title: Identification and characterization of a novel thioredoxin-related transmembrane protein of the endoplasmic reticulum.
abstract: The endoplasmic reticulum (ER) contains a number of thiol-disulfide oxidoreductases of the protein-disulfide isomerase (PDI) family that catalyze the formation of disulfide bonds in newly synthesized proteins. Here we describe the identification and characterization of a novel member of the human PDI family, TMX3 (thioredoxin-related transmembrane protein 3). The TMX3 gene encodes a protein of 454 amino acid residues that contains a predicted N-terminal signal sequence, a single domain with sequence similarity to thioredoxin and a CGHC active site sequence, a potential transmembrane domain, and a C-terminal KKKD tetrapeptide sequence that matches the classical KKXX-type consensus sequence for ER retrieval of type I transmembrane proteins. Endogenous TMX3 contains endoglycosidase H-sensitive glycans, localizes to the ER by immunofluorescence microscopy, and is present in the membrane fraction after alkaline extraction of the ER luminal content. The TMX3 transcript is found in a variety of tissues and is not up-regulated by the unfolded protein response. Circular dichroism spectroscopy of the recombinantly expressed luminal domain of TMX3 showed features typical of a properly folded protein of the alpha/beta type. The redox potential of recombinant luminal TMX3 was determined to -0.157 V, similar to the values previously found for PDI and ERp57. Interestingly, TMX3 showed oxidase activity, and in human tissue-culture cells the protein was found partially in the oxidized form, potentially suggesting a function of the protein as a dithiol oxidase.
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zürich CH-8093, Switzerland.
authors.forename: Johannes
authors.initials: J
authors.lastname: Haugstetter
authors.forename: Thomas
authors.initials: T
authors.lastname: Blicher
authors.forename: Lars
authors.initials: L
authors.lastname: Ellgaard
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D015152:Blotting, Northern; D015153:Blotting, Western; D002384:Catalysis; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D002942:Circular Dichroism; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005091:Exons; D005822:Genetic Vectors; D005978:Glutathione / Q000737:chemistry; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D007438:Introns; D007700:Kinetics; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D010455:Peptides / Q000737:chemistry; D010957:Plasmids / Q000378:metabolism; D011489:Protein Denaturation; D019704:Protein Disulfide-Isomerases / Q000096:biosynthesis* / Q000737:chemistry*; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013347:Subcellular Fractions; D013879:Thioredoxins / Q000737:chemistry*; D013997:Time Factors; D014018:Tissue Distribution; D014158:Transcription, Genetic; D014466:Ultraviolet Rays; D015854:Up-Regulation; D014709:Vero Cells
chemical_list: D010455:Peptides; D012333:RNA, Messenger; D011994:Recombinant Proteins; D013879:Thioredoxins; D010088:Oxidoreductases; D006026:Glycoside Hydrolases; D019704:Protein Disulfide-Isomerases; C498879:TMX3 protein, human; D005978:Glutathione; D010100:Oxygen