ACS Chemical Biology

Tuberculosis (TB) remains the worlds leading infectious cause of death. Trehalose-based fluorogenic probes have emerged as powerful tools for labeling and studying mycobacteria, including Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. However, existing probes occupy a limited spectral range and require compromise between brightness, specificity, and functional readouts. Here, we report the design and characterization of two trehalose conjugates derived from Janelia Fluor(R) dyes, JF635-Tre and JF646-Tre, which extend the trehalose-based platform into the far-red region. Following NHS ester-mediated synthesis, the trehalose-conjugated analogs displayed strong far-red fluorescence, with excitation/emission maxima at 638/654 nm and 648/663 nm for JF635-Tre and JF646-Tre, respectively. Both probes exhibited concentration- and time-dependent labeling of Mycobacterium smegmatis (Msmeg) and Mtb with minimal background fluorescence from the corresponding unconjugated dyes. Furthermore, we observed reduced labeling in heat-killed cells compared to live Msmeg, particularly for JF646-Tre, consistent with sensitivity to metabolic activity. Both JF-Tre derivatives produced significant cellular labeling and JF635-Tre distinguished untreated from INH-treated samples in drug susceptible Mtb, demonstrating the potential of the JF-Tre probes to report on INH susceptibility and resistance. Together, these findings expand the toolkit of trehalose-based probes and highlight how fluorophore identity influences probe performance in mycobacterial fluorescence imaging and drug susceptibility testing.

Non-canonical amino acids (ncAAs) are valuable tools in chemical biology and biochemistry for labeling, probing, and tracking biomolecules. ncAAs that can be recombinantly incorporated using native E. coli machinery are particularly useful because they allow for global protein incorporation and avoid complex genetic code expansion. Here, we demonstrate successful incorporation of a methionine analog, L-cyanohomoalanine (Cha), by the methionyl-tRNA synthetase of E. coli into mutant superfolder GFP (sfGFP) expressed in methionine auxotroph bacterial cultures. We compare to methionine auxotroph bacterial cultures supplemented with L-methionine (Met) or L-azidohomoalanine (Aha). In control prototrophic E. coli, bacterial growth rates are inhibited with high concentrations of Aha but not Cha. However, less sfGFP is produced in auxotrophic cells supplemented with Cha compared to Aha and Met. Thus, while Cha is non-toxic to E. coli it is incorporated less efficiently into proteins than Aha or Met. Mass spectrometry confirmed that N-terminal Cha, Aha, and Met are cleaved, as expected for the sfGFP mutants. Other sites of Cha and Aha incorporation were confirmed by mass spectrometry, with labeling efficiency varying by position. Thermal melts of purified sfGFPs demonstrate that Cha and Aha labeling does not significantly perturb the protein stability. In the future, Cha may be useful for proteome labeling by wild-type methionyl-tRNA synthetase and could be implemented in metabolic pulse-labeling of newly synthesized proteins with other methionine analogs. Additionally, the nitrile moiety of Cha may be used to perform reactions orthogonal to azide/alkyne click chemistry or could serve as a vibrational reporter of the environment.

Small-molecule drug discovery relies on identifying compounds that modulate specific protein targets, a process often hindered by cellular complexity. Through phenotypic screening of a kinase-focused diazaquinazoline library, we serendipitously identified CEM198 as the first high-affinity ligand of tubulin-tyrosine ligase (TTL). Functional assays combining live-cell TTL inhibition, microtubule polymerization, cell cycle analysis, and proteomics revealed that CEM198 acts through a dual mechanism: directly binding to TTL and altering /{beta}-tubulin conformation. This interaction restricts -tubulin tyrosination and disrupts tubulin polymerization, leading to microtubule destabilization. The differential effects observed between SH-SY5Y and HEK293T cells indicate that effective TTL inhibition depends on both direct binding and structural modulation of the tubulin heterodimer. These findings introduce CEM198 as a chemical probe for investigating the tubulin tyrosination-detyrosination and demonstrate the potential of chemoproteomics to uncover novel modulators of microtubule dynamics.

