Supernatant Protein Factor in Complex with RRR-α-Tocopherylquinone: A Link Between Oxidized Vitamin E and Cholesterol Biosynthesis

The vast majority of monomeric lipid transport in nature is performed by lipid-specific protein carriers. This class of proteins can enclose cognate lipid molecules in a hydrophobic cavity and transport them across the aqueous environment. Supernatant protein factor (SPF) is an enigmatic representative of monomeric lipid transporters belonging to the SEC14 family. SPF stimulates squalene epoxidation, a downstream step of the cholesterol biosynthetic pathway, by an unknown mechanism. Here, we present the three-dimensional crystal structure of human SPF in complex with RRR-α-tocopherylquinone, the major physiological oxidation product of RRR-α-tocopherol, at a resolution of 1.95 Å. The structure of the complex reveals how SPF sequesters RRR-α-tocopherylquinone (RRR-α-TQ) in its protein body and permits a comparison with the recently solved structure of human α-tocopherol transfer protein (α-TTP) in complex with RRR-α-tocopherol. Recent findings have shown that RRR-α-TQ is reduced in vivo to RRR-α-TQH2, the latter has been suggested to protect low-density lipoprotein (LDL) particles from oxidation. Hence, the antioxidant function of the redox couple RRR-α-TQ/RRR-α-TQH2 in blocking LDL oxidation may reduce cellular cholesterol uptake and thus explain how SPF upregulates cholesterol synthesis.