Abstract
Fluorescent probes such as thiol-reactive and Ni2+-nitrilotriacetate (NTA) based probes provide a powerful toolbox for real-time visualization of a protein and a proteome in living cells. Herein, we first went through basic principles and applications of thiol-reactive based probes in protein imaging and recognition. We then summarize a family of metal-NTA based fluorescence probes in the visualization of His6-tagged protein and identification of metalloproteins at proteome-wide scale. The pros and cons of the probes, as well as ways to optimize them, are discussed.
Introduction
In the past decades, small molecule-based probes have been extensively used in protein labeling, allowing in situ studies of function, subcellular localization, and dynamics of the proteins of interest (POIs). Because of their relatively small sizes, these probes have less perturbation on the investigated proteins (Giepmans et al., ; Sletten and Bertozzi, ). Particularly, the small molecule-based tagging system has received growing attention, the POI is genetically fused with a short peptide of specific motifs, which is subsequently recognized by the probes. The recognition of POI by the probe either through chemical reaction or metal chelation usually results in fluorescent responses, which is applicable for biological imaging and quantification. Metal-chelation labeling of a protein is particularly attractive owing to its simplicity and high specificity. Pioneered by Griffin et al. (), a family of biarsenical probes have been developed, which specifically recognize a tetracysteine tag. Given that the (histidine)6-tag is more widely used in protein purification, enormous efforts have been made to develop fluorescence probes to selectively recognize the His6-tag (Kapanidis et al., ; Hauser and Tsien, ). These probes have been further utilized to label endogenous metal-binding proteins, providing a useful tool for metalloproteomics (Zhang et al., 2015; Jiang et al., ).
In this mini-review, we focus on metal chelation-based protein labeling, typically on the development of fluorescence probes that specifically recognize tetra-cysteine- and His6-tags in live cells. We highlight the optimization of the probes and their utilities in various systems. We also discuss the optimizations of fluorophores and molecule delivery for probes in tracking intracellular proteins.
The Thiol-Reactive Fluorescent Probes
Protein Recognition Based on Thiol Reaction
Cysteine residues are quite abundant and vital in protein structures to link two cysteine-containing peptides. Among 3,758 identified human proteins, more than 15,000 cysteine residues were reported (Gasser, ). The high abundance of cysteine residues has inspired scientists to develop strategies to label proteins via these sites. Although both As(III) and Sb(III) have high affinity to cysteine residues, only As(III) has been selected in the development of sensors. As-S coordination bonds are generated through thiol exchange, enabling As(III) to recognize cysteine residues. Compared with the liner structures created by As(III) and monothiols, the ring structures consisting of As(III) and two closely spaced thiols appear to be much more stable due to the factor of entropy (Stocken and Thompson, ). This explains why the formation of dithiol-reactive As(III) complexes is the main trend and the distance between the two cysteine residues was estimated to be 3–4 Å (Adams et al., ; Bhattacharjee and Rosen, ). When As(III) meets di-vicinal cysteine residues, chelation of As(III) to cysteines is achieved (Figure 1A).
Figure 1
Designs of Arsenic-Based Fluorescent Probes
A family of arsenic(III)-based probes was designed and further modified for in situ visualizations of proteins with genetically fused tetra-cysteine residues. Roger Tsien et al. firstly reported a membrane-permeable and non-fluorescent As(III)-complex, i.e., FLAsH-EDT2 (EDT = ethanedithiol), to site-specifically recognize proteins containing four cysteines at the i, i + 1, i + 4, and i + 5 positions of an α-helix in 1998 (Griffin et al.,
Both FLAsH-EDT2 and ReAsH-EDT2 are turn-on fluorescent probes as the 1,3,2-dithiarsolane (EDT) results in weak fluorescence before thiol-reaction with cysteines. Such a benefit was inherited in the design of other As-based probes, NPE, and CTNPE. Due to the structural features of vicinal-dithiol-containing proteins (VDPs), only one As(III) was kept for protein recognition, whilst two biocompatible diglycol amine groups were integrated into these probes to improve their water solubility and biological compatibility (Huang et al.,
Targeting Cysteine in Diverse Systems
The first generation of As(III)-based fluorescent probes, FLAsH-EDT2 and ReAsH-EDT2, mainly target proteins with genetically fused tetracysteine tag since this motif seems to be rare among native proteins (Femia et al.,
Besides those tetra-cysteine fused proteins, the thiol-groups of native proteins play a principal role in the balance of intracellular redox environment (Ying et al., 2007), whilst those proteins usually have the dithiol-motif such as -CXnC- rather than tetracysteine. Acting as the reductive end of this oxidation-reduction network, VDPs can be labeled by NPE both in vitro and in vivo. Staining of NPE finally lit up the VDPs in live cells without invasiveness (Huang et al.,
