Work in the Ait-Si-Ali lab was supported by the Association Fran&#231;aise contre les Myopathies T&#233;l&#233;thon (AFM-T&#233;l&#233;thon); Institut National du Cancer (INCa); Agence Nationale de la Recherche (ANR), Fondation Association pour la Recherche sur le Cancer (Fondation ARC); Groupement des Entreprises Fran&#231;aises pour la Lutte contre le Cancer (GEFLUC); CNRS; Universit&#233; Paris Diderot and the ''Who Am I?'' Laboratory of Excellence #ANR-11- LABX-0071 funded by the French Government through its ''Investments for the Future'' program operated by the ANR under grant #ANR-11-IDEX-0005-01. EB was supported by an INCa grant.

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</ol>

The authors have nothing to disclose.

The presented method permits exhaustive identification of partners of a transcription factor, MyoD. It revealed MyoD partners in a heterologous system resistant to MyoD-induced differentiation, namely HeLa-S3 cells. Thus, by definition, identified MyoD partners are ubiquitously expressed. These include general and sequence-specific transcription factors, chromatin-modifying enzymes, RNA processing proteins, kinases (Tables 1 and 2). Since HeLa-S3 cells are resistant to MyoD-induced trans...

Eukaryotic multi-cellular organisms are composed of different organs and tissues. Each functional tissue has a specific gene pattern expression, which is determined at each differentiation step. Cellular differentiation involves activation of specific genes, maintenance of their expression and, generally, silencing of a set of genes such those involved in cell proliferation. Skeletal muscle differentiation, or myogenesis, is thus a multi-step process, that begins with the determination of mesodermal stem cells into myoblasts, and then leads to the terminal differentiation of these myoblasts into first mono-nucleated, and then multi-nucleated, myotubes. Thus, myoblasts are &#34;determined&#34; cells, that are still able to proliferate, but they are committed to the skeletal muscle lineage, and thus can differentiate solely into skeletal muscle cells either during embryonic development or in adult muscle regeneration. The process of skeletal muscle terminal differentiation is orchestrated by a specific genetic program that begins with the permanent exit from the cell cycle of myoblast precursor cells that leads to a definitive silencing of proliferation associated genes, such as E2F target genes1. Indeed, during the process of terminal differentiation, myoblast proliferation arrest is a crucial step that precedes the expression of skeletal muscle specific genes and the fusion of myoblasts into myotubes2. Such a program permits adult muscle stem cells, also called satellite cells, to differentiate during the regeneration process following skeletal muscle injury.
Mammalian myogenesis is critically dependent on a family of myogenic basic helix-loop-helix (bHLH) transcription factors MyoD, Myf5, MRF4 and Myogenin, frequently referred to as the family of skeletal muscle determination factors or MRFs (Muscle Regulatory Factors)3. Each of them plays an essential role in specification and differentiation of skeletal muscle cells and has a specific expression pattern45-7. The activation of Myf5 and MyoD constitutes the determinative step that commits cells to the myogenic lineage, and subsequent expression of Myogenin triggers myogenesis with activation of skeletal muscle specific genes, such as MCK (Muscle Creatine Kinase). Myogenic bHLH transcription factors cooperate with members of the MEF2 family in the activation of muscle genes from previously silent loci8. They also stimulate skeletal muscle gene transcription as heterodimers with ubiquitous bHLH proteins, E12 and E47, known as E proteins, which bind so-called E-boxes in various gene-regulatory regions8. Twist, Id (inhibitor of differentiation) and other factors negatively regulate this process, by competing with MyoD for E proteins binding8.
MyoD is considered as the major player in triggering muscle terminal differentiation9 since it has the capacity to induce a myogenic determination/differentiation (trans-differentiation) program in many fully differentiated non-muscle systems10-13. Indeed, forced expression of MyoD induces the trans-differentiation of different cellular types even those derived from another embryologic origin12. For example, MyoD can convert hepatocytes, fibroblasts, melanocytes, neuroblasts, and adipocytes into muscle-like cells. The trans-differentiative action of MyoD involves an abnormal activation of the myogenic genetic program (notably its target genes) in a non-muscular environment, concomitant to the silencing of the original genetic program (notably, proliferation genes).
In proliferating myoblasts, MyoD is expressed but is unable to activate its target genes even when it binds to their promoters14-16. Therefore, the requirement for MyoD to be continuously expressed in undifferentiated myoblasts remains quite elusive. MyoD could repress its target genes due to recruitment of repressive chromatin-modifying enzymes in proliferating myoblasts prior to loading of activating chromatin remodeling enzymes14,17. For example, in proliferating myoblasts, MyoD is associated with transcriptional co-repressor KAP-1, histone deacetylases (HDACs) and repressive lysine methyltransferases (KMTs), including histone H3 lysine 9 or H3K9 and H3K27 KMTs, and actively suppress its target genes expression by establishing a locally repressive chromatin structure14,17. Importantly, a recent report indicated that MyoD is itself directly methylated by the H3K9 KMT G9a resulting in inhibition of its transactivating activity16.
The epigenetic mechanisms involved in this trans-differentiation of non-muscle cells by MyoD are largely unknown. Notably, some cell lines are resistant to MyoD-induced trans-differentiation. Thus, in HeLa cells, MyoD is either inactive or even might function as repressor rather than activator of transcription due to lack of expression of the BAF60C subunit of the chromatin remodeling complex SWI/SNF18. This model can thus be of choice to better characterize the mechanisms of MyoD-induced gene repression. It is also suitable to assay the ability of MyoD to induce repressive chromatin environment at its target loci with its associated partners and therefore uncover the MyoD-dependent repressive mechanisms in proliferating myoblasts to fine-tune terminal differentiation.
Here we describe the protocol for the identification of MyoD partners by using Tandem Affinity Purification (TAP-Tag) coupled to mass spectrometry (MS) characterization. The use of HeLa-S3 cells stably expressing Flag-HA-MyoD permitted to get enough material to purify the MyoD complexes from fractionated nuclear extracts. The identification of MyoD partners in the heterologous system was followed by validation in a relevant system.

