title: Biosynthesis of intestinal microvillar proteins. Further characterization of the intracellular processing and transport.
abstract: The effect of tunicamycin on synthesis and intracellular transport of pig small intestinal aminopeptidase N (EC 3.4.11.2), sucrase-isomaltase (EC 3.2.1.48-10) and maltase-glucoamylase (EC 3.2.1.20) was studied by labelling of mucosal explants with [35S]methionine. The expression of the microvillar enzymes was greatly reduced by tunicamycin but could be partially restored by leupeptin, suggesting the existence of a mechanism whereby newly synthesized, malprocessed enzymes are recognized and degraded. In the presence of tunicamycin, polypeptides likely to represent non-glycosylated forms of the enzymes persisted in the Mg2+-precipitated membrane fraction, indicating that high mannose glycosylation is essential for transport to the microvillar membrane. Treatment of aminopeptidase N and sucrase-isomaltase with endo F reduced the size of the high mannose forms approximately to those seen in the presence of tunicamycin. The complex forms were also sensitive to endo F but did not coincide with the high mannose forms after treatment, indicating that the size difference cannot alone be ascribed to processing of N-linked carbohydrate.
mesh_terms: D000626:Aminopeptidases / Q000096:biosynthesis*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D005944:Glucosamine / Q000031:analogs & derivatives*; D005959:Glucosidases / Q000096:biosynthesis*; D006023:Glycoproteins / Q000096:biosynthesis; D007413:Intestinal Mucosa / Q000201:enzymology*; D007976:Leupeptins / Q000494:pharmacology; D008871:Microvilli / Q000378:metabolism; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000096:biosynthesis*; D013394:Sucrase-Isomaltase Complex / Q000096:biosynthesis*; D013552:Swine; D014415:Tunicamycin / Q000494:pharmacology*; D000520:alpha-Glucosidases / Q000096:biosynthesis*
chemical_list: D006023:Glycoproteins; D007976:Leupeptins; D009097:Multienzyme Complexes; D014415:Tunicamycin; D005959:Glucosidases; D013394:Sucrase-Isomaltase Complex; D000520:alpha-Glucosidases; D000626:Aminopeptidases; C032854:leupeptin; D005944:Glucosamine