The multiple facets of glutaredoxins: one fold, several partners and functions
Résumé
Glutaredoxins (Grxs) are oxidoreductases structurally related to thioredoxins (Trxs). They have two major proposed biochemical roles, the reduction of disulfide bonds and the binding of iron-sulfur (Fe-S) clusters, both of which usually involve glutathione. In photosynthetic organisms, Grxs are distributed into six classes. Several Grxs from classes I and II can indeed exist either as apoforms which display deglutathionylation activity or as holoforms which bind Fe-S clusters.
Using biochemical, spectroscopic and structural approaches, we have characterized four plastidial Grxs, showing that three of these can bind an Fe-S cluster. Site-directed mutagenesis experiments and resolution of the crystal structure of class I Grxs (GrxC5 and S12) revealed the critical role of some active site residues for cluster formation and for protein activity. The deglutathionylation activity of these two Grxs proceeds through a monothiol mechanism, which is important for the regeneration of the thiol-peroxidase and methionine sulfoxide reductase families. The two plastidial class II Grxs (GrxS14 and S16) can bind more labile Fe-S clusters that can serve for the maturation of other proteins as demonstrated by in vitro Fe-S cluster transfer experiments and by the complementation of a yeast strain deleted for the mitochondrial Grx5. Using binary yeast two hybrid and bimolecular fluorescence complementation experiments, we have shown that class II but not class I Grxs interact with plastidial BolA proteins, including SufE1, a sulfurtransferase required for the maturation of plastidial Fe-S proteins. Finally, using spectroscopic and structural approaches, we have determined that the Grx-BolA couples form two types of complexes involving distinct regions of both partners.
Hence, we propose (i) that, based on its peculiar catalytic and thermodynamic properties, GrxS12 and potentially GrxC5 could rather act as oxidoreductases and/or redox sensors, and (ii) that GrxS14 and GrxS16 could participate to the maturation of iron-sulfur proteins either by being directly involved in Fe-S cluster trafficking or by having regulatory roles in particular via their interaction with BolA proteins.