Mammalian glutaredoxin 3 (Grx3/PICOT) is an essential protein involved in the regulation of signal transduction, for instance during immune cell activation and development of cardiac hypertrophy, presumably in response to redox signals. This function requires the sensing of such stresses by a hitherto unknown mechanism. Here, we characterized Grx3/PICOT as iron-sulfur protein. The protein binds two bridging [2Fe-2S] clusters in a homodimeric complex with the active site cysteinyl residues of its two monothiol glutaredoxin domains and glutathione bound non-covalently to the Grx domains. Co-immunoprecipitation of 55-iron with Grx3/PICOT from Jurkat cells suggested the presence of these cofactors under physiological conditions. The [2Fe-2S](2+) clusters were not redox active, instead they were lost upon treatment of the holo protein with ferricyanide or S-nitroso glutathione. This redox-induced dissociation of the Grx3/PICOT holo complex may be a mechanism of Grx3/PICOT activation in response to reactive oxygen and nitrogen species. (C) 2010 Elsevier Inc. All rights reserved.