receptor for endogenous regulatory oxysterols. The binding protein from cultured mouse fibroblasts (L cells) has been partially purified and characterized. Changes in its molecular form occur when a ligand is bound and further changes in form and binding kinetics occur at acid pH and in the presence of urea. Based on these changes a subunit model for the binding protein is presented.