Supported by the NIH/NIDA DA034783. We thank Bryan A. Killinger for technical assistance with the multimer-PAGE.

1. Tissue Preparation
<ol>
	<li>Prepare 10 mL of 4x BN-PAGE sample buffer (200 mM Bis(2-hydroxyethyl)amino-tris(hydroxymethyl)methane (Bis-Tris), 200 mM NaCl, 40% w/v glycerol, 0.004% Ponceau S, pH 7.2). 
	NOTE: This stock solution may be made in advance and stored at 4 &#176;C.</li>
	<li>Dilute 250 &#181;L of 4x sample buffer in 750 &#181;L dH2O containing 1x commercial protease inhibitor cocktail. Vortex and chill on ice.</li>
	<li>Homogenize 20 mg of target tissue in the 1 mL 1x ice-cold BN-PAGE sample buffer with 30 strokes of a clean dounce homogenizer. 
	NOTE: For this demonstration experiment, the target tissue is whole rat brain tissue. After homogenization, samples may be treated with mild detergent such as 2% digitonin to solubilize and permit electrophoresis of membrane proteins.</li>
	<li>Transfer the homogenate to a 1.5 mL microcentrifuge tube, and centrifuge at 14,000 x g for 30 min to pellet insoluble cellular contents. After centrifugation, decant the supernatant into a clean tube on ice.</li>
	<li>Follow manufacturer instructions to measure supernatant protein concentration using bicinchoninic acid (BCA) or similar protein quantitation assay.</li>
	<li>If the homogenate sample(s) contain detergent, add a quantity of 5% Coomassie Blue G-250 in aqueous solution sufficient to bring the homogenate solution to 0.25% Coomassie.</li>
</ol>
2. BN-PAGE
<ol>
	<li>Dilute 25 mL 20x blue native (BN) electrophoresis buffer stock (1.0 M Bis-Tris, 1.0 M tricine, pH 6.8) into 475 mL dH2O containing 0.002% Coomassie Blue to make 500 mL of 1x running buffer.
	<ol>
		<li>If samples contain detergent, instead make 250 mL of 1x running buffer containing 0.02% Coomassie and another 250 mL containing none.</li>
	</ol>
	</li>
	<li>Chill buffer(s) to 4 &#176;C.</li>
	<li>Clean and assemble a polyacrylamide gel pouring cassette according to manufacturer instructions.</li>
	<li>Pour BN polyacrylamide gel (3% T stacking layer, 6% T resolving layer) according to a standard recipe7, and place the well comb.</li>
	<li>After the gels have polymerized, rinse the cassette with dH2O and assemble the electrophoresis apparatus according to manufacturer instructions.</li>
	<li>Remove the well comb and fill the wells with 1x BN-PAGE running buffer. Avoid filling the entire inner chamber with buffer until samples are loaded.
	<ol>
		<li>If samples contain detergent, fill the wells with the buffer containing 0.02% Coomassie Blue. 
		NOTE: Perform steps 2.7-2.9 at 4 &#176;C.</li>
	</ol>
	</li>
	<li>Using gel-loading tips, pipette a volume of homogenate containing 20 &#181;g of protein into the desired wells. Pipette an equivalent volume of 1x BN-PAGE sample buffer into any unused wells. 
	NOTE: Use the protein concentration determined from the BCA assay to calculate the appropriate volume of homogenate to add to the wells.</li>
	<li>After samples are loaded, fill the inner chamber with 1x BN-PAGE running buffer. Be sure the gel is entirely submerged in buffer. Next, fill the outer chamber with 1x running buffer to the level indicated by the manufacturer.
	<ol>
		<li>If samples contain detergent, fill the inner chamber with the 1x buffer containing 0.02% Coomassie Blue, and the outer chamber with dye-free 1x running buffer.</li>
	</ol>
	</li>
	<li>Connect the electrodes to the power supply, and electrophorese the proteins in the gel at 150 V until the dye band progresses ~2 cm into resolving layer. Stop and disconnect the power supply.</li>
</ol>
3. Cross-linking
NOTE: Perform Steps 3.1-3.6 at 4 &#176;C.
<ol>
	<li>Disassemble the electrophoresis apparatus, and separate the glass panes of the gel cassette. Remove and discard the stacking layer.</li>
	<li>Carefully cut the gel just below the bottom edge of the dye front. Take care to make this cut as smooth and straight as possible, and then discard the unused piece of gel. This will leave a ~2 cm wide, horizontal strip of polyacrylamide gel, which will contain all the protein in the homogenate sample.</li>
	<li>Trim away any unused portions along the edges of the gel strip.</li>
