Probing the target-specific inhibition of sensitized protein tyrosine phosphatases with biarsenical probes

The connection of α- and β-domains in mammalian metallothionein-2 differentiates Zn(II) binding affinities, affects folding, and determines zinc buffering properties

Mammalian metallothioneins (MTs) are small Cys-rich proteins involved in Zn(II) and Cu(I) homeostasis. They bind seven Zn(II) ions in two distinct β- and α-domains, forming Zn3Cys9 and Zn4Cys11 clusters, respectively. After six decades of research, their role in cellular buffering of Zn(II) ions has begun to be understood recently. This is because of different affinities of bound ions and the proteins' coexistence in variously Zn(II)-loaded Zn4-7MT species in the cell. To date, it has remained unclear how these mechanisms of action occur and how the affinities are differentiated despite the Zn(S-Cys)4 coordination environment being the same. Here, we dissect the molecular basis of these phenomena by using several MT2 mutants, hybrid protein, and isolated domains. Through a combination of spectroscopic and stability studies, thiol(ate) reactivity, and steered molecular dynamics, we demonstrate that both protein folding and thermodynamics of Zn(II) ion (un)binding significantly differ between isolated domains and the whole protein. Close proximity reduces the degrees of freedom of separated domains, making them less dynamic. It is caused by the formation of intra- and interdomain electrostatic interactions. The energetic consequence of domains connection has a critical impact on the role of MTs in the cellular environment, where they function not only as a zinc sponge but also as a zinc buffering system keeping free Zn(II) in the right concentrations. Any change of that subtle system affects the folding mechanism, zinc site stabilities, and cellular zinc buffer components.

In vitro and in vivo characterization of erythrosin B and derivatives against Zika virus

Zika virus (ZIKV) causes significant human diseases without specific therapy. Previously we found erythrosin B, an FDA-approved food additive, inhibited viral NS2B−NS3 interactions, leading to inhibition of ZIKV infection in cell culture. In this study, we performed pharmacokinetic and in vivo studies to demonstrate the efficacy of erythrosin B against ZIKV in 3D mini-brain organoid and mouse models. Our results showed that erythrosin B is very effective in abolishing ZIKV replication in the 3D organoid model. Although pharmacokinetics studies indicated that erythrosin B had a low absorption profile, mice challenged by a lethal dose of ZIKV showed a significantly improved survival rate upon oral administration of erythrosin B, compared to vehicle control. Limited structure−activity relationship studies indicated that most analogs of erythrosin B with modifications on the xanthene ring led to loss or reduction of inhibitory activities towards viral NS2B−NS3 interactions, protease activity and antiviral efficacy. In contrast, introducing chlorine substitutions on the isobenzofuran ring led to slightly increased activities, suggesting that the isobenzofuran ring is well tolerated for modifications. Cytotoxicity studies indicated that all derivatives are nontoxic to human cells. Overall, our studies demonstrated erythrosin B is an effective antiviral against ZIKV both in vitro and in vivo.

Biarsenical fluorescent probes for multifunctional site-specific modification of proteins applicable in life sciences: an overview and future outlook

Fluorescent modification of proteins of interest (POI) in living cells is desired to study their behaviour and functions in their natural environment. In a perfect setting it should be easy to perform, inexpensive, efficient and site-selective. Although multiple chemical and biological methods have been developed, only a few of them are applicable for cellular studies thanks to their appropriate physical, chemical and biological characteristics. One such successful system is a tetracysteine tag/motif and its selective biarsenical binders (e.g. FlAsH and ReAsH). Since its discovery in 1998 by Tsien and co-workers, this method has been enhanced and revolutionized in terms of its efficiency, formed complex stability and breadth of application. Here, we overview the whole field of knowledge, while placing most emphasis on recent reports. We showcase the improvements of classical biarsenical probes with various optical properties as well as multifunctional molecules that add new characteristics to proteins. We also present the evolution of affinity tags and motifs of biarsenical probes demonstrating much more possibilities in cellular applications. We summarize protocols and reported observations so both beginners and advanced users of biarsenical probes can troubleshoot their experiments. We address the concerns regarding the safety of biarsenical probe application. We showcase examples in virology, studies on receptors or amyloid aggregation, where application of biarsenical probes allowed observations that previously were not possible. We provide a summary of current applications ranging from bioanalytical sciences to allosteric control of selected proteins. Finally, we present an outlook to encourage more researchers to use these magnificent probes.

Optimized allosteric inhibition of engineered protein tyrosine phosphatases with an expanded palette of biarsenical small molecules

Protein tyrosine phosphatases (PTPs), which catalyze the dephosphorylation of phosphotyrosine in protein substrates, are important cell-signaling regulators, as well as potential drug targets for a range of human diseases. Chemical tools for selectively targeting the activities of individual PTPs would help to elucidate PTP signaling roles and potentially expedite the validation of PTPs as therapeutic targets. We have recently reported a novel strategy for the design of non-natural allosteric-inhibition sites in PTPs, in which a tricysteine moiety is engineered within the PTP catalytic domain at a conserved location outside of the active site. Introduction of the tricysteine motif, which does not exist in any wild-type PTP, serves to sensitize target PTPs to inhibition by a biarsenical compound, providing a generalizable strategy for the generation of allosterically sensitized (as) PTPs. Here we show that the potency, selectivity, and kinetics of asPTP inhibition can be significantly improved by exploring the inhibitory action of a range of biarsenical compounds that differ in interarsenical distance, steric bulk, and electronic structure. By investigating the inhibitor sensitivities of five asPTPs from four different subfamilies, we have found that asPTP catalytic domains can be broadly divided into two groups: one that is most potently inhibited by biarsenical compounds with large interarsenical distances, such as AsCy3-EDT₂, and one that is most potently inhibited by compounds with relatively small interarsenical distances, such as FlAsH-EDT₂. Moreover, we show that a tetrachlorinated derivative of FlAsH-EDT₂, Cl4FlAsH-EDT₂, targets asPTPs significantly more potently than the parent compound, both in vitro and in asPTP-expressing cells. Our results show that biarsenicals with altered interarsenical distances and electronic properties are important tools for optimizing the control of asPTP activity and, more broadly, suggest that diversification of biarsenical libraries can serve to increase the efficacy of these compounds in targeted control of protein function.

