This work was supported by a V scholar award (V2014-004) and a V scholar plus award (D2016-030) from the V Foundation for Cancer Research, and an American Cancer Society Institutional Research Grant (ACS IRG Grant 58-006-53) to the Purdue University Center for Cancer Research. E. G. P. was supported by the Borch Graduate Endowment Award to the Purdue University Medicinal Chemistry and Molecular Pharmacology Department.

1. Preparations
<ol>
	<li>Prepare 100 mL of hypotonic solution Buffer A: 0.3 M sucrose, 60 mM KCl, 60 mM Tris pH 8.0, 2 mM EDTA, and 0.5% NP-40. Store at 4 &#186;C. 
	NOTE: Some cell lines, such as HEK293T, require less stringent lysing conditions. If nuclei lyse easily, use Modified Buffer A: 25 mM HEPES pH 7.6, 25 mM KCl, 5 mM MgCl2, 0.05 mM EDTA, 0.1% NP-40, and 10% glycerol.</li>
	<li>Prepare a 250 mL stock solution of 2x mRIPA solution: 100 mM Tris pH 8.0, 2% NP-40, and 0.5% sodium deoxycholate.</li>
	<li>Prepare a 100 mL stock solution of 5 M NaCl.</li>
	<li>Using the 2x mRIPA and 5 M NaCl solutions, prepare 50 mL of 1x mRIPA solution for each of the six salt concentrations: 0 mM, 100 mM, 200 mM, 300 mM, 400 mM, and 500 mM NaCl. Prior to starting the experiment, cool all solutions on ice.</li>
	<li>Grow cell lines under desired conditions. 
	NOTE: When determining the effects of knocking down a protein, SSE for the knockdown line must be compared to wildtype cells. These SSE must be performed side by side. For examples using chemical inhibitors or stimuli, refer to sections 3 - 5.</li>
</ol>
2. Basic Sequential Salt Extraction
<ol>
	<li>Harvest 8 million cells of each condition and wash twice with 5 mL of ice cold PBS. 
	NOTE: It is critical to have equal number of cells to have equivalent protein concentrations. The exact number of cells may need to be optimized for individual cell lines or particular proteins. It is important not to have too many, as the increase in protein concentrations will prevent observation of the elution curve, which is dependent on equilibrium binding. However, with too few&#160;cells there may not be enough protein to detect the curve, and the chromatin pellet is harder to isolate and can be lost between fractions.</li>
	<li>Re-suspend cells in 1 mL of Buffer A + protease inhibitors, transfer into 1.5 mL micro centrifuge tubes, and rotate top over bottom at 4 &#186;C for 10 min.</li>
	<li>Isolate the nuclei by centrifugation at 6000 x g for 5 min at 4 &#186;C . 
	NOTE: After this step the nuclear envelope should still be intact. If the nuclear envelope is intact, the pellet will re-suspend fully; however, when the envelope is lysed and the chromatin is released, the pellet will not re-suspend. If the nuclear envelope is not intact after Buffer A incubation, use Modified Buffer A.</li>
	<li>Add 200 &#181;L of mRIPA 0 mM NaCl + protease inhibitors to each nuclei pellet before re-suspending pellet. Homogenize each sample by pipetting 15 times with a 1 mL pipette. The pellet should re-suspend easily, however, the nuclei will lyse as it is pipetted and the sample will become thick and sticky and difficult to pipette.
	<ol>
		<li>In order to draw up the pellet into the tip, tap the end of the pipette tip against the bottom of the centrifuge tube. The pellet will not fully dissolve once the DNA is released from the nuclei, but pipetting will break it up.</li>
	</ol>
	</li>
	<li>When all the samples have been homogenized, incubate all the samples on ice for 3 min. 
	NOTE: The incubation time may need to be optimized depending on the protein of interest.</li>
	<li>Isolate chromatin pellet by centrifuging the samples for 3 min at 6500 x g at 4 &#186;C.</li>
	<li>Transfer supernatant to a clean 1.5 mL centrifuge tube. This will be the 0 mM fraction. This 0 mM mRIPA solution can act as a wash step to lysis the nuclei and remove any loose proteins not associated with chromatin.</li>
