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 (By similarity). Homodimer; disulfide-linked (By similarity). Occurs in both reduced and oxidized monomeric form (By similarity). Oxidative conditions increase homodimerization (By similarity). 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.