title: A quantitative analysis of apolipoprotein binding to SR-BI: multiple binding sites for lipid-free and lipid-associated apolipoproteins.
abstract: Competitive binding experiments were performed using Y1-BS1 adrenal cells to provide information about the interaction of HDL apolipoproteins with scavenger receptor class B, type I (SR-BI). Exchangeable apolipoproteins apolipoprotein A-I (apoA-I), apoA-II, apoE-2, apoE-3, and apoE-4 as phospholipid complexes bind like HDL3 to SR-BI via their multiple amphipathic alpha-helices; the concentrations required to reduce the binding of HDL3 to SR-BI by 50% (IC50) were similar and in the range of 35-50 microgram protein/ml. In the case of apoA-I, peptides corresponding to segments 1-85, 44-65, 44-87, 149-243, and 209-241 all had the same IC50 as each other (P = 0.86), showing that a specific amino acid sequence in apoA-I is not responsible for the interaction with SR-BI. The distribution of charged residues in the amphipathic alpha-helix affects the interaction, with class A and Y helices binding better than class G* helices. Synthetic alpha-helical peptides composed of either l or d amino acids can bind equally to the receptor. Association with phospholipid increases the amount of apolipoprotein binding to SR-BI without altering the affinity of binding. Lipid-free apolipoproteins compete only partially with the binding of HDL to SR-BI, whereas lipidated apolipoproteins compete fully. These results are consistent with the existence of more than one type of apolipoprotein binding site on SR-BI.
authors.affiliation: Division of GI/Nutrition, Lipid Research Group, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, 19104-4318, USA.
authors.forename: Stephen T
authors.initials: ST
authors.lastname: Thuahnai
authors.forename: Sissel
authors.initials: S
authors.lastname: Lund-Katz
authors.forename: G M
authors.initials: GM
authors.lastname: Anantharamaiah
authors.forename: David L
authors.initials: DL
authors.lastname: Williams
authors.forename: Michael C
authors.initials: MC
authors.lastname: Phillips
mesh_terms: D000818:Animals; D001053:Apolipoproteins / Q000737:chemistry* / Q000378:metabolism; D001667:Binding, Competitive; D018955:CD36 Antigens / Q000737:chemistry* / Q000378:metabolism; D002478:Cells, Cultured; D002788:Cholesterol Esters / Q000378:metabolism; D006801:Humans; D007457:Iodine Radioisotopes; D007700:Kinetics; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000378:metabolism; D008565:Membrane Proteins; D010743:Phospholipids / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; D051117:Scavenger Receptors, Class A; D051122:Scavenger Receptors, Class B
chemical_list: D001053:Apolipoproteins; D018955:CD36 Antigens; D002788:Cholesterol Esters; D007457:Iodine Radioisotopes; D008075:Lipoproteins, HDL; D008565:Membrane Proteins; D010743:Phospholipids; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; C495124:SCARB1 protein, human; C454739:Scarb1 protein, mouse; D051117:Scavenger Receptors, Class A; D051122:Scavenger Receptors, Class B