TRP14 is the rate-limiting enzyme for intracellular cystine reduction and regulates proteome cysteinylation

It has remained unknown how cells reduce cystine taken up from the extracellular space, which is a required step for further utilization of cysteine in key processes such as protein or glutathione synthesis. Here, we show that the thioredoxin-related protein of 14 kDa (TRP14, encoded by TXNDC17 ) is the rate-limiting enzyme for intracellular cystine reduction. When TRP14 is genetically knocked out, cysteine synthesis through the transsulfuration pathway becomes the major source of cysteine in human cells, and knockout of both pathways becomes lethal in C. elegans subjected to proteotoxic stress. TRP14 can also reduce cysteinyl moieties on proteins, rescuing their activities as here shown with cysteinylated peroxiredoxin 2. Txndc17 knockout mice were, surprisingly, protected in an acute pancreatitis model, concomitant with activation of Nrf2-driven antioxidant pathways and upregulation of transsulfuration. We conclude that TRP14 is the evolutionarily conserved enzyme principally responsible for intracellular cystine reduction in C. elegans , mice, and humans. Synopsis Metabolic utilization of cysteine requires reduction of cystine taken up from extracellular sources. Here, TXNDC17 -encoded TRP14 is identified as the main enzyme catalyzing this step, regulating protein cysteinylation and cysteine homeostasis in concert with the transsulfuration pathway. Thioredoxin-related protein TRP14 is the rate-limiting enzyme in mammalian cytosolic cystine reduction. Genetic ablation of TRP14 redirects cysteine biosynthesis towards the transsulfuration pathway as alternative, methionine-based source of cysteine. TRP14 also reduces cysteinylated proteins and potentially modulates protein function, such as reactivation of cysteinylated peroxiredoxin 2 through its decysteinylation. Txndc17 -deficient mice are protected from acute pancreatitis pathophysiology through activation of Nrf2-mediated antioxidant pathways and primed upregulation of transsulfuration. Interplay between TRP14 and transsulfuration in cysteine metabolism appears conserved across species, as knockout of both pathways becomes lethal in C. elegans subjected to proteotoxic stress. A thioredoxin-related protein controls cysteine homeostasis in parallel to the transsulfuration pathway, and can also modulate protein function via protein decysteinylation.