	<li>Add 200 &#181;L of mRIPA 100 mM NaCl + protease inhibitors to each chromatin pellet before re-suspending pellet. Break up the chromatin pellet by pipetting the pellet up and down 15 times. 
	NOTE: It is critical to be consistent with the number of times the pellet in pipetted.</li>
	<li>Incubate on ice for 3 min. This incubation step allows all samples to reach an equilibrium state, which is particularly important when there are multiple samples.</li>
	<li>Centrifuge at 6500 x g for 3 min at 4 &#186;C and transfer supernatant to a clean 1.5 mL tube.</li>
	<li>Repeat 2.6 - 2.10 for the remaining salt concentrations. 
	NOTE: After 400 mM NaCl, mRIPA the pellet should be clear and gloopy and will not stay at the bottom of the tube. The pellet can be placed in the lid of the centrifuge tube while the supernatant isolated.</li>
	<li>Add 70 &#181;L of 4x protein loading dye to each fraction and load 30 &#181;L of each fraction on to an SDS acrylamide gel for western blot analysis. 
	NOTE: To determine the binding profile of the protein, it is critical to load equivalent volumes of lysate, rather than loading equal protein concentrations.</li>
	<li>Perform a standard western blot by transferring onto a membrane and use primary antibodies for proteins of interest.</li>
	<li>To quantitate the protein eluted from the chromatin, incubate the blot in infrared fluorescence IRDye secondary antibodies and image blot with an imager. While other methods of development can be used, we recommend fluorescence or infrared imaging, as it is more quantitative in nature.</li>
	<li>Use ImageJ or similar software to calculate the intensity for the protein eluted at each salt concentration. By graphing the band intensity against the salt concentration, the elution pattern of your protein of interest can be determined.</li>
</ol>
3. Sequential Salt Extraction in the Presence of a &#34;Reader&#34; Inhibitor
<ol>
	<li>Harvest two sets of cells (4 million) and isolate the nuclei as in a standard SSE.</li>
	<li>Re-suspend both sets in 200 &#181;L of mRIPA 0 mM NaCl and incubate for 3 min. This will allow for the removal of any free protein in the nuclei.</li>
	<li>Add 200 &#181;L mRIPA 0 mM NaCl to each pellet. Add the inhibitor (2 &#181;L of 1 mM (+)JQ1) to one sample and DMSO to the control set.</li>
	<li>Agitate the pellet by pipetting 15 times and incubate on ice for 5 min.</li>
	<li>Centrifuge at 6500 x g for 3 min at 4 &#186;C and transfer supernatant to a clean 1.5 mL tube.</li>
	<li>Repeat 3.3 - 4 for all the salt concentrations and perform a standard western blot.</li>
</ol>
4. Sequential Salt Extraction in the Presence of a &#34;Writer&#34; Inhibitor
<ol>
	<li>Treat cells with the inhibitor (10 &#181;M SAHA) or DMSO for 3 h.</li>
	<li>Harvest 4 million cells for each treatment.</li>
	<li>Perform a standard SSE for the samples with the inhibitor added to all the buffers.</li>
</ol>
5. Sequential Salt Extraction Following DNA Damage
<ol>
	<li>Treat cells with 1 &#181;M doxorubicin for 1 h.</li>
	<li>Change the media on the cells and allow them to recover for 3 h.</li>
	<li>Harvest 8 million cells and perform the basic SSE.</li>
</ol>
6. Non-Sequential Salt Extraction
<ol>
	<li>Harvest 12 million cells and wash with PBS.</li>
	<li>Divide cells so that there are 2 million cells per micro centrifuge tube.</li>
	<li>Re-suspend in 500 &#181;L of Buffer A and incubate for 10 min.</li>
	<li>Isolate the nuclei by centrifugation at 6000 x g.</li>
	<li>Re-suspend pellets in a 200 &#181;L of each of the mRIPA buffers + NaCl.</li>
	<li>Homogenize each sample by pipetting 15 times with a 1 mL pipette tip.</li>
	<li>Incubate on ice for 3 min.</li>
	<li>Isolate the chromatin by centrifugation at 6500 x g and transfer the supernatant to clean tubes. Add 70 &#181;L of loading dye and run 30 &#181;L on a SDS-page gel for western blot analysis.</li>
</ol>

