Genetically encoded FRET-based optical sensor for Hg2+ detection and intracellular imaging in living cells

Due to the potential toxicity of mercury, there is an immediate need to understand its uptake, transport and flux within living cells. Conventional techniques used to analyze Hg²⁺ are invasive, involve high cost and are less sensitive. In the present study, a highly efficient genetically encoded mercury FRET sensor (MerFS) was developed to measure the cellular dynamics of Hg²⁺ at trace level in real time. To construct MerFS, the periplasmic mercury-binding protein MerP was sandwiched between enhanced cyan fluorescent protein (ECFP) and venus. MerFS is pH stable, offers a measurable fluorescent signal and binds to Hg²⁺ with high sensitivity and selectivity. Mutant MerFS-51 binds with an apparent affinity (K_d) of 5.09 × 10^-7 M, thus providing a detection range for Hg²⁺ quantification between 0.210 µM and 1.196 µM. Furthermore, MerFS-51 was targeted to Escherichia coli (E. coli), yeast and human embryonic kidney (HEK)-293T cells that allowed dynamic measurement of intracellular Hg²⁺ concentration with a highly responsive saturation curve, proving its potential application in cellular systems.

Identification of Gold Sensing Peptide by Integrative Proteomics and a Bacterial Two-Component System

The proteomics strategy was utilized to analyze and identify the gold adsorption proteins from Tepidimonas fonticaldi AT-A2, due to its outstanding performance in gold-binding and recovery. The results showed that three small proteins, including histidine biosynthesis protein (HisIE), iron donor protein (CyaY) and hypothetical protein_65aa, have a higher ability to adsorb gold ions because of the negatively charged domains or metal binding sites. On the other hand, the Salmonella PmrA/PmrB two-component system first replaces the iron (III)-binding motif using the peptide sequence from hypothetical protein_65aa, and this is then used to reveal the sensing and responsiveness to gold metal ions, which is totally different from the performance of traditional gold binding peptide (GBP) on the crystals on the surface of gold (111). We have successfully demonstrated an integrative proteomics and bacterial two-component system to explore the novel GBP. Finally, the heterologous over-expression of GBP by E. coli and the equilibrium of binding capacity for Au(III) have been conducted.

Unbound position II in MXCXXC metallochaperone model peptides impacts metal binding mode and reactivity: Distinct similarities to whole proteins

The effect of position II in the binding sequence of copper metallochaperones, which varies between Thr and His, was investigated through structural analysis and affinity and oxidation kinetic studies of model peptides. A first Cys-Cu(I)-Cys model obtained for the His peptide at acidic and neutral pH, correlated with higher affinity and more rapid oxidation of its complex; in contrast, the Thr peptide with the Cys-Cu(I)-Met coordination under neutral conditions demonstrated weaker and pH dependent binding. Studies with human antioxidant protein 1 (Atox1) and three of its mutants where S residues were replaced with Ala suggested that (a) the binding affinity is influenced more by the binding sequence than by the protein fold (b) pH may play a role in binding reactivity, and (c) mutating the Met impacted the affinity and oxidation rate more drastically than did mutating one of the Cys, supporting its important role in protein function. Position II thus plays a dominant role in metal binding and transport.

Selenocysteine containing analogues of Atx1-based peptides protect cells from copper ion toxicity

Seleno-substituted model peptides of copper metallochaperone proteins display particularly high Cu(i) affinity andin vitroanti-oxidative reactivity.

Peptide self-assembly triggered by metal ions

This review summarizes the recent development of structures, functions, as well as strategies of a peptide self-assembly induced by metal ions.

Structure and coordination determination of peptide-metal complexes using 1D and 2D ¹H NMR

Copper (I) binding by metallochaperone transport proteins prevents copper oxidation and release of the toxic ions that may participate in harmful redox reactions. The Cu (I) complex of the peptide model of a Cu (I) binding metallochaperone protein, which includes the sequence MTCSGCSRPG (underlined is conserved), was determined in solution under inert conditions by NMR spectroscopy. NMR is a widely accepted technique for the determination of solution structures of proteins and peptides. Due to difficulty in crystallization to provide single crystals suitable for X-ray crystallography, the NMR technique is extremely valuable, especially as it provides information on the solution state rather than the solid state. Herein we describe all steps that are required for full three-dimensional structure determinations by NMR. The protocol includes sample preparation in an NMR tube, 1D and 2D data collection and processing, peak assignment and integration, molecular mechanics calculations, and structure analysis. Importantly, the analysis was first conducted without any preset metal-ligand bonds, to assure a reliable structure determination in an unbiased manner.

