Biosynthesis of human preproapolipoprotein A-II

The primary translation product of human apolipoprotein A-II was purified from wheat germ and ascites cell-free lysates programmed with RNA isolated from either a hepatocellular carcinoma cell line (HepG2) or intestinal epithelium. A-II mRNA represents 0.2% of the translatable RNA in these hepatocytes and in jejunal epithelium. Plasma high density lipoprotein-associated A-II is a 77-amino acid polypeptide. The primary translation product is 100 amino acids long and contains a 23-amino acid NH₂-terminal extension. Cotranslational cleavage of the cell-free product indicated that this NH₂-terminal sequence consists of an 18-amino acid long signal peptide, Met-Lys-Leu-Leu-Ala-Ala-X-Val-Leu-Leu-Leu-X-X-Cys-X-Leu-X-X-, and a 5-amino acid long propeptide, Ala-Leu-Val-Arg-Arg. This functional division was confirmed by sequencing the stable intracellular form of apolipoprotein A-II isolated from HepG2 cells. Approximately 45% of the proapo-A-II is cleaved to the mature form during export from HepG2 cells. The COOH-terminal dipeptide conforms to the rule that prosegments are cleaved after paired basic residues. We have previously shown (Gordon, J.I., Sims, H.F., Lentz, S.R., Edelstein, C., Scanu, A. M., and Strauss, A. W. (1983) J. Biol. Chem. 25, 4037-4044) that proapolipoprotein A-I is not cleaved during export from these cells and contains a prosegment with a COOH-terminal Gln-Gln dipeptide. Therefore, proteolytic processing of the two principal high density lipoprotein-associated apolipoproteins proceeds along different pathways.