Cross-Linking and Lipid Efflux Properties of ApoA-I Mutants Suggest Direct Association between ApoA-I Helices and ABCA1

To explore the functional interactions between apoA-I and ABCA1, we correlated the cross-linking properties of several apoA-I mutants with their ability to promote cholesterol efflux. In a competitive cross-linking assay, amino-terminal deletion and double amino- and carboxy-terminal deletion mutants of apoA-I competed effectively the cross-linking of WT 125I-apoA-I to ABCA1, while the carboxy-terminal deletion mutant apoA-I[Δ(220−243)] competed poorly. Direct cross-linking of WT apoA-I, amino-terminal, and double deletion mutants of apoA-I to ABCA1 showed similar apparent K d values (49−74 nM), whereas the apparent K d values of the carboxy-terminal deletion mutants apoA-I[Δ(185−243)] and apoA-I[Δ(220−243)] were increased 3-fold. Analysis of several internal deletions and point mutants of apoA-I showed that apoA-I[Δ(61−78)], apoA-I[Δ(89−99)], apoA-I[Δ(136−143)], apoA-I[Δ(144−165)], apoA-I[D102A/D103A], apoA-I[E125K/E128K/K133E/E139K], apoA-I[L141R], apoA-I[R160V/H162A], and WT apoA-I had similar ABCA1-mediated lipid efflux, and all competed efficiently the cross-linking of WT 125I-apoA-I to ABCA1. WT apoA-I and ABCA1 could be cross-linked with a 3 Å cross-linker. The WT apoA-I, amino, carboxy and double deletion mutants of apoA-I showed differences in the cross-linking to WT ABCA1 and the mutant ABCA1[W590S]. The findings are consistent with a direct association of different combinations of apoA-I helices with a complementary ABCA1 domain. Mutations that alter ABCA1/apoA-I association affect cholesterol efflux and inhibit biogenesis of HDL.