title: Quantity and quality control of gastric proton pump in the endoplasmic reticulum by ubiquitin/proteasome system.
abstract: The gastric proton pump, H(+),K(+)-ATPase, consists of the catalytic alpha-subunit and the noncatalytic beta-subunit. These subunits are assembled in the endoplasmic reticulum (ER) and leave the ER to reach to the cell surface as a functional holoenzyme. We studied the quantity control mechanism of the H(+),K(+)-ATPase in the ER by using a heterologous expression system in human embryonic kidney 293 cells. The alpha-subunit in the alpha-expressing cells was degraded more rapidly than in the alpha+beta-expressing cells. It was stabilized, however, in the presence of a proteasome inhibitor, lactacystin. Polyubiquitination of the alpha-subunit was observed in the alpha-expressing cells as well as in the alpha+beta-expressing cells. The extent of polyubiquitination was higher in the former alpha-expressing cells especially in the presence of lactacystin. On the other hand, polyubiquitination of the beta-subunit was not observed in the absence and presence of lactacystin. When the alpha-subunit was coexpressed with a mutant beta-subunit that lacks alpha/beta assembly capacity, degradation of the alpha-subunit was accelerated in parallel with increased polyubiquitination of the alpha-subunit. These results indicate that the ubiquitin/proteasome system is involved in degradation of the unassembled alpha-subunits in the ER to control the cell surface expression of the functional alpha/beta holoenzymes.
authors.affiliation: Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama 930-0194, Japan.
authors.forename: Tohru
authors.initials: T
authors.lastname: Kimura
authors.forename: Hokara
authors.initials: H
authors.lastname: Ishizuka
authors.forename: Ayumi
authors.initials: A
authors.lastname: Yoshida
authors.forename: Magotoshi
authors.initials: M
authors.lastname: Morii
authors.forename: Noriaki
authors.initials: N
authors.lastname: Takeguchi
authors.forename: Shinji
authors.initials: S
authors.lastname: Asano
mesh_terms: D002460:Cell Line; D002874:Chromosome Mapping; D002882:Chromosomes, Human, Pair 13; D002888:Chromosomes, Human, Pair 19; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D005753:Gastric Mucosa / Q000201:enzymology*; D017506:H(+)-K(+)-Exchanging ATPase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D014176:Protein Biosynthesis; D021122:Protein Subunits / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: D009097:Multienzyme Complexes; D021122:Protein Subunits; D011994:Recombinant Proteins; D025801:Ubiquitin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D017506:H(+)-K(+)-Exchanging ATPase