The N-linked oligosaccharides at the amino terminus of human apoB are important for the assembly and secretion of VLDL.
We determined the role of N-linked glycosylation of apolipoprotein B (apoB) in the assembly and secretion of lipoproteins using transfected rat hepatoma McA-RH7777 cells expressing human apoB-17, apoB-37, and apoB-50, three apoB variants with different ability to recruit neutral lipids. Substituting Asn residue with Gln at the single glycosylation site within apoB-17 (N(158)) decreased its secretion efficiency to a level equivalent to that of wild-type apoB-17 treated with tunicamycin, but had little effect on its synthesis or intracellular distribution. When selective N-to-Q substitution was introduced at one or more of the five N-linked glycosylation sites within apoB-37 (N(158), N(956), N(1341), N(1350), and N(1496)), secretion efficiency of apoB-37 from transiently transfected cells was variably affected. When all five N-linked glycosylation sites were mutated within apoB-37, the secretion efficiency and association with lipoproteins were decreased by >50% as compared with wild-type apoB-37. Similarly, mutant apoB-50 with all of its N-linked glycosylation sites mutagenized showed decreased secretion efficiency and decreased lipoprotein association in both d < 1.02 and d > 1.02 g/ml fractions. The inability of mutant apoB-37 and apoB-50 to associate with very low-density lipoproteins was attributable to impaired assembly and was not due to the limitation of lipid availability. The decreased secretion of mutant apoB-17 and apoB-37 was not accompanied by accumulation within the cells, suggesting that the proportion of mutant apoB not secreted was rapidly degraded. However unlike apoB-17 or apoB-37, accumulation of mutant apoB-50 was observed within the endoplasmic reticulum and Golgi compartments. These data imply that the N-glycans at the amino terminus of apoB play an important role in the assembly and secretion of lipoproteins containing the carboxyl terminally truncated apoB.
Vukmirica
Jelena
J
Lipoprotein and Atherosclerosis Group, Department of Pathology, University of Ottawa Heart Institute, Ottawa, Ontario, Canada, K1Y 4W7.
Nishimaki-Mogami
Tomoko
T
Tran
Khai
K
Shan
Jing
J
McLeod
Roger S
RS
Yuan
Jane
J
Yao
Zemin
Z
D019943:Amino Acid Substitution / Q000235:genetics; D001055:Apolipoproteins B / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001216:Asparagine / Q000378:metabolism; D005973:Glutamine / Q000378:metabolism; D006801:Humans; D008079:Lipoproteins, VLDL / Q000096:biosynthesis* / Q000378:metabolism*; D009844:Oligosaccharides / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014415:Tunicamycin / Q000494:pharmacology
D001055:Apolipoproteins B; D008079:Lipoproteins, VLDL; D009844:Oligosaccharides; C052679:apolipoprotein B-37; C416027:apolipoprotein b-17; D005973:Glutamine; D014415:Tunicamycin; D001216:Asparagine