How FlAsH got its sparkle: historical recollections of the biarsenical-tetracysteine tag

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.

Distance-Matched Tagging Sequence Optimizes Live-Cell Protein Labeling by a Biarsenical Fluorescent Reagent AsCy3_E

Cell permeable biarsenical fluorescent dyes built around a cyanine scaffold (AsCy3) create the ability to monitor the structural dynamics of tagged proteins in living cells. To extend the capability of this photostable and bright biarsenical probe to site-specifically label cellular proteins, we have compared the ability of AsCy3 to label two different tagging sequences (i.e., CCKAEAACC and CCKAEAAKAEAAKCC), which were separately engineered onto enhanced green fluorescent proteins (EGFPs) and expressed in Escherichia coli. The cysteine pairs within the shorter protein tag (i.e., Cy3TAG) are designed to specifically match the 14.5 Å interarsenic atomic separation within AsCy3, whereas the longer protein tag (Cy3TAG+6) was identified using a peptide screening approach and reported to enhance the binding affinity and brightness. We report that AsCy3 binds both the tagged proteins with similar high affinities (K_d < 1 μM) under both in vivo labeling conditions and following isolation and labeling of the tagged EGFP protein. Greater experimental reproducibility and substantially larger AsCy3 labeling stoichiometries are observed under in vivo conditions using the shorter Cy3TAG in comparison to the Cy3TAG+6. These results suggest that the use of the distance-matched and conformationally restricted Cy3TAG avoids nonspecific protein interactions, thereby enabling routine measurements of protein localization and conformational dynamics in living cells.

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.