<ol>
	<li>Ramakrishnan, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Histone+structure+and+the+organization+of+the+nucleosome.">Histone structure and the organization of the nucleosome.</a> Annu. Rev. Biophys. Biomol. Struct. 26, 83-112 (1997).</li><li>Bannister, A. J., Kouzarides, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Regulation+of+chromatin+by+histone+modifications.">Regulation of chromatin by histone modifications.</a> Cell Rese. 21 (3), 381-395 (2011).</li><li>Musselman, C. A., Lalonde, M. -. E., C&#244;t&#233;, J., Kutateladze, T. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perceiving+the+epigenetic+landscape+through+histone+readers.">Perceiving the epigenetic landscape through histone readers.</a> Nat Struct Mol Biol. 19 (12), 1218-1227 (2012).</li><li>Furey, T. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ChIP-seq+and+beyond:+new+and+improved+methodologies+to+detect+and+characterize+protein-DNA+interactions.">ChIP-seq and beyond: new and improved methodologies to detect and characterize protein-DNA interactions.</a> Nat Rev Genet. 13 (12), 840-852 (2012).</li><li>Skene, P. J., Henikoff, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+efficient+targeted+nuclease+strategy+for+high-resolution+mapping+of+DNA+binding+sites.">An efficient targeted nuclease strategy for high-resolution mapping of DNA binding sites.</a> eLife. 6, (2017).</li><li>Wu, S. Y., Lee, A. Y., Lai, H. T., Zhang, H., Chiang, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phospho+switch+triggers+brd4+chromatin+binding+and+activator+recruitment+for+gene-specific+targeting.">Phospho switch triggers brd4 chromatin binding and activator recruitment for gene-specific targeting.</a> Mol Cell. 49 (5), 843-857 (2013).</li><li>Porter, E. G., Dykhuizen, E. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Individual+bromodomains+of+Polybromo-1+contribute+to+chromatin+association+and+tumor+suppression+in+clear+cell+renal+carcinoma.">Individual bromodomains of Polybromo-1 contribute to chromatin association and tumor suppression in clear cell renal carcinoma.</a> J Biol Chem. 292 (7), 2601-2610 (2017).</li><li>Kerppola, T. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polycomb+group+complexes--many+combinations,+many+functions.">Polycomb group complexes--many combinations, many functions.</a> Trends cell biol. 19 (12), 692-704 (2009).</li><li>Filippakopoulos, P., 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+inhibition+of+BET+bromodomains.">Selective inhibition of BET bromodomains.</a> Nature. 468 (7327), 1067-1073 (2010).</li><li>Filippakopoulos, P., 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=Histone+recognition+and+large-scale+structural+analysis+of+the+human+bromodomain+family.">Histone recognition and large-scale structural analysis of the human bromodomain family.</a> Cell. 149 (1), 214-231 (2012).</li><li>Sanchez, R., Zhou, M. -. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+role+of+human+bromodomains+in+chromatin+biology+and+gene+transcription.">The role of human bromodomains in chromatin biology and gene transcription.</a> Curr opin drug disc dev. 12 (5), 659-665 (2009).</li><li>Yang, F., Teves, S. S., Kemp, C. J., Henikoff, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Doxorubicin,+DNA+torsion,+and+chromatin+dynamics.">Doxorubicin, DNA torsion, and chromatin dynamics.</a> Biochim. Biophys. Acta - Rev Cancer. 1845 (1), 84-89 (2014).</li><li>Kakarougkas, A., 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=Requirement+for+PBAF+in+Transcriptional+Repression+and+Repair+at+DNA+Breaks+in+Actively+Transcribed+Regions+of+Chromatin.">Requirement for PBAF in Transcriptional Repression and Repair at DNA Breaks in Actively Transcribed Regions of Chromatin.</a> Mol Cell. 55 (5), 723-732 (2014).</li><li>Lee, S. Y., Park, J. -. H., Kim, S., Park, E. -. J., Yun, Y., Kwon, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+proteomics+approach+for+the+identification+of+nucleophosmin+and+heterogeneous+nuclear+ribonucleoprotein+C1/C2+as+chromatin-binding+proteins+in+response+to+DNA+double-strand+breaks.">A proteomics approach for the identification of nucleophosmin and heterogeneous nuclear ribonucleoprotein C1/C2 as chromatin-binding proteins in response to DNA double-strand breaks.</a> Biochem J. 388 (Pt 1), (2005).</li><li>Donaldson, M. M., Boner, W., Morgan, I. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=TopBP1+Regulates+Human+Papillomavirus+Type+16+E2+Interaction+with+Chromatin.">TopBP1 Regulates Human Papillomavirus Type 16 E2 Interaction with Chromatin.</a> J Virol. 81 (8), 4338-4342 (2007).</li></ol>

