Highly purified scavenger receptor class B,type I reconstituted into phosphatidylcholine/cholesterol liposomes mediates high affinity high density lipoprotein binding and selective lipid uptake

The murine class B, type I scavenger receptor mSR-BI is a high and low density lipoprotein (HDL and LDL) receptor that mediates selective uptake of cholesteryl esters. Here we describe a reconstituted phospholipid/cholesterol liposome assay of the binding and selective uptake activities of SR-BI derived from detergent-solubilized cells. The assay, employing lysates from epitope-tagged receptor (mSR-BI-t1)-expressing mammalian and insect cells, recapitulated many features of SR-BI activity in intact cells, including high affinity and saturable (125)I-HDL binding, selective lipid uptake from (3)H]cholesteryl ether-labeled HDL, and poor inhibition of HDL receptor activity by LDL. The novel properties of a mutated receptor (Q402R/Q418R, normal LDL binding but loss of most HDL binding) were reproduced in the assay, as was the ability of the SR-BI homologue CD36 to bind HDL but not mediate efficient lipid uptake. In this assay, essentially homogeneously pure mSR-BI-t1, prepared by single-step immunoaffinity chromatography, mediated high affinity HDL binding and efficient selective lipid uptake from HDL. Thus, SR-BI-mediated HDL binding and selective lipid uptake are intrinsic properties of the receptor that do not require the intervention of other proteins or specific cellular structures or compartments.