title: Synthesis, intracellular processing, and signal peptide of human apolipoprotein E.
abstract: Northern blotting analysis has shown apo-E mRNA synthesis by human liver, HepG2 cells, and primary cultures of human monocyte macrophages but not by the macrophage-like cell line U937 and normal or transformed human fibroblasts. Cell-free translation has shown that the primary translation product of apo-E consists of one major and one minor isoprotein of apparent Mr = 28,500 and isoelectric points 6.20 and 6.02, respectively. These isoproteins differ by +1 and 0 charges from apo-E3 and have been designated preapo-E. Co-translational treatment of mRNA with dog pancreatic membranes converts both preapo-E isoproteins to a form which is undistinguishable by two-dimensional gel electrophoresis from plasma apo-E3. The isolation and nucleotide sequence analysis of a full length apo-E cDNA clone has shown that preapo-E contains an 18-amino acid NH2-terminal signal peptide compared to plasma apo-E. The signal peptide sequence is: MetLysValLeuTrpAlaAlaLeuLeuValThrPheLeuAlaGlyCysGlnAla. Comparison of co-translationally modified apo-E with intracellular, secreted, and plasma forms indicates that after the intracellular cleavage of the signal peptide, the protein is glycosylated with carbohydrate chains containing sialic acid, secreted as sialoapo-E (apo-Es), and subsequently desialated in plasma. These findings demonstrate that apo-E is synthesized as preprotein and undergoes intracellular proteolysis and glycosylation and extracellular desialation to attain the major asialoapo-E isoprotein form observed in plasma.
mesh_terms: D000595:Amino Acid Sequence; D001053:Apolipoproteins / Q000235:genetics*; D001057:Apolipoproteins E; D002294:Carcinoma, Squamous Cell; D002460:Cell Line; D002478:Cells, Cultured; D004262:DNA Restriction Enzymes; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D008075:Lipoproteins, HDL / Q000235:genetics*; D008099:Liver / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism*; D009693:Nucleic Acid Hybridization; D010455:Peptides / Q000378:metabolism*; D014176:Protein Biosynthesis; D021382:Protein Sorting Signals; D012333:RNA, Messenger / Q000235:genetics; D014158:Transcription, Genetic
chemical_list: D001053:Apolipoproteins; D001057:Apolipoproteins E; D008075:Lipoproteins, HDL; D010455:Peptides; D021382:Protein Sorting Signals; D012333:RNA, Messenger; D004262:DNA Restriction Enzymes