Ayako Takemori

Top‐down proteomics (TDP) is a powerful analytical approach for the highly sensitive measurement of intact proteoforms by mass spectrometry. However, its application to high molecular weight proteoforms remains challenging. Middle‐down proteomics (MDP) offers a practical solution but requires pre‐fractionation of the complex peptide mixture generated by limited digestion to successfully achieve trace‐level peptide detection. Here, we present 2D‐GeLC‐FAIMS‐MS, an innovative gel‐based sample pre‐fractionation workflow for in‐depth MDP. This workflow integrates limited Glu‐C digestion with a two‐dimensional sample fractionation strategy that combines a BAC (N,N′‐bis(acryloyl)cystamine)‐cross‐linked dissolvable polyacrylamide gel electrophoresis (BAC‐PAGE) with PEPPI‐MS, a highly efficient passive protein extraction method. Samples are first size‐fractionated by BAC‐PAGE and subsequently subjected to in‐gel Glu‐C digestion. The resulting middle‐down peptides (< 50 kDa) undergo a second fractionation via SDS‐PAGE, followed by peptide recovery using PEPPI‐MS and LC‐FAIMS‐MS analysis. The dissolution properties of BAC gels enable efficient sample transfer between the two PAGE steps with minimal loss, ensuring high‐resolution pre‐fractionation. This novel workflow provides a robust and efficient strategy for the comprehensive characterization of middle‐down peptides, facilitating improved sensitivity and depth in proteome analysis.

The quantification of proteoforms, i.e., all molecular forms in which proteins can be present, by top-down proteomics provides essential insights into biological processes at the molecular level. Isobaric labeling-based quantification strategies are suitable for multidimensional separation strategies and allow for multiplexing of the samples. Here, we investigated cysteine-directed isobaric labeling by iodoTMT in combination with a gel- and gas-phase fractionation (GeLC-FAIMS-MS) for in-depth quantitative proteoform analysis. We optimized the acquisition workflow (i.e., the FAIMS compensation voltages, isolation windows, acquisition strategy, and fragmentation method) using a two-proteome mix to increase the number of quantified proteoforms and reduce ratio compression. Additionally, we implemented a mass feature-based quantification strategy in the widely used deconvolution algorithm FLASHDeconv, which improves and facilitates data analysis. The optimized iodoTMT GeLC-FAIMS-MS workflow was applied to quantitatively analyze the proteome of Escherichia coli grown under glucose or acetate as the sole carbon source, resulting in the identification of 726 differentially abundant proteoforms.

Precise prefractionation of proteome samples is a potent method for realizing in-depth analysis in top-down proteomics. PEPPI-MS (Passively Eluting Proteins from Polyacrylamide gels as Intact species for MS), a gel-based sample fractionation method, enables high-resolution proteome fractionation based on molecular weight by highly efficient extraction of proteins from polyacrylamide gels after SDS-PAGE separation. Thereafter it is essential to effectively remove contaminants such as CBB and SDS from the PEPPI fraction prior to mass spectrometry. In this study, we developed a complete, robust, and simple sample preparation workflow named PEPPI-SP3 for top-down proteomics by combining PEPPI-MS with the magnetic bead-based protein purification approach used in SP3 (single-pot, solid-phase-enhanced sample preparation), now one of the standard sample preparation methods in bottom-up proteomics. In PEPPI-SP3, proteins extracted from the gel are collected on the surface of SP3 beads, washed with organic solvents, and recovered intact with 100 mM ammonium bicarbonate containing 0.05% (w/v) SDS. The recovered proteins are subjected to mass spectrometry after additional purification using an anion-exchange StageTip. Performance validation using human cell lysates showed a significant improvement in low-molecular-weight protein recovery with a lower coefficient of variation compared to conventional PEPPI workflows using organic solvent precipitation or ultrafiltration.

