Direct visualization of protein association in living cells with complex-edited electron microscopy

Detection of oligonucleotides by PNA-peptide conjugates recognizing the biarsenical fluorescein complex FlAsH-EDT2

We report the application of the arsenical complex FlAsH-EDT₂ for the identification of oligonucleotide sequences. We designed PNA sequences conjugated to either a tetracysteine motif and to split tetracysteine sequences, that are recognized by FlAsH. The effect of conjugation of the PNA to the tetracysteine peptide and RNA hybridization on the fluorescence of the arsenical complex has been investigated. The reconstitution of the tetracysteine motif, starting from 15-mer PNAs conjugated to split tetracysteine sequences and hybridized to a complementary oligonucleotide was also explored.

Advances in chemical labeling of proteins in living cells

The pursuit of quantitative biological information via imaging requires robust labeling approaches that can be used in multiple applications and with a variety of detectable colors and properties. In addition to conventional fluorescent proteins, chemists and biologists have come together to provide a range of approaches that combine dye chemistry with the convenience of genetic targeting. This hybrid-tagging approach amalgamates the rational design of properties available through synthetic dye chemistry with the robust biological targeting available with genetic encoding. In this review, we discuss the current range of approaches that have been exploited for dye targeting or for targeting and activation and some of the recent applications that are uniquely permitted by these hybrid-tagging approaches.

Visualizing protein partnerships in living cells and organisms

In recent years, scientists have expanded their focus from cataloging genes to characterizing the multiple states of their translated products. One anticipated result is a dynamic map of the protein association networks and activities that occur within the cellular environment. While in vitro-derived network maps can illustrate which of a multitude of possible protein-protein associations could exist, they supply a falsely static picture lacking the subtleties of subcellular location (where) or cellular state (when). Generating protein association network maps that are informed by both subcellular location and cell state requires novel approaches that accurately characterize the state of protein associations in living cells and provide precise spatiotemporal resolution. In this review, we highlight recent advances in visualizing protein associations and networks under increasingly native conditions. These advances include second generation protein complementation assays (PCAs), chemical and photo-crosslinking techniques, and proximity-induced ligation approaches. The advances described focus on background reduction, signal optimization, rapid and reversible reporter assembly, decreased cytotoxicity, and minimal functional perturbation. Key breakthroughs have addressed many challenges and should expand the repertoire of tools useful for generating maps of protein interactions resolved in both time and space.

Surveying protein structure and function using bis-arsenical small molecules

Exploration across the fields of biology, chemical biology, and medicine has led to an increasingly complex, albeit incomplete, view of the interactions that drive life's processes. The ability to monitor and track the movement, activity, and interactions of biomolecules in living cells is an essential part of this investigation. In our laboratory, we have endeavored to develop tools that are capable not only of monitoring protein localization but also reporting on protein structure and function. Central to our efforts is a new strategy, bipartite tetracysteine display, that relies on the specific and high-affinity interaction between a fluorogenic, bis-arsenical small molecule and a unique protein sequence, conformation, or assembly. In 1998, a small-molecule analogue of fluorescein with two arsenic atoms, FlAsH, was shown by Tsien and coworkers to fluoresce upon binding to a linear amino acid sequence, Cys-Cys-Arg-Glu-Cys-Cys. Later work demonstrated that substituting Pro-Gly for Arg-Glu optimized both binding and fluorescence yield. Our strategy of bipartite tetracysteine display emanated from the idea that it would be possible to replace the intervening Pro-Gly dipeptide in this sequence with a protein or protein partnership, provided the assembled protein fold successfully reproduced the approximate placement of the two Cys-Cys pairs. In this Account, we describe our recent progress in this area, with an emphasis on the fundamental concepts that underlie the successful use of bis-arsenicals such as FlAsH and the related ReAsH for bipartite display experiments. In particular, we highlight studies that have explored how broadly bipartite tetracysteine display can be employed and that have navigated the conformational boundary conditions favoring success. To emphasize the utility of these principles, we outline two recently reported applications of bipartite tetracysteine display. The first is a novel, encodable, selective, Src kinase sensor that lacks fluorescent proteins but possesses a fluorescent readout exceeding that of most sensors based on Förster resonance energy transfer (FRET). The second is a unique method, called complex-edited electron microscopy (CE-EM), that facilitates visualization of protein-protein complexes with electron microscopy. Exciting as these applications may be, the continued development of small-molecule tools with improved utility in living cells, let alone in vivo, will demand a more nuanced understanding of the fundamental photophysics that lead to fluorogenicity, as well as creative approaches toward the synthesis and identification of new and orthogonal dye-tag pairs that can be applied facilely in tandem. We describe one example of a dye-sequence tag pair that is chemically distinct from bis-arsenical chemistry. Through further effort, we expect that that bipartite tetracysteine display will find successful use in the study of sophisticated biological questions that are essential to the fields of biochemistry and biology as well as to our progressive understanding of human disease.

Exploration of biarsenical chemistry--challenges in protein research

The fluorescent modification of proteins (with genetically encoded low-molecular-mass fluorophores, affinity probes, or other chemically active species) is extraordinarily useful for monitoring and controlling protein functions in vitro, as well as in cell cultures and tissues. The large sizes of some fluorescent tags, such as fluorescent proteins, often perturb normal activity and localization of the protein of interest, as well as other effects. Of the many fluorescent-labeling strategies applied to in vitro and in vivo studies, one is very promising. This requires a very short (6- to 12-residue), appropriately spaced, tetracysteine sequence (-CCXXCC-); this is either placed at a protein terminus, within flexible loops, or incorporated into secondary structure elements. Proteins that contain the tetracysteine motif become highly fluorescent upon labeling with a nonluminescent biarsenical probe, and form very stable covalent complexes. We focus on the development, growth, and multiple applications of this protein research methodology, both in vitro and in vivo. Its application is not limited to intact-cell protein visualization; it has tremendous potential in other protein research disciplines, such as protein purification and activity control, electron microscopy imaging of cells or tissue, protein-protein interaction studies, protein stability, and aggregation studies.