title: A vesicular intermediate in the transport of hepatoma secretory proteins from the rough endoplasmic reticulum to the Golgi complex.
abstract: We have identified a vesicle fraction that contains alpha 1-antitrypsin and other human HepG2 hepatoma secretory proteins en route from the rough endoplasmic reticulum (RER) to the cis face of the Golgi complex. [35S]Methionine pulse-labeled cells were chased for various periods of time, and then a postnuclear supernatant fraction was resolved on a shallow sucrose-D2O gradient. This intermediate fraction has a density lighter than RER or Golgi vesicles. Most alpha 1-antitrypsin in this fraction (P1) bears N-linked oligosaccharides of composition similar to that of alpha 1-antitrypsin within the RER; mainly Man8GlcNac2 with lesser amounts of Man7GlcNac2 and Man9GlcNac2; this suggests that the protein has not yet reacted with alpha-mannosidase-I on the cis face of the Golgi complex. This light vesicle species is the first post-ER fraction to be filled by labeled alpha 1-antitrypsin after a short chase, and newly made secretory proteins enter this compartment in proportion to their rate of exit from the RER and their rate of secretion from the cells: alpha 1-antitrypsin and albumin faster than preC3 and alpha 1-antichymotrypsin, faster, in turn, then transferrin. Deoxynojirimycin, a drug that blocks removal of glucose residues from alpha 1-antitrypsin in the RER and blocks its intracellular maturation, also blocks its appearance in this intermediate compartment. Upon further chase of the cells, we detect sequential maturation of alpha 1-antitrypsin to two other intracellular forms: first, P2, a form that has the same gel mobility as P1 but that bears an endoglycosidase H-resistant oligosaccharide and is found in a compartment--probably the medial Golgi complex--of density higher than that of the intermediate that contains P1; and second, the mature sialylated form of alpha 1-antitrypsin.
mesh_terms: D017485:1-Deoxynojirimycin; D001692:Biological Transport; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002460:Cell Line; D003594:Cytoplasmic Granules / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005944:Glucosamine / Q000031:analogs & derivatives / Q000494:pharmacology; D006056:Golgi Apparatus / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms / Q000378:metabolism*; D008358:Mannose / Q000378:metabolism; D009363:Neoplasm Proteins / Q000096:biosynthesis / Q000378:metabolism*; D000515:alpha 1-Antitrypsin / Q000096:biosynthesis / Q000378:metabolism*
chemical_list: D009363:Neoplasm Proteins; D000515:alpha 1-Antitrypsin; D017485:1-Deoxynojirimycin; D005944:Glucosamine; D008358:Mannose