Synthesis, bioactivity, 3D-QSAR studies of novel dibenzofuran derivatives as PTP-MEG2 inhibitors

PTP-MEG2 plays a critical role in the diverse cell signalling processes, so targeting PTP-MEG2 is a promising strategy for various human diseases treatments. In this study, a series of novel dibenzofuran derivatives was synthesized and assayed for their PTP-MEG2 inhibitory activities. 10a with highest inhibitory activity (320 nM) exhibited significant selectivity for PTP-MEG2 over its close homolog SHP2, CDC25 (IC₅₀ > 50 μM). By means of the powerful “HipHop” technique, a 3D-QSAR study was carried out to explore structure activity relationship of these molecules. The generated pharmacophore model revealed that the one RA, three Hyd, and two HBA features play an important role in binding to the active site of the target protein-PTP-MEG2. Docking simulation study indicated that 10a achieved its potency and specificity for PTP-MEG2 by targeting unique nearby peripheral binding pockets and the active site. The absorption, distribution, metabolism and excretion (ADME) predictions showed that the 11 compounds hold high potential to be novel lead compounds for targeting PTP-MEG2. Our findings here can provide a new strategy or useful insights for designing the effective PTP-MEG2 inhibitors.

Fluorescent probes for selective protein labeling in lysosomes: a case of α-galactosidase A

Fluorescence-based live-cell imaging (LCI) of lysosomal glycosidases is often hampered by unfavorable pH and redox conditions that reduce fluorescence output. Moreover, most lysosomal glycosidases are low-mass soluble proteins that do not allow for bulky fluorescent protein fusions. We selected α-galactosidase A (GALA) as a model lysosomal glycosidase involved in Anderson-Fabry disease (AFD) for the current LCI approach. Examination of the subcellular localization of AFD-causing mutants can reveal the mechanism underlying cellular trafficking deficits. To minimize genetic GALA modification, we employed a biarsenical labeling protocol with tetracysteine (TC-tag) detection. We tested the efficiency of halogen-substituted biarsenical probes to interact with C-terminally TC-tagged GALA peptide at pH 4.5 in vitro and identified F2FlAsH-EDT₂ as a superior detection reagent for GALA. This probe provides improved signal/noise ratio in labeled COS-7 cells transiently expressing TC-tagged GALA. The investigated fluorescence-based LCI technology of TC-tagged lysosomal protein using an improved biarsenical probe can be used to identify novel compounds that promote proper trafficking of mutant GALA to lysosomal compartments and rescue the mutant phenotype.-Bohl, C., Pomorski, A., Seemann, S., Knospe, A.-M., Zheng, C., Krężel, A., Rolfs, A., Lukas, J. Fluorescent probes for selective protein labeling in lysosomes: a case of α-galactosidase A.

Rotamer-Restricted Fluorogenicity of the Bis-Arsenical ReAsH

Fluorogenic dyes such as FlAsH and ReAsH are used widely to localize, monitor, and characterize proteins and their assemblies in live cells. These bis-arsenical dyes can become fluorescent when bound to a protein containing four proximal Cys thiols-a tetracysteine (Cys4) motif. Yet the mechanism by which bis-arsenicals become fluorescent upon binding a Cys4 motif is unknown, and this nescience limits more widespread application of this tool. Here we probe the origins of ReAsH fluorogenicity using both computation and experiment. Our results support a model in which ReAsH fluorescence depends on the relative orientation of the aryl chromophore and the appended arsenic chelate: the fluorescence is rotamer-restricted. Our results do not support a model in which fluorogenicity arises from the relief of ring strain. The calculations identify those As-aryl rotamers that support fluorescence and those that do not and correlate well with prior experiments. The rotamer-restricted model we propose is supported further by biophysical studies: the excited-state fluorescence lifetime of a complex between ReAsH and a protein bearing a high-affinity Cys4 motif is longer than that of ReAsH-EDT2, and the fluorescence intensity of ReAsH-EDT2 increases in solvents of increasing viscosity. By providing a higher resolution view of the structural basis for fluorogenicity, these results provide a clear strategy for the design of more selective bis-arsenicals and better-optimized protein targets, with a concomitant improvement in the ability to characterize previously invisible protein conformational changes and assemblies in live cells.

Synthesis of Arylazide- and Diazirine-Containing CrAsH-EDT2 Photoaffinity Probes

Two photo-crosslinking biarsenical (CrAsH-EDT2 )-modified probes were synthesized that are expected to be useful tools for tetracysteine-labeled proteins to facilitate the co-affinity purification of their DNA binding sequences and interacting proteins. In addition, improvements for the synthesis of CrAsH-EDT2 and N(1) -(4-azido-2-nitrophenyl)hexane-1,6-diamine are reported. Both photoprobes effectively entered HeLa cells (and the nucleus) and were dependent on the tetracysteine motif in recombinant DMRT1 (doublesex and Mab3-related transcription factor) to induce fluorescence, suggesting that their crosslinking abilities can be exploited for the identification of nucleic acids and proteins associated with a protein of interest.