The Structure of Human Lipoprotein A-I

Abstract

The two main competing models for the structure of discoidal lipoprotein A-I complexes both presume that the protein component is helical and situated around the perimeter of a lipid bilayer disc. However, the more popular “picket fence” model orients the protein helices perpendicular to the surface of the lipid bilayer, while the alternative “belt” model orients them parallel to the bilayer surface. To distinguish between these models, we have investigated the structure of human lipopro- tein A-I using a novel form of polarized internal reflec- tion infrared spectroscopy that can characterize the rel- ative orientation of protein and lipid components in the lipoprotein complexes under native conditions. Our re- sults verify lipid bilayer structure in the complexes and point unambiguously to the belt model.