Identification of deubiquitinase targets of isothiocyanates using SILAC-assisted quantitative mass spectrometry.

Cruciferous vegetables such as broccoli and kale have well documented chemopreventative and anticancer effects that are attributed to the presence of isothiocyanates (ITCs). ITCs modulate the levels of many oncogenic proteins, but the molecular mechanisms of ITC action are not understood. We previously reported that phenethyl isothiocyanate (PEITC) inhibits two deubiquitinases (DUBs), USP9x and UCH37. DUBs regulate many cellular processes and DUB dysregulation is linked to the pathogenesis of human diseases including cancer, neurodegeneration, and inflammation. Using SILAC assisted quantitative mass spectrometry, here we identify 9 new PEITC-DUB targets: USP1, USP3, USP10, USP11, USP16, USP22, USP40, USP48 and VCPIP1. Seven of these PEITC-sensitive DUBs have well-recognized roles in DNA repair or chromatin remodeling. PEITC both inhibits USP1 and increases its ubiquitination and degradation, thus decreasing USP1 activity by two mechanisms. The loss of USP1 activity increases the level of mono-ubiquitinated DNA clamp PCNA, impairing DNA repair. Both the inhibition/degradation of USP1 and the increase in mono-ubiquitinated PCNA are new activities for PEITC that can explain the previously recognized ability of ITCs to enhance cancer cell sensitivity to cisplatin treatment. Our work also demonstrates that PEITC reduces the mono-ubiquityl histones H2A and H2B. Understanding the mechanism of action of ITCs should facilitate their use as therapeutic agents.

Covalent protein modification: the current landscape of residue-specific electrophiles.

Functional amino acids that play critical roles in catalysis and regulation are known to display elevated nucleophilicity and can be selectively targeted for covalent modification by reactive electrophiles. Chemical-proteomic platforms, such as activity-based protein profiling (ABPP), exploit this reactivity by utilizing chemical probes to covalently modify active-site residues to inform on the functional state of enzymes within complex proteomes. These and other applications rely on the availability of a diverse array of electrophiles and detailed knowledge of the reactivity and amino-acid selectivity of these groups. Here, we survey the current landscape of electrophiles that covalently target various nucleophilic amino acids in proteins and highlight proteomic applications that have benefited from the unique properties of these electrophiles.

Investigating the proteome reactivity and selectivity of aryl halides.

Protein-reactive electrophiles are critical to chemical proteomic applications including activity-based protein profiling, site-selective protein modification, and covalent inhibitor development. Here, we explore the protein reactivity of a panel of aryl halides that function through a nucleophilic aromatic substitution (S(N)Ar) mechanism. We show that the reactivity of these electrophiles can be finely tuned by varying the substituents on the aryl ring. We identify p-chloro- and fluoronitrobenzenes and dichlorotriazines as covalent protein modifiers at low micromolar concentrations. Interestingly, investigating the site of labeling of these electrophiles within complex proteomes identified p-chloronitrobenzene as highly cysteine selective, whereas the dichlorotriazine favored reactivity with lysines. These studies illustrate the diverse reactivity and amino-acid selectivity of aryl halides and enable the future application of this class of electrophiles in chemical proteomics.

Orphan PTMs: Rare, yet functionally important modifications of cysteine.

The enhanced nucleophilicity and redox sensitivity of the thiol group renders cysteine residues susceptible to numerous electrophilic and oxidative post-translational modifications to form disulfides, sulfenic acids, nitrosothiols, and lipid-modified species. Outside of these well-characterized modifications of cysteine, there are reports of cysteine modification through phosphorylation, methylation, and ubiquitination. Although these post-translational modifications are highly abundant on other amino acids, they play a less pervasive role in cysteine biology. Despite the rarity of these modifications of cysteine, they have been shown to play critical roles in catalysis and regulation. Here we describe these rare post-translational modifications of cysteine in detail, by describing their discovery and functional characterization on diverse proteins. Furthermore, we highlight potential proteomic tools that may aid in globally identifying these modifications to fully elucidate their abundance in biological systems.

Chemoproteomic discovery of cysteine-containing human short open reading frames.

The application of ribosome profiling and mass spectrometry technologies has recently revealed that the human proteome is larger than previously appreciated. Short open reading frames (sORFs), which are difficult to identify using traditional gene-finding algorithms, constitute a significant fraction of unknown protein-coding genes. Thus, experimental approaches to identify sORFs provide invaluable insight into the protein-coding potential of genomes. Here, we report an affinity-based approach to enrich and identify cysteine-containing human sORF-encoded polypeptides (ccSEPs) from cells. This approach revealed 16 novel ccSEPs, each derived from an uncharacterized sORF, demonstrating its potential for discovering new genes. We validated expression of a SEP from its endogenous RNA, and demonstrated the specificity of our labeling approach using synthetic SEP. The discovery of additional human SEPs and their conservation indicate the potential importance of these molecules in biology.

Chemical proteomics with sulfonyl fluoride probes reveals selective labeling of functional tyrosines in glutathione transferases.

Chemical probes have great potential for identifying functional residues in proteins in crude proteomes. Here we studied labeling sites of chemical probes based on sulfonyl fluorides (SFs) on plant and animal proteomes. Besides serine proteases and many other proteins, SF-based probes label Tyr residues in glutathione transferases (GSTs). The labeled GSTs represent four different GST classes that share less than 30% sequence identity. The targeted Tyr residues are located at similar positions in the promiscuous substrate binding site and are essential for GST function. The high selectivity of SF-based probes for functional Tyr residues in GSTs illustrates how these probes can be used for functional studies of GSTs and other proteins in crude proteomes.

Sulfonyl fluoride analogues as activity-based probes for serine proteases.

Enriched with fluoride: To expand on the available tools to interrogate proteases, we explored sulfonyl fluorides as activity-based probes. An alkyne-tagged sulfonyl fluoride covalently modifies members of the S1 family of serine proteases. By applying click chemistry, avidin enrichment and mass spectrometry, we can enrich and identify active endogenous serine proteases from a complex proteome.