L-Cysteine Desulfhydrase 1 modulates the generation of the signaling molecule sulfide in plant cytosol

Abstract

Consistent with data in animal systems, experimental evidence highlights sulfide as a signaling molecule of equal importance to NO and H₂O₂ in plant systems. In mammals, two cytosolic enzymes, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), have been shown to be responsible for the endogenous production of sulfide. L-cysteine desulfhydrase 1 (DES1) has been recently established as the only enzyme that is involved in the generation of hydrogen sulfide in plant cytosol. Although plants have an available source of sulfide within chloroplasts, the basic stromal pH prevents sulfide release into the cytosol. Therefore, DES1 is essential for the production of sulfide for signaling purposes.

For a long time, sulfide has been known primarily as a toxic molecule that is hazardous to the environment. Recently, however, sulfide has been recognized as a gasotransmitter (gaseous molecule that transmits information between cells) of equal importance to CO or NO in animal systems. Numerous reports describing the biological effects of sulfide highlight its physiological importance, and there is no doubt that most cells in mammals are able to produce and metabolize sulfide in a precise, regulated manner.¹ However, identifying the molecular targets of sulfide and deciphering its mechanism of action remain challenging. A small number of direct targets, including ion channels, glyceraldehyde phosphate dehydrogenase (GAPDH), actin, tubulin, complex IV of the mitochondrial electron transport chain and tyrosine phosphatase PTP1B, have been identified so far. In these cases, sulfhydration of reactive cysteines has been observed.² Therefore, the sulfhydration can be considered a physiologic posttranslational modification for proteins, and previous studies have indicated that this modification can be regulated by competition between the nitrosylation and sulfhydration of the same cysteine residues. This competition has been observed on GAPDH protein.³

In recent years, experimental evidence emerging from numerous plant biology studies has shown H₂S to be a signaling molecule of equal importance to NO and H₂O₂. Sulfide has been shown to be involved in protection against copper, aluminum and boron stress,^4-7 in the improvement of drought resistance and in the promotion of heat and salinity tolerance.^8-10 Furthermore, sulfide has been suggested to play a role in regulating photosynthesis and flower senescence and in prolonging the postharvest shelf life of fruits.^11-13 This molecule has also been identified as a component of the ABA signaling pathway in guard cells.^14,15

Our recent research has demonstrated that sulfide per se, but not sulfide as sulfur nutrient, exerts a general effect on autophagy through negative regulation.¹⁶ Thus, it seems that independently of the sulfur nutrition status of the plant, in this case Arabidopsis thaliana, sulfide that is specifically generated inside the cytosol and that results from the degradation of cysteine acts as a signaling molecule and regulates the process of autophagy.¹⁷ Another interesting observation that warrants further investigation is that sulfide seems to behave as a modulator of the transcriptional profile of the plant at its mature developmental stage. This hypothesis was proposed because when the capacity of the cytosol to release sulfide is reduced, the plant transcriptome is dramatically altered. This situation can be reversed by restoring the plant’s capacity to generate sulfide (GSE32566).

It has recently been established that the main sources of endogenous production of H₂S in mammals are the enzymatic reactions catalyzed by cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE).^1,18 Both enzymes are known for their participation in the transsulfuration pathway, which is critical for the synthesis of cysteine from methionine (the latter being the source of sulfur from diet in animals). The two enzymes have also been demonstrated to be the main proteins that are responsible for generating hydrogen sulfide from cysteine (Fig. 1). Both CBS and CSE use pyridoxal 5′-phosphate (PLP) as a cofactor and are exclusively located in the cytosol. We recently identified an analogous PLP-dependent enzyme with L-cysteine desulfhydrase activity, DES1, located in the cytosol in Arabidopsis thaliana plants.^16,19 DES1 is the only enzyme unequivocally established to be involved in the degradation of cysteine and the concomitant generation of hydrogen sulfide in the cytosol (Fig. 1). We have reached the conclusion that DES1 in the plant cell could be responsible for modulating the generation of sulfide for signaling in important processes, such as the progression of autophagy.¹⁷ Doubt regarding this role could arise from the fact that in plants, the chloroplast is the main source of sulfide, which results from sulfate reduction in the sulfur assimilation pathway.²⁰ This chloroplastic sulfide is thought to overflow into the cytosol. However, hydrogen sulfide is weakly acidic and dissociates in aqueous solution into H⁺ and SH^-. This ionized form cannot permeate membranes.²¹ Thus, at the pH of 8.5 maintained by the chloroplast stroma under illumination, sulfide is mainly present in its charged form and is therefore unable to transport across the chloroplast envelope.

Figure 1. Enzymatic generation of sulfide from cysteine. Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) catalyze this process in mammals; in plants, L-cysteine desulfhydrase 1 (DES1) performs this function.

In conclusion, we suggest that DES1 modulates the production of sulfide in the plant cytosol and that this sulfide is utilized by the plant for signaling purposes. This hypothesis yields many areas of potential investigation, such as the mechanism underlying this modulation, the specific function of DES1 in the signaling and control of different plant processes, the subcellular/tissue localization and regulation of the enzyme and the mode of action for and specific targets of the DES1-generated sulfide. Our present investigation is committed to answering some of these important questions.

Álvarez C, García I, Moreno I, Pérez-Pérez ME, Crespo JL, Romero LC, et al. Cysteine-generated sulfide in the cytosol negatively regulates autophagy and modulates the transcriptional profile in Arabidopsis. Plant Cell. 2012;24:4621–34. doi: 10.1105/tpc.112.105403.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.