Analysis of the endoplasmic reticular Ca2+ requirement for alpha1-antitrypsin processing and transport competence.
Depletion of Ca2+ sequestered within the endoplasmic reticulum (ER) of HepG2 hepatoma cells results in the luminal accumulation of immature alpha1-antitrypsin possessing Man8-9 GlcNAc2 oligosaccharide side chains. This study explores the basis for this arrest and describes consequent alterations in the size and rate of secretion of the complex endoglycosidase H-resistant form of the protein. Inhibition of glucosidase I and II with castanospermine or alpha-1,2-mannosidase with 1-deoxymannojirimycin produced altered ER processing intermediates that were rapidly secreted. Subsequent mobilization of ER Ca2+ stores resulted in the appearance and retention of slightly larger related forms of these intermediates. Retention of glycosylated intermediates was not ascribable to an association with alpha1,2-mannosidase or lectin-like chaperones, the intermediates were not degraded and all evidence of ER retention or size alterations produced by Ca2+ depletion was quickly reversed by Ca2+ restoration. Cells that were Ca2+ depleted for 2 h slowly secreted an abnormal slightly smaller complex oligosaccharide form of alpha1-antitrypsin at approximately the same rate as the non-glycosylated protein generated by treatment with tunicamycin. The hypothesis that Ca2+ affects the folding and ER transport competence of glycosylated forms of alpha1-antitrypsin is discussed.
Cooper
G R
GR
Department of Pharmacology, Robert Wood Johnson Medical School, Piscataway, NJ 08854, USA.
Brostrom
C O
CO
Brostrom
M A
MA
D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D015759:Ionomycin / Q000494:pharmacology; D011499:Protein Processing, Post-Translational; D014407:Tumor Cells, Cultured; D000515:alpha 1-Antitrypsin / Q000378:metabolism*
D000515:alpha 1-Antitrypsin; D015759:Ionomycin; D002118:Calcium