O-linked glycosylation modifies the association of apolipoprotein A-II to high density lipoproteins.

O-linked glycosylation is a common post-translational modification of apolipoproteins, but no structural or functional role for it has been identified. We examined the biosynthesis of apolipoprotein (apo) A-II in Hep G2 cells and in glycosylation-defective Chinese hamster ovary (CHO) cell mutants transfected with apoA-II cDNA. Three monomeric isoforms of apoA-II with an apparent molecular mass of 8.5, 9.8, and 11.4 kDa were synthesized by Hep G2 cells and transfected wild-type CHO cells. The 9.8- and 11.4-kDa isoforms were sialylated but not the 8.5-kDa isoform. Transfected 1dlD cells, which are defective in the biosynthesis of galactose and N-acetylgalactosamine, only produced the 8.5-kDa isoform; however, when grown in media supplemented with these sugars, ldlD cells produced all three isoforms of apoA-II. Pulse-chase analysis of ldlD cells showed that glycosylation was not necessary for secretion of apoA-II. Glycosylation did modify the association of apoA-II with nascent high density lipoprotein (HDL) secreted by Hep G2 cells. The sialylated isoforms were lipid-poor and were present in the lipoprotein-deficient density range, whereas the nonsialylated 8.5-kDa isoform was associated with LpA-I, A-II lipoprotein particles in the HDL density range. ApoA-II from transfected ldlD cells, regardless of glycosylation, were lipid-poor. When preincubated with HDL from serum, however, sialylated apoA-II from both ldlD cells and Hep G2 cells associated with lipoprotein particles within the HDL3 density, whereas nonsialylated apoA-II was found throughout the HDL density range. In summary, O-linked glycosylation is not necessary for the secretion of apoA-II but does modify the association of apoA-II to HDL and may, therefore, play an important role in the metabolism of HDL.