Biomimetic α-selective ribosylation enables two-step modular synthesis of biologically important ADP-ribosylated peptides

Uncommon posttranslational modifications in proteomics: ADP-ribosylation, tyrosine nitration, and tyrosine sulfation

Posttranslational modifications (PTMs) are covalent modifications of proteins that modulate the structure and functions of proteins and regulate biological processes. The development of various mass spectrometry-based proteomics workflows has facilitated the identification of hundreds of PTMs and aided the understanding of biological significance in a high throughput manner. Improvements in sample preparation and PTM enrichment techniques, instrumentation for liquid chromatography-tandem mass spectrometry (LC-MS/MS), and advanced data analysis tools enhance the specificity and sensitivity of PTM identification. Highly prevalent PTMs like phosphorylation, glycosylation, acetylation, ubiquitinylation, and methylation are extensively studied. However, the functions and impact of less abundant PTMs are not as well understood and underscore the need for analytical methods that aim to characterize these PTMs. This review focuses on the advancement and analytical challenges associated with the characterization of three less common but biologically relevant PTMs, specifically, adenosine diphosphate-ribosylation, tyrosine sulfation, and tyrosine nitration. The advantages and disadvantages of various enrichment, separation, and MS/MS techniques utilized to identify and localize these PTMs are described.

Four of a Kind: A Complete Collection of ADP-Ribosylated Histidine Isosteres Using Cu(I)- and Ru(II)-Catalyzed Click Chemistry

Adenosine diphosphate ribosylation (ADP-ribosylation) is a crucial post-translational modification involved in important regulatory mechanisms of numerous cellular pathways including histone maintenance and DNA damage repair. To study this modification, well-defined ADP-ribosylated peptides, proteins, and close analogues thereof have been invaluable tools. Recently, proteomics studies have revealed histidine residues to be ADP-ribosylated. We describe here the synthesis of a complete set of triazole-isosteres of ADP-ribosylated histidine to serve as probes for ADP-ribosylating biomachinery. By exploiting Cu(I)- and Ru(II)-catalyzed click chemistry between a propargylglycine building block and an α- or β-configured azidoribose, we have successfully assembled the α- and β-configured 1,4- and 1,5-triazoles, mimicking N(τ)- and N(π)-ADP-ribosylated histidine, respectively. The ribosylated building blocks could be incorporated into a peptide sequence using standard solid-phase peptide synthesis and transformed on resin into the ADP-ribosylated fragments to provide a total of four ADP-ribosyl triazole conjugates, which were evaluated for their chemical and enzymatic stability. The 1,5-triazole analogues mimicking the N(π)-substituted histidines proved susceptible to base-induced epimerization and the ADP-ribosyl α-1,5-triazole linkage could be cleaved by the (ADP-ribosyl)hydrolase ARH3.

Mimetics of ADP-Ribosylated Histidine through Copper(I)-Catalyzed Click Chemistry

A convergent synthesis provided nearly perfect τ-ADP-ribosylated histidine isosteres (His*-τ-ADPr) via a copper(I)-catalyzed cycloaddition between an azido-ADP-ribosyl analogue and an oligopeptide carrying a propargyl glycine. Both α- and β-configured azido-ADP-ribosyl analogues have been synthesized. The former required participation of the C-2 ester functionality during glycosylation, while the latter was obtained in high stereoselectivity from an imidate donor with a nonparticipating para-methoxy benzyl ether. Four His*-τ-ADPr peptides were screened against a library of human ADP-ribosyl hydrolases.

Serine ADP-ribosylation marks nucleosomes for ALC1-dependent chromatin remodeling

Serine ADP-ribosylation (ADPr) is a DNA damage-induced post-translational modification catalyzed by the PARP1/2:HPF1 complex. As the list of PARP1/2:HPF1 substrates continues to expand, there is a need for technologies to prepare mono- and poly-ADP-ribosylated proteins for biochemical interrogation. Here, we investigate the unique peptide ADPr activities catalyzed by PARP1 in the absence and presence of HPF1. We then exploit these activities to develop a method that facilitates installation of ADP-ribose polymers onto peptides with precise control over chain length and modification site. Importantly, the enzymatically mono- and poly-ADP-ribosylated peptides are fully compatible with protein ligation technologies. This chemoenzymatic protein synthesis strategy was employed to assemble a series of full-length, ADP-ribosylated histones and show that ADPr at histone H2B serine 6 or histone H3 serine 10 converts nucleosomes into robust substrates for the chromatin remodeler ALC1. We found ALC1 preferentially remodels 'activated' substrates within heterogeneous mononucleosome populations and asymmetrically ADP-ribosylated dinucleosome substrates, and that nucleosome serine ADPr is sufficient to stimulate ALC1 activity in nuclear extracts. Our study identifies a biochemical function for nucleosome serine ADPr and describes a new, highly modular approach to explore the impact that site-specific serine mono- and poly-ADPr have on protein function.