The authors have no competing financial interests.

Characterization of protein and chromatin interactions through salt extractions is a common method that has been employed for decades14,15; however, it has not been systematically optimized before to reveal its full utility. We demonstrate how this sequential method provides a rapid and inexpensive way to distinguish changes in chromatin binding when the protein or the environment is altered. SSE is highly adaptable and optimizable, and importantly, it is technic...

DNA regulation in eukaryotic cells is an intricate and sophisticated system that is tightly controlled by an assortment of proteins that coordinate responses to intracellular and extracellular stimuli. DNA is wrapped around histone octamers to form nucleosomes, which can be loosely distributed along DNA or compacted into tight coils1. This structural arrangement of DNA and histones is known as chromatin, which is regulated by a network of proteins that read, write, and erase post translational modifications (PTM) on histones2. Some histone PTMs, such as acetylation, change the charge of the amino acid they are deposited on, altering interactions between histones and DNA2. Histone PTMs also serve to recruit transcriptional regulators, chromatin remodelers, DNA damage repair machinery, and DNA replication machinery to specific regions of the genome3.
Most methods for studying chromatin interactions either probe small scale interactions or involve large genome-wide analyses. In vitro binding studies often utilize individual recombinant domains with histone peptides or DNA in assays such as electrophoresis mobility shift assays (EMSA), isothermal titration calorimetry, fluorescence polarization, and peptide pulldowns. Because these assays typically focus on an individual protein domain, they facilitate the understanding of a small piece of the puzzle, but do not allow us to understand the cooperative nature of multi-domain proteins, let alone their role in multi-protein complexes. Another layer of intricacy is added by the heterogeneous composition of most mammalian chromatin-modifying complexes. This protein heterogeneity, in combination with the dynamic nature of the chromatin landscape, makes it challenging to recapitulate the in vivo binding interactions of chromatin proteins to chromatin in vitro.
In vivo methods have made significant advances; however, they are often expensive, time consuming, and technically challenging. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is very useful for determining the localization of proteins and histone modifications across the genome, however it requires substantial optimization4. Proteins are often crosslinked to chromatin to preserve interactions; however, this can produce artificial interactions and may cause epitope masking5. Furthermore, the immunoprecipitations (IP) require highly specific antibodies, and extensive optimization of DNA shearing and IP conditions by ChIP-qPCR using a known binding site, which is often not available a priori. After optimization of ChIP conditions, processing of the samples is costly and requires several weeks to months to sequence and analyze. Though this method is invaluable for identifying the localization of chromatin bound proteins across the genome, the cost and time commitment make it prohibitive to use this method to generate hypotheses about how small changes may affect global binding properties.
In this paper, we describe how a sequential salt extraction (SSE) assay can be used to examine global binding profiles of chromatin-bound proteins and distinguish how changes in a protein, complex, or global PTM profile can alter interactions. Though salt extractions are a commonly and broadly used technique, we demonstrate how this sequential method is highly reproducible and versatile. SSE allows us to characterize how a single subunit of a complex or even a single domain contributes to the complex&#39;s overall affinity for bulk chromatin. SSE can also be used to determine if the binding of a protein is influenced by changes in chromatin landscape, providing interesting hypotheses for histone mark targeting that can be confirmed using ChIP-seq and other genome wide studies.
We originally adapted this method from Wu et al., to examine of the function of Polybromo1 (PBRM1) in the binding of the PBAF chromatin remodeler6,7. Using this technique, we determined the role of PBRM1 for chromatin binding within the PBAF chromatin remodeling complex and then determined the relative contribution of the six individual bromodomains to this function7.
Here we describe how to optimize this method to explore chromatin binding in different cell types, to assess the relative binding affinity of similar chromatin modifying complexes, to examine the displacement of a protein from chromatin by a chemical inhibitor, and to determine the effects of chromatin binding after alterations to the chromatin landscape.

