Thioredoxin-related protein of 14 kDa may directly reduce protein cysteinylation motifs

Disulfide stress has been associated with inflammation and characterized by an increase in cystine levels and protein cysteinylation. Furthermore, it was recently discovered that thioredoxin-related protein of 14 kDa (TRP14, encoded by TXNDC17) exhibits efficient cystine reductase activity. The aim of our research was to elucidate if TRP14 is also able to reduce cysteinylated proteins in mammalian cells. Thus, protein cysteinylation was assessed in control and TRP14 knockdown cells in vitro through their pre-treatment with 25 µg/ml cycloheximide for 30 min and incubation with 250 µM biotinylated cysteine for 1 h. Moreover, such TRP14 knockdown cell lysates were tested as cysteinylated substrates for TRP14 activity through their incubation with 20 µM human TRP14, 1 uM thioredoxin reductase 1 (TrxR1) and 1 mM NADPH for 15 min at 25oC. TRP14 knockdown cells showed more protein cysteinylation than control cells. Human TRP14, in a reaction together with TrxR1 and NADPH, could also reduce cysteinylated protein substrates in cell lysates. In conclusion, TRP14 is able to directly reduce protein cysteinylation motifs. The potential in vivo significance of this enzymatic activity awaits further clarification.