	<li>Carefully place the strip into 10 mL phosphate buffered saline (PBS) in a small container, and gently mix by nutation for 30 min to equilibrate.</li>
	<li>After equilibration, discard and replace the PBS with another 10 mL. Pipette 500 &#181;L of 25 mM DSP dissolved in dimethyl sulfoxide into the PBS, and continue mixing as above (step 3.4) for 30 min.</li>
	<li>Pour off the DSP solution. Add 10 mL of 0.375 M tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl), pH 8.8, containing 2% sodium dodecyl sulfate (SDS) to quench the unreacted DSP. Continue nutation for 15 min.</li>
</ol>
4. SDS-PAGE
<ol>
	<li>While gel strip is quenching, prepare SDS-PAGE gel solutions according to standard methods9. Do not add polymerization reagents.</li>
	<li>After quenching, return the &#914;&#925; gel strip to room temperature, and cast the strip into a new gel cassette.
	<ol>
		<li>To do this, carefully pick up the gel and place it onto a clean gel cassette spacer plate.</li>
		<li>Orient the strip so the bottom of the dye front is nearest to the top of new cassette (i.e., the side that travelled furthest during BN-PAGE should be closest to the top of the new cassette; the gel strip should be flipped from its prior orientation). See Figure 3.</li>
		<li>Place the strip such that its top edge lies even with where the top edge of the cover plate will be (i.e., it will be at the top of the new gel). Make sure the dye front is parallel to the horizontal edges of the glass plate.</li>
		<li>Push one side of the excised strip against one of the spacer walls, leaving room on the other side for gel to be poured and a protein standard or ladder to be loaded.</li>
		<li>If the bottom edge of the gel strip contains any jagged or uneven areas, carefully cut them away. If present, they will trap bubbles during gel pouring.</li>
		<li>Once the gel strip is positioned correctly, lay the cover plate over the spacer plate. Apply gentle pressure to push out any trapped air bubbles.</li>
		<li>Continue to assemble the gel-pouring apparatus according to manufacturer instructions.</li>
	</ol>
	</li>
	<li>Add polymerization reagents to the resolving gel buffer, and pour it into the prepared gel cassette, using a serological pipette. Fill the gel cassette to ~2 cm below the excised BN-PAGE gel strip, to leave room for the stacking layer.</li>
	<li>Add 100 &#181;L butanol over the top of the poured gel, and allow 30 min for polymerization of the resolving layer. Pour off the butanol.</li>
	<li>Add polymerization reagents to the stacking gel solution. Using a serological pipette, pour the stacking layer to fill all remaining empty space in the gel cassette.
	<ol>
		<li>Tilt the gel cassette as the stacking layer is poured so it fills the space below the gel strip, and air bubbles are not trapped.</li>
		<li>As the stacking gel buffer fills the empty space below the gel strip, gradually return the gel cassette to level footing.</li>
		<li>Continue to fill the empty space next to the excised gel strip with the stacking gel buffer, until it nearly overflows.</li>
	</ol>
	</li>
	<li>Allow the stacking layer to polymerize for 30 min. 
	NOTE: Make 10x SDS-PAGE running buffer (250 mM tris(hydroxymethyl)aminomethane (tris), 1.9 M glycine, 1% SDS) while the gel is polymerizing. This can also be made beforehand and stored at room temperature.</li>
	<li>Dilute 50 mL 10X SDS-PAGE running buffer stock into 450 mL dH2O to make 1x working buffer.</li>
	<li>After the stacking layer has polymerized, remove the gel cassette from the pouring apparatus, rinse with dH2O, and assemble the electrophoresis apparatus according to manufacturer instructions.</li>
	<li>Fill the inner chamber completely with 1x SDS-PAGE running buffer, then fill the outer chamber to the level indicated by the manufacturer.</li>
	<li>Load the space next to the excised gel strip with a molecular weight ladder or appropriate protein standard.</li>
	<li>Attach the electrodes to the power supply, and electrophorese the samples at 120 V. When the Coomassie dye runs off the gel, stop and disconnect the power supply.</li>
	<li>Analyze the gel using standard methods of electroblotting and protein detection by antibody binding10.</li>
</ol>

