The Chloroplast-Localized Plant sHsp in Arabidopsis Thaliana: Role of Its Oligomeric Conformation and Its Translocation into Membranes

Abstract

Recent advances in quantitative proteomics show that small heat shock proteins (sHsps) are among the most highly upregulated proteins in cellular stress response. In plants there are numerous paralogous sHsps expressed in various cellular compartments. The chloroplast-localized sHsp, named Hsp21 with reference to its monomeric size, has an N-terminal region that is some 40 amino acids longer compared to the cytosolic sHsps, and increases plant stress tolerance as shown by Arabidopsis thaliana plants which overexpress Hsp21. Recombinantly expressed and purified Hsp21-protein shows the features expected for a chaperone protein in rescuing temperature-sensitive model substrate proteins from aggregation. Hsp21 is a dodecameric protein, with C-terminal tails that keep the dodecamer together but are highly flexible in solution. The N-terminal domains, which resemble intrinsically disordered proteins and are located in the interior of the dodecamer, contain conserved methionine residues of crucial importance for function. Methionine sulfoxidation abolishes the chaperone activity in vitro, but in vivo such oxidized methioniones appear to be continuously re-reduced thanks to a chloroplast-localized form of peptide methionine sulfoxide reductase, an important enzyme expressed ubiquitously and belonging to the minimal gene set for life. There is not a clear picture of which the endogenous substrate proteins of Hsp21 are. For more than a decade it has been observed that cyanobacterial sHsps enter the membranes at increased temperatures. In a quantitative proteomics approach we have recently analyzed and compared Hsp21 with hundreds of other chloroplast proteins, and found that Hsp21 is fairly unique with respect to its translocation into membrane in heat-stressed plants. One reason for the oligomericity of Hsp21 is suggested to be the possibility to rapidly supply hydrophobic surfaces with chaperoning capacity in response to stress, while still preventing membrane lysis by such hydrophobic surfaces under non-stress conditions.