<table><tbody><tr><td>&lt;strong&gt;Cell lines&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>HeLa-S3</td><td>ATCC</td><td>CCL-2.2</td><td></td></tr><tr><td>C2C12</td><td>ATCC</td><td>CRL-1772</td><td></td></tr><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Spinner</td><td>Corning</td><td>778531</td><td></td></tr><tr><td>Homogenizer:&amp;nbsp; Dounce homogenizer&amp;nbsp;</td><td>Wheaton</td><td>432-1273 and 432-1271</td><td></td></tr><tr><td>Agitating device for spinners</td><td>Bellco</td><td>778531</td><td></td></tr><tr><td>Sonicator</td><td>Diagenode</td><td>UCD 200</td><td></td></tr><tr><td>Hemocytometer</td><td>Marienfeld Superior&amp;nbsp;</td><td>640610</td><td></td></tr><tr><td>Low-binding tubes</td><td>Sorenson</td><td>27210</td><td></td></tr><tr><td>Empty spin column</td><td>Bio-Rad</td><td>7326204</td><td></td></tr><tr><td>&lt;strong&gt;Reagents&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>SDS-polyacrylamide 4-12%&amp;nbsp; gel</td><td>Life technologies</td><td>NP0336BOX</td><td></td></tr><tr><td>4x loading buffer&amp;nbsp;</td><td>Life technologies</td><td>NP0007</td><td></td></tr><tr><td>10x reducing agent</td><td>Life technologies</td><td>NP0004</td><td></td></tr><tr><td>Silver staining kit</td><td>Life technologies</td><td>A8592</td><td></td></tr><tr><td>Centrifugal filter units, 10K</td><td>Millipore</td><td>UFC501024</td><td></td></tr><tr><td>Protein G agarose beads</td><td>Sigma-Aldrich</td><td>P4691</td><td></td></tr><tr><td>Protein A/G&amp;nbsp; Resin</td><td>Thermo Scientific</td><td>53132</td><td></td></tr><tr><td>Flag resin (anti-Flag M2-agarose affinity gel)</td><td>Sigma-Aldrich</td><td>A2220</td><td></td></tr><tr><td>HA resin (monoclonal Anti-HA agarose)</td><td>Sigma-Aldrich</td><td>A2095</td><td></td></tr><tr><td>MNase</td><td>Sigma-Aldrich</td><td>N3755</td><td></td></tr><tr><td>Flag free peptide (DYKDDDDK)</td><td>Ansynth Service BV</td><td>Custom synthesis</td><td>Resuspend up to 4 mg/ml in 50 mM Tris-HCl, pH 7.8</td></tr><tr><td>HA free peptide (YPYDVPDYA)</td><td>Ansynth Service BV</td><td>Custom synthesis</td><td>Resuspend up to 4 mg/ml in 50 mM Tris-HCl, pH 7.8</td></tr><tr><td>Bicinchoninic acid based protein assay kit : BCA kit</td><td>Thermo Scientific</td><td>23225</td><td></td></tr><tr><td>Protease inhibitors</td><td>Sigma-Aldrich</td><td>S8830</td><td></td></tr><tr><td>Luminol-based enhanced chemiluminescence (ECL) HRP substrate</td><td>Life technologies</td><td>34075</td><td></td></tr><tr><td>BSA</td><td>Sigma-Aldrich</td><td>A9647</td><td></td></tr><tr><td>Sheared salmon sperm DNA (Deoxyribonucleic acid sodium salt from salmon testes)</td><td>Sigma-Aldrich</td><td>D1626</td><td></td></tr><tr><td>Spermine tetrahydrochloride</td><td>Sigma-Aldrich</td><td>S1141</td><td></td></tr><tr><td>Spermidine</td><td>Sigma-Aldrich</td><td>S0266</td><td></td></tr><tr><td>Glycerol</td><td>Sigma-Aldrich</td><td>G5516</td><td></td></tr><tr><td>PBS</td><td>Sigma-Aldrich</td><td>D8537</td><td></td></tr><tr><td>MOPS running buffer</td><td>Life technologies</td><td>NP0001</td><td></td></tr><tr><td>DMEM (high glucose)</td><td>Sigma-Aldrich</td><td>D0822</td><td>Pre-warm&amp;nbsp; at 37 &amp;deg;C before use</td></tr><tr><td>Trypsin-EDTA (0.05% phenol red</td><td>Life technologies</td><td>25300-054</td><td></td></tr><tr><td>Serum</td><td>GE healthcare life sciences&amp;nbsp;</td><td>PAA A15-102</td><td>Each lot of serum has to be first tested for your cells.</td></tr><tr><td>Penicillin and Streptomycin&amp;nbsp;</td><td>Life technologies</td><td>15140-122</td><td></td></tr><tr><td>Trypan Blue Solution, 0.4%</td><td>Life technologies</td><td>15250-061</td><td></td></tr><tr><td>Water (sterile-filtered)</td><td>Sigma-Aldrich</td><td>W3500</td><td></td></tr><tr><td>&lt;strong&gt;Antibodies&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>HA from rat (12CA5)</td><td>Roche</td><td>11583816001</td><td></td></tr><tr><td>Flag</td><td>Sigma-Aldrich</td><td>A8592</td><td></td></tr><tr><td>MyoD</td><td>Santa Cruz</td><td>sc-32758</td><td>To use for western blotting</td></tr><tr><td>MyoD</td><td>Santa Cruz</td><td>SC-760</td><td>To use for immunoprecipitation and western blotting</td></tr><tr><td>CBFbeta</td><td>Santa Cruz</td><td>FL-182&amp;nbsp;</td><td></td></tr><tr><td>HP1alpha</td><td>Euromedex</td><td>2HP2G9</td><td></td></tr><tr><td>HP1beta</td><td>Euromedex</td><td>1MOD1A9AS</td><td></td></tr><tr><td>HP1gamma</td><td>Euromedex</td><td>2MOD1GC</td><td></td></tr><tr><td>Suv39h1</td><td>Cell Signaling Technology</td><td>8729</td><td></td></tr><tr><td>BAF47</td><td>BD Biosciences</td><td>612111</td><td></td></tr><tr><td>Myf5</td><td>Santa Cruz</td><td>SC-302</td><td></td></tr><tr><td>Tubulin</td><td>Sigma-Aldrich</td><td>T9026</td><td></td></tr><tr><td>Actin</td><td>Sigma-Aldrich</td><td>A5441</td><td></td></tr><tr><td>IgG Mouse</td><td>Santa Cruz</td><td>SC-2025</td><td></td></tr><tr><td>IgG Rabbit</td><td>Santa Cruz</td><td>SC-2027</td><td></td></tr><tr><td>Goat anti-rat -HRP</td><td>Sigma-Aldrich</td><td>A9037</td><td></td></tr><tr><td>Goat anti-rabbit -HRP</td><td>Sigma-Aldrich</td><td>A6154&amp;nbsp;</td><td></td></tr><tr><td>Goat anti-mouse -HRP</td><td>Sigma-Aldrich</td><td>A4416</td><td></td></tr></tbody></table>

