This study was supported by the Astellas Foundation for Research on Metabolic Disorders (HT), the Takeda Science Foundation (HT), and by a Japan Society for the Promotion of Science Grants-in-Aid for Young Scientists (B) to HT.

1. Transfection
<ol>
	<li>Seed HEK293 cells in 100-mm culture plates (2 x 106 cells/dish). Culture the cells in Dulbecco&#39;s modified Eagle&#39;s medium supplemented with 10% fetal calf serum, 100 &#181;g/ml streptomycin, and 100 units/ml penicillin. Incubate the cells in a humidified incubator at 5% CO2 and 37 &#176;C.</li>
	<li>The next day, transfect the cells with 10 &#181;g of FLAG-tagged ARIP4 plasmid using 40 &#181;l of transfection reagent according to the manufacturer&#39;s directions11.</li>
</ol>
2. Protein Extraction
<ol>
	<li>Approximately 36 hr post-transfection, wash the cells with ice-cold PBS and harvest the cells using a scraper. Transfer the cells into a 1.5 mltube and centrifuge it at 8,000 x g for 2 min at 4 &#176;C. Aspirate the supernatant and resuspend the cell pellet in 5x the pellet volume of 0.4 M KCl buffer (High salt buffer: 20 mM Tris-HCl (pH 8.0), 0.4 M KCl, 5 mM MgCl2, 10% glycerol, 0.1% NP-40, 10 mM &#946;-mercaptethanol, 1x protease inhibitor cocktail). Rotate the mixture for 20 min at 4 &#176;C.</li>
	<li>Centrifuge the tube at 17,700 x g for 10 min at 4 &#176;C.</li>
	<li>Transfer the supernatant (whole-cell extract) to a new 2-mL tube and add 3x the initial volume of 0 M KCl buffer (Dilution buffer: 20 mM Tris-HCl (pH 8.0), 5 mM MgCl2, 10% glycerol, 0.1% NP-40, 10 mM &#946;-Mercaptoethanol, 1x protease inhibitor cocktail).</li>
	<li>Perform another centrifugation (17,700 x g for 10 min at 4 &#176;C) and transfer the supernatant (whole-cell extract) to a new 2 ml tube.</li>
</ol>
3. Immunoprecipitation
<ol>
	<li>During the centrifugation (2.4), wash 50 &#181;l of anti-FLAG beads with PBS, 0.1 M glycine-hydrochloride, and then 1 M Tris-hydrochloride, followed by washing twice with 0.1M KCl buffer (Low salt buffer: 20 mM Tris-HCl (pH 8.0), 0.1 M KCl, 5 mM MgCl2, 10% glycerol, 0.1% NP-40, 10 mM &#946;-mercaptoethanol, 1x protease inhibitor cocktail). The centrifugation for this washing process is performed at 2,000 x g for 1 min at 4 &#176;C.</li>
	<li>Mix 50 &#181;l of FLAG beads with the whole-cell extracts and gently rotate the mixture for 4 hr at 2-4 &#176;C (1% of the whole-cell extract from step 2.4 should also be kept at -80 &#176;C for future use as an input).</li>
	<li>Transfer the sample to a micro spin column (gravity drop). Keep the flow-through in a 1.5 ml tube, freeze it using liquid nitrogen, and store it at -80 &#176;C (check the protein levels of both the input and flow-through using Western blot analysis).</li>
	<li>Add 1 ml of 0.1 M KCl buffer (Low salt buffer) to wash the FLAG beads (gravity drop).</li>
	<li>Repeat step 3.4 at least four more times (for a total of five washes).</li>
</ol>
4. Elution
<ol>
	<li>Place the column into a 1.5 ml tube and centrifuge it at 2,000 x g for 1 min; the beads will dry.</li>
	<li>Cap the bottom of the column.</li>
	<li>Add 50 &#181;l (equal to the volume of FLAG beads) of 0.1 M glycine-HCl (pH 2.5).</li>
	<li>Gently shake the mixture for 10 min at room temperature.</li>
	<li>Place the column into a new 1.5 ml tube and centrifuge it at 2,000 x g for 1 min.</li>
	<li>The eluted solution is neutralized with 10 &#181;l of 1 M Tris-HCl (pH 8.0); mix well by pipetting or by using a vortex machine.</li>
</ol>
5. Alkylation and Trypsinization
<ol>
	<li>Add 50 &#181;l of 100 mM NH4HCO3, 10 &#181;l of CH3CN, and 2 &#181;l of dithiothreitol to the mixture (112 &#181;l total volume).</li>
	<li>Incubate the sample for 30 min at 56 &#176;C.</li>
	<li>Place the sample at room temperature for 10 min.</li>
	<li>Add 10 &#181;l of 333 mM iodoacetamide to the sample and incubate it in the dark for 30 min at 37 &#176;C.</li>
	<li>Add 10 &#181;l of trypsin (33 ng/&#181;l) to the mixture and incubate it overnight at 37 &#176;C.</li>
</ol>
6. LC-MS/MS Analysis
<ol>
	<li>Following overnight trypsinization, transport the final product to an LC-MS/MS facility or to a third-party mass spectrometry lab for analysis12.</li>
</ol>

