CBSXs are sensor relay proteins sensing adenosine-containing ligands in Arabidopsis.

We recently determined that CBSX proteins, which have only one pair of cystathionine β-synthase (CBS) domains, directly regulate the activation of thioredoxins and thereby control cellular H2O2 levels and modulate both plant development and growth. The Arabidopsis genome contains six CBSXs, and these are localized to different subcellular compartments‑ CBSX1 and CBSX2 in the chloroplast, CBSX3 in the mitochondria, CBSX4 in the cytosol, and CBSX5 and CBSX6 in the endoplasmic reticulum. The CBSXs have been identified in prokaryotes and plants, but not in animals. The considerable differences in length and amino acid sequence between CBSX members may result in variations in protein structure and in their specificity to interact with ligands and/or target proteins. Here, we discuss the possibility that the CBSXs are novel sensor relay proteins that use adenosine-containing molecules as a ligand.