Cleavable biotin probes for labeling of biomolecules via azide-alkyne cycloaddition.
ABSTRACT: The azide-alkyne cycloaddition provides a powerful tool for bio-orthogonal labeling of proteins, nucleic acids, glycans, and lipids. In some labeling experiments, e.g., in proteomic studies involving affinity purification and mass spectrometry, it is convenient to use cleavable probes that allow release of labeled biomolecules under mild conditions. Five cleavable biotin probes are described for use in labeling of proteins and other biomolecules via azide-alkyne cycloaddition. Subsequent to conjugation with metabolically labeled protein, these probes are subject to cleavage with either 50 mM Na(2)S(2)O(4), 2% HOCH(2)CH(2)SH, 10% HCO(2)H, 95% CF(3)CO(2)H, or irradiation at 365 nm. Most strikingly, a probe constructed around a dialkoxydiphenylsilane (DADPS) linker was found to be cleaved efficiently when treated with 10% HCO(2)H for 0.5 h. A model green fluorescent protein was used to demonstrate that the DADPS probe undergoes highly selective conjugation and leaves a small (143 Da) mass tag on the labeled protein after cleavage. These features make the DADPS probe especially attractive for use in biomolecular labeling and proteomic studies.
Project description:Labeling proteins with biotin is a widely used method to identify target proteins due to biotin's strong binding affinity for streptavidin. Combined with alkyne-azide cycloaddition, which enables the coupling of probes to targeted proteins, biotin tags linked to an alkyne or azide have become a powerful tool for purification and analysis of proteins in proteomics. However, biotin requires harsh elution conditions to release the captured protein from the bead matrix. Use of these conditions reduces signal to noise and complicates the analysis. To improve affinity capture, cleavable linkers have been introduced. Here, we demonstrate the use of a cyclic acetal biotin probe that is prepared easily from commercially available starting materials, is stable to cell lysates, yet is cleaved under mildly acidic conditions, and which provides an aldehyde for further elaboration of the protein, if desired.
Project description:Cysteine residues on proteins serve a variety of catalytic and regulatory functions due to the high nucleophilicity and redox activity of the thiol group. Quantitative proteomic platforms for profiling cysteine reactivity can provide valuable information related to the post-translational modification state and inhibitor occupancy of functional cysteine residues within a complex proteome. Cysteine-reactivity profiling typically monitors changes in the extent of cysteine labeling by cysteine-reactive chemical probes, such as iodoacetamide (IA)-alkyne. To enable accurate measurements of cysteine reactivity changes, isotopic labels are introduced into the two proteomes of interest using either isotopically tagged proteomes (SILAC) or cleavable linkers (isoTOP-ABPP) that are installed using copper-catalyzed azide-alkyne cycloaddition (CuAAC). Here we provide an alternative strategy for isotopic tagging of two proteomes for cysteine-reactivity profiling by developing IA-light and IA-heavy, a pair of isotopically labeled iodoacetamide-alkyne probes. These probes can be utilized for proteome samples that are not amenable to SILAC labeling and are facile to synthesize, especially when compared to the isotopically tagged cleavable linkers. We confirm the quantitative accuracy of IA-light and IA-heavy by assessing cysteine reactivity in a purified thioredoxin protein, as well as globally within a complex proteome where IA-light treatment generates mass-spectrometry identification of 992 cysteine residues. Importantly, these isotopically tagged probes can also be utilized for quantifying the percentage of cysteine modification within a single sample. Preliminary data supports the use of these tags to quantify the stoichiometry of TCEP-susceptible cysteine oxidation events in cell lysates.
Project description:The ability to bind strongly to complementary nucleic acid sequences, invade complex nucleic acid structures, and resist degradation by cellular enzymes has made peptide nucleic acid (PNA) oligomers as very useful hybridization probes in molecular diagnosis. For such applications, the PNA oligomers have to be labeled with appropriate reporters as they lack intrinsic labels that can be used in biophysical assays. Although solid-phase synthesis is commonly used to attach reporters onto PNA, development of milder and modular labeling methods will provide access to PNA oligomers labeled with a wider range of biophysical tags. Here, we describe the establishment of a postsynthetic modification strategy based on bioorthogonal chemical reactions in functionalizing PNA oligomers in solution with a variety of tags. A toolbox composed of alkyne- and azide-modified monomers were site-specifically incorporated into PNA oligomers and postsynthetically click-functionalized with various tags, ranging from sugar, amino acid, biotin, to fluorophores, by using copper(I)-catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne cycloaddition, and Staudinger ligation reactions. As a proof of utility of this method, fluorescent PNA hybridization probes were developed and used in imaging human telomeres in chromosomes and poly(A) RNAs in cells. Taken together, this simple approach of generating a wide range of functional PNA oligomers will expand the use of PNA in molecular diagnosis.
Project description:Transglutaminase 2 (TG2) is a Ca(2+)-dependent enzyme able to catalyze the formation of ?(?-glutamyl)-lysine crosslinks between polypeptides, resulting in high molecular mass multimers. We have developed a bioorthogonal chemical method for the labeling of TG2 glutamine-donor proteins. As amine-donor substrates we used a set of azide- and alkyne-containing primary alkylamines that allow, after being crosslinked to glutamine-donor proteins, specific labeling of these proteins via the azide-alkyne cycloaddition. We demonstrate that these azide- and alkyne-functionalized TG2 substrates are cell permeable and suitable for specific labeling of TG2 glutamine-donor substrates in HeLa and Movas cells. Both the Cu(I)-catalyzed and strain promoted azide-alkyne cycloaddition proved applicable for subsequent derivatization of the TG2 substrate proteins with the desired probe. This new method for labeling TG2 substrate proteins introduces flexibility in the detection and/or purification of crosslinked proteins, allowing differential labeling of cellular proteins.
