One-Pot Quantitative Top- and Middle-Down Analysis of GluC-Digested Histone H4

How to Deal With Internal Fragment Ions?

Tandem mass spectrometry of peptides and proteins generates 3mass spectra of their gas-phase fragmentation product ions, including N-terminal, C-terminal, and internal fragment ions. While N- and C-terminal ions are routinely assigned and identified using computational methods, internal fragment ions are often difficult to annotate correctly. They become particularly relevant for long peptides and full proteoforms where the peptide backbone is more likely to be fragmented multiple times. Internal fragment ions potentially offer tremendous information regarding amino acid sequences and positions of post-translational modifications of peptides and intact proteins. However, their practical application is challenged by the vast number of theoretical internal fragments that exist for long amino acid sequences, leading to a high risk of false-positive annotations. We analyze the mass spectral contributions of internal fragment ions in spectra from middle-down and top-down experiments and introduce a novel graph-based annotation approach designed to manage the complexity of internal fragments. Our graph-based representation allows us to compare multiple candidate proteoforms in a single graph, and to assess different candidate annotations in a fragment ion spectrum. We demonstrate cases from middle-down and top-down data where internal ions enhance amino acid sequence coverage of polypeptides and proteins and accurate localization of post-translational modifications. We conclude that our graph-based method provides a general approach to process complex tandem mass spectra, enhance annotation of internal fragment ions, and improve proteoform sequencing and characterization by mass spectrometry.

Filling the gaps in peptide maps with a platform assay for top-down characterization of purified protein samples

Liquid chromatography-mass spectrometry (LC-MS) intact mass analysis and LC-MS/MS peptide mapping are decisional assays for developing biological drugs and other commercial protein products. Certain PTM types, such as truncation and oxidation, increase the difficulty of precise proteoform characterization owing to inherent limitations in peptide and intact protein analyses. Top-down MS (TDMS) can resolve this ambiguity via fragmentation of specific proteoforms. We leveraged the strengths of flow-programmed (fp) denaturing online buffer exchange (dOBE) chromatography, including robust automation, relatively high ESI sensitivity, and long MS/MS window time, to support a TDMS platform for industrial protein characterization. We tested data-dependent (DDA) and targeted strategies using 14 different MS/MS scan types featuring combinations of collisional- and electron-based fragmentation as well as proton transfer charge reduction. This large, focused dataset was processed using a new software platform, named TDAcquireX, that improves proteoform characterization through TDMS data aggregation. A DDA-based workflow provided objective identification of αLac truncation proteoforms with a two-termini clipping search. A targeted TDMS workflow facilitated the characterization of αLac oxidation positional isomers. This strategy relied on using sliding window-based fragment ion deconvolution to generate composite proteoform spectral match (cPrSM) results amenable to fragment noise filtering, which is a fundamental enhancement relevant to TDMS applications generally.

Histone variant-specific post-translational modifications

Post-translational modifications (PTMs) of histones play a key role in DNA-based processes and contribute to cell differentiation and gene function by adding an extra layer of regulation. Variations in histone sequences within each family of histones expands the chromatin repertoire and provide further mechanisms for regulation and signaling. While variants are known to be present in certain genomic loci and carry out important functions, much remains unknown about variant-specific PTMs and their role in regulating chromatin. This ambiguity is in part due to the limited technologies and appropriate reagents to identify and quantitate variant-specific PTMs. Nonetheless, histone variants are an integral portion of the chromatin system and the understanding of their modifications and resolving how PTMs function differently on specific variants is paramount to the advancement of the field. Here we review the current knowledge on post-translational modifications specific to histone variants, with an emphasis on well-characterized PTMs of known function. While not every possible PTM is addressed, we present key variant-specific PTMs and what is known about their function and mechanisms in convenient reference tables.

