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J. Biol. Chem., Vol. 275, Issue 39, 30465-30470, September 29, 2000
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and
From the Departments of Biophysics and Biochemistry, Center for
Advanced Biomedical Research, Boston University School of Medicine,
Boston, Massachusetts 02118
The low density lipoprotein (LDL) receptor is a
transmembrane glycoprotein performing "receptor-mediated
endocytosis" of cholesterol-rich lipoproteins. At the N terminus, the
LDL receptor has modular cysteine-rich repeats in both the ligand
binding domain and the epidermal growth factor (EGF) precursor homology
domain. Each repeat contains six disulfide-bonded cysteine residues,
and this structural motif has also been found in many other proteins.
The bovine LDL receptor has been purified and reconstituted into egg yolk phosphatidylcholine vesicle bilayers. Using gel electrophoresis and cryoelectron microscopy (cryoEM), the ability of the reconstituted LDL receptor to bind its ligand LDL has been demonstrated. After reduction of the disulfide-bonds in the N-terminal domain of the receptor, the reduced LDL receptor was visualized using cryoEM; reduced
LDL receptors showed images with a diffuse density region at the distal
end of the extracellular domain. Gold labeling of the reduced cysteine
residues was achieved with monomaleimido-Nanogold, and the bound
Nanogold was visualized in cryoEM images of the reduced, gold-labeled
receptor. Multiple gold particles were observed in the diffuse density
region at the distal end of the receptor. Thus, the location of the
ligand binding domain of the LDL receptor has been determined, and a
model is suggested for the arrangement of the seven cysteine-rich
repeats of the ligand binding domain and two EGF-like cysteine-rich
repeats of the EGF precursor homology domain.
Present address: Dept. of Pathology, Harvard Medical School,
Boston, MA 02115.
§
To whom correspondence should be addressed: Biophysics Dept.,
Center for Advanced Biomedical Research, Boston University School of
Medicine, 715 Albany St., Boston, MA 02118. Tel.: 617-638-4009; Fax:
617-638-4041; E-mail: shipley@med-biophd.bu.edu.
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