Web Release Date: February 17,
The C-Terminal Lipid-Binding Domain of Apolipoprotein E Is a Highly Efficient
Mediator of ABCA1-Dependent Cholesterol Efflux that Promotes the Assembly of
High-Density Lipoproteins
and
GI/Nutrition Division, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-4318, Center for the Prevention of Obesity, Cardiovascular Disease and Diabetes, Children's Hospital Oakland Research Institute, Oakland, California 94609, and Molecular Imaging Department, Donner Laboratory MS1-224, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received November 21, 2006
Revised Manuscript Received January 9, 2007
Abstract:
This study was undertaken to identify the 
-helical domains of human apoE that mediate
cellular cholesterol efflux and HDL assembly via ATP-binding cassette transporter A1 (ABCA1). The
C-terminal (CT) domain (residues 222-299) of apoE was found to stimulate ABCA1-dependent cholesterol
efflux in a manner similar to that of intact apoE2, -E3, and -E4 in studies using J774 macrophages and
HeLa cells. The N-terminal (NT) four-helix bundle domain (residues 1-191) was a relatively poor mediator
of cholesterol efflux. On a per molecule basis, the CT domain stimulated cholesterol efflux with the same
efficiency (Km ~ 0.2 
M) as intact apoA-I and apoE. Gel filtration chromatography of conditioned medium
from ABCA1-expressing J774 cells revealed that, like the intact apoE isoforms, the CT domain promoted
the assembly of HDL particles with diameters of 8 and 13 nm. Removal of the CT domain abolished the
formation of HDL-sized particles, and only larger particles eluting in the void volume were formed. Studies
with CT truncation mutants of apoE3 and peptides indicated that hydrophobic helical segments governed
the efficiency of cellular cholesterol efflux and that conjoined class A and G amphipathic -helices were
required for optimal efflux activity. Collectively, the data suggest that the CT lipid-binding domain of
apoE encompassing amino acids 222-299 is necessary and sufficient for mediating ABCA1 lipid efflux
and HDL particle assembly.
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