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 μg 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 μg 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.