Differential reactivity of closely related zinc(II)-binding metallothioneins from the plant Arabidopsis thaliana

Changes in physio-biochemical parameters and expression of metallothioneins in Avena sativa L. in response to drought

Drought is one of the major threats to food security. Among several mechanisms involved in plant stress tolerance, one protein family-the plant metallothioneins (MTs)-shows great promise for enhancing drought resistance. Plant metallothioneins in oat (Avena sativa L.) have not yet been deeply analysed, and the literature lacks a comprehensive study of the whole family of plant MTs in response to drought. In this study, we showed that the number and nature of cis-elements linked with stress response in promoters of AsMTs1-3 differed depending on the MT type. Drought stress in oat plants caused an increase in the expression of AsMT2 and AsMT3 and a decrease in the expression of AsMT1 compared to well-watered plants. Moreover, the low values of relative water content, water use efficiency, net photosynthesis (P_N), transpiration (E), stomatal conductance (g_s), chlorophyll a, and carotenoid were accompanied by high levels of electrolyte leakage, internal CO₂ concentration (C_i) and abscisic acid content, and high activity of antioxidants enzymes in plants under drought stress. The present study puts forward the idea that AsMTs are crucial for oat response to drought stress not only by regulating antioxidant activity but also by changing the plant water regime and photosynthesis. Our results support the hypothesis that structural differences among types of plant MTs reflect their diversified physiological roles.

An ion mobility-mass spectrometry study of copper-metallothionein-2A: binding sites and stabilities of Cu-MT and mixed metal Cu-Ag and Cu-Cd complexes

The presence of Cu, a highly redox active metal, is known to damage DNA as well as other cellular components, but the adverse effects of cellular Cu can be mitigated by metallothioneins (MT), small cysteine rich proteins that are known to bind to a broad range of metal ions. While metal ion binding has been shown to involve the cysteine thiol groups, the specific ion binding sites are controversial as are the overall structure and stability of the Cu-MT complexes. Here, we report results obtained using nano-electrospray ionization mass spectrometry and ion mobility-mass spectrometry for several Cu-MT complexes and compare our results with those previously reported for Ag-MT complexes. The data include determination of the stoichiometries of the complex (Cui-MT, i = 1-19), and Cu+ ion binding sites for complexes where i = 4, 6, and 10 using bottom-up and top-down proteomics. The results show that Cu+ ions first bind to the β-domain to form Cu4MT then Cu6MT, followed by addition of four Cu+ ions to the α-domain to form a Cu10-MT complex. Stabilities of the Cui-MT (i = 4, 6 and 10) obtained using collision-induced unfolding (CIU) are reported and compared with previously reported CIU data for Ag-MT complexes. We also compare CIU data for mixed metal complexes (CuiAgj-MT, where i + j = 4 and 6 and CuiCdj, where i + j = 4 and 7). Lastly, higher order Cui-MT complexes, where i = 11-19, were also detected at higher concentrations of Cu+ ions, and the metalated product distributions observed are compared to previously reported results for Cu-MT-1A (Scheller et al., Metallomics, 2017, 9, 447-462).

Zinc deficiency responses: bridging the gap between Arabidopsis and dicotyledonous crops

Zinc (Zn) deficiency is a widespread phenomenon in agricultural soils worldwide and has a major impact on crop yield and quality, and hence on human nutrition and health. Although dicotyledonous crops represent >30% of human plant-based nutrition, relatively few efforts have been dedicated to the investigation of Zn deficiency response mechanisms in dicotyledonous, in contrast to monocotyledonous crops, such as rice or barley. Here, we describe the Zn requirement and impact of Zn deficiency in several economically important dicotyledonous crops, Phaseolus vulgaris, Glycine max, Brassica oleracea, and Solanum lycopersicum. We briefly review our current knowledge of the Zn deficiency response in Arabidopsis and outline how this knowledge is translated in dicotyledonous crops. We highlight commonalities and differences between dicotyledonous species (and with monocotyledonous species) regarding the function and regulation of Zn transporters and chelators, as well as the Zn-sensing mechanisms and the role of hormones in the Zn deficiency response. Moreover, we show how the Zn homeostatic network intimately interacts with other nutrients, such as iron or phosphate. Finally, we outline how variation in Zn deficiency tolerance and Zn use efficiency among cultivars of dicotyledonous species can be leveraged for the design of Zn biofortification strategies.

Ag+ Ion Binding to Human Metallothionein-2A Is Cooperative and Domain Specific

Metallothioneins (MTs) constitute a family of cysteine-rich proteins that play key biological roles for a wide range of metal ions, but unlike many other metalloproteins, the structures of apo- and partially metalated MTs are not well understood. Here, we combine nano-electrospray ionization-mass spectrometry (ESI-MS) and nano-ESI-ion mobility (IM)-MS with collision-induced unfolding (CIU), chemical labeling using N-ethylmaleimide (NEM), and both bottom-up and top-down proteomics in an effort to better understand the metal binding sites of the partially metalated forms of human MT-2A, viz., Ag4-MT. The results for Ag4-MT are then compared to similar results obtained for Cd4-MT. The results show that Ag4-MT is a cooperative product, and data from top-down and bottom-up proteomics mass spectrometry analysis combined with NEM labeling revealed that all four Ag+ ions of Ag4-MT are bound to the β-domain. The binding sites are identified as Cys13, Cys15, Cys19, Cys21, Cys24, and Cys26. While both Ag+ and Cd2+ react with MT to yield cooperative products, i.e., Ag4-MT and Cd4-MT, these products are very different; Ag+ ions of Ag4-MT are located in the β-domain, whereas Cd2+ ions of Cd4-MT are located in the α-domain. Ag6-MT has been reported to be fully metalated in the β-domain, but our data suggest the two additional Ag+ ions are more weakly bound than are the other four. Higher order Agi-MT complexes (i = 7-17) are formed in solutions that contain excess Ag+ ions, and these are assumed to be bound to the α-domain or shared between the two domains. Interestingly, the excess Ag+ ions are displaced upon addition of NEM to this solution to yield predominantly Ag4NEM14-MT. Results from CIU suggest that Agi-MT complexes are structurally more ordered and that the energy required to unfold these complexes increases as the number of coordinated Ag+ increases.

The type 4 metallothionein from Brassica napus seeds folds in a metal-dependent fashion and favours zinc over other metals

Rapeseed MT4 only folds properly in the presence of Zn²⁺ and thus may serve as a selectivity filter for metal accumulation in plant embryos.