Glutathione- and glutaredoxin-dependent reduction of methionine sulfoxide reductase A.

A natural fusion occurring between two tandemly repeated glutaredoxin (Grx) modules and a methionine sulfoxide reductase A (MsrA) has been detected in Gracilaria gracilis. Using an in vivo yeast complementation assay and in vitro activity measurements, we demonstrated that this fusion enzyme was able to reduce methionine sulfoxide into methionine using glutathione as a reductant. Consistently, a poplar cytosolic MsrA can be regenerated in vitro by glutaredoxins with an efficiency comparable to that of thioredoxins, but using a different mechanism. We hypothesize that the glutathione/glutaredoxin system could constitute an evolutionary conserved alternative regeneration system for MsrA.