Apolipoprotein E: Phospholipid Binding Studies with Synthetic Peptides Containing the Putative Receptor Binding Region†

To define the lipid and receptor binding regions of apolipoprotein E (apoE), we have synthesized four peptides beginning at residue 169 and continuing through the putative receptor binding region and ending at residue 129 so as to include a proposed lipid binding domain. The peptides were synthesized by solid-phase techniques, cleaved with anhydrous HF, and purified by ion-exchange and semipreparative reversed-phase high-performance liquid chromatography (HPLC). The peptides had the correct amino acid composition and were greater than 99% pure by analytical reversed-phase HPLC. The circular dichroic spectrum of each peptide was recorded before and after mixing with dimyristoylphosphatidylcholine. With apoE(148-169), apoE(144-169), and apoE(139-169), no changes were observed in the ellipticity at 222 nm. However, with apoE(129-169), an increase in α-helicity to ∼42% was observed. Density gradient ultracentrifugation of the lipid-peptide mixture permitted isolation of a complex with apoE(129-169) with a molar ratio of lipid to peptide of 125:1, which was stable to recentrifugation. The α-helicity of the peptide in the complex was estimated to be 56%. No complexes were isolated from the gradients of the shorter peptides. Therefore, we conclude that the amphipathic helix formed by residues 130-150 contains one of the lipid binding regions of apoE.