Potential unrecognized redox-active atypical thioredoxins in eukaryote photosynthetic organisms.

Photosynthetic organisms have a high diversity of proteins belonging to the thioredoxin (TRX) superfamily. It comprises more than 150 proteins distributed in different families and classes, including in particular thioredoxins, glutaredoxins, protein disulfide isomerases, thiol peroxidases or glutathione transferases, which share the thioredoxin structural fold. Many of them have one or two redox-active cysteines and a characteristic cis-proline at specific positions, but also additional domains or secondary structures at either end or inserted into the protein core. With the aim of further describing the TRX family in plants, we identified a set of 17 atypical TRX-like proteins from Arabidopsis thaliana, which have not been considered before despite having both a TRX fold and the CxxC/S signature typical of redox-active TRX. The in silico sequence and structure analyses revealed that they are distributed in eight distinct classes with unique active site signatures and structures, some with DsbA and peroxiredoxin-like folds. The distinct subcellular localizations (plastids, mitochondria, extracellular space) and gene expression profiles suggest that these proteins are involved in diverse cellular processes, further expanding the set of proteins involved in redox regulation and/or stress adaptation. These results further reveal the diversity in structure and function of atypical TRXs in plants.