Accelerating Reaction Rates of Biomolecules by Using Shear Stress in Artificial Capillary Systems

Biomimetics is a design principle within chemistry, biology, and engineering, but chemistry biomimetic approaches have been generally limited to emulating nature's chemical toolkit while emulation of nature's physical toolkit has remained largely unexplored. To begin to explore this, we designed biophysically mimetic microfluidic reactors with characteristic length scales and shear stresses observed within capillaries. We modeled the effect of shear with molecular dynamics studies and showed that this induces specific normally buried residues to become solvent accessible. We then showed using kinetics experiments that rates of reaction of these specific residues in fact increase in a shear-dependent fashion. We applied our results in the creation of a new microfluidic approach for the multidimensional study of cysteine biomarkers. Finally, we used our approach to establish dissociation of the therapeutic antibody trastuzumab in a reducing environment. Our results have implications for the efficacy of existing therapeutic antibodies in blood plasma as well as suggesting in general that biophysically mimetic chemistry is exploited in biology and should be explored as a research area.

Integrative Chemical Biology Approaches to Deciphering the Histone Code: A Problem-Driven Journey

The hereditary blueprint of a eukaryotic cell is encoded in its genomic DNA that is tightly compacted into chromatin together with histone proteins. The basic repeating units of chromatin fibers are nucleosomes, in which approximately 1.7 turns of DNA wrap around a proteinaceous octamer consisting of two copies of histones H2A, H2B, H3, and H4. Histones are extensively decorated by a variety of posttranslational modifications (PTMs, e.g., methylation, acetylation, ubiquitylation, phosphorylation, etc.), serving as one of the cellular mechanisms that regulates DNA-templated processes, including but not limited to gene transcription, DNA replication, and DNA damage repair. Most of the histone PTMs exist in dynamic fluctuations, and their on and off states are exquisitely regulated by enzymes known as "writers" and "erasers", respectively. When installed at certain sites, histone PTMs can change the local physicochemical environment and thereby directly influence the nucleosome and chromatin structures. Alternatively, histone PTMs can recruit effectors (or "readers") to signal the downstream events. A "histone code" hypothesis has been proposed in which the combinatory actions of different histone PTMs orchestrate the epigenetic landscape of cells, modulating the activity of the underlying DNA and maintaining the genome stability between generations. Accumulating evidence also suggests that malfunctions of histone PTMs are associated with the pathogenesis of human diseases, such as cancer. It is therefore important to fully decipher the histone code, namely, to dissect the regulatory mechanisms and biological functions of histone PTMs.Owing to the advances in state-of-the-art mass spectrometry, dozens of novel histone modifications have been archived during the past decade. However, most of these newly identified histone PTMs remain poorly explored. To unravel the roles played by these PTMs in histone code, key questions that have driven our study are (i) how to detect the novel histone PTMs; (ii) how to identify the enzymes that catalyze the addition (writers) and removal (erasers) of the histone PTMs along with the regulating mechanisms; (iii) what is the biological significance of the histone PTMs and how do they function, by affecting the nucleosome and chromatin dynamics or by recruiting readers; and (iv) how to develop chemical probes to interrogate the histone PTMs or even serve as potential leads for the drug discovery campaigns to treat diseases caused by abnormalities in the regulation of histone PTMs.This Account focuses on our efforts in developing and applying chemical tools and methods to answer the above questions. Specifically, we review the detection of negatively charged histone acylations by developing and applying chemical reporters; preparing homogeneous nucleosomes carrying negatively charged acylations by protein chemistry approaches and the in vitro biophysical analyses of the effects of the acylations on nucleosome structures; investigating the negatively charged acylations' influence on chromatin dynamics in vivo using yeast genetic approaches; identifying and characterizing protein-protein interactions (PPIs) mediated by histone PTMs in different biological contexts (i.e., to identify the readers and erasers) by establishing a chemical proteomics platform that is enabled by photo-cross-linking chemistry and quantitative proteomics strategies; and manipulating PTM-mediated PPIs by the structure-guided design of inhibitors. We also discuss possible future directions in our journey to fully decipher the histone code.

Uncovering the Invisible: Mono-ADP-ribosylation Moved into the Spotlight

Adenosine diphosphate (ADP)-ribosylation is a nicotinamide adenine dinucleotide (NAD+)-dependent post-translational modification that is found on proteins as well as on nucleic acids. While ARTD1/PARP1-mediated poly-ADP-ribosylation has extensively been studied in the past 60 years, comparably little is known about the physiological function of mono-ADP-ribosylation and the enzymes involved in its turnover. Promising technological advances have enabled the development of innovative tools to detect NAD+ and NAD+/NADH (H for hydrogen) ratios as well as ADP-ribosylation. These tools have significantly enhanced our current understanding of how intracellular NAD dynamics contribute to the regulation of ADP-ribosylation as well as to how mono-ADP-ribosylation integrates into various cellular processes. Here, we discuss the recent technological advances, as well as associated new biological findings and concepts.