<ol>
	<li>De Las Rivas, J., Fontanillo, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein-protein+interactions+essentials:+key+concepts+to+building+and+analyzing+interactome+networks.">Protein-protein interactions essentials: key concepts to building and analyzing interactome networks.</a> PLoS Comput.Biol. 6 (6), e1000807 (2010).</li><li>Westermarck, J., Ivaska, J., Corthals, G. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+protein+interactions+involved+in+cellular+signaling.">Identification of protein interactions involved in cellular signaling.</a> Mol.Cell.Proteomics. 12 (7), 1752-1763 (2013).</li><li>MacPherson, R. E., Ramos, S. V., Vandenboom, R., Roy, B. D., Peters, S. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Skeletal+muscle+PLIN+proteins,+ATGL+and+CGI-58,+interactions+at+rest+and+following+stimulated+contraction.">Skeletal muscle PLIN proteins, ATGL and CGI-58, interactions at rest and following stimulated contraction.</a> Am.J.Physiol.Regul.Integr.Comp.Physiol. 304 (8), 644-650 (2013).</li><li>Qin, K., Dong, C., Wu, G., Lambert, N. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inactive-state+preassembly+of+G(q)-coupled+receptors+and+G(q)+heterotrimers.">Inactive-state preassembly of G(q)-coupled receptors and G(q) heterotrimers.</a> Nat.Chem.Biol. 7 (10), 740-747 (2011).</li><li>Ogawa, H., Ishiguro, K., Gaubatz, S., Livingston, D. M., Nakatani, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+complex+with+chromatin+modifiers+that+occupies+E2F-+and+Myc-responsive+genes+in+G0+cells.">A complex with chromatin modifiers that occupies E2F- and Myc-responsive genes in G0 cells.</a> Science. 296 (5570), 1132-1136 (2002).</li><li>Shi, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coordinated+histone+modifications+mediated+by+a+CtBP+co-repressor+complex.">Coordinated histone modifications mediated by a CtBP co-repressor complex.</a> Nature. 422 (6933), 735-738 (2003).</li><li>Huh, K. W., DeMasi, J., Ogawa, H., Nakatani, Y., Howley, P. M., Munger, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Association+of+the+human+papillomavirus+type+16+E7+oncoprotein+with+the+600-kDa+retinoblastoma+protein-associated+factor,+p600.">Association of the human papillomavirus type 16 E7 oncoprotein with the 600-kDa retinoblastoma protein-associated factor, p600.</a> Proc.Natl.Acad.Sci.U.S.A. 102 (32), 11492-11497 (2005).</li><li>Huang, B. X., Kim, H. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effective+identification+of+Akt+interacting+proteins+by+two-step+chemical+crosslinking,+co-immunoprecipitation+and+mass+spectrometry.">Effective identification of Akt interacting proteins by two-step chemical crosslinking, co-immunoprecipitation and mass spectrometry.</a> PLoS One. 8 (4), 61430 (2013).</li><li>ten Have, S., Boulon, S., Ahmad, Y., Lamond, A. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mass+spectrometry-based+immuno-precipitation+proteomics+-+the+user's+guide.">Mass spectrometry-based immuno-precipitation proteomics - the user's guide.</a> Proteomics. 11 (6), 1153-1159 (2011).</li><li>Ogawa, H., Komatsu, T., Hiraoka, Y., Morohashi, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transcriptional+Suppression+by+Transient+Recruitment+of+ARIP4+to+Sumoylated+nuclear+receptor+Ad4BP/SF-1.">Transcriptional Suppression by Transient Recruitment of ARIP4 to Sumoylated nuclear receptor Ad4BP/SF-1.</a> Mol.Biol.Cell. 20 (19), 4235-4245 (2009).</li><li>Tsuchiya, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+autophagic+receptor+p62+regulates+the+abundance+of+transcriptional+coregulator+ARIP4+during+nutrient+starvation.">Selective autophagic receptor p62 regulates the abundance of transcriptional coregulator ARIP4 during nutrient starvation.</a> Sci.Rep. 5, 14498 (2015).</li><li>Watanabe, T., Matsuo, I., Maruyama, J., Kitamoto, K., Ito, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+and+characterization+of+an+intracellular+lectin,+calnexin,+from+Aspergillus+oryzae+using+N-glycan-conjugated+beads.">Identification and characterization of an intracellular lectin, calnexin, from Aspergillus oryzae using N-glycan-conjugated beads.</a> Biosci.Biotechnol.Biochem. 71 (11), 2688-2696 (2007).</li><li>Sitz, J. H., Tigges, M., Baumgartel, K., Khaspekov, L. G., Lutz, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dyrk1A+potentiates+steroid+hormone-induced+transcription+via+the+chromatin+remodeling+factor+Arip4.">Dyrk1A potentiates steroid hormone-induced transcription via the chromatin remodeling factor Arip4.</a> Mol.Cell.Biol. 24 (13), 5821-5834 (2004).</li><li>Mohammed, H., Taylor, C., Brown, G. D., Papachristou, E. K., Carroll, J. S., D'Santos, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+immunoprecipitation+mass+spectrometry+of+endogenous+proteins+(RIME)+for+analysis+of+chromatin+complexes.">Rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME) for analysis of chromatin complexes.</a> Nat.Protoc. 11 (2), 316-326 (2016).</li><li>Fonslow, B. R., Yates, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+electrophoresis+applied+to+proteomic+analysis.">Capillary electrophoresis applied to proteomic analysis.</a> J.Sep.Sci. 32 (8), 1175-1188 (2009).</li></ol>

