Rational Design for High Bioorthogonal Fluorogenicity of Tetrazine-Encoded Green Fluorescent Proteins

Tetrazine Amino Acid Encoding for Rapid and Complete Protein Bioconjugation

Generating protein conjugates using the bioorthogonal ligation between tetrazines and trans-cyclooctene groups avoids the need to manipulate cysteine amino acids; this ligation is rapid, site-specific, and stoichiometric and allows for labeling of proteins in complex biological environments. Here, we provide a protocol for the expression of conjugation-ready proteins at high yields in Escherichia coli with greater than 95% encoding and labeling fidelity. This protocol focuses on installing the Tet2 tetrazine amino acid using an optimized genetic code expansion (GCE) machinery system, Tet2 pAJE-E7, to direct Tet2 encoding at TAG stop codons in BL21 E. coli strains, enabling reproducible expression of Tet2-proteins that quantitatively react with trans-cyclooctene (TCO) groups within 5 min at room temperature and physiological pH. The use of the BL21 derivative B95(DE3) minimizes premature truncation byproducts caused by incomplete suppression of TAG stop codons, which makes it possible to use more diverse protein construct designs. Here, using a superfolder green fluorescent protein construct as an example protein, we describe in detail a four-day process for encoding Tet2 with yields of ~200 mg per liter of culture. Additionally, a simple and fast diagnostic gel electrophoretic mobility shift assay is described to confirm Tet2-Et encoding and reactivity. Finally, strategies are discussed to adapt the protocol to alternative proteins of interest and optimize expression yields and reactivity for that protein. Key features • Protocol describes site-specific encoding of the tetrazine amino acid Tet2-Et into proteins for bioorthogonal, quantitative, and rapid attachment of trans-cyclooctene-containing labels. • Protocol uses auto-induction methods for the production Tet2-Et protein in E. coli. • This protocol focuses on Tet-protein expressions in BL21(DE3) and B95(DE3) strains, which take approximately 4 days to complete. • SDS-PAGE mobility shift assay using a strained TCO-PEG5000 (sTCO-PEG5000) reagent provides a simple, generalizable method for testing Tet-protein reactivity.

Truncation-Free Genetic Code Expansion with Tetrazine Amino Acids for Quantitative Protein Ligations

Quantitative labeling of biomolecules is necessary to advance areas of antibody-drug conjugation, super-resolution microscopy imaging of molecules in live cells, and determination of the stoichiometry of protein complexes. Bio-orthogonal labeling to genetically encodable noncanonical amino acids (ncAAs) offers an elegant solution; however, their suboptimal reactivity and stability hinder the utility of this method. Previously, we showed that encoding stable 1,2,4,5-tetrazine (Tet)-containing ncAAs enables rapid, complete conjugation, yet some expression conditions greatly limited the quantitative reactivity of the Tet-protein. Here, we demonstrate that reduction of on-protein Tet ncAAs impacts their reactivity, while the leading cause of the unreactive protein is near-cognate suppression (NCS) of UAG codons by endogenous aminoacylated tRNAs. To overcome incomplete conjugation due to NCS, we developed a more catalytically efficient tRNA synthetase and developed a series of new machinery plasmids harboring the aminoacyl tRNA synthetase/tRNA pair (aaRS/tRNA pair). These plasmids enable robust production of homogeneously reactive Tet-protein in truncation-free cell lines, eliminating the contamination caused by NCS and protein truncation. Furthermore, these plasmid systems utilize orthogonal synthetic origins, which render these machinery vectors compatible with any common expression system. Through developing these new machinery plasmids, we established that the aaRS/tRNA pair plasmid copy-number greatly affects the yields and quality of the protein produced. We then produced quantitatively reactive soluble Tet-Fabs, demonstrating the utility of this system for rapid, homogeneous conjugations of biomedically relevant proteins.

Capturing excited-state structural snapshots of evolutionary green-to-red photochromic fluorescent proteins

Photochromic fluorescent proteins (FPs) have proved to be indispensable luminous probes for sophisticated and advanced bioimaging techniques. Among them, an interplay between photoswitching and photoconversion has only been observed in a limited subset of Kaede-like FPs that show potential for discovering the key mechanistic steps during green-to-red photoconversion. Various spectroscopic techniques including femtosecond stimulated Raman spectroscopy (FSRS), X-ray crystallography, and femtosecond transient absorption were employed on a set of five related FPs with varying photoconversion and photoswitching efficiencies. A 3-methyl-histidine chromophore derivative, incorporated through amber suppression using orthogonal aminoacyl tRNA synthetase/tRNA pairs, displays more dynamic photoswitching but greatly reduced photoconversion versus the least-evolved ancestor (LEA). Excitation-dependent measurements of the green anionic chromophore reveal that the varying photoswitching efficiencies arise from both the initial transient dynamics of the bright cis state and the final trans-like photoswitched off state, with an exocyclic bridge H-rocking motion playing an active role during the excited-state energy dissipation. This investigation establishes a close-knit feedback loop between spectroscopic characterization and protein engineering, which may be especially beneficial to develop more versatile FPs with targeted mutations and enhanced functionalities, such as photoconvertible FPs that also feature photoswitching properties.

