NMR Structure of Carcinoscorpius rotundicauda Thioredoxin-related Protein 16 and Its Role in Regulating Transcription Factor NF-κB Activity

Thioredoxins (Trxs), which play a key role in maintaining a redox environment in the cell, are found in almost all organisms. Trxs act as potential reducing agents of disulfide bonds and contain two vicinal cysteines in a CXXC motif at the active site. Trx is also known to activate the DNA binding activity of NF-kappa B, an important transcription factor. Previously, Trx-related protein 16 from Carcinoscorpius rotundicauda (Cr-TRP16), a 16-kDa Trx-like protein that contains a WCPPC motif, was reported. Here we present the NMR structure of the reduced form of Cr-TRP16, along with its regulation of NF-kappa B activity. Unlike other 16-kDa Trx-like proteins, Cr-TRP16 contains an additional Cys residue (Cys-15, at the N terminus), through which it forms a homodimer. Moreover, we have explored the molecular basis of Cr-TRP16-mediated activation of NF-kappa B and showed that Cr-TRP16 exists as a dimer under physiological conditions, and only the dimeric form binds to NF-kappa B and enhances its DNA binding activity by directly reducing the cysteines in the DNA-binding motif of NF-kappa B. The C15S mutant of Cr-TRP16 was unable to dimerize and hence does not bind to NF-kappa B. Based on our finding and combined with the literature, we propose a model of how Cr-TRP16 is likely to bind to NF-kappa B. These findings elucidate the molecular mechanism by which NF-kappa B activation is regulated through Cr-TRP16.