Vesicle-reconstituted low density lipoprotein receptor - Visualization by cryoelectron microscopy

The low density lipoprotein (LDL) receptor is a key protein for maintaining cellular cholesterol homeostasis by binding cholesterol-rich lipoproteins through their apoB and apoE apoproteins. The LDL receptor is a transmembrane glycoprotein of M-r similar to 115 kDa; based on its primary sequence, five distinct structural domains have been identified (Yamamoto, T., Davis, C. G., Brown, M. S., Schneider, W. J., Casey, M. L., Goldstein, J. L., and Russell, D. W. (1984) Cell 39, 27-38). As a first step toward providing a structural description of the intact LDL receptor, the receptor has been purified from bovine adrenal cortices, reconstituted into unilamellar egg yolk phosphatidylcholine vesicles, and imaged using cryoelectron microscopy (cryoEM). CryoEM has the advantage of providing images of the reconstituted LDL receptor in its frozen, fully hydrated state. LDL receptor molecules were visualized as elongated, stick-like projections from the vesicle surface with maximum dimensions similar to 120-Angstrom length by similar to 45-Angstrom width. In some of the images, a short arm (or arms) was visible at the distal end of the stick-like projections. The LDL receptor was labeled via accessible free cysteine residues, probably including that corresponding to Cys-431 of the known full-length sequence of the human LDL receptor. The accessible cysteine was demonstrated using a maleimide-biotin streptavidin conjugate and confirmed by labeling with monomaleimido-Nanogold. Images obtained by cryoEM showed that the extracellular stick-like domain of the reconstituted LDL receptor was labeled by Nanogold. This combined cryoEM-Nanogold labeling study has provided the first low resolution structural images of the reconstituted, full-length bovine LDL receptor.