Genomic Structure, Biosynthesis, and Processing of Preproapolipoprotein C-II
Apolipoprotein C-II plays a major role in lipid metabolism as a cofactor for lipoprotein lipase, the enzyme involved in the hydrolysis of plasma triglycerides. Patients with deficiency of apo C-II have marked elevations of plasma triglyceride-rich lipoproteins and are at increased risk of pancreatitis. Apolipoprotein C-II has been cloned, and the complete genomic structure elucidated. The apo C-II gene consists of four exons interrupted by three introns and encodes a 22-amino-acid signal peptide that undergoes cotranslational cleavage. The posttranslational processing of apo C-II was analyzed by two-dimensional gel electrophoresis followed by immunoblotting of apo C-II isoforms in the media of Hep G2 cells and in plasma. Four major isoforms have been identified and designated apo C-II-2, apo C-II-1, apo C-II-1/2, and apo C-II0. Neuroaminidase studies have shown that apo C-II-2 and apo C-II-1 are sialic-acid-containing glycoproteins. There is a relative enrichment of these two isoforms of apo C-II in Hep G2 cell media, but they represent minor apo C-II isoforms in normal fasting plasma. Apolipoprotein C-II0, the major plasma isoform of apo C-II, is a proprotein that undergoes proteolytic cleavage of the amino-terminal hexapeptide to form mature apo C-II (apo C-II-1/2)-Amino acid composition and amino-terminal analysis of apo C-II-1/2 confirms the loss of the six terminal amino acids of apo C-II0. In summary: (1) apo C-II has been cloned, and its complete genomic sequence determined; (2) apo C-II is synthesized as preproapo C-II, which undergoes cleavage of a 22-amino-acid signal peptide to form proapo C-II; (3) proapo C-II is glycosylated to generate the sialic-acid-containing glycoproteins apo C-II-2 and apo C-II-1; (4) apo C-II-2 and apo C-II-1 are deglycosylated to form apo C-II0; (5) apo C-II0, the major plasma isoform of apo C-II, is a proprotein; (6) proteolytic processing of apo C-IIo results in the loss of six amino terminal residues to form apo C-II-1/2, the mature apo C-II isoform. A better understanding of the structural relationship of the various plasma isoforms of apo C-II will help to elucidate the mechanisms involved in normal as well as defective processing of apo C-II.
KeywordsLipoprotein Lipase Human Apolipoprotein Primer Extension Product Dideoxynucleotide Chain Termination Method Amino Terminal Residue
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