High-throughput profiling of histone post-translational modifications and chromatin modifying proteins by reverse phase protein array

Epigenetic variation plays a significant role in normal development and human diseases including cancer, in part through post-translational modifications (PTMs) of histones. Identification and profiling of changes in histone PTMs, and in proteins regulating PTMs, are crucial to understanding diseases, and for discovery of epigenetic therapeutic agents. In this study, we have adapted and validated an antibody-based reverse phase protein array (RPPA) platform for profiling 20 histone PTMs and expression of 40 proteins that modify histones and other epigenomic regulators. The specificity of the RPPA assay for histone PTMs was validated with synthetic peptides corresponding to histone PTMs and by detection of histone PTM changes in response to inhibitors of histone modifier proteins in cell cultures. The useful application of the RPPA platform was demonstrated with two models: induction of pluripotent stem cells and a mouse mammary tumor progression model. Described here is a robust platform that includes a rapid microscale method for histone isolation and partially automated workflows for analysis of histone PTMs and histone modifiers that can be performed in a high-throughput manner with hundreds of samples. This RPPA platform has potential for translational applications through the discovery and validation of epigenetic states as therapeutic targets and biomarkers. SIGNIFICANCE: Our study has established an antibody-based reverse phase protein array platform for global profiling of a wide range of post-translational modifications of histones and histone modifier proteins. The high-throughput platform provides comprehensive analyses of epigenetics for biological research and disease studies and may serve as screening assay for diagnostic purpose or therapy development.

A dynamic and combinatorial histone code drives malaria parasite asexual and sexual development

Histone posttranslational modifications (PTMs) frequently co-occur on the same chromatin domains or even in the same molecule. It is now established that these “histone codes” are the result of cross talk between enzymes that catalyze multiple PTMs with univocal readout as compared with these PTMs in isolation. Here, we performed a comprehensive identification and quantification of histone codes of the malaria parasite, Plasmodium falciparum. We used advanced quantitative middle-down proteomics to identify combinations of PTMs in both the proliferative, asexual stages and transmissible, sexual gametocyte stages of P. falciparum. We provide an updated, high-resolution compendium of 77 PTMs on H3 and H3.3, of which 34 are newly identified in P. falciparum. Coexisting PTMs with unique stage distinctions were identified, indicating that many of these combinatorial PTMs are associated with specific stages of the parasite life cycle. We focused on the code H3R17me2K18acK23ac for its unique presence in mature gametocytes; chromatin proteomics identified a gametocyte-specific SAGA-like effector complex including the transcription factor AP2-G2, which we tied to this specific histone code, as involved in regulating gene expression in mature gametocytes. Ultimately, this study unveils previously undiscovered histone PTMs and their functional relationship with coexisting partners. These results highlight that investigating chromatin regulation in the parasite using single histone PTM assays might overlook higher-order gene regulation for distinct proliferation and differentiation processes.

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First middle-down chromatin proteomics compendium of the malaria parasite, Plasmodium falciparum.

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Novel histone PTMs (including arginine methylation) in both asexual parasites and transmissible gametocytes.

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Histone PTM cross talk is dynamic life cycle stage stratified.

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Gametocytes rely on histone PTM connectivity to allow onward transmission.

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AP2-G2 is an important effector of H3K18acK23ac in mature gametocytes.

An Improved Top-Down Mass Spectrometry Characterization of Chlamydomonas reinhardtii Histones and Their Post-translational Modifications

We present an updated analysis of the linker and core histone proteins and their proteoforms in the green microalga Chlamydomonas reinhardtii by top-down mass spectrometry (TDMS). The combination of high-resolution liquid chromatographic separation, robust fragmentation, high mass spectral resolution, the application of a custom search algorithm, and extensive manual analysis enabled the characterization of 86 proteoforms across all four core histones H2A, H2B, H3, and H4 and the linker histone H1. All canonical H2A paralogs, which vary in their C-termini, were identified, along with the previously unreported noncanonical variant H2A.Z that had high levels of acetylation and C-terminal truncations. Similarly, a majority of the canonical H2B paralogs were identified, along with a smaller noncanonical variant, H2B.v1, that was highly acetylated. Histone H4 exhibited a novel acetylation profile that differs significantly from that found in other organisms. A majority of H3 was monomethylated at K4 with low levels of co-occuring acetylation, while a small fraction of H3 was trimethylated at K4 with high levels of co-occuring acetylation.