<table>
	<tbody>
		<tr>
			<td>Sodium Chloride, Crystal 2.5 Kg AR (ACS)</td>
			<td>Avantor/Macron</td>
			<td>7581-06</td>
			<td></td>
		</tr>
		<tr>
			<td>Sucrose, Crystal 500 G AR&#168; (ACS)</td>
			<td>Avantor/Macron</td>
			<td>8360-04</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium Chloride, Granular 500 G AR&#168; (ACS)</td>
			<td>Avantor/Macron</td>
			<td>6858-04</td>
			<td></td>
		</tr>
		<tr>
			<td>Trizma(R) base,Primary Standard and Buffer, &#62;=99.9% (titration), crystalline</td>
			<td>Sigma-Aldrich</td>
			<td>T1503-1kg</td>
			<td></td>
		</tr>
		<tr>
			<td>EDTA (Ethylenedinitrilo) Tetraacetic Acid, Disodium Salt, Dihydrate 125 G AR (ACS)</td>
			<td>Avantor/Macron</td>
			<td>4931-02</td>
			<td></td>
		</tr>
		<tr>
			<td>NONIDET P-40 SUBSTRATE, 100mL UN3082</td>
			<td>Amresco</td>
			<td>E109-100ML</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES Buffer Solution (1M)</td>
			<td>Gibco</td>
			<td>15630-080</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium Chloride, 6-Hydrate, Crystal 500 G AR&#168; (ACS)</td>
			<td>Avantor/Macron</td>
			<td>5958-04</td>
			<td></td>
		</tr>
		<tr>
			<td>GLYCEROL, 1L</td>
			<td>Amresco</td>
			<td>0854-1L</td>
			<td></td>
		</tr>
		<tr>
			<td>DEOXYCHOLIC ACID SODIUM SALT, 100g</td>
			<td>Amresco</td>
			<td>0613-100G</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf Research plus pipette</td>
			<td>Fisher Scientific</td>
			<td>13-690-032</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf 5424 R refrigerated microcentrifuge</td>
			<td>Eppendorf</td>
			<td>5424 R</td>
			<td></td>
		</tr>
		<tr>
			<td>Secondary mouse IgG HRP-linked</td>
			<td>Cell Signaling</td>
			<td>7076</td>
			<td></td>
		</tr>
		<tr>
			<td>Secondary rabbit IgG HRP-linked</td>
			<td>Cell Signaling</td>
			<td>7074</td>
			<td></td>
		</tr>
		<tr>
			<td>BMI-1 antibody</td>
			<td>Millipore</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PBRM1 antibody</td>
			<td>Bethyl</td>
			<td>A301-591A</td>
			<td></td>
		</tr>
		<tr>
			<td>ARID1a antibody</td>
			<td>Santa Cruz</td>
			<td>sc-32761</td>
			<td></td>
		</tr>
		<tr>
			<td>BRD4 antibody</td>
			<td>Bethyl</td>
			<td>A301-9852a</td>
			<td></td>
		</tr>
		<tr>
			<td>(+) JQ1</td>
			<td>Cayman Chemical Company</td>
			<td>11187</td>
			<td></td>
		</tr>
		<tr>
			<td>SAHA</td>
			<td>Cayman Chemical Company</td>
			<td>10009929</td>
			<td></td>
		</tr>
		<tr>
			<td>Doxorubicin</td>
			<td>AK Scientific</td>
			<td>25316-40-9</td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethyl Sulfoxide (DMSO)</td>
			<td>Macron</td>
			<td>4948-02</td>
			<td></td>
		</tr>
		<tr>
			<td>Mini Trans-Blot Cell</td>
			<td>BioRad</td>
			<td>1703930</td>
			<td></td>
		</tr>
		<tr>
			<td>Mini Gel Tank</td>
			<td>ThermoFisher</td>
			<td>A25977</td>
			<td></td>
		</tr>
		<tr>
			<td>Albumin, Bovine (BSA)</td>
			<td>Amresco</td>
			<td>0332-100g</td>
			<td>Used in as a 5% solution in PBS-T as blocking solution for Western blots</td>
		</tr>
		<tr>
			<td>Bolt MOPS SDS Running Buffer (20x)</td>
