HSP33 in eukaryotes – an evolutionary tale of a chaperone adapted to photosynthetic organisms

HSP33 was originally identified in bacteria as a redox‐sensitive chaperone that protects unfolded proteins from aggregation. Here, we describe a eukaryote ortholog of HSP33 from the green algae Chlamydomonas reinhardtii, which appears to play a protective role under light‐induced oxidizing conditions. The algal HSP33 exhibits chaperone activity, as shown by citrate synthase aggregation assays. Studies from the Jakob laboratory established that activation of the bacterial HSP33 upon its oxidation initiates by the release of pre‐bound Zn from the well conserved Zn‐binding motif Cys–X–Cys–X_n–Cys–X–X–Cys, and is followed by significant structural changes (Reichmann et al., 2012 ). Unlike the bacterial protein, the HSP33 from C. reinhardtii had lost the first cysteine residue of its center, diminishing Zn‐binding activity under all conditions. As a result, the algal protein can be easily activated by minor structural changes in response to oxidation and/or excess heat. An attempt to restore the missing first cysteine did not have a major effect on Zn‐binding and on the mode of activation. Replacement of all remaining cysteines abolished completely any residual Zn binding, although the chaperone activation was maintained. A phylogenetic analysis of the algal HSP33 showed that it clusters with the cyanobacterial protein, in line with its biochemical localization to the chloroplast. Indeed, expression of the algal HSP33 increases in response to light‐induced oxidative stress, which is experienced routinely by photosynthetic organisms. Despite the fact that no ortholog could be found in higher eukaryotes, its abundance in all algal species examined could have a biotechnological relevance.