Development and validation of a purification system for functional full-length human SR-B1 and CD36

Scavenger receptor class B type 1 (SR-B1) and CD36 are both members of the class B scavenger receptor family that play important roles in lipoprotein metabolism and atherosclerotic disease. SR-B1 is the primary receptor for high-density lipoproteins, while CD36 is the receptor responsible for the internalization of oxidized low-density lipoproteins. Despite their importance, class B scavenger receptor structure has only been studied by functional domain or peptide fragments-there are currently no reports of utilizing purified full-length protein. Here we report the successful expression and purification of full-length human SR-B1 and CD36 using an Spodoptera frugiperda insect cell system. We demonstrate that both SR-B1 and CD36 retained their normal functions in Spodoptera frugiperda cells, including lipoprotein binding, lipid transport, and the formation of higher order oligomers in the plasma membrane. Purification schemes for both scavenger receptors were optimized and their purity was confirmed by SDS-PAGE. Both purified scavenger receptors were assessed for stability by thermal shift assay and shown to maintain stable melting temperatures up to 6 weeks post-purification. Microscale thermophoresis was used to demonstrate that purified SR-B1 and CD36 were able to bind their native lipoprotein ligands. Further, there was no difference in affinity of SR-B1 for high-density lipoprotein or CD36 for oxidized low-density lipoprotein, when comparing glycosylated and deglycosylated receptors. These studies mark a significant step forward in creating physiologically relevant tools to study scavenger receptor function and lay the groundwork for future functional studies and determination of receptor structure.