identification of myod interactome using tandem affinity purification coupled to mass spectrometry

Skeletal muscle terminal differentiation starts with the commitment of pluripotent mesodermal precursor cells to myoblasts. These cells have still the ability to proliferate or they can differentiate and fuse into multinucleated myotubes, which maturate further to form myofibers. Skeletal muscle terminal differentiation is orchestrated by the coordinated action of various transcription factors, in particular the members of the Muscle Regulatory Factors or MRFs (MyoD, Myogenin, Myf5, and MRF4), also called the myogenic bHLH transcription factors family. These factors cooperate with chromatin-remodeling complexes within elaborate transcriptional regulatory network to achieve skeletal myogenesis. In this, MyoD is considered the master myogenic transcription factor in triggering muscle terminal differentiation. This notion is strengthened by the ability of MyoD to convert non-muscle cells into skeletal muscle cells. Here we describe an approach used to identify MyoD protein partners in an exhaustive manner in order to elucidate the different factors involved in skeletal muscle terminal differentiation. The long-term aim is to understand the epigenetic mechanisms involved in the regulation of skeletal muscle genes, i.e., MyoD targets. MyoD partners are identified by using Tandem Affinity Purification (TAP-Tag) from a heterologous system coupled to mass spectrometry (MS) characterization, followed by validation of the role of relevant partners during skeletal muscle terminal differentiation. Aberrant forms of myogenic factors, or their aberrant regulation, are associated with a number of muscle disorders: congenital myasthenia, myotonic dystrophy, rhabdomyosarcoma and defects in muscle regeneration. As such, myogenic factors provide a pool of potential therapeutic targets in muscle disorders, both with regard to mechanisms that cause disease itself and regenerative mechanisms that can improve disease treatment. Thus, the detailed understanding of the intermolecular interactions and the genetic programs controlled by the myogenic factors is essential for the rational design of efficient therapies.

