title: The large C-terminal region of the integral pore membrane protein, POM121, is facing the nuclear pore complex.
abstract: POM121 is an integral membrane protein that has been specifically localized to the "pore membrane" domain of the nuclear envelope. Based on its cDNA-deduced primary structure it was suggested that POM121 contains one or two transmembrane segments and that its major C-terminal portion faces the pore side rather than the cisternal side of the pore membrane. We have investigated the membrane topology of POM121 by studying the accessibility of a C-terminal and an N-terminal epitope of POM121 for epitope-specific antibodies. The accessibility of POM121 in unfixed, semi-intact or permeabilized tissue culture cells was analyzed by indirect immunofluorescence. We found that the C-terminal epitope was accessible for antibodies in both semi-intact and permeabilized cells, whereas the N-terminal epitope was only accessible in the permeabilized cells. The results show that the large C-terminal region of POM121, containing more than 90% of its total mass, is exposed on the pore side of the nuclear membrane and suggest that the N-terminal portion is most likely localized in the perinuclear space. The data also show that at least part of the C-terminal epitopes are localized on the cytoplasmic side of the nuclear envelope. The topology suggests that the C-terminal portion of POM121, which contains a nucleoporin-like domain, interacts with the nuclear pore complex and thus, may play a role in biogenesis of the nuclear envelope and the nuclear pore complex.
authors.affiliation: Department of Biochemistry, Arrhenius Laboratories for Natural Sciences, Stockholm University, Sweden.
authors.forename: H
authors.initials: H
authors.lastname: Söderqvist
authors.forename: E
authors.initials: E
authors.lastname: Hallberg
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002463:Cell Membrane Permeability / Q000187:drug effects; D002478:Cells, Cultured; D018076:DNA, Complementary / Q000235:genetics; D004072:Digitonin / Q000494:pharmacology; D000939:Epitopes / Q000276:immunology; D008099:Liver; D008565:Membrane Proteins / Q000737:chemistry* / Q000276:immunology; D008969:Molecular Sequence Data; D009685:Nuclear Envelope; D009687:Nuclear Proteins; D010446:Peptide Fragments / Q000276:immunology; D011817:Rabbits; D051381:Rats; D011915:Rats, Inbred BUF
chemical_list: D018076:DNA, Complementary; D000939:Epitopes; D008565:Membrane Proteins; D009687:Nuclear Proteins; D010446:Peptide Fragments; C082264:Pom121 protein, rat; D004072:Digitonin