Molecular dynamics simulations of discoidal bilayers assembled from truncated human lipoproteins.

Human apolipoprotein A-1 (apo A-1) is the major protein component of high-density lipoproteins. The apo A-1 lipid-binding domain was used as a template for the synthesis of amphipathic helical proteins termed membrane scaffold proteins, employed to self-assemble soluble monodisperse discoidal particles called Nanodiscs. In these particles, membrane scaffold proteins surround a lipid bilayer in a belt-like fashion forming bilayer disks of discrete size and composition. Here we investigate the structure of Nanodiscs through molecular dynamics simulations in which Nanodiscs were built from scaffold proteins of various lengths. The simulations showed planar or deformed Nanodiscs depending on optimal length and alignment of the scaffold proteins. Based on mean surface area per lipid calculations, comparison of small-angle x-ray scattering curves, and the relatively planar shape of Nanodiscs made from truncated scaffold proteins, one can conclude that the first 17 to 18 residues of the 200-residue apo A-1 lipid-binding domain are not involved in formation of the protein "belts" surrounding the lipid bilayer. To determine whether the addition of an integral membrane protein has an effect on the overall structure of a Nanodisc, bacteriorhodopsin was embedded into a Nanodisc and simulated using molecular dynamics, revealing a planar disk with a slightly rectangular shape.