Chemical proteomics focuses on the drug–target–phenotype relationship for target deconvolution and elucidation of the mechanism of action—key and bottleneck in drug development and repurposing. Majorly due to the limits of using chemically modified ligands in affinity-based methods, new, unbiased, proteome-wide, and MS-based chemical proteomics approaches have been developed to perform drug target deconvolution, using full proteome profiling and no chemical modification of the studied ligand. Of note among them, thermal proteome profiling (TPP) aims to identify the target(s) by measuring the difference in melting temperatures between each identified protein in drug-treated versus vehicle-treated samples, with the thermodynamic interpretation of “protein melting” and curve fitting of all quantified proteins, at all temperatures, in each biological replicate. Including TPP, all the other chemical proteomics approaches often fail to provide target deconvolution with sufficient proteome depth, statistical power, throughput, and sustainability, which could hardly fulfill the final purpose of drug development. The proteome integral solubility alteration (PISA) assay provides no thermodynamic interpretation, but a throughput 10–100-fold compared to the other proteomics methods, high sustainability, much lower time of analysis and sample amount requirements, high confidence in results, maximal proteome coverage (~10,000 protein IDs), and up to five drugs / test molecules in one assay, with at least biological triplicates of each treatment. Each drug-treated or vehicle-treated sample is split into many fractions and exposed to a gradient of heat as solubility perturbing agent before being recomposed into one sample; each soluble fraction is isolated, then deep and quantitative proteomics is applied across all samples. The proteins interacting with the tested molecules (targets and off-targets), the activated mechanistic factors, or proteins modified during the treatment show reproducible changes in their soluble amount compared to vehicle-treated controls. As of today, the maximal multiplexing capability is 18 biological samples per PISA assay, which enables statistical robustness and flexible experimental design accommodation for fuller target deconvolution, including integration of orthogonal chemical proteomics methods in one PISA assay. Living cells for studying target engagement in vivo or, alternatively, protein extracts to identify in vitro ligand-interacting proteins can be studied, and the minimal need in sample amount unlocks target deconvolution using primary cells and their derived cultures.

Graphical abstract:

Biochemical investigations into DNA-binding and DNA-cutting proteins often benefit from the specific attachment of a radioactive label to one of the two DNA termini. In many cases, it is essential to perform two versions of the same experiment: one with the 5′ DNA end labeled and one with the 3′ DNA end labeled. While homogeneous 5′-radiolabeling can be accomplished using a single kinase-catalyzed phosphorylation step, existing procedures for 3′-radiolabeling often result in probe heterogeneity, prohibiting precise DNA fragment identification in downstream experiments. We present here a new protocol to efficiently attach a ³²P-phosphate to the 3′ end of a DNA oligonucleotide of arbitrary sequence, relying on inexpensive DNA oligonucleotide modifications (2′-O-methylribonucleotide and ribonucleotide sugar substitutions), two enzymes (T4 polynucleotide kinase and T4 RNA ligase 2), and the differential susceptibility of DNA and RNA to hydroxide treatment. Radioactive probe molecules produced by this protocol are homogeneous and oxidant-compatible, and they can be used for precise cleavage-site mapping in the context of both DNase enzyme characterization and DNA footprinting assays.

Graphic abstract

Yeast are an ideal system to study Heat Shock Protein 70 (Hsp70) function in a cellular context. This protocol was generated to analyze the function of non-native Hsp70 proteins by expressing them as the sole cytosolic Hsp70 in yeast. As an initial step, Hsp70 variants (such as Ssa1 point mutants and non-yeast versions such as Nematostella vectensis NvHsp70A, B and D) are cloned into an appropriate expression plasmid. Next, these plasmids are transformed into ssa1-4∆ yeast [expressing native Ssa1 from an uracil-based (URA3) plasmid] which are subsequently cured of the original yeast on 5-Fluroorotic Acid (5-FOA). The resulting cells can be screened for a variety of phenotypes which match to the activity of well-studied cellular pathways.

Circadian rhythms are biological processes displaying an endogenous oscillation with a period of ~24 h. They allow organisms to anticipate and get prepared for the environmental changes caused mainly by the rotation of Earth. Circadian rhythms are driven by circadian clocks that consist of proteins, DNA, and/or RNA. Circadian clocks of cyanobacteria are the simplest and one of the best studied models. They contain the three clock proteins KaiA, KaiB, and KaiC which can be used for in vitro reconstitution experiments and determination of the auto-phosphatase activity of KaiC as described in this protocol.

This is a detailed protocol of an autophosphorylation and phosphotransfer activities of Synechocystis sp. PCC 6803 full-length Histidine Kinase 2 (Hik2) protein described by Ibrahim et al., 2016. In this protocol, radioactively labelled ATP was used to study an autophosphorylation and phosphotransfer activity of the full-length Hik2 protein.

The enzyme γ-glutamyltranspeptidase (GGT, EC 2.3.2.2) is highly conserved among eukaryotic and prokaryotic organisms (Heisterkamp et al., 2008) and has a key function in glutathione metabolism. Although the enzyme is highly conserved and found throughout organisms ranging from bacteria to plants and animals several major difference between eukaryotic and prokaryotic GGT can be noticed. They mainly concern the enzyme localization and posttranslational modification. Eukaryotic GGT is cell membrane anchored and highly glycosylated whereas prokaryotic GGT does not undergo this posttranslational modification and is a soluble periplasmic protein. GGT amino acids sequences of diverse origin exhibit high amino acid similarity (Ong et al., 2008). The prokaryotic GGT enzymes are produced as proenzyme, equipped with a typical prokaryotic signal sequence and transported through the inner membrane into the periplasm where the enzyme undergoes autocatalytic cleavage. This proteolysis yields a mature dimer which transfers the γ-glutamyl moieties from extracellular glutathione and related compounds to amino acids or peptides (Hanigan et al., 1998). The GGT enzyme activity can be easily measured as it catalyzes the transfer of a γ-glutamyl group from a colorless substrate, L-γ-glutamyl-3-carboxy-4-nitroanilide, to the acceptor, glycylglycine with leads to the production of yellow colored product, p-nitroaniline (Figure 1) which can be measured by a spectrophotometer (Figure 2). Here we describe a protocol to measure the GGT activity in the Gram-negative bacterium Campylobacter jejuni, with some minor modifications; this protocol also works for other Gram-negative bacterial species.


Figure 1. Yellow colored product, p-nitroaniline formed during the GGT enzyme assay


Figure 2. Spectral curve of pNA in Tris/HCl buffer, recorded on a Biodrop µLite (Isogen)