Characterization of the lipid-binding properties and lipoprotein lipase inhibition of a novel apolipoprotein C-III variant Ala23Thr.
We have identified a G-to-A transition in exon 3 of the APOC3 gene resulting in a novel Ala23Thr apolipoprotein (apo) C-III variant, associated with apoC-III deficiency in three unrelated Yucatan Indians. The Ala23Thr substitution modifies the hydrophobic/hydrophilic repartition of the helical N-terminal peptide and hence could disturb the lipid association. In vitro expression in Escherichia coli of wild-type and mutant apoC-III enabled the characterization of the variant. Compared with wild-type apoC-III-Ala23, the mutant apoC-III-Thr23 showed reduced affinity for dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles with higher amounts of free apoC-III. Displacement of apoE from discoidal apoE:dipalmitoylphosphatidycholine (DPPC) complex by apoC-III-Thr23 was comparable to wild type but the less efficient binding of the apoC-III-Thr23 to the discoidal complex resulted in a higher apoE/apoC-III (mol/mol) ratio (34%) than with wild-type/apoE:DPPC mixtures. The inhibition of lipoprotein lipase (LPL) by apoC-III-Thr23 was comparable to that of wild type, and therefore effects on LPL activity could not explain the lower triglyceride (Tg) levels in Thr-23 carriers. Thus, these in vitro results suggest that in vivo the less efficient lipid binding of apoC-III-Thr23 might lead to a faster catabolism of free apoC-III, reflected in the reduced plasma apoC-III levels identified in Thr-23 carriers, and poorer competition with apoE, which might enhance clearance of Tg-rich lipoproteins and lower plasma Tg levels seen in Thr-23 carriers.
Liu
H
H
Cardiovascular Genetics Division, Department of Medicine, Royal Free and University College London Medical School, London WC1E 6JJ, UK.
Labeur
C
C
Xu
C F
CF
Ferrell
R
R
Lins
L
L
Brasseur
R
R
Rosseneu
M
M
Weiss
K M
KM
Humphries
S E
SE
Talmud
P J
PJ
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism; D000595:Amino Acid Sequence; D053305:Apolipoprotein C-III; D001056:Apolipoproteins C / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D001057:Apolipoproteins E / Q000378:metabolism; D002489:Central America; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004252:DNA Mutational Analysis; D004134:Dimyristoylphosphatidylcholine / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007197:Indians, Central American; D050356:Lipid Metabolism; D008071:Lipoprotein Lipase / Q000037:antagonists & inhibitors*; D008297:Male; D008875:Middle Aged; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D016133:Polymerase Chain Reaction; D011994:Recombinant Proteins / Q000494:pharmacology
D053305:Apolipoprotein C-III; D001056:Apolipoproteins C; D001057:Apolipoproteins E; D004791:Enzyme Inhibitors; D011994:Recombinant Proteins; D015060:1,2-Dipalmitoylphosphatidylcholine; D008071:Lipoprotein Lipase; D004134:Dimyristoylphosphatidylcholine