Top-down protein identification using isotopic envelope fingerprinting

Structural N-glycoproteomics characterization of cell-surface N-glycosylation of MCF-7/ADR cancer stem cells

Breast cancer is responsible for the highest mortality all over the world. Cancer stem cells (CSCs) along with epithelial mesenchymal transition (EMT) are identified as a driver of cancer which are responsible for cancer metastasis and drug resistance. Several signaling pathways are associated with drug resistance. Additionally, glycosyltransferases regulate different types of glycosylation which are involved in drug resistance. To the end, it is urgent to figure out the knowledge on cell-surface altered N-glycosylation and putative markers. Here, differential cell-surface intact N-glycopeptides in adriamycin (ADR)-resistant michigan breast cancer foundation-7 stem cells (MCF-7/ADR CSCs) relative to ADR-sensitive MCF-7 CSCs were analyzed with site- and structure-specific quantitative N-glycoproteomics. The intact N-glycopeptides and differentially expressed intact N-glycopeptides (DEGPs) were determined and quantified via intact N-glycopeptide search engine GPSeeker. Totally, 4777 intact N-glycopeptides were identified and N-glycan sequence structures among 2764 IDs were distinguished from their isomers by structure-diagnostic fragment ions. Among 1717 quantified intact N-glycopeptides, 104 DEGPs were determined (fold change ≥ 1.5 and p value < 0.05). Annotation of protein-protein interaction and biological processes among others of DEGPs were finally carried out; down-regulated intact N-glycopeptide with bisecting GlcNAc from p38-interacting protein and up-regulated intact N-glycopeptide with β1,6-branching N-glycan from integrin beta-5 were found.

Proteoform Identification and Quantification Using Intact Protein Database Search Engine ProteinGoggle

Proteomics studies the proteome of organisms, especially proteins that are differentially expressed under certain physiological or pathological conditions; qualitative identification of protein sequences and posttranslational modifications (PTMs) and their positions can help us systematically understand the structure and function of proteoforms. With the development and relative popularity of soft ionization technology (such as electrospray ionization technology) and high mass measurement accuracy and high-resolution mass spectrometers (such as orbitrap), the mass spectrometry (MS) characterization of complete proteins (the so-called top-down proteomics) has become possible and has gradually become popular. Corresponding database search engines and protein identification bioinformatics tools have also been greatly developed. This chapter provides a brief overview of intact protein database search algorithm "isotopic mass-to-charge ratio and envelope fingerprinting" and search engine ProteinGoggle.

Recent Advances in Analytical Approaches for Glycan and Glycopeptide Quantitation

Growing implications of glycosylation in physiological occurrences and human disease have prompted intensive focus on revealing glycomic perturbations through absolute and relative quantification. Empowered by seminal methodologies and increasing capacity for detection, identification, and characterization, the past decade has provided a significant increase in the number of suitable strategies for glycan and glycopeptide quantification. Mass-spectrometry-based strategies for glycomic quantitation have grown to include metabolic incorporation of stable isotopes, deposition of mass difference and mass defect isotopic labels, and isobaric chemical labeling, providing researchers with ample tools for accurate and robust quantitation. Beyond this, workflows have been designed to harness instrument capability for label-free quantification, and numerous software packages have been developed to facilitate reliable spectrum scoring. In this review, we present and highlight the most recent advances in chemical labeling and associated techniques for glycan and glycopeptide quantification.

Sensitive Top-Down Proteomics Analysis of a Low Number of Mammalian Cells Using a Nanodroplet Sample Processing Platform

Top-down proteomics is a powerful tool for characterizing genetic variations and post-translational modifications at intact protein level. However, one significant technical gap of top-down proteomics is the inability to analyze a low amount of biological samples, which limits its access to isolated rare cells, fine needle aspiration biopsies, and tissue substructures. Herein, we developed an ultrasensitive top-down platform by incorporating a microfluidic sample preparation system, termed nanoPOTS (nanodroplet processing in one pot for trace samples), into a top-down proteomic workflow. A unique combination of a nonionic detergent dodecyl-β-d-maltopyranoside (DDM) with urea as protein extraction buffer significantly improved both protein extraction efficiency and sample recovery. We hypothesize that the DDM detergent improves protein recovery by efficiently reducing nonspecific adsorption of intact proteins on container surfaces, while urea serves as a strong denaturant to disrupt noncovalent complexes and release intact proteins for downstream analysis. The nanoPOTS-based top-down platform reproducibly and quantitatively identified ∼170 to ∼620 proteoforms from ∼70 to ∼770 HeLa cells containing ∼10 to ∼115 ng of total protein. A variety of post-translational modifications including acetylation, myristoylation, and iron binding were identified using only less than 800 cells. We anticipate the nanoPOTS top-down proteomics platform will be broadly applicable in biomedical research, particularly where clinical specimens are not available in amounts amenable to standard workflows.

