Lipoprotein Association of Human Apolipoprotein E/A-I Chimeras
Both apolipoprotein (apo) E and apoA-I are associated with lipoproteins, although with different particle classes. ApoE is associated with very low density lipoprotein (VLDL) and with the larger high density lipoprotein (HDL) subspecies, while apoA-I is found predominantly in association with most H...
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Published in | The Journal of biological chemistry Vol. 271; no. 11; pp. 6062 - 6070 |
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Main Authors | , , |
Format | Journal Article |
Language | English |
Published |
American Society for Biochemistry and Molecular Biology
15.03.1996
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Online Access | Get full text |
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Summary: | Both apolipoprotein (apo) E and apoA-I are associated with lipoproteins, although with different particle classes. ApoE is
associated with very low density lipoprotein (VLDL) and with the larger high density lipoprotein (HDL) subspecies, while apoA-I
is found predominantly in association with most HDL subclasses. The genes encoding these proteins have a similar overall structure
with the nucleotide sequences of the third and fourth exons coding for the mature protein. In an effort to understand the
difference in lipoprotein association patterns of these two apoproteins, we have constructed and expressed chimeric apoproteins
using cDNAs in which the third (n) and fourth (c) exons of human apoE and apoA-I are exchanged. McArdle rat hepatoma cells
(McA-RH7777), which secrete VLDL- and HDL-like particles, were stably transfected with these cDNAs, and the cDNAs for human
apoE and human apoA-I. Single spin NaBr gradient fractions of lipoprotein deficient serum-treated cell medium from transfected
McA-RH7777 cells were analyzed. The distributions of transfected human apoE and apoA-I and endogenous rat apoE and apoA-I
were compared with those of the chimeras. Among HDL subspecies, human apoE expressed by these cells is associated with particles
of density 1.108 g/ml. Similarly, chimera apoA-InEc (exon 3 of apoA-I and exon 4 of apoE) is found in particles of density
1.111 g/ml. Human apoA-I, however, distributes in a broader range of particles with peak densities of 1.111 g/ml and 1.164
g/ml. The distribution of the complementary chimera, apoEnA-Ic, follows this same pattern, with peak particle densities of
1.098 and 1.137 g/ml. This is in contrast to the narrow distributions of endogenous rat apoE and apoA-I, which were found
in particles of density 1.099 and 1.089 g/ml, respectively. When metabolically labeled medium was fractionated via gel filtration
column chromatography, apoA-InEc was found to associate with the VLDL fractions; apoEnA-Ic was absent from these same fractions.
These results suggest that the fourth exon largely determines the distinctive lipoprotein distribution patterns of these two
human apoproteins and that the human apoA-I fourth exon sequence may account for the polydisperse HDL pattern as observed
by others in transgenic mice expressing human apoA-I. |
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ISSN: | 0021-9258 1083-351X |
DOI: | 10.1074/jbc.271.11.6062 |