Crystal structure of chloroplastic thioredoxin z defines a novel type-specific target recognition

Thioredoxins (TRXs) are ubiquitous disulfide oxidoreductases structured according to a highly conserved fold. TRXs are involved in a myriad of different processes through a common chemical mechanism. Plant thioredoxins evolved into seven types with diverse subcellular localization and distinct protein targets selectivity. Five TRX types coexist in the chloroplast, with yet scarcely described specificities. We solved the first crystal structure of a chloroplastic z-type TRX, revealing a conserved TRX fold with an original electrostatic surface potential surrounding the redox site. This recognition surface is distinct from all other known TRX types from plant and non-plant sources and is exclusively conserved in plant z-type TRXs. We show that this electronegative surface endows TRXz with a capacity to activate the photosynthetic Calvin-Benson cycle enzyme phosphoribulokinase. TRXz distinct electronegative surface thereby extends the repertoire of TRX-target recognitions.