To understand the regulation of MyoD activity, we undertook the exhaustive characterization of the MyoD complexes using biochemical purification, based on the immunopurification of a double tagged form of MyoD followed by mass spectrometry (MS). The use of HeLa-S3 cell line expressing Flag-HA-tagged MyoD and a control cell line expressing Flag-HA permits to get enough material to purify the MyoD complex by performing double-affinity purification of Flag-HA-MyoD.
<p class="jove_content...

Watch this Scientific Journal Video about Identification of MyoD Interactome Using Tandem Affinity Purification Coupled to Mass Spectrometry at JoVE.com

Identification of MyoD Interactome Using Tandem Affinity Purification Coupled to Mass Spectrometry

MyoD is a myogenic transcription factor with a strong capacity to induce myogenic transdifferentiation of many fully differentiated non-muscle cell lines. The epigenetic mechanisms involved in this transdifferentiation are largely unknown. Here we describe a double-affinity purification method followed by mass spectrometry to exhaustively characterize MyoD partners.

Skeletal muscle terminal differentiation starts with the commitment of pluripotent mesodermal precursor cells to myoblasts. These cells have still the ...

identification-myod-interactome-using-tandem-affinity-purification

Research

JoVE Journal

Biology

9.8K Views.  Centre National de la Recherche Scientifique CNRS - Université Paris Diderot, Sorbonne Paris Cité. MyoD is a myogenic transcription factor with a strong capacity to induce myogenic transdifferentiation of many fully differentiated non-muscle cell lines. The epigenetic mechanisms involved in this transdifferentiation are largely unknown. Here we describe a double-affinity purification method followed by mass spectrometry to exhaustively characterize MyoD partners.

Video: Identification of MyoD Interactome Using Tandem Affinity Purification Coupled to Mass Spectrometry

Resolving Affinity Purified Protein Complexes by Blue Native PAGE and Protein Correlation Profiling

Most proteins act in association with others; hence, it is crucial to characterize these functional units in order to fully understand biological processes. Affinity purification coupled to mass spectrometry (AP-MS) has become the method of choice for identifying protein-protein interactions. However, conventional AP-MS studies provide information on protein interactions, but the organizational information is lost. To address this issue, we developed a strategy to unravel the distinct functional assemblies a protein might be involved in, by resolving affinity-purified protein complexes prior to their characterization by mass spectrometry. Protein complexes isolated through affinity purification of a bait protein using an epitope tag and competitive elution are separated through blue native electrophoresis. Comparison of protein migration profiles through correlation profiling using quantitative mass spectrometry allows assignment of interacting proteins to distinct molecular entities. This method is able to resolve protein complexes of close molecular weights that might not be resolved by traditional chromatographic techniques such as gel filtration. With little more work than conventional AP-geLC-MS/MS, we demonstrate this strategy may in many cases be adequate for obtaining protein complex topological information concomitantly to identifying protein interactions.

Here we present protocols for affinity purification of protein complexes and their separation by blue native PAGE, followed by protein correlation profiling using label free quantitative mass spectrometry. This method is useful to resolve interactomes into distinct protein complexes.

Most proteins act in association with others; hence, it is crucial to characterize these functional units in order to fully understand biological ...

Biochemistry

Deep Proteome Profiling by Isobaric Labeling, Extensive Liquid Chromatography, Mass Spectrometry, and Software-assisted Quantification

Many exceptional advances have been made in mass spectrometry (MS)-based proteomics, with particular technical progress in liquid chromatography (LC) coupled to tandem mass spectrometry (LC-MS/MS) and isobaric labeling multiplexing capacity. Here, we introduce a deep-proteomics profiling protocol that combines 10-plex tandem mass tag (TMT) labeling with an extensive LC/LC-MS/MS platform, and post-MS computational interference correction to accurately quantitate whole proteomes. This protocol includes the following main steps: protein extraction and digestion, TMT labeling, 2-dimensional (2D) LC, high-resolution mass spectrometry, and computational data processing. Quality control steps are included for troubleshooting and evaluating experimental variation. More than 10,000 proteins in mammalian samples can be confidently quantitated with this protocol. This protocol can also be applied to the quantitation of post translational modifications with minor changes. This multiplexed, robust method provides a powerful tool for proteomic analysis in a variety of complex samples, including cell culture, animal tissues, and human clinical specimens.

We present a protocol to accurately quantitate proteins with isobaric labelling, extensive fractionation, bioinformatics tools, and quality control steps in combination with liquid chromatography interfaced to a high-resolution mass spectrometer.

Many exceptional advances have been made in mass spectrometry (MS)-based proteomics, with particular technical progress in liquid chromatography (LC) ...