			<td>Life Technologies</td>
			<td>B0001</td>
			<td></td>
		</tr>
		<tr>
			<td>Bolt LDS Sample Buffer (4x)</td>
			<td>Life Technologies</td>
			<td>B0007</td>
			<td></td>
		</tr>
		<tr>
			<td>PageRuler Plus Prestained Protein Ladder, 10 to 250 kDa</td>
			<td>ThermoFisher</td>
			<td>26619</td>
			<td></td>
		</tr>
		<tr>
			<td>Methyl Alcohol, Anhydrous</td>
			<td>Macron</td>
			<td>3041-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin kEDTA, 1X</td>
			<td>Cellgro</td>
			<td>25-053-CI</td>
			<td></td>
		</tr>
		<tr>
			<td>SODIUM AZIDE, 250g UN1687</td>
			<td>Amresco</td>
			<td>0639-250G</td>
			<td></td>
		</tr>
		<tr>
			<td>SODIUM DODECYL SULFATE (SDS)500g UN1325</td>
			<td>Amresco</td>
			<td>0227-500G</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycine,for electrophoresis, &#62;=99%</td>
			<td>Sigma-Aldrich</td>
			<td>G8898-1kg</td>
			<td></td>
		</tr>
		<tr>
			<td>BigTop Microcentrifuge Tubes, Polypropylene</td>
			<td>VWR</td>
			<td>20170-333</td>
			<td></td>
		</tr>
		<tr>
			<td>1000 &#181;L Pipet Tips</td>
			<td>VWR</td>
			<td>83007-382</td>
			<td></td>
		</tr>
		<tr>
			<td>NuPAGE Bis-Tris Precast Gels 4-12%</td>
			<td>Invitrogen</td>
			<td>NW04125BOX</td>
			<td></td>
		</tr>
		<tr>
			<td>Leupeptin Hemisulfate, 5 MG</td>
			<td>RPI Research Products</td>
			<td>22035-0.005</td>
			<td>Used for Protease inhibitor solution.</td>
		</tr>
		<tr>
			<td>APROTININ, 10 MG</td>
			<td>RPI Research Products</td>
			<td>20550-0.01</td>
			<td>Used for Protease inhibitor solution.</td>
		</tr>
		<tr>
			<td>PEPSTATIN A, 5 MG</td>
			<td>RPI Research Products</td>
			<td>30100-0.005</td>
			<td>Used for Protease inhibitor solution.</td>
		</tr>
		<tr>
			<td>OVCA429 cells</td>
			<td>Gift from Karen Cowden Dahl. Ph.D.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>HEK293T</td>
			<td>ATCC</td>
			<td>CRL-3216</td>
			<td></td>
		</tr>
		<tr>
			<td>HeLa cells</td>
			<td>ATCC</td>
			<td>CCL-2</td>
			<td></td>
		</tr>
		<tr>
			<td>McCoy&#39;s 5A media</td>
			<td>Corning Mediatech</td>
			<td>10-050-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>DMEM</td>
			<td>Corning Mediatech</td>
			<td>10-013-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>Minimum Essential Medium (MEM), Powder</td>
			<td>Corning Mediatech</td>
			<td>50-011-PC</td>
			<td></td>
		</tr>
		<tr>
			<td>MEM NEAA (100X) non-essential amino acid</td>
			<td>Gibco</td>
			<td>11140-050</td>
			<td></td>
		</tr>
		<tr>
			<td>FB-11: Fetal Bovine Serum, U.S. Source</td>
			<td>Omega Scientific</td>
			<td>FB-11</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin Solution</td>
			<td>Life Technologies</td>
			<td>15140-122</td>
			<td></td>
		</tr>
		<tr>
			<td>Glutamax</td>
			<td>Life Technologies</td>
			<td>35050-061</td>
			<td></td>
		</tr>
		<tr>
			<td>sodium pyruvate</td>
			<td>Life Technologies</td>
			<td>11360070</td>
			<td></td>
		</tr>
		<tr>
			<td>VWR Power Source</td>
			<td>VWR</td>
			<td>13-690-032</td>
			<td></td>
		</tr>
	</tbody>
</table>