The biosynthetic machineries of ribosomally synthesized and post-translationally modified peptides (RiPPs) are often substrate tolerant. A remarkable example is the class II lanthipeptide synthetase ProcM, which naturally functions as a generalist enzyme that has not evolved to use a specific substrate during its evolutionary history. Although ProcM has been studied extensively, the sequence features associated with productive modification remain underexplored. In this study, we use the ultrahigh-throughput mRNA display technique to map the sequence compatibility of ProcM across a focused library. This approach expands the landscape of ProcM reactivity beyond native substrates and individually characterized variants. Machine learning (ML) is used as a tool to demonstrate that the selected dataset contains learnable signatures and classification architectures revealed a balanced accuracy of 0.73. This performance contrasts sharply with the near-perfect accuracy of specialized enzyme models as the sequence-fitness landscape of the generalist enzymes are characterized by class imbalance and limited by intrinsic dataset features. Our results provide a high-throughput view of ProcM reactivity and highlight differences with previous high-throughput studies on substrate selectivity of RiPP modification enzymes. Future studies will need to assess whether these differences are common when comparing generalist with specialist enzymes.

Modulating autophagy has emerged as a potential strategy for treating age-related diseases. However, commonly used pharmacological approaches to induce autophagy, particularly inhibition of mechanistic target of rapamycin complex 1 (mTORC1), can be associated with adverse effects, including immunosuppression and insulin resistance. This has prompted interest in autophagy modulators that act without directly inhibiting mTORC1. 2,5-Diketopiperazines (DKPs) are bioactive cyclic dipeptide scaffolds with diverse biological activities. However, systematic evaluation of their structure-activity relationships has been hindered by racemization during conventional chemical synthesis, leaving the contribution of stereochemistry to autophagy regulation poorly understood. Here, we used a stereoselective one-pot chemoenzymatic synthesis based on the adenylation domain of tyrocidine synthetase A to generate a DKP library with defined stereochemistry. Phenotypic screening in Caco-2 cells stably expressing the GFP-LC3-RFP autophagic flux probe identified four DKPs that increased autophagic flux: c(DW-DP), c(DW-LP), c(DF-DP), and c(DM-LP). Structure-activity analysis revealed stereochemistry-dependent effects associated with amino acid side-chain properties: D-configured residues were favored among DKPs containing aromatic amino acids or methionine, whereas L-configured residues were favored among those containing branched-chain amino acids. Substitution of the proline residue further altered activity, with glycine substitution tending to increase autophagic flux in some DKP scaffolds. Importantly, the active DKPs did not detectably reduce the phosphorylation of the mTORC1 downstream targets p70 S6K and 4EBP1, indicating that their autophagy-inducing effects do not require detectable suppression of canonical mTORC1 signaling. These findings establish stereochemically defined DKPs as candidate scaffolds for the development of autophagy inducers that act through mechanisms distinct from direct mTORC1 inhibition.

Glycosylation depends on tightly regulated pools of nucleotide-sugars (NS), yet the mechanisms controlling mammalian NS homeostasis and their downstream effects on glycoprotein biosynthesis remain poorly understood. UDP-galactose 4'-epimerase (GALE) catalyzes the reversible interconversion of UDP-galactose/UDP-glucose and UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine, making it a central regulator of glycan precursor pools and an excellent model enzyme for studying NS metabolism. Here, we report the discovery of a cell-active small molecule inhibitor of human GALE through a high-throughput chemical screening strategy. Using a coupled luminescence-based assay, we identified the FDA-approved drug disulfiram as a GALE inhibitor. Biochemical analyses demonstrated that disulfiram directly inhibits GALE through covalent modification of cysteine residues, including C153, likely via its reactive metabolite diethyldithiocarbamate. In cultured human cells, disulfiram treatment phenocopied genetic GALE deletion, reducing terminally sialylated glycans, mucin-type O-glycans, and properly glycosylated mucin-domain glycoproteins. These effects were rescued by galactose supplementation, consistent with a mechanism of on-target GALE inhibition. Similar phenotypes were observed in human lung adenocarcinoma cells, supporting a broader role for GALE in regulating glycosylation and mucin biosynthesis across tissue types. Together, these studies establish a platform for the discovery of pharmacological GALE inhibitors as new research tools, identify disulfiram as a cell-active chemical probe for studying NS regulation, and suggest that targeting GALE might modulate mucin hypersecretion in muco-obstructive diseases and mucinous cancers.