Separation and detection of minimal length glycopeptide neoantigen epitopes centering the GSTA region of MUC1 by liquid chromatography/mass spectrometry

RATIONALE

We previously isolated antibodies binding to glycopeptide neoantigen epitopes centering the GSTA sequence of the highly glycosylated tandem repeat region of MUC1. Epitopes centering the GSTA sequence are also predicted by NetMHC programs to bind to MHC molecules. Detecting MUC1 glycopeptide epitopes remains a challenge since antigenic epitopes are often shorter than 10 amino acids.

METHODS

In this study, we used pronase from Streptomyces griseus, which has no amino acid sequence preference for enzymatic cleavage sites, to digest synthetic glycopeptides RPAPGST (Tn)APPAHG and RPAPGS (Tn)TAPPAHG, and analyzed the digests by liquid chromatography/mass spectrometry (LC/MS) using electron transfer dissociation (ETD) and higher-energy collisional dissociation (HCD) methods with an Orbitrap Fusion Lumos Tribid mass spectrometer.

RESULTS

We found that short glycopeptides containing 8 to 11 amino acids could be efficiently generated by pronase digestion. Such glycopeptides of minimal epitope lengths were clearly distinguished by characteristic MS/MS ion patterns and LC elution profiles. A glycopeptide library was generated which may serve as a standard for measuring neoantigen epitopes centering the GSTA sequence.

CONCLUSIONS

Our data established the LC/MS/MS identities of a clinically relevant MUC1 glycopeptide neoantigen epitope centering the GSTA motif. A library of short MUC1 glycopeptides centered on the GSTA motif was created, which is a critical step for analysis of such antigen epitopes in real biological samples.

Mass spectrometry-based qualitative and quantitative N-glycomics: An update of 2017-2018

N-glycosylation is one of the most frequently occurring protein post-translational modifications (PTMs) with broad cellular, physiological and pathological relevance. Mass spectrometry-based N-glycomics has become the state-of-the-art instrumental analytical pipeline for sensitive, high-throughput and comprehensive characterization of N-glycans and N-glycomes. Improvement and new development of methods in N-glycan release, enrichment, derivatization, isotopic labeling, separation, ionization, MS, tandem MS and informatics accompany side-by-side wider and deeper application. This review provides a comprehensive update of mass spectrometry-based qualitative and quantitative N-glycomics in the years of 2017-2018.

A perspective view of top-down proteomics in snake venom research

The venom produced by snakes contains complex mixtures of pharmacologically active proteins and peptides which play a crucial role in the pathophysiology of snakebite diseases. The deep understanding of venom proteomes can help to improve the treatment of this "neglected tropical disease" (as expressed by the World Health Organization [WHO]) and to develop new drugs. The most widely used technique for venom analysis is liquid chromatography/tandem mass spectrometry (LC/MS/MS)-based bottom-up (BU) proteomics. Considering the fact that multiple multi-locus gene families encode snake venom proteins, the major challenge for the BU proteomics is the limited sequence coverage and also the "protein inference problem" which result in a loss of information for the identification and characterization of toxin proteoforms (genetic variation, alternative mRNA splicing, single nucleotide polymorphism [SNP] and post-translational modifications [PTMs]). In contrast, intact protein measurements with top-down (TD) MS strategies cover almost complete protein sequences, and prove the ability to identify venom proteoforms and to localize their modifications and sequence variations.

Top-down characterization of mouse core histones

Histone post-translational modifications (PTMs) play various roles in chromatin-related cellular processes, and comprehensive analysis of these combinatorial PTMs at the intact protein level by top-down proteomics is the method of choice to reveal their crosstalk and biological functions. Here, we report our top-down characterization of the core histones from mouse fibroblasts cells NIH/3T3, which is a classic model used in many kinds of research. With nanoRPLC-MS/MS analysis and ProteinGoggle database search, 547 protein species were identified with spectrum-level FDR ≤ 1%, where PTMs in 51 protein species were unambiguously localized with PTM scores ≥1. High-resolution MS/MS data also allowed the unambiguous identification of acetylation instead of trimethylation. This study presents a general picture of combinatorial PTMs of mouse core histones, which serves as a basic reference for all future related biological studies.

Selective fragmentation of the N-glycan moiety and protein backbone of ribonuclease B on an Orbitrap Fusion Lumos Tribrid mass spectrometer

RATIONALE

The functional study and application of an intact glycoprotein require the structural characterization of both the protein backbone and the glycan moiety; the former has been successfully demonstrated with selective fragmentation of the protein backbone in CID and ExD; whether the latter can be achieved with selective fragmentation of the glycan moiety remains to be explored.

