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

Abstract

SUMMARYHSP33 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.

Exploring intrinsically disordered proteins in Chlamydomonas reinhardtii

“…Moreover, as reported in the literature, IDPs found in plants are associated with many stress-response processes, acting as protein chaperones 56 or protecting other cellular components 2 . We have identified 20 IDPs related to unfolded protein binding or chaperone-function, with some illustrative examples, Hsp33 57 – 60 , Hsp70 and Hsp90 61 , belonging to the family of heat-shock proteins. Hsp70 and Hsp90 were also found in the Chlamydomonas phosphoproteome 14 indicating that they are phosphorylated.…”

Protein plasticity underlines activation and function of ATP-independent chaperones

“…The bacterial (Ilbert et al, 2007 ; Reichmann et al, 2012 ) and algae (Segal and Shapira, 2015 ) chaperone Hsp33 operates a similar mechanism to that of HdeA. Under oxidative stress, this redox-regulated chaperone undergoes significant unfolding and reveals a flexible redox-sensing domain, which triggers its chaperone activity (Ilbert et al, 2007 ; Reichmann et al, 2012 ).…”

TrypOx, a Novel Eukaryotic Homolog of the Redox-Regulated Chaperone Hsp33 in Trypanosoma brucei

“…A majority of the archaeal species do not contain DnaK/J homologs, which may be an acceptor of Hsp33 client proteins. This finding suggests potential co-evolution of DnaK/J-Hsp33 chaperones as seen in bacteria and algae (Segal and Shapira, 2015).…”

“…Hsp33 is a first line of defense chaperone, protecting organisms ranging from bacteria (Jakob et al, 1999;Wholey and Jakob, 2012) to green algae (Segal and Shapira, 2015) against the toxic effects of oxidative stress. By conducting a PSI-BLAST analysis, we identified 1579 homologous sequences with identity scores below 85.…”

“…Redox-regulated chaperone Hsp33 was experimentally shown to serve as a first line of defense chaperone during oxidative stress in pathogenic bacteria (E. coli and V. cholera), and algae C. reinhardtii (Segal and Shapira, 2015). Here, we explored sequence conservation of Hsp33 among the tree of life and revealed phylogenetically extended sequence conservation from the bacteria class (harboring more than 1500 homologs) to eukaryotes.…”