Label-Free Immunoprecipitation Mass Spectrometry Workflow for Large-scale Nuclear Interactome Profiling

Immunoaffinity purification mass spectrometry (IP-MS) has emerged as a robust quantitative method of identifying protein-protein interactions. This publication presents a complete interaction proteomics workflow designed for identifying low abundance protein-protein interactions from the nucleus that could also be applied to other subcellular compartments. This workflow includes subcellular fractionation, immunoprecipitation, sample preparation, offline cleanup, single-shot label-free mass spectrometry, and downstream computational analysis and data visualization. Our protocol is optimized for detecting compartmentalized, low abundance interactions that are difficult to identify from whole cell lysates (e.g., transcription factor interactions in the nucleus) by immunoprecipitation of endogenous proteins from fractionated subcellular compartments. The sample preparation pipeline outlined here provides detailed instructions for the preparation of HeLa cell nuclear extract, immunoaffinity purification of endogenous bait protein, and quantitative mass spectrometry analysis. We also discuss methodological considerations for performing large-scale immunoprecipitation in mass spectrometry-based interaction profiling experiments and provide guidelines for evaluating data quality to distinguish true positive protein interactions from nonspecific interactions. This approach is demonstrated here by investigating the nuclear interactome of the CMGC kinase, DYRK1A, a low abundance protein kinase with poorly defined interactions within the nucleus.

Described is a proteomics workflow for identifying protein interaction partners from a nuclear subcellular fraction using immunoaffinity enrichment of a given protein of interest and label-free mass spectrometry. The workflow includes subcellular fractionation, immunoprecipitation, filter aided sample preparation, offline cleanup, mass spectrometry, and a downstream bioinformatics pipeline.

Immunoaffinity purification mass spectrometry (IP-MS) has emerged as a robust quantitative method of identifying protein-protein interactions. This ...

A Mass Spectrometry-Based Approach to Identify Phosphoprotein Phosphatases and their Interactors

Most cellular processes are regulated by dynamic protein phosphorylation. More than three-quarters of proteins are phosphorylated, and phosphoprotein phosphatases (PPPs) coordinate over 90% of all cellular serine/threonine dephosphorylation. Deregulation of protein phosphorylation has been implicated in the pathophysiology of various diseases, including cancer and neurodegeneration. Despite their widespread activity, the molecular mechanisms controlling PPPs and those controlled by PPPs are poorly characterized. Here, a proteomic approach termed phosphatase inhibitor beads and mass spectrometry (PIB-MS) is described to identify and quantify PPPs, their posttranslational modifications, and their interactors in as little as 12 h using any cell line or tissue. PIB-MS utilizes a non-selective PPP inhibitor, microcystin-LR (MCLR), immobilized on sepharose beads to capture and enrich endogenous PPPs and their associated proteins (termed the PPPome). This method does not require the exogenous expression of tagged versions of PPPs or the use of specific antibodies. PIB-MS offers an innovative way to study the evolutionarily conserved PPPs and expand our current understanding of dephosphorylation signaling.

Here, we present a protocol for the enrichment of endogenous phosphoprotein phosphatases and their interacting proteins from cells and tissues and their identification and quantification by mass spectrometry-based proteomics.

Most cellular processes are regulated by dynamic protein phosphorylation. More than three-quarters of proteins are phosphorylated, and phosphoprotein ...

Identification of Protein Interacting Partners Using Tandem Affinity Purification

A critical and often limiting step in understanding the function of host and viral proteins is the identification of interacting cellular or viral protein partners. There are many approaches that allow the identification of interacting partners, including the yeast two hybrid system, as well as pull down assays using recombinant proteins and immunoprecipitation of endogenous proteins followed by mass spectrometry identification1. Recent studies have highlighted the utility of double-affinity tag mediated purification, coupled with two specific elution steps in the identification of interacting proteins. This approach, termed Tandem Affinity Purification (TAP), was initially used in yeast2,3 but more recently has been adapted to use in mammalian cells4-8.
As proof-of-concept we have established a tandem affinity purification (TAP) method using the well-characterized eukaryotic translation initiation factor eIF4E9,10.The cellular translation factor eIF4E is a critical component of the cellular eIF4F complex involved in cap-dependent translation initiation10. The TAP tag used in the current study is composed of two Protein G units and a streptavidin binding peptide separated by a Tobacco Etch Virus (TEV) protease cleavage sequence. The TAP tag used in the current study is composed of two Protein G units and a streptavidin binding peptide separated by a Tobacco Etch Virus (TEV) protease cleavage sequence8. To forgo the need for the generation of clonal cell lines, we developed a rapid system that relies on the expression of the TAP-tagged bait protein from an episomally maintained plasmid based on pMEP4 (Invitrogen). Expression of tagged murine eIF4E from this plasmid was controlled using the cadmium chloride inducible metallothionein promoter.
Lysis of the expressing cells and subsequent affinity purification via binding to rabbit IgG agarose, TEV protease cleavage, binding to streptavidin linked agarose and subsequent biotin elution identified numerous proteins apparently specific to the eIF4E pull-down (when compared to control cell lines expressing the TAP tag alone). The identities of the proteins were obtained by excision of the bands from 1D SDS-PAGE and subsequent tandem mass spectrometry. The identified components included the known eIF4E binding proteins eIF4G and 4EBP-1. In addition, other components of the eIF4F complex, of which eIF4E is a component were identified, namely eIF4A and Poly-A binding protein. The ability to identify not only known direct binding partners as well as secondary interacting proteins, further highlights the utility of this approach in the characterization of proteins of unknown function.