Design of Metal-NTA Based Fluorescent Probes
His6-Ni2+-NTA System for Imaging Proteins
The first report on using the His6-Ni2+-NTA system for protein purification was dated back to 1975 (Porath et al.,
The major drawback of the Ni-NTA system appeared to be the relatively low affinity of NTA toward Ni2+ with a dissociation constant of 1–20 μM (Soh,
Figure 2

The probe family based upon NTA-metal coordination with their applications in protein tracking. Typical Ni-NTA-based probes were presented in (A) whilst our three generations of homemade probes were listed as (B), including Ni-NTA-AC, Ni-NTA-AF, and Ni-NTA-AB. Then the explorations for metal-associated proteomes in diverse system by using M-TRACER were presented as (C) the covalent labeling ability of M-TRACER toward intracellular proteins with UV-activation, (D) Fe-associated proteome tracked by Fe-TRACER in P. gingivalis and the protein-binding model released by X-ray crystallography, (E) Bi-associated proteome labeled by Bi-TRACER in H. pylori. (C,D) were reproduced from Jiang et al. (
Recently, a new family of Ni-NTA based fluorescent probes, which can rapidly enter cells to label His6-tagged protein covalently, was developed. The first probe, Ni-NTA-AC, consists of a mono-NTA group, a coumarin fluorophore, four-carbon connecting chain, and an arylazide incorporated into the fluorophore (Lai et al.,
Exploring Metalloproteomes
The importance of metals in biology is notable, mostly because metals can play critical roles in life processes, especially acting as the catalytic or structural cofactors (Hoppert,
The specific recognition of metal-associated proteins by the probe was demonstrated by using Fe-TRACER and human serum transferrin as a showcase. The x-ray structure of the protein adduct shows that Fe-TRACER binds to the specific ferric iron binding site in the C-lobe of transferrin, with Fe3+ coordinating to tyrosine 188 (Y188), while asparagic acid 63 (D63), tyrosine 95 (Y95), and lysine 206 (K206) participate in the formation of H-bonding to stabilize the structure. Interestingly, the probe folds a sandwich-like structure and Fe3+ coordinates to only two carboxylates and one nitrogen of the probe and one oxygen from Y188, leaving enough vacant site for interacting with proteins (Figure 2D).
Using Ni-TRACER, Ni-associated proteome in Helicobacter pylori (H. pylori) was firstly mined and 44 Ni2+-associated proteins were identified (Lai et al.,
Molecular Modifications and Delivery Majorization
Development of Fluorophores
The development of delocalized electronic structure, i.e., the π system, just leads to the evaluation of fluorophores from blue to red fluorescence for the energy matching with π/π* energy gap finally resulted in fluorescent emission. Among numerous blue fluorophores, the coumarins appeared to be the oldest ones (Kumar et al.,
Delivery of Probes Into Cells
To track intracellular POI, the delivery of fluorescent probes appeared to be a critical aspect. To break the limitations of lipophilic cell membrane toward large, charged molecules, about five kinds of approaches have been reported for living cell imaging (Lymperopoulos et al.,
Concluding Remarks
Two families of representative probes based on metal-oriented protein recognition, i.e., As(III)-based probes and metal-NTA-based probes, were introduced and summarized from their binding mechanisms, molecule structures, to biological applications. These probes initially targeted fused proteins with either tetracysteine or His6-tags. For imaging of extracellular and intracellular proteins, almost all As(III)-based probes entered cells successfully whilst an array of Ni-NTA-based probes lack cellular membrane permeability. Despite diverse structure optimizations on these Ni-NTA-based probes, no clear clue of cellular membrane permeability was presented. Consequently, the different permeability between these two classes of probes resulted in their different applications in biology besides labeling relevant peptide-fused proteins. As(III)-based probes light up the native As(III)-binding proteins (Huang et al.,
Our probes were built upon metal-NTA coordination chemistry, and Ni2+ was selected as a showcase to inherit the success of Ni-NTA-based probes. In total, three probes were synthesized, i.e., Ni-NTA-AC, Ni-NTA-AF, and Ni-NTA-AB, with fluorescent emission ranging from blue to red. Importantly, they can track intracellular His-tagged proteins in live cells (Lai et al.,
Statements
Author contributions
NJ drafted this manuscript. HL revised this whilst. HS supervised this.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
His-tag, metallomics, metalloproteomics, molecular imaging, thiol-reaction
Citation
Jiang N, Li H and Sun H (2019) Recognition of Proteins by Metal Chelation-Based Fluorescent Probes in Cells. Front. Chem. 7:560. doi: 10.3389/fchem.2019.00560
Received
09 May 2019
Accepted
23 July 2019
Published
09 August 2019
Volume
7 - 2019
Edited by
Huib Ovaa, Leiden University Medical Center, Netherlands
Reviewed by
Giovanni Signore, Scuola Normale Superiore di Pisa, Italy; Alexander Shekhtman, University at Albany, United States
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© 2019 Jiang, Li and Sun.
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*Correspondence: Hongzhe Sun hsun@hku.hk
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