Achieving predictable, tunable, and temporal control over mRNA function would grant direct regulation of gene expression, facilitating the development of new therapeutics and biotechnologies. Although several approaches for stimuli-responsive control of nucleic acids have been explored, most are limited to short oligonucleotides, lack a timed-release mechanism, or both. We envisioned a complementary method using glyoxal as a caging reagent. Glyoxal readily reacts with amidine groups found on the faces of nucleobases to give stable bis-hemiaminal adducts, directly disrupting hydrogen bonding. Fortuitously, this reaction is readily reversible, enabling spontaneous time-release decaging that varies with temperature. However, when applied previously to full-length mRNAs, the sequence length and excessive adduct formation resulted in no reactivation under relevant physiological conditions. To address this challenge, we developed chemical lithography in which portions of longer RNAs are "masked" through hybridization to complementary DNAs, permitting selective caging on only non-masked regions and preventing excessive adduct formation. We present an optimized glyoxalation protocol applied to EGFP as a model mRNA sequence and evaluate masking effectiveness through qualitative and quantitative studies. Using EGFP fluorescence, we monitored and assessed the ability of selective glyoxalation to control gene expression over time in vitro. We demonstrate the direct dependence of both the initial inhibited expression and the respective activity recovery based on the amount and location of glyoxalation. We also highlight distinct caging patterns exhibiting total inhibition upon initial treatment and complete reactivation following decaging. We anticipate that this approach will improve the mechanistic study of mRNA and gene expression and also facilitate new investigations and methods within chemical biology and biomedicine.

The cysteine to serine mutation at residue 481 of Bruton's tyrosine kinase (BTK) is the most common mechanism of clinical resistance against ibrutinib for the treatment of mantle cell lymphoma and chronic lymphocytic leukemia. We report small molecule ligands containing chiral {beta}-lactone electrophiles to address this challenge. The asymmetric warhead enabled stereoselective covalent modification of wild-type and ibrutinib-resistant mutant BTK(C481S) through distinct sites of reactivity. Building on these findings, we developed kinase-directed {beta}-lactone probes and demonstrated that individual enantiomers preferentially engage distinct subsets of the kinome. These studies establish {beta}-lactones as stereochemically encodable covalent warheads whose stereochemistry can serve as a selectivity filter in covalent drug discovery.

Selenocysteine (Sec), the 21st amino acid, is a rare non-canonical amino acid that represents an attractive target for protein engineering due to its desirable chemical properties such as high affinity for metals, strong nucleophilicity, and reversible covalent bond formation. To bypass the natural constraints on Sec placement within proteins, several strategies have been developed to rewire the native translational machinery to enable site-specific incorporation. However, these usually abolish the quality control mechanism that excludes the serine-charged selenocysteinyl-tRNA (Ser-tRNASec), the immediate biosynthetic precursor, from translation resulting in heterogenous protein species. This challenge is confounded by a lack of genetic tools to accurately report the selenylation state of the tRNA pool as most are blind to competing process of Ser incorporation, which can only be observed using analytical methods. To resolve this issue, we have developed a new fluorescent reporter, Selenocysteine Adjusted Ratiometric Chromophore (SeARCh), which exhibits two distinct spectral outputs dependent on the incorporation of either Ser (red) or Sec (green). Using SeARCh, we define several factors which influence the observed Sec:Ser ratio and construct a new hybrid biosynthetic pathway with improved performance, achieving 90% Sec incorporation. Furthermore, SeARCh displays unusually complex mass spectra due to the isotope distribution of selenium and heterogenous nature of the protein in solution and we report specific methods to account for this behaviour and precisely quantify the rare Ser-containing species found at high Sec incorporation efficiencies. Our findings suggest that the equilibrium between selenoprotein and tRNASec expression levels is a key driver of incorporation efficiency and implies a process that is broadly biosynthetically constrained. Collectively these tools represent a significant advance in the metrology of selenocysteine biosynthesis and incorporation and can be used to inform and standardize future engineering efforts.