The authors have nothing to disclose.

Efficient transfection is essential to obtain a successful result with this protocol. Accordingly, we recommend using Western blot analysis to determine the immunoprecipitated FLAG-tagged protein levels. This step enables users to verify that their protein of interest is properly overexpressed and, moreover, that the IP was performed successfully. FLAG-tagged protein levels should also be checked prior to the mass spectrometry analysis.
A mock sample should be used as a negative control to avo...

Protein-protein interactions play an important role in many biological functions. As such, these interactions have been implicated in signal transduction; protein transport across cell membranes; cell metabolism; and several nuclear processes, including DNA replication, DNA damage repair, recombination, and transcription1,2,3,4. Identifying the proteins involved in these interactions is therefore critical to advancing our understanding of these cellular processes.
Immunoprecipitation (IP) is a validated technique used to analyze protein-protein interactions. In order to facilitate the identification of co-immunoprecipitated proteins, mass spectrometry is often utilized5,6,7,8,9. By targeting a known member of a protein complex with an antibody, it is possible to isolate the protein complex and to subsequently identify its unknown components via mass spectrometry analysis. ARIP4 (androgen receptor-interacting protein 4), a transcriptional coregulator, interacts with nuclear receptor proteins in order to activate or repress its target promoters in a context-dependent manner9,10. To better understand the transcriptional regulatory mechanisms governing these nuclear factors, we describe a comprehensive method to purify and identify ARIP4 interacting proteins by using the LC-MS/MS system.

