The chloroplast‐localized small heat shock protein Hsp21 associates with the thylakoid membranes in heat‐stressed plants

Abstract

The small heat shock protein (sHsp) chaperones are crucial for cell survival and can prevent aggregation of client proteins that partially unfold under destabilizing conditions. Most investigations on the chaperone activity of sHsps are based on a limited set of thermosensitive model substrate client proteins since the endogenous targets are often not known. There is a high diversity among sHsps with a single conserved β‐sandwich fold domain defining the family, the α‐crystallin domain, whereas the N‐terminal and C‐terminal regions are highly variable in length and sequence among various sHsps and conserved only within orthologues. The endogenous targets are probably also varying among various sHsps, cellular compartments, cell type and organism. Here we have investigated Hsp21, a non‐metazoan sHsp expressed in the chloroplasts in green plants which experience huge environmental fluctuations not least in temperature. We describe how Hsp21 can also interact with the chloroplast thylakoid membranes, both when isolated thylakoid membranes are incubated with Hsp21 protein and when plants are heat‐stressed. The amount of Hsp21 associated with the thylakoid membranes was precisely determined by quantitative mass spectrometry after metabolic 15N‐isotope labeling of either recombinantly expressed and purified Hsp21 protein or intact Arabidopsis thaliana plants. We found that Hsp21 is among few proteins that become associated with the thylakoid membranes in heat‐stressed plants, and that approximately two thirds of the pool of chloroplast Hsp21 is affected. We conclude that for a complete picture of the role of sHsps in plant stress resistance also their association with the membranes should be considered.

VIPP2 interacts with VIPP1 and HSP22E/F at chloroplast membranes and modulates a retrograde signal for HSP22E/F gene expression

“…Oxidized methionines in VIPP1/2 and HSP22E/F might get re‐reduced by the action of a peptide methionine sulfoxide reductase (PMSR). This scenario has been proposed previously for Arabidopsis Hsp21 under heat stress conditions, during which two‐thirds of the otherwise stromal proteins re‐localize to thylakoid membranes (Bernfur, Rutsdottir, & Emanuelsson, ). Hsp21 has an N‐terminal amphipathic α‐helix harbouring six methionines that can get oxidized by H 2 O 2 and re‐reduced by a plastid PMSR (Gustavsson et al, ; Harndahl et al, ), suggesting an Hsp21 methionine sulfoxidation‐reduction cycle to quench reactive oxygen species (Sundby, Harndahl, Gustavsson, Ahrman, & Murphy, ).…”

Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants

“…Importantly, studies using the transgenic tomato plants overexpressing HSP21 have shown that this protein associates with proteins of Photosystem II and does not reactivate heat-denatured Photosystem II, but instead protects this complex from damage during heat stress [ 150 ]. Interestingly, around two thirds of chloroplast HSP21 proteins are translocated into the thylakoid membranes in response to heat treatment in plants, suggesting that the association with membranes should be considered to fully understand the role of sHsps in physiological adaptation processes under stress conditions [ 151 ]. Despite extensive studies on HSP21, the specific roles of HSP21 in protecting PSII from heat stress remain elusive.…”

Pesticide application has little influence on coding and non-coding gene expressions in rice

“…Hence, upregulation of AT5MAT may be an adaptive strategy of plants to cope with the toxic effects of insecticide [26]. Heat Shock Proteins (HSPs) are involved in the regulation of specific substrate proteins in stressful conditions especially under high temperature [27]. Under TXM application, some HSPs are also inhibited ( Fig.…”