Proline residues in scavenger receptor-BI's C-terminal region support efficient cholesterol transport

High-density lipoproteins (HDLs) facilitate reverse cholesterol transport, a process in which HDL removes cholesterol from circulation and carries it to the liver for biliary excretion. Reverse cholesterol transport is also facilitated by HDL's high-affinity receptor, scavenger receptor-BI (SR-BI), by mechanisms that are not fully understood. To improve our understanding of SR-BI function, we previously solved the NMR (nuclear magnetic resonance) structure of a peptide encompassing amino acids 405-475 of SR-BI. This segment of SR-BI, that includes the functionally critical C-terminal transmembrane domain and part of the extracellular domain, also contains four conserved proline (Pro) residues. We hypothesized that these proline residues support SR-BI in a conformation that allows for efficient cholesterol transport. To test this, we generated individual Pro-to-alanine mutations in full-length SR-BI and transiently expressed the mutant receptors in COS-7 cells to measure the effects on SR-BI-mediated cholesterol transport functions. Our findings reveal that HDL cell association and uptake of HDL-cholesteryl esters are impaired by mutation of Pro-412, Pro-438, or the transmembrane proline kink residue (Pro-459). In addition, SR-BI-mediated cholesterol efflux and membrane cholesterol distribution are impaired by mutation of Pro-412 or Pro-438, indicating that these residues are essential for a fully functional SR-BI receptor. Furthermore, we demonstrate that Pro-408 is necessary for proper SR-BI expression, but mutation of Pro-408 does not cause SR-BI to become misfolded or rapidly degraded by the proteasome or the lysosome. We conclude that key proline residues play an important role in SR-BI function by allowing for the efficient transport of cholesterol between cells and HDL.