title: A mutation in a highly conserved region in brush-border sucrase-isomaltase and lysosomal alpha-glucosidase results in Golgi retention.
abstract: A point mutation in the cDNA of human intestinal sucrase-isomaltase has been recently identified in phenotype II of congenital sucrase-isomaltase deficiency. The mutation results in a substitution of glutamine by proline at position 1098 (Q1098P) in the sucrase subunit. Expression of this mutant sucrase-isomaltase cDNA in COS-1 cells results in an accumulation of sucrase-isomaltase in the ER, intermediate compartment and the cis-Golgi cisternae similar to the accumulation in phenotype II intestinal cells. An interesting feature of the Q1098P substitution is its location in a region of the sucrase subunit that shares striking similarities with the isomaltase subunit and other functionally related enzymes, such as human lysosomal acid alpha-glucosidase and Schwanniomyces occidentalis glucoamylase. We speculated that the Q-->P substitution in these highly conserved regions may result in a comparable accumulation. Here we examined this hypothesis using lysosomal alpha-glucosidase as a reporter gene. Mutagenesis of the glutamine residue at position 244 in the homologous region of alpha-glucosidase to proline results in a protein that is neither transported to the lysosomes nor secreted extracellularly but accumulates in the ER, intermediate compartment and cis-Golgi as a mannose-rich polypeptide similar to mutant sucrase-isomaltase in phenotype II. We propose that the Q1098P and Q244P mutations (in sucrase-isomaltase and alpha-glucosidase, respectively) generate structural alterations that are recognized by a control mechanism, operating beyond the ER in the intermediate compartment or cis-Golgi.
authors.affiliation: Protein Secretion Group, Institute of Microbiology, Heinrich-Heine-University of Düsseldorf, Germany.
authors.forename: C E
authors.initials: CE
authors.lastname: Moolenaar
authors.forename: J
authors.initials: J
authors.lastname: Ouwendijk
authors.forename: M
authors.initials: M
authors.lastname: Wittpoth
authors.forename: H A
authors.initials: HA
authors.lastname: Wisselaar
authors.forename: H P
authors.initials: HP
authors.lastname: Hauri
authors.forename: L A
authors.initials: LA
authors.lastname: Ginsel
authors.forename: H Y
authors.initials: HY
authors.lastname: Naim
authors.forename: J A
authors.initials: JA
authors.lastname: Fransen
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D019556:COS Cells; D018938:Caco-2 Cells; D017124:Conserved Sequence; D006056:Golgi Apparatus / Q000201:enzymology* / Q000648:ultrastructure; D006801:Humans; D008247:Lysosomes / Q000201:enzymology* / Q000648:ultrastructure; D016253:Microscopy, Immunoelectron; D008871:Microvilli / Q000201:enzymology; D008969:Molecular Sequence Data; D017354:Point Mutation; D017510:Protein Folding; D017386:Sequence Homology, Amino Acid; D013394:Sucrase-Isomaltase Complex / Q000235:genetics*; D000520:alpha-Glucosidases / Q000235:genetics*
chemical_list: D013394:Sucrase-Isomaltase Complex; D000520:alpha-Glucosidases