<table><tbody><tr><td>Lipofectamine 2000 Transfection Reagent</td><td>Thermo Fisher Scientific</td><td>11668019</td><td></td></tr><tr><td>Protease Inhibitor Cocktail (EDTA free) (100x)</td><td>nacalai tesque</td><td>03969-21</td><td></td></tr><tr><td>ANTI-FLAG M2 Affinity Gel</td><td>Sigma-Aldrich</td><td>A2220</td><td></td></tr><tr><td>Micro Bio-Spin Columns</td><td>BIO-RAD</td><td>732-6204</td><td></td></tr></tbody></table>

a protein preparation method for the high-throughput identification of proteins interacting with a nuclear cofactor using lc-ms/ms analysis

Transcriptional coregulators are vital to the efficient transcriptional regulation of nuclear chromatin structure. Coregulators play a variety of roles in regulating transcription. These include the direct interaction with transcription factors, the covalent modification of histones and other proteins, and the occasional chromatin conformation alteration. Accordingly, establishing relatively quick methods for identifying proteins that interact within this network is crucial to enhancing our understanding of the underlying regulatory mechanisms. LC-MS/MS-mediated protein binding partner identification is a validated technique used to analyze protein-protein interactions. By immunoprecipitating a previously-identified member of a protein complex with an antibody (occasionally with an antibody for a tagged protein), it is possible to identify its unknown protein interactions via mass spectrometry analysis. Here, we present a method of protein preparation for the LC-MS/MS-mediated high-throughput identification of protein interactions involving nuclear cofactors and their binding partners. This method allows for a better understanding of the transcriptional regulatory mechanisms of the targeted nuclear factors.

We identified a strong ARIP4 signal, around 160 KDa, as well as those of a mock sample control and several other unknown proteins (Figure 1). LC-MS/MS analysis identified both the ARIP4 complex peptides and the potential peptide cofactors within the fraction of FLAG beads (Table 1). p62 (Sequestosome1), a known ARIP4 cofactor11 was identified in the analysis (Table 1), which confirms the efficacy of this system<sup...

Watch this Scientific Journal Video about A Protein Preparation Method for the High-throughput Identification of Proteins Interacting with a Nuclear Cofactor Using LC-MS/MS Analysis at JoVE.com

A Protein Preparation Method for the High-throughput Identification of Proteins Interacting with a Nuclear Cofactor Using LC-MS/MS Analysis

We have established a method for the purification of coregulatory interaction proteins using the LC-MS/MS system.

Transcriptional coregulators are vital to the efficient transcriptional regulation of nuclear chromatin structure. Coregulators play a variety of roles in ...

a-protein-preparation-method-for-high-throughput-identification

Research

JoVE Journal

Biochemistry

8.3K Views.  Osaka University.  We hebben een werkwijze voor het zuiveren van eiwitten mederegulerende interactie met de LC-MS / MS systeem. 

Video: A Protein Preparation Method for the High-throughput Identification of Proteins Interacting with a Nuclear Cofactor Using LC-MS/MS Analysis

Sample Preparation for Mass Spectrometry-based Identification of RNA-binding Regions

Noncoding RNAs play important roles in several nuclear processes, including regulating gene expression, chromatin structure, and DNA repair. In most cases, the action of noncoding RNAs is mediated by proteins whose functions are in turn regulated by these interactions with noncoding RNAs. Consistent with this, a growing number of proteins involved in nuclear functions have been reported to bind RNA and in a few cases the RNA-binding regions of these proteins have been mapped, often through laborious, candidate-based methods.
Here, we report a detailed protocol to perform a high-throughput, proteome-wide unbiased identification of RNA-binding proteins and their RNA-binding regions. The methodology relies on the incorporation of a photoreactive uridine analog in the cellular RNA, followed by UV-mediated protein-RNA crosslinking, and mass spectrometry analyses to reveal RNA-crosslinked peptides within the proteome. Although we describe the procedure for mouse embryonic stem cells, the protocol should be easily adapted to a variety of cultured cells.