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	<li>Alberts, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+cell+as+a+collection+of+protein+machines:+preparing+the+next+generation+of+molecular+biologists.">The cell as a collection of protein machines: preparing the next generation of molecular biologists.</a> Cell. 92 (3), 291-294 (1998).</li><li>Pawson, T., Nash, P. <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+define+specificity+in+signal+transduction.">Protein-protein interactions define specificity in signal transduction.</a> Genes Dev. 14 (9), 1027-1047 (2000).</li><li>Rual, J. F., 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=Towards+a+proteome-scale+map+of+the+human+protein-protein+interaction+network.">Towards a proteome-scale map of the human protein-protein interaction network.</a> Nature. 437 (7062), 1173-1178 (2005).</li><li>Phizicky, E. M., Fields, S. <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:+methods+for+detection+and+analysis.">Protein-protein interactions: methods for detection and analysis.</a> Microbiol Rev. 59 (1), 94-123 (1995).</li><li>Ho, 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=Systematic+identification+of+protein+complexes+in+Saccharomyces+cerevisiae+by+mass+spectrometry.">Systematic identification of protein complexes in Saccharomyces cerevisiae by mass spectrometry.</a> Nature. 415 (6868), 180-183 (2002).</li><li>Krogan, N. J., 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=Global+landscape+of+protein+complexes+in+the+yeast+Saccharomyces+cerevisiae.">Global landscape of protein complexes in the yeast Saccharomyces cerevisiae.</a> Nature. 440 (7084), 637-643 (2006).</li><li>Wittig, I., Braun, H. P., Schagger, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Blue+native+PAGE.">Blue native PAGE.</a> Nat Protoc. 1 (1), 418-428 (2006).</li><li>Schagger, H., Cramer, W. A., von Jagow, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+molecular+masses+and+oligomeric+states+of+protein+complexes+by+blue+native+electrophoresis+and+isolation+of+membrane+protein+complexes+by+two-dimensional+native+electrophoresis.">Analysis of molecular masses and oligomeric states of protein complexes by blue native electrophoresis and isolation of membrane protein complexes by two-dimensional native electrophoresis.</a> Anal Biochem. 217 (2), 220-230 (1994).</li><li>Laemmli, U. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cleavage+of+structural+proteins+during+the+assembly+of+the+head+of+bacteriophage+T4.">Cleavage of structural proteins during the assembly of the head of bacteriophage T4.</a> Nature. 227 (5259), 680-685 (1970).</li><li>Beisiegel, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+Blotting.">Protein Blotting.</a> Electrophoresis. 7 (1), 1-18 (1986).</li><li>Killinger, B. A., Moszczynska, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterization+of+alpha-Synuclein+Multimer+Stoichiometry+in+Complex+Biological+Samples+by+Electrophoresis.">Characterization of alpha-Synuclein Multimer Stoichiometry in Complex Biological Samples by Electrophoresis.</a> Anal Chem. 88 (7), 4071-4084 (2016).</li><li>Newman, A. J., Selkoe, D., Dettmer, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+method+for+quantitative+immunoblotting+of+endogenous+alpha-synuclein.">A new method for quantitative immunoblotting of endogenous alpha-synuclein.</a> PLoS One. 8 (11), e81314 (2013).</li><li>Wong, S. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Chemistry of protein conjugation and cross-linking. , (1991).</li><li>Seddon, A. M., Curnow, P., Booth, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Membrane+proteins,+lipids+and+detergents:+not+just+a+soap+opera.">Membrane proteins, lipids and detergents: not just a soap opera.</a> Biochim Biophys Acta. 1666 (1-2), 105-117 (2004).</li><li>Tanford, C., Reynolds, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterization+of+membrane+proteins+in+detergent+solutions.">Characterization of membrane proteins in detergent solutions.</a> Biochim Biophys Acta. 457 (2), 133-170 (1976).</li><li>Peters, K., Richards, F. 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The authors have nothing to disclose.