Thiomethyltetrazines Are Reversible Covalent Cysteine Warheads Whose Dynamic Behavior can be "Switched Off" via Bioorthogonal Chemistry Inside Live Cells

Electrophilic small molecules that can reversibly modify proteins are of growing interest in drug discovery. However, the ability to study reversible covalent probes in live cells can be limited by their reversible reactivity after cell lysis and in proteomic workflows, leading to scrambling and signal loss. We describe how thiomethyltetrazines function as reversible covalent warheads for cysteine modification, and this dynamic labeling behavior can be "switched off" via bioorthogonal chemistry inside live cells. Simultaneously, the tetrazine serves as a bioorthogonal reporter enabling the introduction of tags for fluorescent imaging or affinity purification. Thiomethyltetrazines can label isolated proteins, proteins in cellular lysates, and proteins in live cells with second-order rate constants spanning 2 orders of magnitude (k2, 1-100 M-1 s-1). Reversible modification by thiomethyltetrazines can be switched off upon the addition of trans-cyclooctene in live cells, converting the dynamic thiomethyltetrazine tag into a Diels-Alder adduct which is stable to lysis and proteomic workflows. Time-course quenching experiments were used to demonstrate temporal control over electrophilic modification. Moreover, it is shown that "locking in" the tag through Diels-Alder chemistry enables the identification of protein targets that are otherwise lost during sample processing. Three probes were further evaluated to identify unique pathways in a live-cell proteomic study. We anticipate that discovery efforts will be enabled by the trifold function of thiomethyltetrazines as electrophilic warheads, bioorthogonal reporters, and switches for "locking in" stability.

Delineating Ultrafast Structural Dynamics of a Green-Red Fluorescent Protein for Calcium Sensing

Fluorescent proteins (FPs) are indispensable tools for noninvasive bioimaging and sensing. Measuring the free cellular calcium (Ca²⁺) concentrations in vivo with genetically encodable FPs can be a relatively direct measure of neuronal activity due to the complex signaling role of these ions. REX-GECO1 is a recently developed red-green emission and excitation ratiometric FP-based biosensor that achieves a high dynamic range due to differences in the chromophore response to light excitation with and without calcium ions. Using steady-state electronic measurements (UV/Visible absorption and emission), along with time-resolved spectroscopic techniques including femtosecond transient absorption (fs-TA) and femtosecond stimulated Raman spectroscopy (FSRS), the potential energy surfaces of these unique biosensors are unveiled with vivid details. The ground-state structural characterization of the Ca²⁺-free biosensor via FSRS reveals a more spacious protein pocket that allows the chromophore to efficiently twist and reach a dark state. In contrast, the more compressed cavity within the Ca²⁺-bound biosensor results in a more heterogeneous distribution of chromophore populations that results in multi-step excited state proton transfer (ESPT) pathways on the sub-140 fs, 600 fs, and 3 ps timescales. These results enable rational design strategies to enlarge the spectral separation between the protonated/deprotonated forms and the Stokes shift leading to a larger dynamic range and potentially higher fluorescence quantum yield, which should be broadly applicable to the calcium imaging and biosensor communities.

Affinity Bioorthogonal Chemistry (ABC) Tags for Site-Selective Conjugation, On-Resin Protein-Protein Coupling, and Purification of Protein Conjugates

The site-selective functionalization of proteins has broad application in chemical biology, but can be limited when mixtures result from incomplete conversion or the formation of protein containing side products. It is shown here that when proteins are covalently tagged with pyridyl-tetrazines, the nickel-iminodiacetate (Ni-IDA) resins commonly used for His-tags can be directly used for protein affinity purification. These Affinity Bioorthogonal Chemistry (ABC) tags serve a dual role by enabling affinity-based protein purification while maintaining rapid kinetics in bioorthogonal reactions. ABC-tagging works with a range of site-selective bioconjugation methods with proteins tagged at the C-terminus, N-terminus or at internal positions. ABC-tagged proteins can also be purified from complex mixtures including cell lysate. The combination of site-selective conjugation and clean-up with ABC-tagged proteins also allows for facile on-resin reactions to provide protein-protein conjugates.