METHODS

RNase B solution was electrosprayed and its intact glycoforms of GlcNAc₂ Man_n (n = 5-9) with the highest abundance (charge state z = 16) were isolated individually and fragmented using CID, ETD, HCD, ETciD, and EThcD on the Orbitrap Fusion Lumos Tribrid mass spectrometer; the dissociation parameters were optimized for selective fragmentation of the N-glycan moiety and protein backbone as well as high sequence coverage. The obtained spectra were interpreted using the protein and N-glycan database search engines ProteinGoggle and GlySeeker, respectively.

RESULTS

With exploration of different dissociation parameters for all the five methods, selective fragmentation of the N-glycan moiety (the protein backbone staying intact) was observed in both HCD and EThcD at low collisional energies, but only a few matched product ions were observed; more comprehensive fragmentation was observed at high collisional energies (the protein backbone lost). Selective protein backbone fragmentation was observed in all the five dissociation methods.

CONCLUSIONS

For comprehensive structural characterization of intact N-glycoproteins using tandem mass spectrometry, the composition and topology of the N-glycan moiety can be identified using HCD and EThcD complementarily at low and high energies; while the amino acid sequence and glycosite can be identified using CID, ETD, HCD, ETciD, and EThcD with their optimal dissociation parameters.

Top-down characterization of chicken core histones

Core histones and their PTMs play important roles in regulating gene transcription and other DNA-related processes. The study of core histones PTMs, their cross-talk and functional roles is not only of broad biological significance but also of wide pathological and clinical relevance. Having the strength of comprehensive proteoform identification with 100% amino acid sequence coverage and combinatorial PTMs, top-down proteomics has become the state-of-the-art analytical tool for combinatorial PTM characterization of core histones. In this study, we report our top-down characterization of chicken (Gallus gallus domesticus) core histones, which have been widely used as models for chromosome re-construction among others because of easy availability and not-so-dense PTMs. With nanoRPLC-MS/MS analysis and ProteinGoggle database search, a total of 58 proteoforms were identified for the core histone families of H4, H2B, H2A, and H3.

Large-scale identification and visualization of N-glycans with primary structures using GlySeeker

RATIONALE

Most of the current popular tandem mass spectrometers have the capability of resolving the primary structures (monosaccharide composition, sequence and linkage) of N-glycans; however, compositions or putative structures have mostly been reported so far. Identification and visualization tools of N-glycans are needed.

METHODS

The isotopic mass-to-charge ratio and envelope fingerprinting algorithm, which has been successfully used for intact protein database search and identification, was adapted for N-glycan database search, and a stand-alone N-glycan database search engine, GlySeeker, for automated N-glycan identification and visualization was developed and successfully benchmarked. Both pseudo 2D graph and one-line text formats with one-letter symbols for monosaccharides were proposed for representing N-glycans. N-glycans were identified with comprehensive interpretation of product ions and false discovery rate (FDR) control.

RESULTS

In a database search of reversed-phase liquid chromatography/tandem mass spectrometry (RPLC/MS/MS) datasets of the N-glycome enriched from OVCAR-3 ovarian cancer cells, with FDR ≤1% and number of best hits (NoBHs) = 1-30, 1525 N-glycans with comprehensive primary structural information (composition, sequence and linkage) were identified and visualized; among these 1525 N-glycans, 559 had NoBHs = 1, i.e. their structures were uniquely identified. This represents a large-scale identification and visualization of N-glycans with primary structures from tandem mass spectra.

CONCLUSIONS

A stand-alone N-glycan database search engine called GlySeeker has been developed for large-scale identification and visualization of N-glycans with comprehensive interpretation of tandem mass spectra and FDR control.

Accurate phosphorylation site localization using phospho-brackets

Phosphorylation is one of the most important and widely studied protein post-translational modifications. Tandem mass spectrometry using higher-energy collisional dissociation has evolved into a state-of-the-art analytical platform for both phosphorylation identification and site localization. Tens of thousands of phosphopeptides can now be routinely identified from a single shotgun proteomics study; site localization, however, is much more complicated and many challenges still exist. Here, we report our development of P-bracket using direct experimental evidence of phospho-containing site-determining product ions for accurate site localization without the need for additional FLR control. A P-bracket is defined as a complementary product ion pair that forms a bracket to confine a phosphorylation event to a unique site. P-bracket has been successfully benchmarked with a set of six synthetic phosphopeptides with a single phosphorylation event, a set of 96 synthetic peptides and phosphopeptide reference libraries, and two HeLa phosphopeptide LC-MS/MS (HCD) datasets; Accurate phosphosite localization by P-bracket will greatly enhance identification confidence of phosphopeptides and contribute to structural and functional studies of phosphoproteins.