We describe a protocol to identify RNA-binding proteins and map their RNA-binding regions in live cells using UV-mediated photocrosslinking and mass spectrometry.

Noncoding RNAs play important roles in several nuclear processes, including regulating gene expression, chromatin structure, and DNA repair. In most ...

Evaluation of Protein&#8211;Protein Interactions using an On-Membrane Digestion Technique

Numerous intracellular proteins physically interact in accordance with their intracellular and extracellular circumstances. Indeed, cellular functions largely depend on intracellular protein&#8211;protein interactions. Therefore, research regarding these interactions is indispensable to facilitating the understanding of physiologic processes. Co-precipitation of associated proteins, followed by mass spectrometry (MS) analysis, enables the identification of novel protein interactions. In this study, we have provided details of the novel technique of immunoprecipitation-liquid chromatography (LC)-MS/MS analysis combined with on-membrane digestion for the analysis of protein&#8211;protein interactions. This technique is suitable for crude immunoprecipitants and can improve the throughput of proteomic analyses. Tagged recombinant proteins were precipitated using specific antibodies; next, immunoprecipitants blotted onto polyvinylidene difluoride membrane pieces were subjected to reductive alkylation. Following trypsinization, the digested protein residues were analyzed using LC-MS/MS. Using this technique, we were able to identify several candidate associated proteins. Thus, this method is convenient and useful for the characterization of novel protein&#8211;protein interactions.

Here, we present a protocol for an on-membrane digestion technique for the preparation of samples for mass spectrometry. This technique facilitates the convenient analysis of protein&#8211;protein interactions.

Numerous intracellular proteins physically interact in accordance with their intracellular and extracellular circumstances. Indeed, cellular functions ...

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 ...

Microsampling in Targeted Mass Spectrometry-Based Protein Analysis of Low-Abundance Proteins

This paper presents a protocol with detailed descriptions for efficient sample cleanup of low-abundance proteins from dried samples. This is performed using bead-based proteolysis prior to proteotypic peptide affinity-capture and liquid chromatography tandem mass spectrometry (LC-MS/MS) determination. The procedure can be applied to both conventional dried samples using paper cards (e.g., dried blood spots [DBSs] and dried serum spots [DSSs]), as well as samples collected with newer sampling methods such as volumetric absorptive microsampling (VAMS). In addition to describing this procedure, the preparation of both trypsin beads and antibody-coated beads is presented in a step-by-step manner in this work. The advantages of the presented procedure are time-efficient proteolysis using beads and selective robust cleanup using peptide affinity-capture. The current procedure describes the determination of the low-abundance small-cell lung cancer (SCLC) biomarker, progastrin-releasing peptide (ProGRP), in dried serum (both DSSs and VAMS). Detailed procedures for bead preparation make it easier to implement the workflow in new applications or other laboratories. It is demonstrated that the results may be dependent on the sampling material; for the present project, higher signal intensities were seen for samples collected using VAMS compared to DSSs.

A protocol is presented for the determination of low-abundance biomarkers from dried serum samples exemplified with the biomarker progastrin-releasing peptide (ProGRP). Antibody-coated magnetic beads are used for the selective cleanup and enrichment of a proteotypic ProGRP peptide. The captured peptide is subsequently analyzed by liquid chromatography-tandem mass spectrometry.

This paper presents a protocol with detailed descriptions for efficient sample cleanup of low-abundance proteins from dried samples. This is performed ...