Cell penetrating thyclotides (CPTs) are synthetic molecules that promote highly efficient cellular uptake and endosomal escape of bioactive peptides. While peptides are valuable as medicinal agents, their translation to therapies is often limited by their inability to cross cell membranes. CPTs have a unique combination of chiral tetrahydrofurans and polar sidechains within a molecular scaffold that can be optimized to efficiently deliver peptide cargo into cells. The cellular uptake and endosomal escape of two peptides with anticancer biological activities but low bioavailabilities were remarkably improved after conjugation to a CPT. Using CPTs to overcome barriers to cellular uptake represents a new direction for the intracellular delivery of bioactive molecules, and will accelerate drug development for new medical therapies.

The incorporation of non-canonical amino acids (ncAAs) into proteins, developed in the past 20 years, has opened new avenues with respect to protein structure, protein modification, protein-protein interaction or enzyme catalysis beyond what is possible with the 20 proteinogenic AAs. Although >300 unusual building blocks including several ncAAs have been described in nonribosomal peptides (NRPs) naturally, we aimed to further expand the scope of the underlying nonribosomal peptide synthetases (NRPS) to incorporate ncAAs beyond the naturally available ones. We have therefore systematically screened for ncAA accepting NRPS systems, applied NRPS engineering to transfer the respective ncAA-accepting parts into other NRPSs and thereby created novel peptides that were further derivatized in post-enzymatic chemical synthesis reactions directly in bacterial culture extracts. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=177 SRC="FIGDIR/small/738027v1_ufig1.gif" ALT="Figure 1"> View larger version (38K): org.highwire.dtl.DTLVardef@90552forg.highwire.dtl.DTLVardef@1c8a5e0org.highwire.dtl.DTLVardef@2549dorg.highwire.dtl.DTLVardef@1012911_HPS_FORMAT_FIGEXP M_FIG C_FIG

The Na+-dependent citrate transporter NaCT (SLC13A5) is a key regulator of citrate homeostasis and has emerged as a therapeutic target for metabolic and neurological disease, including the SLC13A5 Epilepsy, a rare disease marked by severe sezures and neurodevelopmental delays. Current NaCT inhibitors are substrate-like molecules that competitively bind the substrate binding site. In this study, we identify previously unknown small molecule inhibitors of NaCT by targeting a putative allosteric site located at the dimer interface. We performed a virtual screen of 3.5 million compounds from the ZINC20 database against this site and selected 54 candidates for experimental testing using a cell-based citrate uptake assay. Through initial experiments, we identified three weak inhibitors, and subsequent evaluation of 26 structurally related analogs yielded six compounds with improved potency (IC50 = 12.78 M and 15.49 M). We then performed further analysis of the putative binding site by integrating structural data with deep mutational scanning evidence and comparisons with homolog structures. This analysis highlighted the importance of key residues (e.g., Phe362) in ligand modulation. These findings reveal a promising allosteric pocket and establish a chemically distinct series of NaCT inhibitors, providing a foundation for rational development of pharmacological modulators of NaCT function.

