Journal of Lipid Research, Vol 25, 111-120, Copyright © 1984 by Lipid Research, Inc.
Lipid binding properties of the Tangier apolipoprotein A-I and its isoproteins
M Rosseneu, G Assmann, MJ Taveirne and G Schmitz
The apolipoprotein A-I was isolated from the plasma of normal individuals
and of three homozygous patients with Tangier disease by
immunoprecipitation. The apoA-I isoforms were further fractionated by
isofocusing on polyacrylamide gels. The physicochemical behavior of normal
and Tangier apoA-I and of the isoproteins-2 and -4 was studied by
monitoring the tryptophanyl fluorescence emission as a function of
temperature, pH, and under exposure to guanidinium (guanidine)
hydrochloride (GdmCl). Lipid-apoprotein complexes were generated by
incubation with dimyristoylphosphatidylcholine and isolated by density
gradient ultracentrifugation. Our results show that normal apoA-I and its
isoprotein-4 associate with lipids to yield a complex containing 150-200
mol lecithin/mol apoA-I. The isoprotein-2 of normal apoA-I and the
isoprotein-4 of Tangier apoA-I generate lipid-rich complexes with lecithin,
while the isoprotein-2 of Tangier apoA-I shows only a limited association
with lipids. ApoA-I normal and Tangier and their isoproteins-4 undergo a
structural transition around 45 degrees C, which is not observed in the
lecithin-apoA-I complexes. This transition is accompanied by an increased
exposure of the tryptophanyl residues to the solvent. This transition was
observed for the isoprotein-2 of apoA- I Tangier both in its lipid-free
form and in the presence of lecithin. The pH denaturation of apoA-I and of
the isoprotein-4 between pH 9 and 13 and between pH 7 and 2 is accompanied
by a similar conformational transition. The transition occurs around pH
10.8 for the native apoproteins and is shifted towards respectively higher
and lower pH's as result of the protective action of lipid binding on the
protein conformation. Such an effect was not observed with the isoprotein-2
of apoA-I Tangier which is denatured at lower pH's both in its native form
and in a lipid-protein mixture. Finally the denaturation of apoA-I by GdmCl
indicates that apoA-I normal and Tangier undergo structural changes around
1 M GdmCl, whereas the apoA-I-Tangier-lecithin complex is more susceptible
to denaturation than the complex with apoA-I normal. These data suggest
that the apoA-I normal and Tangier and their isoproteins-4 are able to
associate with lipids although the association between apoA-I Tangier with
lecithin is weaker than that of apoA-I normal. The isoprotein-2 of normal
apoA-I associates to a greater extent with lipids than the isoprotein-2 of
Tangier apoA-I, whose structure differs from that of the
isoprotein-4.(ABSTRACT TRUNCATED AT 400 WORDS)
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