Redox Modification of the Iron-Sulfur Glutaredoxin GRXS17 Activates Holdase Activity and Protects Plants from Heat Stress

Heat stress induces misfolding and aggregation of proteins unless they are guarded by chaperone systems. Here, we examined the function of the glutaredoxin GRXS17, a member of thiol reductase families in the model plant Arabidopsis (Arabidopsis thaliana). GRXS17 is a nucleocytosolic monothiol glutaredoxin consisting of an N-terminal thioredoxin domain and three CGFS active-site motif-containing GRX domains that coordinate three iron-sulfur (Fe-S) clusters in a glutathione-dependent manner. As an Fe-S cluster-charged holoenzyme, GRXS17 is likely involved in the maturation of cytosolic and nuclear Fe-S proteins. In addition to its role in cluster biogenesis, GRXS17 presented both foldase and redox-dependent holdase activities. Oxidative stress in combination with heat stress induced loss of its Fe-S clusters followed by subsequent formation of disulfide bonds between conserved active-site cysteines in the corresponding thioredoxin domains. This oxidation led to a shift of GRXS17 to a high-molecular-weight complex and thus activated its holdase activity in vitro. Moreover, GRXS17 was specifically involved in plant tolerance to moderate high temperature and protected root meristematic cells from heat-induced cell death. Finally, GRXS17 interacted with a different set of proteins upon heat stress, possibly protecting them from heat injuries. Therefore, we propose that the Fe-S cluster enzyme GRXS17 is an essential guard that protects proteins against moderate heat stress, likely through a redox-dependent chaperone activity. We reveal the mechanism of an Fe-S cluster-dependent activity shift that converts the holoenzyme GRXS17 into a holdase, thereby preventing damage caused by heat stress.

Inventory, evolution and expression profiling diversity of the LEA (late embryogenesis abundant) protein gene family in Arabidopsis thaliana

We analyzed the Arabidopsis thaliana genome sequence to detect Late Embryogenesis Abundant (LEA) protein genes, using as reference sequences proteins related to LEAs previously described in cotton or which present similar characteristics. We selected 50 genes representing nine groups. Most of the encoded predicted proteins are small and contain repeated domains that are often specific to a unique LEA group. Comparison of these domains indicates that proteins with classical group 5 motifs are related to group 3 proteins and also gives information on the possible history of these repetitions. Chromosomal gene locations reveal that several LEA genes result from whole genome duplications (WGD) and that 14 are organized in direct tandem repeats. Expression of 45 of these genes was tested in different plant organs, as well as in response to ABA and in mutants (such as abi3, abi5, lec2 and fus3) altered in their response to ABA or in seed maturation. The results demonstrate that several so-called LEA genes are expressed in vegetative tissues in the absence of any abiotic stress, that LEA genes from the same group do not present identical expression profile and, finally, that regulation of LEA genes with apparently similar expression patterns does not systematically involve the same regulatory pathway.

Characterization of a ribonuclease III-like protein required for cleavage of the pre-rRNA in the 3'ETS in Arabidopsis

Ribonuclease III (RNaseIII) is responsible for processing and maturation of RNA precursors into functional rRNA, mRNA and other small RNA. In contrast to bacterial and yeast cells, higher eukaryotes contain at least three classes of RNaseIII, including class IV or dicer-like proteins. Here, we describe the functional characterization of AtRTL2, an Arabidopsis thaliana RNaseIII-like protein that belongs to a small family of genes distinct from the dicer family. We demonstrate that AtRTL2 is required for 3′external transcribed spacer (ETS) cleavage of the pre-rRNA in vivo. AtRTL2 localizes in the nucleus and cytoplasm, a nuclear export signal (NES) in the N-terminal sequence probably controlling AtRTL2 cellular localization. The modeled 3D structure of the RNaseIII domain of AtRTL2 is similar to the bacterial RNaseIII domain, suggesting a comparable catalytic mechanism. However, unlike bacterial RNaseIII, the AtRTL2 protein forms a highly salt-resistant homodimer that is only disrupted on treatment with DTT. These data indicate that AtRTL2 may use a dimeric mechanism to cleave double-stranded RNA, but unlike bacterial or yeast RNase III proteins, AtRTL2 forms homodimers through formation of disulfide bonds, suggesting that redox conditions may operate to regulate the activity of RNaseIII.