Glutathione degradation activity of γ‐glutamyl peptidase 1 manifests its dual roles in primary and secondary sulfur metabolism in Arabidopsis

Abstract

Glutathione (GSH) functions as a major sulfur repository and hence occupies an important position in primary sulfur metabolism. GSH degradation results in sulfur reallocation and is believed to be carried out mainly by c-glutamyl cyclotransferases (GGCT2;1, GGCT2;2, and GGCT2;3), which, however, do not fully explain the rapid GSH turnover. Here, we discovered that c-glutamyl peptidase 1 (GGP1) contributes to GSH degradation through a yeast complementation assay. Recombinant proteins of GGP1, as well as GGP3, showed high degradation activity of GSH, but not of oxidized glutathione (GSSG), in vitro. Notably, the GGP1 transcripts were highly abundant in rosette leaves, in agreement with the ggp1 mutants constantly accumulating more GSH regardless of nutritional conditions. Given the lower energy requirements of the GGP-than the GGCT-mediated pathway, the GGP-mediated pathway could be a more efficient route for GSH degradation than the GGCT-mediated pathway. Therefore, we propose a model wherein cytosolic GSH is degraded chiefly by GGP1 and likely also by GGP3. Another noteworthy fact is that GGPs are known to process GSH conjugates in glucosinolate and camalexin synthesis; indeed, we confirmed that the ggp1 mutant contained higher levels of O-acetyl-L-Ser, a signaling molecule for sulfur starvation, and lower levels of glucosinolates and their degradation products. The predicted structure of GGP1 further provided a rationale for this hypothesis. In conclusion, we suggest that GGP1 and possibly GGP3 play vital roles in both primary and secondary sulfur metabolism.

Near-Infrared Fluorescence Probe with a New Recognition Moiety for the Specific Detection of Cysteine to Study the Corresponding Physiological Processes in Cells, Zebrafish, and Arabidopsis thaliana

“…The sulfur metabolic process has been widely considered as an important part of plant growth and development. − Cys, regarded as an intermediate product of sulfur metabolism, plays essential roles. However, no NIR fluorescent probes have been reported for monitoring Cys in plants.…”

Natural Variation in OASC Gene for Mitochondrial O-Acetylserine Thiollyase Affects Sulfate Levels in Arabidopsis

“…The transcript levels of the three isoforms of APR, the key enzyme in the reduction of sulfate, were not significantly affected by the disruption of the OASC gene, neither in the shoot nor in the root ( Figure 3 ). The expression of two markers of sulfur deficiency, SDI1 , which is involved in GLS regulation, and GGTC2;1 , which contributes to GSH degradation, were also not different in oasC compared to Col-0 [ 4 , 18 ]. In addition, the expression levels of the major isoforms of OAS-TL were compared.…”

Characterization of γ-glutamyltransferase- and phytochelatin synthase-mediated catabolism of glutathione and glutathione <i>S</i>-conjugates in <i>Arabidopsis thaliana</i>

“…Taking together, our results indicate that AtGGTs are involved in the apoplastic (AtGGT1) and vacuolar (AtGGT4) catabolism of GSH Sconjugates of xenobiotics (GS-bimane), whereas AtPCSs are involved in their cytoplasmic degradation (Figure 7). While GGTs control GSH and GSH S-conjugate degradation in the apoplastic and vacuolar compartments, alternative cytosolic pathways of GSH catabolism by means of γ-glutamylcyclotransferase (GGC T, EC2.3.2.4) and γ-glutamylpeptidase (GGP,EC3.4.19.16) have been proposed to dominate these processes in the cytosol (Ito et al 2022;Kumar et al 2015;Ohkama-Ohtsu et al 2008;Paulose et al 2013). Arabidopsis contains three potential GGC T genes (GGCT1=At1g44790, GGCT2;1=At5g26220, GGCT2;2=At4g31290) encoding proteins that cleave GSH when overexpressed in yeast (Kumar et al 2015;Paulose et al 2013).…”

“…GGP1 and GGP3 were originally shown to be involved in the processing of GSH S-conjugates in the glucosinolate and camalexin pathways (Geu-Flores et al 2011. However, Ito et al (2022) have recently reported that recombinant GGP1 and GGP3 are capable of hydrolyzing the γ-glutamyl residue of GSH in vitro. Moreover, yeast complementation assays, and the characterization of ggp1 mutants suggest that GGP1 play a major role in cytoplasmic degradation of GSH in Arabidopsis (Ito et al 2022).…”

“…However, Ito et al (2022) have recently reported that recombinant GGP1 and GGP3 are capable of hydrolyzing the γ-glutamyl residue of GSH in vitro. Moreover, yeast complementation assays, and the characterization of ggp1 mutants suggest that GGP1 play a major role in cytoplasmic degradation of GSH in Arabidopsis (Ito et al 2022). The complex biological significance of the different Arabidopsis GGT, GGC T, and GGP isoforms, and the mechanism of interaction between GGT, GGC T, and GGP mediated GSH catabolism should be determined in further studies.…”