Endoplasmic reticulum and mitochondria-associated protein that probably functions as a regulator of cellular redox state and thereby regulates protein post-translational modification, protein folding and mitochondrial activity. Monomer. Homodimer; disulfide-linked. Occurs in both reduced and oxidized monomeric form. Oxidative conditions increase homodimerization. Localizes to endoplasmic reticulum mitochondria-associated membrane (MAMs) that connect the endoplasmic reticulum and the mitochondria. The thioredoxin domain lacks the 2 redox-active cysteines, suggesting that it lacks thioredoxin activity. The di-lysine motif confers endoplasmic reticulum localization for type I membrane proteins.