Helix orientation of the functional domains in apolipoprotein e in discoidal high density lipoprotein particles

Human apolipoprotein E (apoE) mediates high affinity binding to the low density lipoprotein receptor when present on a lipidated complex. In the absence of lipid, however, apoE does not bind the receptor. Whereas the x-ray structure of lipid-free apoE3 N-terminal (NT) domain is known, the structural organization of its lipid-associated, receptor-active conformation is poorly understood. To study the organization of apoE amphipathic alpha-helices in a lipid-associated state, single tryptophan-containing apoE3 variants were employed in fluorescence quenching studies. The relative positions of the Trp residues with respect to the phospholipid component of apoE/lipid particles were established from the degree of quenching by phospholipids bearing nitroxide groups at various positions along their fatty acyl chains. Four apoE3-NT variants bearing Trp reporter groups at positions 141, 148, 155, or 162 within helix 4 and two apoE3 variants containing single Trp at positions 257 or 264 in the C-terminal (CT) domain, were reconstituted into phospholipid-containing discoidal complexes. Parallax analysis revealed that each engineered Trp residue in helix 4 of apoE3-NT, as well as those in the CT domain of apoE, localized approximately 5 A from the center of the bilayer. Circular dichroism studies revealed that lipid association induces additional helix formation in apoE. Protease protection assays suggest the flexible loop segment between the NT and CT domains may transition from unstructured to helix upon lipid association. Taken together, these data support a model wherein the alpha-helices in the receptor-binding region and the CT domain of apoE align perpendicular to the fatty acyl chains of the phospholipid bilayer. In this alignment, the residues of helix 4 are arrayed in a positively charged, curved helical segment for optimal receptor interaction.