title: Biosynthesis and transport of lysosomal alpha-glucosidase in the human colon carcinoma cell line Caco-2: secretion from the apical surface.
abstract: The human adenocarcinoma cell line Caco-2 was used for studies on the biosynthesis and transport of lysosomal acid alpha-glucosidase in polarized epithelial cells. Metabolic labelling revealed that in Caco-2 cells alpha-glucosidase is synthesized as a precursor form of 110 x 10(3) Mr. This form is converted into a precursor of slightly higher Mr (112 x 10(3)) by the addition of complex oligosaccharide chains. Via an intermediate form of 95 x 10(3) Mr, this precursor is processed into a mature form of 76 x 10(3) Mr. Combination of metabolic labelling with subcellular fractionation showed that the 112 x 10(3) Mr precursor of alpha-glucosidase is transported to the lysosomes. However, the same form is secreted into the culture medium (20% of newly synthesized enzyme after 4 h of chase). Immunoprecipitation of alpha-glucosidase from culture medium derived from either the apical or basolateral site of radiolabelled Caco-2 cells, showed that 70-80% of the total amount of precursor form present in the medium is secreted from the apical membrane. Measurement of enzyme activities also showed that alpha-glucosidase, unlike other lysosomal enzymes, is mainly secreted via the apical pathway. Furthermore, immunocytochemistry showed the presence of a precursor form of alpha-glucosidase on the apical, but not the basolateral, membrane of the Caco-2 cells. We conclude that alpha-glucosidase is, unlike all other secretory proteins studied so far, secreted preferentially from the apical membrane of Caco-2 cells.
authors.affiliation: Laboratory for Electron Microscopy, University of Leiden, The Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Klumperman
authors.forename: J A
authors.initials: JA
authors.lastname: Fransen
authors.forename: T C
authors.initials: TC
authors.lastname: Boekestijn
authors.forename: R P
authors.initials: RP
authors.lastname: Oude Elferink
authors.forename: K
authors.initials: K
authors.lastname: Matter
authors.forename: H P
authors.initials: HP
authors.lastname: Hauri
authors.forename: J M
authors.initials: JM
authors.lastname: Tager
authors.forename: L A
authors.initials: LA
authors.lastname: Ginsel
mesh_terms: D016764:Cell Polarity; D003110:Colonic Neoplasms / Q000201:enzymology* / Q000648:ultrastructure; D006801:Humans; D008247:Lysosomes / Q000201:enzymology* / Q000648:ultrastructure; D008871:Microvilli / Q000201:enzymology / Q000648:ultrastructure; D011499:Protein Processing, Post-Translational; D013347:Subcellular Fractions / Q000201:enzymology; D014407:Tumor Cells, Cultured; D000520:alpha-Glucosidases / Q000378:metabolism*
chemical_list: D000520:alpha-Glucosidases