Tandem affinity purification is a robust approach for the identification of protein binding partners. As proof of concept, this methodology was applied to the well-characterized translation initiation factor eIF4E to co-precipitate the host cell factors involved in translation initiation. This method is easily adapted to any cellular or viral protein.

A critical and often limiting step in understanding the function of host and viral proteins is the identification of interacting cellular or viral protein ...

Identification of Protein Complexes in Escherichia coli using Sequential Peptide Affinity Purification in Combination with Tandem Mass Spectrometry

Since most cellular processes are mediated by macromolecular assemblies, the systematic identification of protein-protein interactions (PPI) and the identification of the subunit composition of multi-protein complexes can provide insight into gene function and enhance understanding of biological systems1, 2. Physical interactions can be mapped with high confidence vialarge-scale isolation and characterization of endogenous protein complexes under near-physiological conditions based on affinity purification of chromosomally-tagged proteins in combination with mass spectrometry (APMS). This approach has been successfully applied in evolutionarily diverse organisms, including yeast, flies, worms, mammalian cells, and bacteria1-6. In particular, we have generated a carboxy-terminal Sequential Peptide Affinity (SPA) dual tagging system for affinity-purifying native protein complexes from cultured gram-negative Escherichia coli, using genetically-tractable host laboratory strains that are well-suited for genome-wide investigations of the fundamental biology and conserved processes of prokaryotes1, 2, 7. Our SPA-tagging system is analogous to the tandem affinity purification method developed originally for yeast8, 9, and consists of a calmodulin binding peptide (CBP) followed by the cleavage site for the highly specific tobacco etch virus (TEV) protease and three copies of the FLAG epitope (3X FLAG), allowing for two consecutive rounds of affinity enrichment. After cassette amplification, sequence-specific linear PCR products encoding the SPA-tag and a selectable marker are integrated and expressed in frame as carboxy-terminal fusions in a DY330 background that is induced to transiently express a highly efficient heterologous bacteriophage lambda recombination system10. Subsequent dual-step purification using calmodulin and anti-FLAG affinity beads enables the highly selective and efficient recovery of even low abundance protein complexes from large-scale cultures. Tandem mass spectrometry is then used to identify the stably co-purifying proteins with high sensitivity (low nanogram detection limits).
Here, we describe detailed step-by-step procedures we commonly use for systematic protein tagging, purification and mass spectrometry-based analysis of soluble protein complexes from E. coli, which can be scaled up and potentially tailored to other bacterial species, including certain opportunistic pathogens that are amenable to recombineering. The resulting physical interactions can often reveal interesting unexpected components and connections suggesting novel mechanistic links. Integration of the PPI data with alternate molecular association data such as genetic (gene-gene) interactions and genomic-context (GC) predictions can facilitate elucidation of the global molecular organization of multi-protein complexes within biological pathways. The networks generated for E. coli can be used to gain insight into the functional architecture of orthologous gene products in other microbes for which functional annotations are currently lacking.

Identification of Protein Complexes in Escherichia coli using Sequential Peptide Affinity Purification in Combination with Tandem Mass Spectrometry

Affinity purification of tagged proteins in combination with mass spectrometry (APMS) is a powerful method for the systematic mapping of protein interaction networks and for investigating the mechanistic basis of biological processes. Here, we describe an optimized sequential peptide affinity (SPA) APMS procedure developed for the bacterium Escherichia coli that can be used to isolate and characterize stable multi-protein complexes to near homogeneity even starting from low copy numbers per cell.

Since most cellular processes are mediated by macromolecular assemblies, the systematic identification of protein-protein interactions (PPI) and the ...

A Protocol for the Identification of Protein-protein Interactions Based on 15N Metabolic Labeling, Immunoprecipitation, Quantitative Mass Spectrometry and Affinity Modulation

Protein-protein interactions are fundamental for many biological processes in the cell. Therefore, their characterization plays an important role in current research and a plethora of methods for their investigation is available1. Protein-protein interactions often are highly dynamic and may depend on subcellular localization, post-translational modifications and the local protein environment2. Therefore, they should be investigated in their natural environment, for which co-immunoprecipitation approaches are the method of choice3. Co-precipitated interaction partners are identified either by immunoblotting in a targeted approach, or by mass spectrometry (LC-MS/MS) in an untargeted way. The latter strategy often is adversely affected by a large number of false positive discoveries, mainly derived from the high sensitivity of modern mass spectrometers that confidently detect traces of unspecifically precipitating proteins. A recent approach to overcome this problem is based on the idea that reduced amounts of specific interaction partners will co-precipitate with a given target protein whose cellular concentration is reduced by RNAi, while the amounts of unspecifically precipitating proteins should be unaffected. This approach, termed QUICK for QUantitative Immunoprecipitation Combined with Knockdown4, employs Stable Isotope Labeling of Amino acids in Cell culture (SILAC)5 and MS to quantify the amounts of proteins immunoprecipitated from wild-type and knock-down strains. Proteins found in a 1:1 ratio can be considered as contaminants, those enriched in precipitates from the wild type as specific interaction partners of the target protein. Although innovative, QUICK bears some limitations: first, SILAC is cost-intensive and limited to organisms that ideally are auxotrophic for arginine and/or lysine. Moreover, when heavy arginine is fed, arginine-to-proline interconversion results in additional mass shifts for each proline in a peptide and slightly dilutes heavy with light arginine, which makes quantification more tedious and less accurate5,6. Second, QUICK requires that antibodies are titrated such that they do not become saturated with target protein in extracts from knock-down mutants.
Here we introduce a modified QUICK protocol which overcomes the abovementioned limitations of QUICK by replacing SILAC for 15N metabolic labeling and by replacing RNAi-mediated knock-down for affinity modulation of protein-protein interactions. We demonstrate the applicability of this protocol using the unicellular green alga Chlamydomonas reinhardtii as model organism and the chloroplast HSP70B chaperone as target protein7 (Figure 1). HSP70s are known to interact with specific co-chaperones and substrates only in the ADP state8. We exploit this property as a means to verify the specific interaction of HSP70B with its nucleotide exchange factor CGE19.