Protein-protein interactions are important for every task living things carry out. Because of this, they are the subject of intense scrutiny and research. Multimer-PAGE is a novel method for capturing, separating, and analyzing a wide range of protein complexes. We have previously demonstrated its applicability to studying oligimerization of the disease-associated protein &#945;-synuclein11. However, it is extensible to many protein complexes, as demonstrated in Figure 2. When com...

Normal cellular function is dependent on protein-protein interactions1,2. As a result, human diseases are often marked by perturbations in the assembly and behavior of various protein complexes3. The ability to characterize such interactions is therefore critical. Current means of detecting these interactions require purification of target proteins, often followed by pull-down of their interacting partners. Conventional purification is accomplished by differential centrifugation, precipitation, and/or chromatography4. These methods are time-consuming, must be altered for each target protein, and often result in low yields. Modern purification methods involve the fusion of peptide tags to target proteins, followed by immunoprecipitation or extraction on columns loaded with beads bound to a capture molecule5,6. While this is extensible to many proteins, it requires sequence modification of the target, potentially resulting in constructs that differ in affinity for their usual binding partners. The delicate nature of some protein-protein interactions means this method may not be applicable to many scenarios. Additionally, the pull-down assays used to map protein-protein interactions may not capture an accurate cellular picture, due to restricted degrees of freedom and non-native levels of the bait protein.
Ideally, protein complexes could be detected in their native states, without the need for purification or pull-down. Blue native polyacrylamide gel electrophoresis (BN-PAGE) was developed as a less-denaturing alternative to sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and allows for the separation of some proteins and complexes from biological samples7. However, proteins in BN-PAGE separate based on a large number of variables, including size, charge, three-dimensional structure, and association with other molecules. The interactions of these factors often result in co-separation of proteins, aggregate formation, and poor protein band resolution. Two-dimensional native-polyacrylamide gel electrophoresis resolves some, but not all, of these problems8.
To circumvent the complications associated with native separation, some authors use amine-reactive cross-linking reagents, such as dithiobis(succinimidyl propionate) (DSP), to capture protein complexes in tissue lysates4. These treated lysates can then be denatured and separated by SDS-PAGE, while preserving the native size and makeup of protein complexes. However, since cross-linking reagents react based on proximity of one molecule to another, and proteins in tissue lysates have many degrees of freedom and can stochastically interact, nonspecific background cross-linking can be high, especially in concentrated samples. This can lead to difficult-to-interpret results.
Here, we demonstrate the use of a hybrid BN-PAGE/SDS-PAGE method, termed multimer-polyacrylamide gel electrophoresis (multimer-PAGE), to separate and detect protein assemblies in complex mixtures. Initially, cell lysate is suspended in polyacrylamide gel via BN-PAGE. The lysate-containing gel is then reacted with the cross-linking reagent DSP. Pseudo-immobilized and slightly separated on the gel, proteins are much less likely to react nonspecifically, meaning background cross-linker reactivity is reduced. After cross-linking, the gel bands are excised and separated via SDS-PAGE. The resultant gel can then be analyzed by any means typically associated with polyacrylamide gel electrophoresis. This method allows for the separation and detection of native protein complexes in unmodified tissue lysate, without the need for additional purification or pull-down.