sequential salt extractions for the analysis of bulk chromatin binding properties of chromatin modifying complexes

Elucidation of the binding properties of chromatin-targeting proteins can be very challenging due to the complex nature of chromatin and the heterogeneous nature of most mammalian chromatin-modifying complexes. In order to overcome these hurdles, we have adapted a sequential salt extraction (SSE) assay for evaluating the relative binding affinities of chromatin-bound complexes. This easy and straightforward assay can be used by non-experts to evaluate the relative difference in binding affinity of two related complexes, the changes in affinity of a complex when a subunit is lost or an individual domain is inactivated, and the change in binding affinity after alterations to the chromatin landscape. By sequentially re-suspending bulk chromatin in increasing amounts of salt, we are able to profile the elution of a particular protein from chromatin. Using these profiles, we are able to determine how alterations in a chromatin-modifying complex or alterations to the chromatin environment affect binding interactions. Coupling SSE with other in vitro and in vivo assays, we can determine the roles of individual domains and proteins on the functionality of a complex in a variety of chromatin environments.

In this paper, we demonstrate the advantages and applications of the commonly used sequential salt extraction (SSE) method that we have adapted from the literature6. In Figure 1, we compare the reproducibility of SSE to extracting proteins non-sequentially by detecting the elution patterns of ARID1a and PBRM1. We consistently observe that ARID1a, a BAF subunit, elutes primarily at 200 mM NaCl and PBRM1, an exclusive PBAF subunit, elute...

Watch this Scientific Journal Video about Sequential Salt Extractions for the Analysis of Bulk Chromatin Binding Properties of Chromatin Modifying Complexes at JoVE.com

Sequential Salt Extractions for the Analysis of Bulk Chromatin Binding Properties of Chromatin Modifying Complexes

Sequential salt extraction of chromatin bound proteins is a useful tool for determining the binding properties of large protein complexes. This method can be employed to evaluate the role of individual subunits or domains in the overall affinity of a protein complex to bulk chromatin.