A Protocol for the Identification of Protein-protein Interactions Based on 15N Metabolic Labeling, Immunoprecipitation, Quantitative Mass Spectrometry and Affinity Modulation

We present a variation of the QUICK (QUantitative Immunoprecipitation Combined with Knockdown) approach that was introduced previously to distinguish between true and false protein-protein interactions. Our approach is based on 15N metabolic labeling, the modulation of affinities of protein-protein interactions by the presence/absence of ATP, immunoprecipitation, and quantitative mass spectrometry.

Protein-protein interactions are fundamental for many biological processes in the cell. Therefore, their characterization plays an important role in ...

Simultaneous Affinity Enrichment of Two Post-Translational Modifications for Quantification and Site Localization

Studying multiple post-translational modifications (PTMs) of proteins is a crucial step to understand PTM crosstalk and gain more holistic insights into protein function. Despite the importance of multi-PTM enrichment studies, few studies investigate more than one PTM at a time, due partially to the expenses, time, and large protein quantities required to perform multiple global proteomic analysis of PTMs. The &#34;one-pot&#34; affinity enrichment detailed in this protocol overcomes these barriers by permitting the simultaneous identification and quantification of peptides with lysine residues containing acetylation and succinylation PTMs with low amounts of sample input. The protocol involves preparation of protein lysate from mouse livers of SIRT5 knockout mice, performance of trypsin digestion, enrichment for PTMs, and performance of mass spectrometric analysis using a data-independent acquisition (DIA) workflow. Because this workflow allows for the enrichment of two PTMs from the same sample simultaneously, it provides a practical tool to study PTM crosstalk without requiring large amounts of samples, and it greatly reduces the time required for sample preparation, data acquisition, and analysis. The DIA component of the workflow provides comprehensive PTM-specific information. This is particularly important when studying PTM site localization, as DIA provides comprehensive sets of fragment ions that can be computationally deciphered to differentiate between different PTM localization isoforms.

This workflow describes the performance of time- and cost-efficient enrichment of multiple protein post-translational modifications (PTMs) simultaneously for quantitative global proteomic analysis. The protocol utilizes peptide-level PTM enrichment with multiple conjugated antibodies, followed by data-independent acquisition mass spectrometry analysis to gain biological insights into PTM crosstalk.

Studying multiple post-translational modifications (PTMs) of proteins is a crucial step to understand PTM crosstalk and gain more holistic insights into ...

High-throughput and Deep-proteome Profiling by 16-plex Tandem Mass Tag Labeling Coupled with Two-dimensional Chromatography and Mass Spectrometry

Isobaric tandem mass tag (TMT) labeling is widely used in proteomics because of its high multiplexing capacity and deep proteome coverage. Recently, an expanded 16-plex TMT method has been introduced, which further increases the throughput of proteomic studies. In this manuscript, we present an optimized protocol for 16-plex TMT-based deep-proteome profiling, including protein sample preparation, enzymatic digestion, TMT labeling reaction, two-dimensional reverse-phase liquid chromatography (LC/LC) fractionation, tandem mass spectrometry (MS/MS), and computational data processing. The crucial quality control steps and improvements in the process specific for the 16-plex TMT analysis are highlighted. This multiplexed process offers a powerful tool for profiling a variety of complex samples such as cells, tissues, and clinical specimens. More than 10,000 proteins and posttranslational modifications such as phosphorylation, methylation, acetylation, and ubiquitination in highly complex biological samples from up to 16 different samples can be quantified in a single experiment, providing a potent tool for basic and clinical research.

Presented here is an optimized high-throughput protocol developed with 16-plex tandem mass tag reagents, enabling quantitative proteome profiling of biological samples. Extensive basic pH fractionation and high-resolution LC-MS/MS mitigate ratio compression and provide deep proteome coverage.

Isobaric tandem mass tag (TMT) labeling is widely used in proteomics because of its high multiplexing capacity and deep proteome coverage. Recently, an ...