Chemistry

Profiling of Methyltransferases and Other S-adenosyl-L-homocysteine-binding Proteins by Capture Compound Mass Spectrometry (CCMS)

There is a variety of approaches to reduce the complexity of the proteome on the basis of functional small molecule-protein interactions such as affinity chromatography 1 or Activity Based Protein Profiling 2. Trifunctional Capture Compounds (CCs, Figure 1A) 3 are the basis for a generic approach, in which the initial equilibrium-driven interaction between a small molecule probe (the selectivity function, here S-adenosyl-L-homocysteine, SAH, Figure 1A) and target proteins is irreversibly fixed upon photo-crosslinking between an independent photo-activable reactivity function (here a phenylazide) of the CC and the surface of the target proteins. The sorting function (here biotin) serves to isolate the CC - protein conjugates from complex biological mixtures with the help of a solid phase (here streptavidin magnetic beads). Two configurations of the experiments are possible: "off-bead" 4 or the presently described "on-bead" configuration (Figure 1B). The selectivity function may be virtually any small molecule of interest (substrates, inhibitors, drug molecules).
S-Adenosyl-L-methionine (SAM, Figure 1A) is probably, second to ATP, the most widely used cofactor in nature 5, 6. It is used as the major methyl group donor in all living organisms with the chemical reaction being catalyzed by SAM-dependent methyltransferases (MTases), which methylate DNA 7, RNA 8, proteins 9, or small molecules 10. Given the crucial role of methylation reactions in diverse physiological scenarios (gene regulation, epigenetics, metabolism), the profiling of MTases can be expected to become of similar importance in functional proteomics as the profiling of kinases. Analytical tools for their profiling, however, have not been available. We recently introduced a CC with SAH as selectivity group to fill this technological gap (Figure 1A).
SAH, the product of SAM after methyl transfer, is a known general MTase product inhibitor 11. For this reason and because the natural cofactor SAM is used by further enzymes transferring other parts of the cofactor or initiating radical reactions as well as because of its chemical instability 12, SAH is an ideal selectivity function for a CC to target MTases. Here, we report the utility of the SAH-CC and CCMS by profiling MTases and other SAH-binding proteins from the strain DH5&alpha; of Escherichia coli (E. coli), one of the best-characterized prokaryotes, which has served as the preferred model organism in countless biochemical, biological, and biotechnological studies. Photo-activated crosslinking enhances yield and sensitivity of the experiment, and the specificity can be readily tested for in competition experiments using an excess of free SAH.

Profiling of Methyltransferases and Other S-adenosyl-L-homocysteine-binding Proteins by Capture Compound Mass Spectrometry CCMS

Capture Compounds are trifunctional small molecules to reduce the complexity of the proteome by functional reversible small molecule-protein interaction followed by photo-crosslinking and purification. Here we use a Capture Compound with S-adenosyl-L-homocysteine-binding as selectivity function to isolate methyltransferases from an Escherichia coli whole cell lysate and identify them by MS.

There is a variety of approaches to reduce the complexity of the proteome on the basis of functional small molecule-protein interactions such as affinity ...

Biology

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 ...

A New Approach for the Comparative Analysis of Multiprotein Complexes Based on 15N Metabolic Labeling and Quantitative Mass Spectrometry

The introduced protocol provides a tool for the analysis of multiprotein complexes in the thylakoid membrane, by revealing insights into complex composition under different conditions. In this protocol the approach is demonstrated by comparing the composition of the protein complex responsible for cyclic electron flow (CEF) in Chlamydomonas reinhardtii, isolated from genetically different strains. The procedure comprises the isolation of thylakoid membranes, followed by their separation into multiprotein complexes by sucrose density gradient centrifugation, SDS-PAGE, immunodetection and comparative, quantitative mass spectrometry (MS) based on differential metabolic labeling (14N/15N) of the analyzed strains. Detergent solubilized thylakoid membranes are loaded on sucrose density gradients at equal chlorophyll concentration. After ultracentrifugation, the gradients are separated into fractions, which are analyzed by mass-spectrometry based on equal volume. This approach allows the investigation of the composition within the gradient fractions and moreover to analyze the migration behavior of different proteins, especially focusing on ANR1, CAS, and PGRL1. Furthermore, this method is demonstrated by confirming the results with immunoblotting and additionally by supporting the findings from previous studies (the identification and PSI-dependent migration of proteins that were previously described to be part of the CEF-supercomplex such as PGRL1, FNR, and cyt f). Notably, this approach is applicable to address a broad range of questions for which this protocol can be adopted and e.g. used for comparative analyses of multiprotein complex composition isolated from distinct environmental conditions.

