The Structure of ApoB100 from Human Low-density Lipoprotein

Low-density lipoprotein (LDL) plays a central role in lipid and cholesterol metabolism and is a key molecular agent involved in the development and progression of atherosclerosis, a leading cause of mortality worldwide. Apolipoprotein B100 (apoB100), one of the largest proteins in the genome, is the primary structural and functional component of LDL, yet its size and complex lipid associations have posed major challenges for structural studies. Here we overcome those challenges and present the first structure of apoB100 from human LDL using an integrative approach of cryo-electron microscopy, AlphaFold2, and molecular dynamics-based refinement. The structure consists of a large globular N-terminal domain that leads into a ∼58 nm long x 4 nm wide continuous amphipathic β-sheet that wraps completely around the circumference of the particle, holding it together like a belt. Distributed symmetrically across the two sides of the β-belt are 9 strategically located inserts that vary in size from ∼30-700 residues and appear to have diverse functions. The largest two form long flexible strings of paired amphipathic helices that extend across the lipid surface to provide additional structural support through specific long-range interactions. These results suggest a mechanism for how the various domains of apoB100 act in concert to maintain LDL shape and cohesion across a wide range of particle sizes. More generally, they advance our fundamental understanding of LDL form and function and will help accelerate the design of potential new therapeutics.