Tandem Mass Spectrometry

In tandem mass spectrometry a biomolecule of interest is isolated from a biological sample, and then fragmented into multiple subunits in order to help elucidate its composition and sequence. This is accomplished by having mass spectrometers in series. The first spectrometer ionizes a sample and filter ions of a specific mass to charge ratio. Filtered ions are then fragmented and passed to a second mass spectrometer where the fragments are analyzed.
This video introduces the principles of tandem mass spectrometry, including mass-to-ratio selection and dissociation methods. Also shown is a general procedure for analyzing a biochemical compound using tandem mass spectrometry with collision-induced dissociation. The applications section covers selection reaction monitoring, determination of protein post-translation modifications, and detection of tacrolimus levels in blood.
<div class="chapter_text" id="script_0">
Tandem mass spectrometry links together multiple stages of mass spectrometry to first isolate a biomolecule, and then determine aspects of its chemical makeup. Biomolecules have large, complex structures, making it difficult to determine their molecular composition. Tandem mass spectrometry selects a molecule of interest that is later fragmented into multiple subunits, which can help elucidate its identification and sequence. This video will show the concepts of tandem mass spectrometry, a general procedure, and some of its uses in biochemistry.
</div>
<div class="chapter_text" id="script_1">
Tandem mass spectrometry begins as a typical mass spec instrument: with an ion source, which converts the sample into ions, and a mass analyzer, which separates the ions based on their mass-to-charge ratio. A common mass analyzer, the quadrupole, only allows ions with a specific ratio through, while the others crash into the rods of the apparatus. The species allowed through, called the precursor ion, is the biomolecule of interest. The ion moves into a collision cell, typically another quadrupole, where energy is applied to fragment the ion in a predictable pattern.
These fragments move into another mass analyzer, such as a time-of-flight, which separates these &#34;product ions&#34;. The product ions are then sent to the detector, as in a normal MS instrument. In the case of an unknown protein, the resulting spectrum contains numerous overlapping fragments, making a definitive complete sequence of the biomolecule difficult to generate. However, the spectral pattern is unique for a given protein. Analysis software compares the spectrum to a database of known peptide sequences, elucidating the unknown protein from the overlapping fragments.
Depending on the sample and desired degree of fragmentation, multiple fragmentation methods are possible. Fragmentation patterns depend on how the energy is transferred, its amount, and how it is distributed through the precursor ion. Energy can be transferred via neutral particles, radiation, or electrons. Using neutral atoms, a process called collision-induced dissociation or CID, primarily cleaves at the peptide bond between the amino acids, ideal for their identification.
Now that the basics of the technique have been covered, let&#39;s look at CID tandem mass spectrometry being used to study a component of bacterial cell envelopes.
</div>
<div class="chapter_text" id="script_2">
As with all mass spectrometric experiments, the first step is to ionize the sample. For biomolecules, this is typically done with matrix assisted laser desorption or electrospray ionization. The precursor ion signal is then optimized by tuning of the ion optics. Once done, the target is isolated and the fragmentation method is chosen, such as CID.
The strength of an applied voltage, which accelerates the precursor ion into the collision cell, affects the degree of fragmentation. This voltage is increased until the precursor is roughly 10% abundance compared to the highest product ion. Multiple spectra are acquired and averaged until a sufficient signal-to-noise ratio is achieved. The number of scans needed is dependent on the signal intensity of the original precursor ion and can range from 3 to 300.
The analyte in this example, lipid A from Escherichia coli K-12, had 19 major fragments after CID. Lipid A&#39;s general structure is well known, allowing software to reconstruct the specific composition from the sample.
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Now that we&#39;ve looked the procedure, let&#39;s look at some of the ways tandem mass spectrometry is used in biochemistry.
A common scanning mode in tandem mass spectrometry is selected reaction monitoring, or SRM. In SRM, both mass analyzers are fixed to a selected mass-to-charge ratio, focusing on specific precursor and product ions. Because of SRM&#39;s high degree of sensitivity, the spectra of peptide standards of known concentration can be utilized and compared to that of the unknown samples, allowing proteins of interest to be quantified.
Proteins are commonly modified after translation, typically by the addition of functional groups such as methyl groups, phosphate groups, or sugars, known as glycans. These are important in cell signaling processes, elucidating how cells communicate with one another. Because tandem mass spectrometry fragments the proteins into smaller components, it is possible to determine the location of the PTM to the specific fragment or even an amino acid. Some modifications, such as acetylation and trimethylation, are difficult to differentiate by mass alone, so chromatographic separation is performed before the mass spectrometry.
Many analytes in patient&#39;s blood are found at concentrations below the limit of detection for typical mass spectrometry. Another advantage of SRM is that it discards all but one product ion, increasing the sensitivity and enhancing the lower detection limit by up to 100 fold. In this example, the immunosuppressant drug, tacrolimus, could be detected at levels of 1 ng/mL.
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You&#39;ve just watched JoVE&#39;s video on tandem mass spectrometry. This video described the theory of the instrument, went over a general procedure, and explained some of the ways the technique is currently being utilized. Thanks for watching!
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Tandem Mass Spectrometry: Principle, Instrumentation, Uses

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