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.

热应激会导致蛋白质错误折叠和聚集，除非它们受到分子伴侣系统的保护。在这里，我们检查了模型植物拟南芥（ Arabidopsis thaliana ）中硫醇还原酶家族成员谷氧还蛋白 GRXS17 的功能。 GRXS17 是一种核胞质单硫醇谷氧还蛋白，由 N 端硫氧还蛋白结构域和三个包含 CGFS 活性位点基序的 GRX 结构域组成，以谷胱甘肽依赖性方式协调三个铁硫 (Fe-S) 簇。作为一种带 Fe-S 簇的全酶，GRXS17 可能参与细胞质和核 Fe-S 蛋白的成熟。除了在簇生物发生中的作用外，GRXS17 还具有折叠酶和氧化还原依赖性保持酶活性。氧化应激与热应激相结合诱导其 Fe-S 簇的损失，随后在相应的硫氧还蛋白结构域中的保守活性位点半胱氨酸之间形成二硫键。这种氧化导致 GRXS17 转变为高分子量复合物，从而在体外激活其保持酶活性。此外，GRXS17 特别参与植物对中度高温的耐受性，并保护根分生组织细胞免受热诱导的细胞死亡。最后，GRXS17 在热应激时与一组不同的蛋白质相互作用，可能保护它们免受热损伤。因此，我们认为 Fe-S 簇酶 GRXS17 是保护蛋白质免受中度热应激的重要卫士，可能是通过氧化还原依赖性伴侣活性。我们揭示了 Fe-S 簇依赖性活性转变的机制，该机制将全酶 GRXS17 转化为保持酶，从而防止热应激造成的损害。