Human mitochondrial thioredoxin reductase - cDNA cloning, expression and genomic organization

We have isolated a 1918-bp cDNA from a human adrenal cDNA library which enc odes a novel thioredoxin reductase (TrxR2) of 521 amino acid residues with a calculated molecular mass of 56.2 kDa. It is highly homologous to the pre viously described cytosolic enzyme (TrxR1), including the conserved active site CVNVGC and the FAD-binding and NADPH-binding domains. However, human T rxR2 differs from human TrxR1 by the presence of a 33-amino acid extension at the N-terminus which has properties characteristic of a mitochondrial tr anslocation signal. Northern-blot analysis identified one mRNA species of 2 .2 kb with highest expression in prostate, testis and liver. We expressed h uman TrxR2 as a fusion protein with green fluorescent protein and showed th at in vivo it is localized in mitochondria. Removal of the mitochondrial ta rgeting sequence abolishes the mitochondrial translocation. Finally, we det ermined the genomic organization of the human TrxR2 gene, which consists of 18 exons spanning about 67 kb, and its chromosomal localization at positio n 22q11.2.