Top-down analysis of intact proteins and middle-down analysis of proteins subjected to limited digestion require efficient detection of traces of proteoforms in samples, necessitating the reduction of sample complexity by thorough pre-fractionation of the proteome components in the sample. SDS–PAGE is a simple and inexpensive high-resolution protein-separation technique widely used in biochemical and molecular biology experiments. Although its effectiveness for sample preparation in bottom-up proteomics has been proven, establishing a method for highly efficient recovery of intact proteins from the gel matrix has long been a challenge for its implementation in top-down and middle-down proteomics. As a much-awaited solution to this problem, we present an experimental protocol for efficient proteoform fractionation from complex biological samples using passively eluting proteins from polyacrylamide gels as intact species for mass spectrometry (PEPPI-MS), a rapid method for extraction of intact proteins separated by SDS–PAGE. PEPPI-MS allows recovery of proteins below 100 kDa separated by SDS–PAGE in solution with a median efficiency of 68% within 10 min and, unlike conventional electroelution methods, requires no special equipment, contributing to a remarkably economical implementation. The entire protocol from electrophoresis to protein purification can be performed in <5 h. By combining the resulting PEPPI fraction with other protein-separation techniques, such as reversed-phase liquid chromatography and ion mobility techniques, multidimensional proteome separations for in-depth proteoform analysis can be easily achieved.

Middle‐down proteomics (MDP) is an analytical approach in which protein samples are digested with proteases such as Glu‐C to generate large peptides (>3 kDa) that are analyzed by mass spectrometry (MS). This method is useful for characterizing high‐molecular‐weight proteins that are difficult to detect by top‐down proteomics (TDP), in which intact proteins are analyzed by MS. In this study, we applied GeLC‐FAIMS‐MS, a multidimensional separation workflow that combines gel‐based prefractionation with LC‐FAIMS MS, for deep MDP. Middle‐down peptides generated by optimized limited Glu‐C digestion conditions were first size‐fractionated by polyacrylamide gel electrophoresis, followed by C4 reversed‐phase liquid chromatography separation and additional ion mobility fractionation, resulting in a significant increase in peptide length detectable by MS. In addition to global analysis, the GeLC‐FAIMS‐MS concept can also be applied to targeted MDP, where only proteins in the desired molecular weight range are gel‐fractionated and their Glu‐C digestion products are analyzed, as demonstrated by targeted analysis of integrins in exosomes. In‐depth MDP achieved by global and targeted GeLC‐FAIMS‐MS supports the exploration of proteoform information not covered by conventional TDP by increasing the number of detectable protein groups or post‐translational modifications (PTMs) and improving the sequence coverage.

The combination of liquid chromatography (LC) and gas-phase separation by field-asymmetric ion mobility spectrometry (FAIMS) is a powerful proteoform separation system for top-down proteomics. Here, we present an in-depth top-down proteomics workflow, GeLC-FAIMS-MS, in which a molecular-weight-based proteome fractionation approach using SDS-polyacrylamide gel electrophoresis is performed prior to LC-FAIMS-MS. Since individual bands and their corresponding mass ranges require different compensating voltages (CVs), the MS parameters for each gel band and CV were optimized to increase the number and reliability of proteoform identifications further. We developed an easy-to-implement and inexpensive procedure combining the earlier established Passively Eluting Proteins from Polyacrylamide gels as Intact species (PEPPI) protocol with an optimized Anion-Exchange disk-assisted Sequential sample Preparation (AnExSP) method for the removal of stains and SDS. The protocol was compared with a methanol-chloroform-water (MCW)-based protein precipitation protocol. The results show that the PEPPI-AnExSP procedure is better suited for the identification of low-molecular-weight proteoforms, whereas the MCW-based protocol showed advantages for higher-molecular-weight proteoforms. Moreover, complementary results were observed with the two methods in terms of hydrophobicity and isoelectric points of the identified proteoforms. In total, 8500 proteoforms could be identified in a human proteome standard, showing the effectiveness of the gel-based sample fractionation approaches in combination with LC-FAIMS-MS.

