Identification of a novel tetrameric structure for human apolipoprotein-D

J Struct Biol. 2018 Sep;203(3):205-218. doi: 10.1016/j.jsb.2018.05.012. Epub 2018 Jun 6.


Apolipoprotein-D is a 25 kDa glycosylated member of the lipocalin family that folds into an eight-stranded β-barrel with a single adjacent α-helix. Apolipoprotein-D specifically binds a range of small hydrophobic ligands such as progesterone and arachidonic acid and has an antioxidant function that is in part due to the reduction of peroxidised lipids by methionine-93. Therefore, apolipoprotein-D plays multiple roles throughout the body and is protective in Alzheimer's disease, where apolipoprotein-D overexpression reduces the amyloid-β burden in Alzheimer's disease mouse models. Oligomerisation is a common feature of lipocalins that can influence ligand binding. The native structure of apolipoprotein-D, however, has not been conclusively defined. Apolipoprotein-D is generally described as a monomeric protein, although it dimerises when reducing peroxidised lipids. Here, we investigated the native structure of apolipoprotein-D derived from plasma, breast cyst fluid (BCF) and cerebrospinal fluid. In plasma and cerebrospinal fluid, apolipoprotein-D was present in high-molecular weight complexes, potentially in association with lipoproteins. In contrast, apolipoprotein-D in BCF formed distinct oligomeric species. We assessed apolipoprotein-D oligomerisation using native apolipoprotein-D purified from BCF and a suite of complementary methods, including multi-angle laser light scattering, analytical ultracentrifugation and small-angle X-ray scattering. Our analyses showed that apolipoprotein-D predominantly forms a ∼95 to ∼100 kDa tetramer. Small-angle X-ray scattering analysis confirmed these findings and provided a structural model for apolipoprotein-D tetramer. These data indicate apolipoprotein-D rarely exists as a free monomer under physiological conditions and provide insights into novel native structures of apolipoprotein-D and into oligomerisation behaviour in the lipocalin family.

Keywords: Apolipoprotein structure; Lipid; Lipocalin; Lipocalin structure; Oligomerization; Small-angle X-ray scattering (SAXS).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alzheimer Disease / genetics*
  • Alzheimer Disease / pathology
  • Amyloid beta-Peptides / chemistry
  • Animals
  • Apolipoproteins D / cerebrospinal fluid
  • Apolipoproteins D / chemistry*
  • Apolipoproteins D / genetics
  • Breast Cyst / chemistry
  • Crystallography, X-Ray
  • Disease Models, Animal
  • Humans
  • Ligands
  • Lipocalins / chemistry
  • Mice
  • Protein Binding
  • Protein Conformation*
  • Protein Multimerization*
  • Scattering, Small Angle


  • Amyloid beta-Peptides
  • Apolipoproteins D
  • Ligands
  • Lipocalins