Identification of a novel thioredoxin-related transmembrane protein

We recently identified a series of transforming growth factor-beta -respons ive genes in A549 human adenocarcinoma cell line by a gene trap screening m ethod. Here we report the molecular cloning and characterization of one of these genes, designated TMX, that encodes a novel protein of 280 amino acid residues. The TMX protein possesses an N-terminal signal peptide followed by one thioredoxin (Trx)-like domain with a unique active site sequence, Cy s-Pro-Ala-Cys, and a potential transmembrane domain. There are putative TMX homologs with identical active site sequences in the Caenorhabditis elegan s and Drosophila genomes, Using recombinant proteins expressed in Escherich ia coil, we demonstrated the activity of the Trx domain of TMX to cleave th e interchain disulfide bridges in insulin in vitro, The TMX transcript is w idely expressed in normal human tissues, and subcellular fractionation and immunostaining for an epitope-tagged TMX protein suggest that TMX is predom inantly localized in the endoplasmic reticulum (ER), When TMX was expressed in HEK293 cells, it significantly suppressed the apoptosis induced by bref eldin A, an inhibitor of ER-Golgi transport. This activity was abolished wh en two Cys residues in the active site sequence were mutated to Ser, sugges ting that the Trx-like activity of TMX may help relieve ER stress caused by brefeldin A.