Capturing proteins on anion-exchange discs facilitates concentration of diluted samples and removal of contaminants, allowing more efficient sample pretreatment for bottom-up/cross-linking mass spectrometry than in-solution and in-gel.

We report a new approach to the assembly and construction of QconCATs, quantitative concatamers for proteomic applications that yield stoichiometric quantities of sets of stable isotope-labelled internal standards. The new approach is based on synthetic biology precepts of biobricks, making use of loop assembly to construct larger entities from individual biobricks. It offers a major gain in flexibility of QconCAT implementation and enables rapid and efficient editability that permits, for example, substitution of one peptide for another. The basic building block (a Qbrick) is a segment of DNA that encodes two or more quantification peptides for a single protein, readily held in a repository as a library resource. These Qbricks are then assembled in a one tube ligation reaction that enforces the order of assembly, to yield short QconCATs that are useable for small quantification products. However, the DNA context of the short also allows a second cycle of assembly such that five different short QconCATs can be assembled into a longer QconCAT in a second, single tube ligation. From a library of Qbricks, a bespoke QconCAT can be assembled quickly and efficiently in a form suitable for expression and labelling in vivo or in vitro. We refer to this approach as the ALACAT strategy as it permits à la carte design of quantification standards.

The GeLC–MS workflow, which combines low-cost, easy-to-use sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (SDS-PAGE) with liquid chromatography–mass spectrometry (LC–MS), is very popular in current bottom-up proteomics. However, GeLC–MS requires that PAGE-separated proteins undergo overnight enzymatic digestion in a gel, resulting in more than 20 h of sample preparation for LC–MS. In this study, we overcame the limitations of GeLC–MS by developing a rapid digestion workflow for PAGE separation of proteins using N,N′-bis(acryloyl)cystamine (BAC) cross-linked gels that can be solubilized by reductive treatment. Making use of an established workflow called BAC-DROP (BAC-gel dissolution to digest PAGE-resolved objective proteins), crude proteome samples were fractionated based on molecular weight by BAC cross-linked PAGE. After fractionation, the gel fragments were reductively dissolved in under 5 min, and in-solution trypsin digestion of the protein released from the gel was completed in less than 1 h at 70 °C, equivalent to a 90–95% reduction in time compared to conventional in-gel trypsin digestion. The introduction of the BAC-DROP workflow to the MS assays for inflammatory biomarker CRP and viral marker HBsAg allowed for serum sample preparation to be completed in as little as 5 h, demonstrating successful marker quantification from a 0.5 μL sample of human serum.

Prefractionation of complex mixtures of proteins derived from biological samples is indispensable for proteome analysis via top-down mass spectrometry (MS). Polyacrylamide gel electrophoresis (PAGE), which enables high-resolution protein separation based on molecular size, is a widely used technique in biochemical experiments and has the potential to be useful in sample fractionation for top-down MS analysis. However, the lack of a means to efficiently recover the separated proteins in-gel has always been a barrier to its use in sample prefractionation. In this study, we present a novel experimental workflow, called Passively Eluting Proteins from Polyacrylamide gels as Intact species for MS (“PEPPI-MS”), which allows top-down MS of PAGE-separated proteins. The optimization of Coomassie brilliant blue staining followed by the passive extraction step in the PEPPI-MS workflow enabled the efficient recovery of proteins, separated on commercial precast gels, from a wide range of molecular weight regions in under 10 min. Two-dimensional separation combining offline PEPPI-MS with online reversed-phase liquid chromatographic separation resulted in identification of over 1000 proteoforms recovered from the target region of the gel (≤50 kDa). Given the widespread availability and relatively low cost of traditional sodium dodecyl sulfate (SDS)-PAGE equipment, the PEPPI-MS workflow will be a powerful prefractionation strategy for top-down proteomics.