title: Novel thioredoxin-related transmembrane protein TMX4 has reductase activity.
abstract: In the endoplasmic reticulum (ER), a number of thioredoxin (Trx) superfamily proteins are present to enable correct disulfide bond formation of secretory and membrane proteins via Trx-like domains. Here, we identified a novel transmembrane Trx-like protein 4 (TMX4), in the ER of mammalian cells. TMX4, a type I transmembrane protein, was localized to the ER and possessed a Trx-like domain that faced the ER lumen. A maleimide alkylation assay showed that a catalytic CXXC motif in the TMX4 Trx-like domain underwent changes in its redox state depending on cellular redox conditions, and, in the normal state, most of the endogenous TMX4 existed in the oxidized form. Using a purified recombinant protein containing the Trx-like domain of TMX4 (TMX4-Trx), we confirmed that this domain had reductase activity in vitro. The redox potential of this domain (-171.5 mV; 30 degrees C at pH 7.0) indicated that TMX4 could work as a reductase in the environment of the ER. TMX4 had no effect on the acceleration of ER-associated degradation. Because TMX4 interacted with calnexin and ERp57 by co-immunoprecipitation assay, the role of TMX4 may be to enable protein folding in cooperation with these proteins consisting of folding complex in the ER.
authors.affiliation: Department of Molecular and Cellular Biology, Institute for Frontier Medical Sciences, Kyoto University, Kyoto 606-8507, Japan.
authors.forename: Yoshimi
authors.initials: Y
authors.lastname: Sugiura
authors.forename: Kazutaka
authors.initials: K
authors.lastname: Araki
authors.forename: Shun-ichiro
authors.initials: S
authors.lastname: Iemura
authors.forename: Tohru
authors.initials: T
authors.lastname: Natsume
authors.forename: Jun
authors.initials: J
authors.lastname: Hoseki
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Nagata
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D004721:Endoplasmic Reticulum / Q000201:enzymology / Q000648:ultrastructure; D006367:HeLa Cells; D006801:Humans; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D011490:Protein Disulfide Reductase (Glutathione) / Q000235:genetics / Q000378:metabolism*; D019704:Protein Disulfide-Isomerases / Q000235:genetics / Q000378:metabolism; D017510:Protein Folding; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D013879:Thioredoxins / Q000235:genetics / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D008562:Membrane Glycoproteins; D034741:RNA, Small Interfering; D011993:Recombinant Fusion Proteins; D013879:Thioredoxins; D010088:Oxidoreductases; D011490:Protein Disulfide Reductase (Glutathione); C546436:TMX4 protein, human; D019704:Protein Disulfide-Isomerases; C085868:PDIA3 protein, human