<table><tbody><tr><td>&lt;strong&gt;Chemicals&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>&amp;epsilon;-Aminocaproic acid</td><td>Sigma</td><td>A2504</td><td></td></tr><tr><td>Acrylamide</td><td>Acros Organics</td><td>164855000</td><td>Toxic.</td></tr><tr><td>Acrylamide/bisacrilamide 37.5:1 (40%T stock solution)</td><td>BioRad</td><td>161-0148</td><td>Toxic.</td></tr><tr><td>Ammonium persulfate</td><td>Sigma</td><td>A3678</td><td></td></tr><tr><td>Anti rabbit IgG-HRP from goat</td><td>Santa Cruz Biotechnology</td><td>sc-2004</td><td></td></tr><tr><td>Bicinchoninic acid assay kit</td><td>Thermofisher</td><td>23225</td><td></td></tr><tr><td>Bis-tris</td><td>Sigma</td><td>B9754</td><td></td></tr><tr><td>Bovine serum albumin</td><td>Sigma</td><td>A9647</td><td></td></tr><tr><td>Butanol</td><td>Fisher Scientific</td><td>A399-1</td><td></td></tr><tr><td>Chemiluminescence substrate kit</td><td>ThermoFisher</td><td>24078</td><td></td></tr><tr><td>Coomassie blue G-250</td><td>Sigma-Aldrich</td><td>B0770</td><td></td></tr><tr><td>Digitonin</td><td>Sigma</td><td>D141</td><td>Toxic.</td></tr><tr><td>Dimethyl sulfoxide</td><td>Fisher Scientific</td><td>D128-1</td><td></td></tr><tr><td>Dithiobis(succinimidylpropionate)</td><td>Thermo Scientific</td><td>22585</td><td></td></tr><tr><td>Dry nonfat milk</td><td>LabScientific</td><td>M0841</td><td></td></tr><tr><td>Glycerol</td><td>Sigma</td><td>G9012</td><td></td></tr><tr><td>Glycine</td><td>Fisher Scientific</td><td>BP381-5</td><td></td></tr><tr><td>Halt Protease Inhibitor Cocktail</td><td>Thermofisher</td><td>78430</td><td></td></tr><tr><td>Hydrochloric acid</td><td>Fisher Scientific</td><td>A144SI-212</td><td>For titration. Caustic.</td></tr><tr><td>Methanol</td><td>Fisher Scientific</td><td>A412-4</td><td>For PVDF membrane activation. Toxic.</td></tr><tr><td>Monoclonal anti &amp;alpha;-synuclein IgG from rabbit</td><td>Santa Cruz Biotechnology</td><td>sc-7011-R</td><td></td></tr><tr><td>N,N,N&#039;,N&#039;-tetramethylethylenediamine</td><td>Sigma</td><td>T9281</td><td></td></tr><tr><td>N,N&#039;-methylenebisacrylamide</td><td>Acros Organics</td><td>16479</td><td>Toxic.</td></tr><tr><td>NP40</td><td>Boston Bioproducts</td><td>P-872</td><td></td></tr><tr><td>Polysorbate 20 (tween-20)</td><td>Fisher Scientific</td><td>BP337-500</td><td></td></tr><tr><td>Polyvinylidene fluoride transfer membranes</td><td>Thermo Scientific</td><td>88518</td><td></td></tr><tr><td>Ponceau S</td><td>Sigma</td><td>P3504</td><td></td></tr><tr><td>Potassium chloride</td><td>Fisher Scientific</td><td>P217-3</td><td></td></tr><tr><td>Potassium phosphate monobasic</td><td>Sigma</td><td>P9791</td><td></td></tr><tr><td>Protease inhibitor cocktail</td><td>Thermo Scientific</td><td>88265</td><td></td></tr><tr><td>SDS solution (10% w/v)</td><td>BioRad</td><td>161-0416</td><td></td></tr><tr><td>Sodium chloride</td><td>Fisher Scientific</td><td>BP358-212</td><td></td></tr><tr><td>Sodium dodecyl sulfate</td><td>Sigma</td><td>L37771</td><td></td></tr><tr><td>Sodium phosphate monobasic</td><td>Fisher Scientific</td><td>BP329-1</td><td></td></tr><tr><td>Tricine</td><td>Sigma</td><td>T0377</td><td></td></tr><tr><td>Tris base</td><td>Fisher Scientific</td><td>BP152-500</td><td></td></tr><tr><td>Tris-HCl (0.5 M buffer, pH 6.8)</td><td>BioRad</td><td>161-0799</td><td></td></tr><tr><td>Tris-HCl (1.5 M buffer, pH 8.8)</td><td>BioRad</td><td>161-0798</td><td></td></tr><tr><td>&lt;strong&gt;Name&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Catalog Number&lt;/strong&gt;</td><td>&lt;strong&gt;Comments&lt;/strong&gt;</td></tr><tr><td>&lt;strong&gt;Instruments&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>GE Imagequant LAS 4000</td><td>GE Healthcare</td><td>28-9558-10</td><td></td></tr><tr><td>ImageJ software</td><td>NIH</td><td></td><td></td></tr><tr><td>Synergy H1 microplate reader</td><td>BioTek</td><td></td><td></td></tr><tr><td>Gel Former + Stand</td><td>Biorad</td><td></td><td></td></tr><tr><td>Microfuge 22R centrifuge</td><td>Beckman Coulter</td><td></td><td></td></tr><tr><td>2 mL dounce homogenizer</td><td></td><td></td><td></td></tr><tr><td>Vortex mixer</td><td>Fisher Scientific</td><td></td><td></td></tr><tr><td>Ultrasonic tissue homogenizer</td><td>Fisher Scientific</td><td>FB120220</td><td></td></tr></tbody></table>

multimer-page: a method for capturing and resolving protein complexes in biological samples

There are many well-developed methods for purifying and studying single proteins and peptides. However, most cellular functions are carried out by networks of interacting protein complexes, which are often difficult to investigate because their binding is non-covalent and easily perturbed by purification techniques. This work describes a method of stabilizing and separating native protein complexes from unmodified tissue using two-dimensional polyacrylamide gel electrophoresis. Tissue lysate is loaded onto a non-denaturing blue-native polyacrylamide gel, then an electric current is applied until the protein migrates a short distance into the gel. The gel strip containing the migrated protein is then excised and incubated with the amine-reactive cross-linking reagent dithiobis(succinimidyl propionate), which covalently stabilizes protein complexes. The gel strip containing cross-linked complexes is then cast into a sodium dodecyl sulfate polyacrylamide gel, and the complexes are separated completely. The method relies on techniques and materials familiar to most molecular biologists, meaning it is inexpensive and easy to learn. While it is limited in its ability to adequately separate extremely large complexes, and has not been universally successful, the method was able to capture a wide variety of well-studied complexes, and is likely applicable to many systems of interest.

In this demonstration experiment, multimer-PAGE was performed on whole rat brain lysate. The resultant separated proteins were blotted onto polyvinylidene diflouride (PVDF) membranes, and then probed with antibodies against proteins that are known to form complexes. Figure 1 shows a validation of the protocol by two means. First, we demonstrate that the cross-linked proteins are cleavable by addition of a reducing agent, meaning the observed higher molecular weight specie...

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Multimer-PAGE: A Method for Capturing and Resolving Protein Complexes in Biological Samples

A method for stabilizing and separating native protein complexes from unmodified tissue lysate using an amine-reactive protein cross-linker coupled to a novel two-dimensional polyacrylamide gel electrophoresis (PAGE) system is presented.