Conversion of cysteine residues to 5-thiooxazole moieties by multinuclear nonheme iron-dependent oxidative enzymes (MNIOs) is a prevalent modification in ribosomally synthesized, post-translationally modified peptide (RiPP) natural products. However, this post-translational modification (PTM) is difficult to distinguish from MNIO-produced oxazolone-coupled thioamides, such as those present in the RiPP methanobactin. The RiPP virulence factor oxazolin contains six copper-binding heterocycles installed by an MNIO. Here, we reassign these PTMs, originally described as oxazolones/thioamides, as 5-thiooxazoles on the basis of detailed comparative chemical and structural characterization of oxazolin and methanobactin. These data establish a benchmark for differentiating these two PTMs in newly discovered RiPPs. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=86 SRC="FIGDIR/small/730506v1_ufig1.gif" ALT="Figure 1"> View larger version (12K): org.highwire.dtl.DTLVardef@d30435org.highwire.dtl.DTLVardef@1b933fforg.highwire.dtl.DTLVardef@4b997org.highwire.dtl.DTLVardef@349ee1_HPS_FORMAT_FIGEXP M_FIG C_FIG

Macrocyclic peptides have gained increased attention amid claims they are a "Goldilocks" therapeutic modality that can encode the selectivity of a biologic in a footprint close to that of a small molecule. Here we attempt to find a peptide that binds selectively to NRAS, sparing HRAS and KRAS, while accessing the cytosol via passive cell permeability. To do so, we combine subtractive affinity selection with mRNA display to identify Compound 1, an 11mer macrocyclic peptide which binds NRAS at a novel allosteric site between Helix 3 and Helix 4 of the GTPase domain. Compound 1 has total isoform selectivity and can be tuned to achieve activation-state selectivity with a single amino acid change. While it has preferential affinity for oncogenic NRAS-specific mutations, it does not inhibit NRAS function or achieve passive membrane permeability.

Triculamin is a potent antibiotic lasso peptide first isolated in 1967. Previous studies have demonstrated that its biosynthesis follows a non-canonical logic unlike any other lasso peptide. In this study, we investigate the role of the unusual follower peptide and demonstrate that it is essential for efficient biosynthesis. Using structural prediction and targeted mutations of key conserved residues, we hypothesize that the interactions between the follower peptide and the macrocyclase create an enzyme-substrate complex that ensures delivery of the core peptide to the enzyme active site. Moreover, we demonstrate that analogs of the lasso peptide can be produced by modifying the core peptide, highlighting the substrate promiscuity of the lasso macrocyclase and identifying lysine-3 in the lasso peptide ring as the site of acetylation. Lastly, we achieve successful heterologous expression in Burkholderia sp. FERM 3421, which proves to be a superior heterologous host.

The discovery of sensory ion channels, such as thermosensitive transient receptor potential (TRP) channels and mechanosensitive PIEZOs, have transformed our understanding of mammalian sensory biology. However, the sensory receptor landscape remains incomplete, as many physiologically relevant sensory stimuli still lack identified molecular targets. Here, we describe a novel screening strategy utilizing FM 1-43, a fluorescent marker for activity of various cation channels, with a CRISPRa library (MPCL) targeting multi-transmembrane domain proteins. We validate this method by focusing on allyl isothiocyanate (AITC) and its putative receptor TRPA1. Specifically, we show that CRISPRa-mediated overexpression of TRPA1 is sufficient for FM 1-43 labeling when co-treated with AITC. Furthermore, we show that using FM 1-43 and AITC, we can efficiently FACS enrich TRPA1-expressing cells from a pool of MPCL-expressing cells. Collectively, this presents a novel method for rapidly screening select cation-dependent sensory stimuli.

Engineered glycosynthases (GSs) are powerful biocatalysts for custom glycan synthesis, yet their optimization via directed evolution is severely constrained by bottlenecks in high-throughput screening for activated azido-sugar donors. Here, we demonstrate that chemical rescue (CR)--the azide-mediated restoration of hydrolytic activity in nucleophile-deficient mutants--serves as a predictive, high-throughput proxy for glycosynthase activity. Applying an azide-responsive Escherichia coli biosensor screen to a site-saturation mutagenesis library of Thermotoga maritima -L-fucosidase (TmAfc), we established a strong rank-order correlation between CR and GS activities in both crude lysates ({rho} = 0.73) and purified enzymes ({rho} = 0.95). Transition path sampling and QM/MM umbrella sampling revealed that both pathways proceed through a shared oxocarbenium-ion-like transition state ({Delta}G{ddagger} {approx} 8.7 kcal/mol), providing a structural and thermodynamic rationale for using CR to select for transition-state-stabilizing mutations. Biochemical characterization of top-performing variants yielded an engineered fucosynthase (TmAfc_D224G_N70D_T392S) exhibiting a nearly 100-fold enhancement in Vmax alongside altered regioselectivity. This two-tiered screening framework leverages cost-effective chemical rescue assays to streamline glycosynthase engineering for tailored glycans synthesis.