A New Approach for the Comparative Analysis of Multiprotein Complexes Based on 15N Metabolic Labeling and Quantitative Mass Spectrometry

The described comparative, quantitative proteomic approach aims at obtaining insights into the composition of multiprotein complexes&#160;under different conditions and is demonstrated by comparing genetically different strains. For quantitative analysis equal volumes of different fractions from a sucrose density gradient are mixed and analyzed by mass spectrometry.

The introduced protocol provides a tool for the analysis of multiprotein complexes in the thylakoid membrane, by revealing insights into complex ...

Identification of Protein Interaction Partners in Mammalian Cells Using SILAC-immunoprecipitation Quantitative Proteomics

Quantitative proteomics combined with immuno-affinity purification, SILAC immunoprecipitation, represent a powerful means for the discovery of novel protein:protein interactions. By allowing the accurate relative quantification of protein abundance in both control and test samples, true interactions may be easily distinguished from experimental contaminants. Low affinity interactions can be preserved through the use of less-stringent buffer conditions and remain readily identifiable. This protocol discusses the labeling of tissue culture cells with stable isotope labeled amino acids, transfection and immunoprecipitation of an affinity tagged protein of interest, followed by the preparation for submission to a mass spectrometry facility. This protocol then discusses how to analyze and interpret the data returned from the mass spectrometer in order to identify cellular partners interacting with a protein of interest. As an example this technique is applied to identify proteins binding to the eukaryotic translation initiation factors: eIF4AI and eIF4AII.

SILAC immunoprecipitation experiments represent a powerful means for discovering novel protein:protein interactions. By allowing the accurate relative quantification of protein abundance in both control and test samples, true interactions may be easily distinguished from experimental contaminants, and low affinity interactions preserved through use of less-stringent buffer conditions.

Quantitative proteomics combined with immuno-affinity purification, SILAC immunoprecipitation, represent a powerful means for the discovery of novel ...

Characterization of Neuronal Lysosome Interactome with Proximity Labeling Proteomics

Lysosomes frequently communicate with a variety of biomolecules to achieve the degradation and other diverse cellular functions. Lysosomes are critical to human brain function, as neurons are postmitotic and rely heavily on the autophagy-lysosome pathway to maintain cellular homeostasis. Despite advancements in the understanding of various lysosomal functions, capturing the highly dynamic communications between lysosomes and other cellular components is technically challenging, particularly in a high-throughput fashion. Here, a detailed protocol is provided for the recently published endogenous (knock-in) lysosome proximity labeling proteomic method in human induced pluripotent stem cell (hiPSC)-derived neurons. 
Both lysosomal membrane proteins and proteins surrounding lysosomes within a 10-20 nm radius can be confidently identified and accurately quantified in live human neurons. Each step of the protocol is described in detail, i.e., hiPSC-neuron culture, proximity labeling, neuron harvest, fluorescence microscopy, biotinylated protein enrichment, protein digestion, LC-MS analysis, and data analysis. In summary, this unique endogenous lysosomal proximity labeling proteomics method provides a high-throughput and robust analytical tool to study the highly dynamic lysosomal activities in live human neurons.

A neuronal lysosome proximity labeling proteomics protocol is described here to characterize the dynamic lysosomal microenvironment in human induced pluripotent stem cell-derived neurons. Lysosomal membrane proteins and proteins that interact with lysosomes (stably or transiently) can be accurately quantified in this method with excellent intracellular spatial resolution in live human neurons.

Lysosomes frequently communicate with a variety of biomolecules to achieve the degradation and other diverse cellular functions. Lysosomes are critical to ...

Neuroscience

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.

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 ...

A Protein Preparation Method for the High-throughput Identification of Proteins Interacting with a Nuclear Cofactor Using LC-MS/MS Analysis

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