There are many well-developed methods for purifying and studying single proteins and peptides. However, most cellular functions are carried out by ...

multimer-page-method-for-capturing-resolving-protein-complexes

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Biochemistry

11.3K Views.  Michigan State University.  Een werkwijze voor het stabiliseren en scheiden van natieve eiwitcomplexen van ongewijzigd weefsellysaat onder toepassing van een amino-reactieve eiwit-crosslinker gekoppeld aan een nieuw tweedimensionaal polyacrylamide gelelektroforese (PAGE) systeem wordt gepresenteerd. 

Video: Multimer-PAGE: A Method for Capturing and Resolving Protein Complexes in Biological Samples

Protein Complex Affinity Capture from Cryomilled Mammalian Cells

Affinity capture is an effective technique for isolating endogenous protein complexes for further study. When used in conjunction with an antibody, this technique is also frequently referred to as immunoprecipitation. Affinity capture can be applied in a bench-scale and in a high-throughput context. When coupled with protein mass spectrometry, affinity capture has proven to be a workhorse of interactome analysis. Although there are potentially many ways to execute the numerous steps involved, the following protocols implement our favored methods. Two features are distinctive: the use of cryomilled cell powder to produce cell extracts, and antibody-coupled paramagnetic beads as the affinity medium. In many cases, we have obtained superior results to those obtained with more conventional affinity capture practices. Cryomilling avoids numerous problems associated with other forms of cell breakage. It provides efficient breakage of the material, while avoiding denaturation issues associated with heating or foaming. It retains the native protein concentration up to the point of extraction, mitigating macromolecular dissociation. It reduces the time extracted proteins spend in solution, limiting deleterious enzymatic activities, and it may reduce the non-specific adsorption of proteins by the affinity medium. Micron-scale magnetic affinity media have become more commonplace over the last several years, increasingly replacing the traditional agarose- and Sepharose-based media. Primary benefits of magnetic media include typically lower non-specific protein adsorption; no size exclusion limit because protein complex binding occurs on the bead surface rather than within pores; and ease of manipulation and handling using magnets.

Here we describe protocols to disrupt mammalian cells by solid-state milling at a cryogenic temperature, produce a cell extract from the resulting cell powder, and isolate protein complexes of interest by affinity capture upon antibody-coupled micron-scale paramagnetic beads.

Affinity capture is an effective technique for isolating endogenous protein complexes for further study. When used in conjunction with an antibody, this ...

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 ...

Analyzing Dynamic Protein Complexes Assembled On and Released From Biolayer Interferometry Biosensor Using Mass Spectrometry and Electron Microscopy

In vivo, proteins are often part of large macromolecular complexes where binding specificity and dynamics ultimately dictate functional outputs. In this work, the pre-endosomal anthrax toxin is assembled and transitioned into the endosomal complex. First, the N-terminal domain of a cysteine mutant lethal factor (LFN) is attached to a biolayer interferometry (BLI) biosensor through disulfide coupling in an optimal orientation, allowing protective antigen (PA) prepore to bind (Kd 1 nM). The optimally oriented LFN-PAprepore complex then binds to soluble capillary morphogenic gene-2 (CMG2) cell surface receptor (Kd 170 pM), resulting in a representative anthrax pre-endosomal complex, stable at pH 7.5. This assembled complex is then subjected to acidification (pH 5.0) representative of the late endosome environment to transition the PAprepore into the membrane inserted pore state. This PApore state results in a weakened binding between the CMG2 receptor and the LFN-PApore and a substantial dissociation of CMG2 from the transition pore. The thio-attachment of LFN to the biosensor surface is easily reversed by dithiothreitol. Reduction on the BLI biosensor surface releases the LFN-PAprepore-CMG2 ternary complex or the acid transitioned LFN-PApore complexes into microliter volumes. Released complexes are then visualized and identified using electron microscopy and mass spectrometry. These experiments demonstrate how to monitor the kinetic assembly/disassembly of specific protein complexes using label-free BLI methodologies and evaluate the structure and identity of these BLI assembled complexes by electron microscopy and mass spectrometry, respectively, using easy-to-replicate sequential procedures.

Here we present a protocol to monitor the assembly and disassembly of the anthrax toxin using biolayer interferometry (BLI). Following assembly/disassembly on the biosensor surface, the large protein complexes are released from the surface for visualization and identification of components of the complexes using electron microscopy and mass spectrometry, respectively.

In vivo, proteins are often part of large macromolecular complexes where binding specificity and dynamics ultimately dictate functional outputs. In this ...