Tunicamycins are potent inhibitors of dolichyl-phosphate N-acetylglucosamine phosphotransferase (DPAGT1) but are unsuitable for therapeutic development due to non-selective cytotoxicity, acid-labile glycosidic linkages, and poor physicochemical properties. Although prior structural modifications reduced the promiscuous toxicity of tunicamycins, the intrinsic 11'-{beta}-1''- trehalose-type glycosidic linkage remains chemically unstable, limiting biological durability. Here, we report a rationally engineered scaffold-stabilization strategy in which the acid-labile linkage is replaced with a chemically robust cyclitol framework, enabling the concise synthesis of chemically stable and water-soluble tunicamycin analogues in only 12 synthetic steps. From this platform, TM-Cy-TBPA (4) was identified as a lead DPAGT1 inhibitor that potently suppresses the proliferation of breast cancer cells by inducing G2-phase arrest followed by apoptosis, while exhibiting minimal cytotoxicity toward nontransformed cells. The compound shows improved solubility, and favorable pharmacokinetic exposure. These results establish tunicamycin cyclitol analogues as a structurally distinct class of selective DPAGT1-targeted anticancer agents and demonstrate that stabilization of the glycosidic linkage is an effective strategy for enhancing pharmacological selectivity, improving in vivo performance, and simplifying the synthetic route.

Glycosylation of proteins is central to cell signaling, immune function, and pathogen interactions, yet existing methods for monitoring glycan changes require specialized instrumentation and primarily report on membrane-anchored, rather than secreted, glycoproteins, with a slow turnover. Here, we present the Enzyme-Linked Cycloaddition Assay (ELCA), a click chemistry-based platform for ultra-sensitive detection and semi-quantitative analysis of secreted sialoglycoproteins. By metabolically incorporating an azide-modified sialic acid into newly synthesized glycoproteins and capturing labeled material via strain-promoted cycloaddition, ELCA quantifies aggregate sialylation using a microplate reader-compatible, ELISA-like workflow. We demonstrate that the secreted glycoproteome responds rapidly to pharmacological perturbation, with changes detectable within hours. Benchmarking against common glycosylation inhibitors and profiling cytokine-driven macrophage polarization further establishes ELCAs sensitivity and temporal resolution. Compatible with serum-containing conditions and requiring no specialized instrumentation, ELCA provides a broadly accessible tool for rapid, cost-effective monitoring of secreted glycoprotein dynamics.

-Synuclein (S), a protein that plays a central role in Parkinsons disease and related synucleinopathies, is an intrinsically disordered protein (IDP) whose functional interactions and aggregation behavior can be strongly influenced by post-translational modifications (PTMs). Phosphorylation, acetylation, and other PTMs regulate Ss interactions with lipid membranes and binding partners, whereas their dysregulation is associated with aggregation and neuronal toxicity. Despite significant progress through chemical and semi-synthetic approaches, investigating the combinatorial effects of PTMs has remained challenging due to the lack of accessible, site-specific methods. Here, we present an integrated strategy combining genetic code expansion, enzymatic modification, and intein-mediated click chemistry to generate S variants bearing multiple defined PTMs and a C-terminal fluorescent label. The resulting constructs enable direct evaluation of how individual and combined PTMs influence S structure, lipid binding, and cellular internalization. Our approach expands the molecular toolkit for dissecting PTM crosstalk in S and other aggregation-prone IDPs, advancing mechanistic understanding and supporting the development of therapeutic strategies for neurodegenerative disease.