Peptide models of Cu(I) and Zn(II) metallochaperones: the effect of pH on coordination and mechanistic implications

The first NMR structures of Cu(I) and Zn(II) peptide complexes as models of metallochaperones were derived with no predetermined binding mode. The cyclic peptide MDCSGCSRPG was reacted with Cu(I) and Zn(II) at low and moderate pH. This peptide features the conserved sequence of copper chaperones but with Asp at position 2 as appears in the zinc binding domain of ZntA. The structures were compared with those of the Cu(I) complexes of the wild-type sequence peptide MTCSGCSRPG. All analyses were conducted first with no metal-binding constraints to ensure accurate binding ligand assignment. Several structures included metal-Met binding, raising a possible role of Met in the metal transport mechanism. Both Cu(I) and Zn(II) gave different complexes when reacted with the peptide of the native-like sequence under different pH conditions, raising the possibility of pH-dependent transport mechanisms. Cu(I) bound the MTCSGCSRPG peptide through one Cys and the Met under acidic conditions and differently under basic conditions; Zn(II) bound the MDCSGCSRPG peptide through two Cys and the Met residues under acidic conditions and through one Cys and the Met under basic conditions, while Cu(I) bound the non-native Asp mutant peptide through the Asp and one Cys under both conditions, suggesting that Asp may inhibit pH-dependent binding for Cu(I). NOESY and ESI-HRMS supported the presence of an aqua ligand for Zn(II), which likely deprotonated under basic conditions to give a hydroxo group. Coordination similarities were detected among the model system and native proteins, which overall suggest that coordination flexibility is required for the function of metallochaperones.

Overexpression of Arabidopsis ATX1 retards plant growth under severe copper deficiency

In a previous study, we demonstrated that Arabidopsis Antioxidant Protein1 (ATX1) plays an essential role in copper (Cu) homeostasis, conferring tolerance to both excess and subclinically deficient Cu. The Cu-binding motif MXCXXC was required for the physiological function of ATX1. In this study, we found that overexpression of ATX1 resulted in hypersensitivity to severe Cu deficiency despite enhancing tolerance to subclinical Cu deficiency. However, overexpression of mutated ATX1, replacing the Cu-binding motif MXCXXC with MXGXXG, abolished the hypersensitivity, for no differences from the wild type under the same conditions. Thus, the expression of ATX1 must be cautiously regulated to avoid homeostatic imbalance with the over-chelation of Cu.

Copper chaperone antioxidant protein1 is essential for copper homeostasis

Copper (Cu) is essential for plant growth but toxic in excess. Specific molecular mechanisms maintain Cu homeostasis to facilitate its use and avoid the toxicity. Cu chaperones, proteins containing a Cu-binding domain(s), are thought to assist Cu intracellular homeostasis by their Cu-chelating ability. In Arabidopsis (Arabidopsis thaliana), two Cu chaperones, Antioxidant Protein1 (ATX1) and ATX1-Like Copper Chaperone (CCH), share high sequence homology. Previously, their Cu-binding capabilities were demonstrated and interacting molecules were identified. To understand the physiological functions of these two chaperones, we characterized the phenotype of atx1 and cch mutants and the cchatx1 double mutant in Arabidopsis. The shoot and root growth of atx1 and cchatx1 but not cch was specifically hypersensitive to excess Cu but not excess iron, zinc, or cadmium. The activities of antioxidant enzymes in atx1 and cchatx1 were markedly regulated in response to excess Cu, which confirms the phenotype of Cu hypersensitivity. Interestingly, atx1 and cchatx1 were sensitive to Cu deficiency. Overexpression of ATX1 not only enhanced Cu tolerance and accumulation in excess Cu conditions but also tolerance to Cu deficiency. In addition, the Cu-binding motif MXCXXC of ATX1 was required for these physiological functions. ATX1 was previously proposed to be involved in Cu homeostasis by its Cu-binding activity and interaction with the Cu transporter Heavy metal-transporting P-type ATPase5. In this study, we demonstrate that ATX1 plays an essential role in Cu homeostasis in conferring tolerance to excess Cu and Cu deficiency. The possible mechanism is discussed.

The fictile coordination chemistry of cuprous-thiolate sites in copper chaperones

Copper plays vital roles in the active sites of cytochrome oxidase and in several other enzymes essential for human health. Copper is also highly toxic when dysregulated; because of this an elaborate array of accessory proteins have evolved which act as intracellular carriers or chaperones for the copper ions. In most cases chaperones transport cuprous copper. This review discusses some of the chemistry of these copper sites, with a view to some of the structural factors in copper coordination which are important in the biological function of these chaperones. The coordination chemistry and accessible geometries of the cuprous oxidation state are remarkably plastic and we discuss how this may relate to biological function. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.