High-Resolution Complexome Profiling by Cryoslicing BN-MS Analysis

Proteins generally exert biological functions through interactions with other proteins, either in dynamic protein assemblies or as a part of stably formed complexes. The latter can be elegantly resolved according to molecular size using native polyacrylamide gel electrophoresis (BN-PAGE). Coupling of such separations to sensitive mass spectrometry (BN-MS) has been well-established and theoretically allows for exhaustive assessment of the extractable complexome in biological samples. However, this approach is rather laborious and provides limited complex size resolution and sensitivity. Also, its application has remained restricted to abundant mitochondrial and plastid proteins. Thus, for a majority of proteins, information regarding integration into stable protein complexes is still lacking. Presented here is an optimized approach for complexome profiling comprising preparative-scale BN-PAGE separation, sub-millimeter sampling of broad gel lanes by cryomicrotome slicing, and mass spectrometric analysis with label-free protein quantification. The procedures and tools for critical steps are described in detail. As an application, the report describes complexome analysis of a solubilized endosome-enriched membrane fraction from mouse kidneys, with 2,545 proteins profiled in total. The results demonstrate identification of uniform, low-abundance membrane proteins such as intracellular ion channels as well as high resolution, complex protein assembly patterns, including glycosylation isoforms. The results are in agreement with independent biochemical analyses. In summary, this methodology allows for comprehensive and unbiased identification of protein (super)complexes and their subunit composition, providing a basis for investigating stoichiometry, assembly, and interaction dynamics of protein complexes in any biological system.

A versatile cryoslicing BN-MS protocol using a microtome is presented for high-resolution complexome profiling.

Proteins generally exert biological functions through interactions with other proteins, either in dynamic protein assemblies or as a part of stably formed ...

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 ...

Isolation of Labile Multi-protein Complexes by in vivo Controlled Cellular Cross-Linking and Immuno-magnetic Affinity Chromatography

The dynamic nature of cellular machineries is frequently built on transient and/or weak protein associations. These low affinity interactions preclude stringent methods for the isolation and identification of protein networks around a protein of interest. The use of chemical crosslinkers allows the selective stabilization of labile interactions, thus bypassing biochemical limitations for purification. Here we present a protocol amenable for cells in culture that uses a homobifunctional crosslinker with a spacer arm of 12 &Aring;, dithiobis-(succinimidyl proprionate) (DSP). DSP is cleaved by reduction of a disulphide bond present in the molecule. Cross-linking combined with immunoaffinity chromatography of proteins of interest with magnetic beads allows the isolation of protein complexes that otherwise would not withstand purification. This protocol is compatible with regular western blot techniques and it can be scaled up for protein identification by mass spectrometry1.
Stephanie A. Zlatic and Pearl V. Ryder contributed equally to this work.

Isolation of Labile Multi-protein Complexes by in vivo Controlled Cellular Cross-Linking and Immuno-magnetic Affinity Chromatography

The cell permeable crosslinker DSP [dithiobis-(succinimidyl propionate)] stabilizes transient and labile interactions in vivo, which allows their isolation using stringent protein complex purification techniques. Here we present a technique for crosslinking cells grown in culture followed by isolation of protein complexes by immunoprecipitation.

The dynamic nature of cellular machineries is frequently built on transient and/or weak protein associations. These low affinity interactions preclude ...

Biology

Analyzing Large Protein Complexes by Structural Mass Spectrometry

Living cells control and regulate their biological processes through the coordinated action of a large number of proteins that assemble themselves into an array of dynamic, multi-protein complexes1. To gain a mechanistic understanding of the various cellular processes, it is crucial to determine the structure of such protein complexes, and reveal how their structural organization dictates their function. Many aspects of multi-protein complexes are, however, difficult to characterize, due to their heterogeneous nature, asymmetric structure, and dynamics. Therefore, new approaches are required for the study of the tertiary levels of protein organization.
One of the emerging structural biology tools for analyzing macromolecular complexes is mass spectrometry (MS)2-5. This method yields information on the complex protein composition, subunit stoichiometry, and structural topology. The power of MS derives from its high sensitivity and, as a consequence, low sample requirement, which enables examination of protein complexes expressed at endogenous levels. Another advantage is the speed of analysis, which allows monitoring of reactions in real time. Moreover, the technique can simultaneously measure the characteristics of separate populations co-existing in a mixture. 
Here, we describe a detailed protocol for the application of structural MS to the analysis of large protein assemblies. The procedure begins with the preparation of gold-coated capillaries for nanoflow electrospray ionization (nESI). It then continues with sample preparation, emphasizing the buffer conditions which should be compatible with nESI on the one hand, and enable to maintain complexes intact on the other. We then explain, step-by-step, how to optimize the experimental conditions for high mass measurements and acquire MS and tandem MS spectra. Finally, we chart the data processing and analyses that follow. Rather than attempting to characterize every aspect of protein assemblies, this protocol introduces basic MS procedures, enabling the performance of MS and MS/MS experiments on non-covalent complexes. Overall, our goal is to provide researchers unacquainted with the field of structural MS, with knowledge of the principal experimental tools.

