CBS Domain Proteins Regulate Redox Homeostasis

Abstract

fluorescent protein fusion localized to the chloroplast. In the chloroplast, CBSX1 interacted with four distinct thioredoxins, as shown by yeast two-hybrid and in vitro pull-down studies. The interaction of CBSX1 and thioredoxin also occurred in living plant cells, as shown by bimolecular fluorescence complementation analysis in tobacco (Nicotiana tabacum) leaves. Based on these interactions, the authors hypothesize that CBSX1 and CBX2 positively regulate thioredoxins, which then reduce target proteins, such as peroxiredoxin, which in turn reduce cellular H2O2 and regulate its level. They show that both CBSX proteins increase the activity of all four thioredoxins, in some cases more effectively than the chemical reductant DTT. Addition of AMP further increased the activation of all four thioredoxins by CBSX1 and CBSX2, but ADP and ATP did not have this enhancing effect. The authors also present a crystal structure of CBSX2 and describe a positively charged ligand binding pocket formed by Lys and Arg residues. This pocket is similar to those in other CBS domain proteins that contain adenosinerelated ligands. In CBSX1, this pocket can accommodate AMP, but not the bulkier ADP or ATP, consistent with the above demonstration that CBSX1induced activation of thioredoxin was en