Mechanism of Lipid-Protein Interaction in the Plasma Lipoproteins: Lipid-Binding Properties of Synthetic Fragments of Apolipoprotein A-II

To delineate the basic structural unit for the binding of phospholipids by the plasma apolipoproteins, we have synthesized peptides of human high-density apolipoprotein A-II (apoA-II) and tested them for their ability to interact with single bilayer vesicles of dimyristoylphosphatidylcholine (DMPC). The fragments corresponding to residues 65-77, 56-77, 47-77, and 40-77 in the apoprotein were prepared by solid-phase peptide synthesis. The binding of phospholipid by these synthetic fragments and by the native tryptic peptide 56-77 was studied by changes in conformation, as determined by circular dichroism and by fractionation of peptide-DMPC mixtures in density gradients of KBr. Fragments 65-77 and 56-77 (synthetic and native) had disordered structures in the absence and presence of DMPC and no significant amount of peptide-phospholipid complexes was isolated. When fragments 47-77 and 40-77 were incubated with DMPC, there were marked conformational changes; the α-helical content of fragment 47-77 increased from 25 to 48% and fragment 40-77 from 23 to 48%. After ultracentrifugation in KBr, phospholipid complexes with fragments 47-77 and 40-77 were isolated between d 1.07 and 1.10 g/mL. The molar ratios of DMPC to fragments 47-77 and 40-77 were 25 and 44, respectively. The results of these studies and examination of space-filling models of apoA-II suggest that an amphipathic α helix which contains a nonpolar face and a polar face with at least one juxtaposed acidic and basic group is required for phospholipid binding by apoA-II.