Project description:Photoconvertible fluorophores can enable the visualization and tracking of a specific biomolecules, complexes, and cellular compartments with precise spatiotemporal control. The field of photoconvertible probes is dominated by fluorescent protein variants, which can introduce perturbations to the target biomolecules due to their large size. Here, we present a photoconvertible small molecule, termed CPX, that can be conjugated to any target through azide-alkyne cycloaddition ("click" reaction). To demonstrate its utility, we have applied CPX to study (1) trafficking of biologically relevant synthetic vesicles and (2) intracellular processes involved in transmission of ?-synuclein (?S) pathology. Our results demonstrate that CPX can serve as a minimally perturbing probe for tracking the dynamics of biomolecules.
Project description:This protocol describes an efficient method to site-specifically label cell-surface or purified proteins with chemical probes in two steps: probe incorporation mediated by enzymes (PRIME) followed by chelation-assisted copper-catalyzed azide-alkyne cycloaddition (CuAAC). In the PRIME step, Escherichia coli lipoic acid ligase (LplA) site-specifically attaches a picolyl azide (pAz) derivative to a 13-aa recognition sequence that has been genetically fused onto the protein of interest. Proteins bearing pAz are chemoselectively derivatized with an alkyne-probe conjugate by chelation-assisted CuAAC in the second step. We describe herein the optimized protocols to synthesize pAz to perform PRIME labeling and to achieve CuAAC derivatization of pAz on live cells, fixed cells and purified proteins. Reagent preparations, including synthesis of pAz probes and expression of LplA, take 12 d, whereas the procedure for performing site-specific pAz ligation and CuAAC on cells or on purified proteins takes 40 min-3 h.
Project description:Glycomics is emerging as a new field for the biology of complex glycoproteins and glycoconjugates. The lack of versatile glycan-labeling methods has presented a major obstacle to visualizing at the cellular level and studying glycoconjugates. To address this issue, we developed a fluorescent labeling technique based on the Cu(I)-catalyzed [3 + 2] cycloaddition, or click chemistry, which allows rapid, versatile, and specific covalent labeling of cellular glycans bearing azide groups. The method entails generating a fluorescent probe from a nonfluorescent precursor, 4-ethynyl-N-ethyl-1,8-naphthalimide, by clicking the fluorescent trigger, the alkyne at the 4 position, with an azido-modified sugar. Using this click-activated fluorescent probe, we demonstrate incorporation of an azido-containing fucose analog into glycoproteins via the fucose salvage pathway. Distinct fluorescent signals were observed by flow cytometry when cells treated with 6-azidofucose were labeled with the click-activated fluorogenic probe or biotinylated alkyne. The intracellular localization of fucosylated glycoconjugates was visualized by using fluorescence microscopy. This technique will allow dynamic imaging of cellular fucosylation and facilitate studies of fucosylated glycoproteins and glycolipids.
Project description:Novel BODIPY-estrone conjugates were synthesized via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). Estrone-alkynes or an estrone-azide as starting compounds were synthesized via Michael addition or Sonogashira reaction as key steps. Fluorescent dyes based on BODIPY-core were provided by azide or alkyne functional groups. Fluorescent labeling of estrone was efficiently achieved at the C-2 or C-15 position. The newly-elaborated coupling procedures might have a broad applicability in the synthesis of fluorescent-labeled estrone conjugates suitable for biological assays.
Project description:Developing tools for investigating the cellular activity of glycans will help to delineate the molecular basis for aberrant glycosylation in pathological processes such as cancer. Metabolic oligosaccharide engineering, which inserts sugar-reporting groups into cellular glycoconjugates, represents a powerful method for imaging the localization, trafficking, and dynamics of glycans and isolating them for glyco-proteomic analysis. Herein, we show that the alkyne-reporting group can be incorporated into cellular glycans. The alkyne group is a small, inert, bio-orthogonal handle that can be chemoselectively labeled by using the Cu(I) catalyzed [3 + 2] azide-alkyne cycloaddition, or click chemistry. Alkynyl sugar monomers, based on fucose (Fuc) and N-acetylmannosamine (ManNAc), were incorporated into fucosylated and sialylated glycans in several cancer cell lines, allowing for cell surface and intracellular visualization of glycoconjugates, as well as, observation of alkyne-bearing glycoproteins. Similarly to our previous results with an azido Fuc/alkynyl probe system, we demonstrated that click-activated fluorogenic probes are practical tools for efficiently and selectively labeling alkynyl-modified glycans. Because Fuc and sialic acid are terminal glycan residues with a notably increased presence in many tumors, we hope that our method will provide useful information about their roles in cancer and ultimately can be used for diagnostic and therapeutic purposes.
Project description:New additions to the bioorthogonal chemistry compendium can advance biological research by enabling multiplexed analysis of biomolecules in complex systems. Here we introduce the quadricyclane ligation, a new bioorthogonal reaction between the highly strained hydrocarbon quadricyclane and Ni bis(dithiolene) reagents. This reaction has a second-order rate constant of 0.25 M(-1) s(-1), on par with fast bioorthogonal reactions of azides, and proceeds readily in aqueous environments. Ni bis(dithiolene) probes selectively labeled quadricyclane-modified bovine serum albumin, even in the presence of cell lysate. We have demonstrated that the quadricyclane ligation is compatible with, and orthogonal to, strain-promoted azide-alkyne cycloaddition and oxime ligation chemistries by performing all three reactions in one pot on differentially functionalized protein substrates. The quadricyclane ligation joins a small but growing list of tools for the selective covalent modification of biomolecules.