The effects of mutations in helices 4 and 6 of ApoA-I on scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux suggest that formation of a productive complex between reconstituted high density lipoprotein and SR-BI is required for efficient lipid transport.
We have studied the effects of mutations in apoA-I on reconstituted high density lipoprotein (HDL) particle (rHDL(apoA-I)) binding to and cholesterol efflux from wild-type (WT) and mutant forms of the HDL receptor SR-BI expressed by ldlA-7 cells. Mutations in helix 4 or helix 6 of the apoA-I reduced efflux by 79 and 51%, respectively, without substantially altering receptor binding (apparent K(d) values of 1.1-4.4 microg of protein/ml). SR-BI with an M158R mutation bound poorly to rHDL with WT and helix 4 mutant apoA-I; the helix 6 mutant restored tight binding to SR-BI(M158R) (K(d) values of 48, 60, and 7 microg of protein/ml, respectively). SR-BI(M158R)-mediated cholesterol efflux rates, normalized for binding, were high for all three rHDLs (71-111% of control). In contrast, absolute (12-19%) and binding-corrected (24-47%) efflux rates for all three rHDLs mediated by SR-BI with Q402R/Q418R mutations were very low. We propose that formation of a productive complex between apoA-I in rHDL and SR-BI, in which the lipoprotein and the receptor must either be precisely aligned or have the capacity to undergo appropriate conformational changes, is required for efficient SR-BI-mediated cholesterol efflux. Some mutations in apoA-I and/or SR-BI can result in high affinity, but non-productive, binding that does not permit efficient cholesterol efflux.
Liu
Tong
T
Section of Molecular Genetics, Whitaker Cardiovascular Institute, Department of Medicine and Biochemistry, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Krieger
Monty
M
Kan
Horng-Yuan
HY
Zannis
Vassilis I
VI
D000818:Animals; D016632:Apolipoprotein A-I / Q000737:chemistry* / Q000378:metabolism; D001692:Biological Transport; D018955:CD36 Antigens / Q000737:chemistry* / Q000235:genetics*; D016466:CHO Cells; D002784:Cholesterol / Q000378:metabolism*; D006224:Cricetinae; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004797:Enzyme-Linked Immunosorbent Assay; D007700:Kinetics; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000378:metabolism; D008565:Membrane Proteins; D008954:Models, Biological; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; D051122:Scavenger Receptors, Class B; D013997:Time Factors
D016632:Apolipoprotein A-I; D018955:CD36 Antigens; D018076:DNA, Complementary; D008075:Lipoproteins, HDL; D008565:Membrane Proteins; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; C454739:Scarb1 protein, mouse; D051122:Scavenger Receptors, Class B; D002784:Cholesterol