Intracellular maturation of apolipoprotein[a] and assembly of lipoprotein[a] in primary baboon hepatocytes.
The glycoprotein apolipoprotein[a] (apo[a]) is present in plasma at highly variable concentrations and appears as a number of genetically determined size isoforms (400-800 kDa), disulfide linked to apoB-100 in low density lipoprotein to produce lipoprotein [a](Lp[a]). Apo[a] is synthesized by the liver, but the site of association of apo[a] and apoB and factors that regulate its production are unknown. To examine the morphogenesis of the Lp[a] particle, baboon hepatocytes expressing a single, low molecular weight isoform of apo[a] were labeled with [35S]cysteine and methionine, and apo[a] was analyzed by immunoprecipitation and SDS-PAGE. Steady-state labeling revealed two molecular weight forms of apo[a] inside the cell. Only the large form was recovered from the culture medium. Pulse-chase studies and endoglycosidase treatment revealed that the lower molecular weight form of apo[a] represented a precursor with a prolonged residence time in the endoplasmic reticulum or an early Golgi compartment, after which it was processed to the mature form. A proportion of the mature form of apo[a] was rapidly secreted after synthesis, whereas the remainder had a prolonged residence time in a late Golgi compartment. In all experiments, apoB co-precipitated with apo[a] from the culture medium, but not from cell lysates. Density gradient ultracentrifugation and immunoblot analysis revealed that the majority of apo[a] was secreted into the medium in a free form, suggesting that the association between apo[a] and apoB occurred after secretion. Regulation of the movement of apo[a] between intracellular compartments may be one mechanism by which the plasma levels of Lp[a] are influenced.
White
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Department of Virology and Immunology, Southwest Foundation for Biomedical Research, San Antonio, TX 78228-0147.
Rainwater
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Lanford
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D000581:Amidohydrolases / Q000378:metabolism; D000818:Animals; D001054:Apolipoproteins A / Q000096:biosynthesis* / Q000378:metabolism; D001055:Apolipoproteins B / Q000096:biosynthesis / Q000378:metabolism; D002478:Cells, Cultured; D002499:Centrifugation, Density Gradient; D003545:Cysteine / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus; D006596:Hexosaminidases / Q000378:metabolism; D015151:Immunoblotting; D007163:Immunosorbent Techniques; D007700:Kinetics; D017270:Lipoprotein(a) / Q000096:biosynthesis* / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008715:Methionine / Q000378:metabolism; D010215:Papio; D043524:Peptide-N4-(N-acetyl-beta-glucosaminyl) Asparagine Amidase; D013462:Sulfur Radioisotopes
D001054:Apolipoproteins A; D001055:Apolipoproteins B; D017270:Lipoprotein(a); D013462:Sulfur Radioisotopes; D008715:Methionine; D006596:Hexosaminidases; D000581:Amidohydrolases; D043524:Peptide-N4-(N-acetyl-beta-glucosaminyl) Asparagine Amidase; D003545:Cysteine