Expeditious Extraction of Histones from Limited Cells or Tissue Samples and Quantitative Top-Down Proteomic Analysis

Histones are the primary protein component of chromatin and are involved in virtually all DNA-templated processes. Histones are abundantly post-translationally modified by a variety of chromatin-modifying machinery. These post-translational modifications (PTMs) are recognized by a range of "reader" proteins, which recruit additional proteins to specific locations on chromatin and impart precise and powerful effects on gene regulation. Each PTM typically exerts a positive or negative effect on transcription, and recent studies have shown that histone PTMs function in a combinatorial histone code: that is, histone PTMs function in combination to exert precise DNA-templated regulation. Thus, there is a need to identify and understand proteoforms, or unambiguously defined single protein molecules with all combinations of modifications. Top-down proteomics is currently the only viable approach for identifying and quantitating histone proteoforms, and mass spectrometry instruments have become sufficiently powerful to perform these quantitative analyses in a robust and high-throughput fashion. These recent innovations have enabled new experimental directions in chromatin research but have also introduced temporal and other constraints. This has led us to develop the protocols described here, which increase throughput, reduce sample requirements, and maintain robust quantitation. Although originally designed for high-throughput quantitative top-down proteomics, the protocols described here are useful for a wide range of chromatin biology applications. Starting with small amounts of cells or tissue, we describe two basic protocols for exceptionally rapid and efficient nuclei isolation, acid extraction of histones, and high-performance liquid chromatography fractionation of histones into histone families. We additionally describe the quantitative top-down proteomic analysis of histone H4 proteoforms. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Nuclei isolation and acid extraction of histones from mammalian cells in culture/tissues Basic Protocol 2: HPLC fractionation of histones and histone H4 HPLC-MS/MS Support Protocol: Preparation of intact H3 histone tails by Glu-C digestion.

Capillary Zone Electrophoresis-Tandem Mass Spectrometry As an Alternative to Liquid Chromatography-Tandem Mass Spectrometry for Top-down Proteomics of Histones

Top-down proteomics (TDP) is an ideal approach for deciphering the histone code and it routinely employs reversed-phase liquid chromatography (RPLC)-tandem mass spectrometry (MS/MS). Because of the extreme complexity of histones regarding the number of proteoforms, new analytical tools with high-capacity separation and highly sensitive detection of proteoforms are required for TDP of histones. Here we present capillary zone electrophoresis (CZE)-MS/MS via the electro-kinetically pumped sheath-flow CE-MS interface for large-scale top-down delineation of histone proteoforms. CZE-MS/MS identified a comparable number of proteoforms to RPLC-MS/MS from a calf histone sample with more than 30-fold less sample consumption (75-ng vs. Three μg), indicating its substantially higher sensitivity. We identified about 400 histone proteoforms from the calf histone sample using two-dimensional size-exclusion chromatography (SEC)-CZE-MS/MS with less than 300-ng proteins consumed. We identified histone proteoforms carrying various tentative post-translational modifications (PTMs), for example, acetylation, methylation (mono-, di-, and tri-), phosphorylation, and succinylation. The electrophoretic mobility (μef) of unmodified histone proteoforms can be predicted accurately (R2 = 0.98) with an optimized semiempirical model based on our recent work. The results render CZE-MS/MS as a useful tool for deciphering the histone code in a proteoform-specific manner and on a global scale.

Investigation of Neutral Losses and the Citrulline Effect for Modified H4 N-Terminal Pentapeptides

Tandem mass spectrometry is an indispensable tool in proteomics used for protein sequencing and quantitation. On the basis of the sequential fragments usually generated from peptide ions via collision-induced dissociation, electron-transfer dissociation, or a combination of the two, probabilistic database search engines could be used for the identification of the peptides. The correct localization of posttranslational modifications (PTMs) poses a more challenging problem than the general identification of proteins. Histones are involved in the regulation of DNA transcription via the wealth of PTMs on their N-terminal tail. In this study, we analyzed the histone H4 peptide SGRGK incorporating four different posttranslational modifications: citrullination, acetylation, phosphorylation, and arginine methylation at various positions. The pentapeptides model the enzymatic cleavage of the N-terminal tail of human histone H4 protein by LysC protease. Fragmentation of the peptides was investigated using higher-energy collisional dissociation (HCD), electron-transfer dissociation (ETD), and electron-transfer higher-energy collisional dissociation (EThcD) on an ultrahigh resolution and mass accuracy instrument. We found that while all three techniques have their unique characteristics, advantages, and pitfalls, EThcD generated the most fragment ion-rich spectra. Despite potential ambiguities regarding exact fragment identities, full sequence coverage and PTM mapping may also be achievable. We also found novel neutral losses from the charge-reduced precursors characteristic to citrullination in ETD and EThcD which may be used in proteomic applications. N-Terminal acetylation and arginine methylation could also be confirmed by their characteristic neutral losses from the charge-reduced precursors.