Mass spectrometry has proven to be a valuable tool for analyzing large protein complexes. This method enables insights into the composition, stoichiometry and overall architecture of multi-subunit assemblies. Here, we describe, step-by-step, how to perform a structural mass spectrometry analysis, and characterize macromolecular structures.

Living cells control and regulate their biological processes through the coordinated action of a large number of proteins that assemble themselves into an ...

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 ...

The MultiBac Protein Complex Production Platform at the EMBL

Proteomics research revealed the impressive complexity of eukaryotic proteomes in unprecedented detail. It is now a commonly accepted notion that proteins in cells mostly exist not as isolated entities but exert their biological activity in association with many other proteins, in humans ten or more, forming assembly lines in the cell for most if not all vital functions.1,2 Knowledge of the function and architecture of these multiprotein assemblies requires their provision in superior quality and sufficient quantity for detailed analysis. The paucity of many protein complexes in cells, in particular in eukaryotes, prohibits their extraction from native sources, and necessitates recombinant production. The baculovirus expression vector system (BEVS) has proven to be particularly useful for producing eukaryotic proteins, the activity of which often relies on post-translational processing that other commonly used expression systems often cannot support.3 BEVS use a recombinant baculovirus into which the gene of interest was inserted to infect insect cell cultures which in turn produce the protein of choice. MultiBac is a BEVS that has been particularly tailored for the production of eukaryotic protein complexes that contain many subunits.4 A vital prerequisite for efficient production of proteins and their complexes are robust protocols for all steps involved in an expression experiment that ideally can be implemented as standard operating procedures (SOPs) and followed also by non-specialist users with comparative ease. The MultiBac platform at the European Molecular Biology Laboratory (EMBL) uses SOPs for all steps involved in a multiprotein complex expression experiment, starting from insertion of the genes into an engineered baculoviral genome optimized for heterologous protein production properties to small-scale analysis of the protein specimens produced.5-8 The platform is installed in an open-access mode at EMBL Grenoble and has supported many scientists from academia and industry to accelerate protein complex research projects.

Protein complexes catalyze key cellular functions. Detailed functional and structural characterization of many essential complexes requires recombinant production. MultiBac is a baculovirus/insect cell system particularly tailored for expressing eukaryotic proteins and their complexes. MultiBac was implemented as an open-access platform, and standard operating procedures developed to maximize its utility.

Proteomics  research revealed the impressive complexity of eukaryotic proteomes in  unprecedented detail. It is now a commonly accepted notion that ...

Discovering Protein Interactions and Characterizing Protein Function Using HaloTag Technology

Research in proteomics has exploded in recent years with advances in mass spectrometry capabilities that have led to the characterization of numerous proteomes, including those from viruses, bacteria, and yeast.&#160; In comparison, analysis of the human proteome lags behind, partially due to the sheer number of proteins which must be studied, but also the complexity of networks and interactions these present. To specifically address the challenges of understanding the human proteome, we have developed HaloTag technology for protein isolation, particularly strong for isolation of multiprotein complexes and allowing more efficient capture of weak or transient interactions and/or proteins in low abundance. &#160;HaloTag is a genetically encoded protein fusion tag, designed for covalent, specific, and rapid immobilization or labelling of proteins with various ligands. Leveraging these properties, numerous applications for mammalian cells were developed to characterize protein function and here we present methodologies including: protein pull-downs used for discovery of novel interactions or functional assays, and cellular localization. We find significant advantages in the speed, specificity, and covalent capture of fusion proteins to surfaces for proteomic analysis as compared to other traditional non-covalent approaches. We demonstrate these and the broad utility of the technology using two important epigenetic proteins as examples, the human bromodomain protein BRD4, and histone deacetylase HDAC1.&#160; These examples demonstrate the power of this technology in enabling &#160;the discovery of novel interactions and characterizing cellular localization in eukaryotes, which will together further understanding of human functional proteomics. &#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;

HaloTag technology is a multifunctional technology which has shown significant success in isolation of both small and large protein complexes from mammalian cells.&#160; Here we highlight the advantages of this technology compared to existing alternatives and demonstrate its utility to study numerous aspects of protein function inside eukaryotic cells.

Research in proteomics has exploded in recent years with advances in mass spectrometry capabilities that have led to the characterization of numerous ...

Multimer-PAGE: A Method for Capturing and Resolving Protein Complexes in Biological Samples

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