Novel atypical thioredoxins with potential redox activity 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 thioredoxins, glutaredoxins, protein disulfide isomerases, thiol peroxidases, and glutathione transferases, which all share the thioredoxin structural fold. Many of them have one or two redox-active cysteines and a characteristic cis-proline at specific positions, and 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 have identified a set of 17 atypical TRX-like proteins from Arabidopsis, which have not been considered before despite having both a TRX fold and the CxxC/S signature typical of redox-active TRXs. In silico sequence and structure analyses revealed that they are divided into eight distinct classes with unique active-site signatures and structures, some with disulfide bond-forming protein A (DsbA) and peroxiredoxin-like folds. Their 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. Our results reveal additional diversity in the structure and function of atypical TRXs in plants.