High-Throughput Quantitative Top-Down Proteomics: Histone H4

Proteins physiologically exist as "proteoforms" that arise from one gene and acquire additional function by post-translational modifications (PTM). When multiple PTMs coexist on single protein molecules, top-down proteomics becomes the only feasible method of characterization; however, most top-down methods have limited quantitative capacity and insufficient throughput to truly address proteoform biology. Here we demonstrate that top-down proteomics can be quantitative, reproducible, sensitive, and high throughput. The proteoforms of histone H4 are well studied both as a challenging proteoform identification problem and due to their essential role in the regulation of all eukaryotic DNA-templated processes. Much of histone H4's function is obfuscated from prevailing methods due to combinatorial mechanisms. Starting from cells or tissues, after an optimized protein purification process, the H4 proteoforms are physically separated by on-line C3 chromatography, narrowly isolated in MS1 and sequenced with ETD fragmentation. We achieve more than 30 replicates from a single 35-mm tissue culture dish by loading 55 ng of H4 on column. Parallelization and automation yield a sustained throughput of 12 replicates per day. We achieve reproducible quantitation (average biological Pearson correlations of 0.89) of hundreds of proteoforms (about 200-300) over almost six orders of magnitude and an estimated LLoQ of 0.001% abundance. We demonstrate the capacity of the method to precisely measure well-established changes with sodium butyrate treatment of SUM159 cells. We show that the data produced by a quantitative top-down method can be amenable to parametric statistical comparisons and is capable of delineating relevant biological changes at the full proteoform level.

A Multiple Protease Strategy to Optimise the Shotgun Proteomics of Mature Medicinal Cannabis Buds

Earlier this year we published a method article aimed at optimising protein extraction from mature buds of medicinal cannabis for trypsin-based shotgun proteomics (Vincent, D., et al. Molecules2019, 24, 659). We then developed a top-down proteomics (TDP) method (Vincent, D., et al. Proteomes2019, 7, 33). This follow-up study aims at optimising the digestion of medicinal cannabis proteins for identification purposes by bottom-up and middle-down proteomics (BUP and MDP). Four proteases, namely a mixture of trypsin/LysC, GluC, and chymotrypsin, which target different amino acids (AAs) and therefore are orthogonal and cleave proteins more or less frequently, were tested both on their own as well as sequentially or pooled, followed by nLC-MS/MS analyses of the peptide digests. Bovine serum albumin (BSA, 66 kDa) was used as a control of digestion efficiency. With this multiple protease strategy, BSA was reproducibly 97% sequenced, with peptides ranging from 0.7 to 6.4 kD containing 5 to 54 AA residues with 0 to 6 miscleavages. The proteome of mature apical buds from medicinal cannabis was explored more in depth with the identification of 27,123 peptides matching 494 unique accessions corresponding to 229 unique proteins from Cannabis sativa and close relatives, including 130 (57%) additional annotations when the list is compared to that of our previous BUP study (Vincent, D., et al. Molecules2019, 24, 659). Almost half of the medicinal cannabis proteins were identified with 100% sequence coverage, with peptides composed of 7 to 91 AA residues with up to 9 miscleavages and ranging from 0.6 to 10 kDa, thus falling into the MDP domain. Many post-translational modifications (PTMs) were identified, such as oxidation, phosphorylations, and N-terminus acetylations. This method will pave the way for deeper proteome exploration of the reproductive organs of medicinal cannabis, and therefore for molecular phenotyping within breeding programs.