Abstract 5477: The Double Helix Model of High Density Lipoprotein
High-density lipoprotein (HDL), the carrier of “good” cholesterol, has remained resistant to structural interrogation by traditional high-resolution approaches. Using an integrative multimodal approach permitting for the first time direct structural visualization of both protein and lipid components individually within HDL, we demonstrate that apolipoprotein A1 (apoA1) of HDL exist in an anti-parallel double helical conformation. This surprising “Double Helix Model” of HDL was built using a combination of contrast matching small angle neutron scattering (SANS), isotopic deuteration of apoA1, hydrogen deuterium exchange tandem mass spectrometry (HD-MS/MS) and novel computational tools. The lipid phase of the HDL particle was directly visualized by contrast matching SANS and is ellipsoidal in shape with phospholipid and cholesterol adopting a micellar structure partially bounded by hydrophobic surfaces of the protein double helix. Independent support for a micellar rather than lamellar (bilayer) packing structure of the phospholipids in both plasma HDL and recombinant HDL disks was obtained by 31P NMR spectroscopy. The remarkable protein double helix and lipid structures observed accommodate SANS scattering data at multiple D2O levels, apoA1 HD-MS/MS data in HDL, and distance constraints reported in prior chemical cross-linking studies. The present results have important implications for particle genesis, maturation and remodeling. They reveal a highly dynamic and adaptive structure capable of accommodating changes in particle shape and composition during HDL maturation and remodeling. Finally, through the use of multiple complementary and synergistic biophysical approaches combined with novel integrative computational tools, we have developed a multidisciplinary methodological platform for solving the structure of HDL and other dynamic macromolecular complexes resistant to traditional structural approaches.
This research has received full or partial funding support from the American Heart Association, AHA Great Rivers Affiliate (Delaware, Kentucky, Ohio, Pennsylvania & West Virginia).