Enhancing the Contrast of ApoB to Locate the Surface Components in the 3D Density Map of Human LDL

摘要

A 26Å resolution map of the structure of human LDL was obtained from cryo-EM and single particle image reconstruction. The structure showed a discoidal shaped LDL particle with high-density regions mainly distributed at the edge of the particle and low-density regions at the flat surface that covers the core region. To determine the chemical components that correspond to these density regions and to delineate the distribution of protein and phospholipid located at the particle surface at the resolution of the map, we used Mono-Sulfo-NHS-Undecagold labeling to increase preferentially the contrast of the apoB protein component on the LDL particle. In the 3D maps from the image reconstruction of the undecagold labeled LDL particles, the high-density region from the undecagold label was distributed mainly at the edge of the particle and lower density regions were found at the flat surfaces that cover the neutral lipid core. This suggests that apoB mainly encircles LDL at the edge of the particle and the phospholipid monolayers are located at the flat surfaces, which are parallel to the cholesterol ester layers in the core and may interact with the core lipid layers through the acyl-chains.