A di-arginine motif contributes to the ER localization of the type I transmembrane ER oxidoreductase TMX4.
The thiol-disulfide oxidoreductases of the PDI (protein disulfide isomerase) family assist in disulfide-bond formation in the ER (endoplasmic reticulum). In the present study, we have shown that the previously uncharacterized PDI family member TMX4 (thioredoxin-like transmembrane 4) is an N-glycosylated type I membrane protein that localizes to the ER. We also demonstrate that TMX4 contains a single ER-luminal thioredoxin-like domain, which, in contrast with similar domains in other PDIs, is mainly oxidized in living cells. The TMX4 transcript displays a wide tissue distribution, and is strongly expressed in melanoma cells. Unlike many type I membrane proteins, TMX4 lacks a typical C-terminal di-lysine retrieval signal. Instead, the cytoplasmic tail has a conserved di-arginine motif of the RXR type. We show that mutation of the RQR sequence in TMX4 to KQK interferes with ER localization of the protein. Moreover, whereas the cytoplasmic region of TMX4 confers ER localization to a reporter protein, the KQK mutant of the same protein redistributes to the cell surface. Overall, features not commonly found in other PDIs characterize TMX4 and suggest unique functional properties of the protein.
Roth
Doris
D
Department of Cell Biology, University of Copenhagen, Ole Maaløes Vej 5, 2200 Copenhagen N, Denmark.
Lynes
Emily
E
Riemer
Jan
J
Hansen
Henning G
HG
Althaus
Nils
N
Simmen
Thomas
T
Ellgaard
Lars
L
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001120:Arginine / Q000235:genetics / Q000378:metabolism*; D015152:Blotting, Northern; D015153:Blotting, Western; D045744:Cell Line, Tumor; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005260:Female; D020869:Gene Expression Profiling; D006031:Glycosylation; D006367:HeLa Cells; D006801:Humans; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D009154:Mutation; D010084:Oxidation-Reduction; D010802:Phylogeny; D011490:Protein Disulfide Reductase (Glutathione) / Q000235:genetics / Q000378:metabolism*; D019704:Protein Disulfide-Isomerases / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D014709:Vero Cells
D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D001120:Arginine; D011490:Protein Disulfide Reductase (Glutathione); C546436:TMX4 protein, human; D019704:Protein Disulfide-Isomerases