Elucidation of the binding properties of chromatin-targeting proteins can be very challenging due to the complex nature of chromatin and the heterogeneous ...

sequential-salt-extractions-for-analysis-bulk-chromatin-binding

Research

JoVE Journal

Genetics

8.2K Views.  Purdue University. Sequential salt extraction of chromatin bound proteins is a useful tool for determining the binding properties of large protein complexes. This method can be employed to evaluate the role of individual subunits or domains in the overall affinity of a protein complex to bulk chromatin.

Video: Sequential Salt Extractions for the Analysis of Bulk Chromatin Binding Properties of Chromatin Modifying Complexes

Analysis of the c-KIT Ligand Promoter Using Chromatin Immunoprecipitation

Multiple cellular processes, including DNA replication and repair, DNA recombination, and gene expression, require interactions between proteins and DNA. Therefore, DNA-protein interactions regulate multiple physiological, pathophysiological, and biological functions, such as cell differentiation, cell proliferation, cell cycle control, chromosome stability, epigenetic gene regulation, and cell transformation. In eukaryotic cells, the DNA interacts with histone and nonhistone proteins and is condensed into chromatin. Several technical tools can be used to analyze DNA-protein interactions, such as the Electrophoresis (gel) Mobility Shift Assay (EMSA) and DNase I footprinting. However, these techniques analyze the protein-DNA interaction in vitro, not within the cellular context. Chromatin immunoprecipitation (ChIP) is a technique that captures proteins at their specific DNA binding sites, thereby allowing for the identification of DNA-protein interactions within their chromatin context. It is done by fixation&#160;of the DNA-protein interaction, followed by&#160;immunoprecipitation of the protein of interest. Subsequently, the genomic site that the protein was bound to is characterized. Here, we describe and discuss ChIP and demonstrate its analytical value for the identification of the Transforming Growth Factor-&#946; (TGF-&#946;)-induced binding of the transcription factor SMAD2 to SMAD Binding Elements (SBE) within the promoter region of the tyrosine-protein kinase Kit (c-KIT) receptor ligand Stem Cell Factor (SCF).

DNA-protein interactions are essential for multiple biological processes. During the evaluation of cellular functions, the analysis of DNA-protein interactions is indispensable for understanding gene regulation. Chromatin immunoprecipitation (ChIP) is a powerful tool to analyze such interactions in vivo.

Multiple cellular processes, including DNA replication and repair, DNA recombination, and gene expression, require interactions between proteins and DNA. ...

Generation of Native Chromatin Immunoprecipitation Sequencing Libraries for Nucleosome Density Analysis

We present a modified native chromatin immunoprecipitation sequencing (ChIP-seq) experimental protocol compatible with a Gaussian mixture distribution based analysis methodology (nucleosome density ChIP-seq; ndChIP-seq) that enables the generation of combined measurements of micrococcal nuclease (MNase) accessibility with histone modification genome-wide. Nucleosome position and local density, and the posttranslational modification of their histone subunits, act in concert to regulate local transcription states. Combinatorial measurements of nucleosome accessibility with histone modification generated by ndChIP-seq allows for the simultaneous interrogation of these features. The ndChIP-seq methodology is applicable to small numbers of primary cells inaccessible to cross-linking based ChIP-seq protocols. Taken together, ndChIP-seq enables the measurement of histone modification in combination with local nucleosome density to obtain new insights into shared mechanisms that regulate RNA transcription within rare primary cell populations.

We present a modified native chromatin immunoprecipitation sequencing (ChIP-seq) methodology for the generation of sequence datasets suitable for a nucleosome density ChIP-seq analytical framework integrating micrococcal nuclease (MNase) accessibility with histone modification measurements.

We present a modified native chromatin immunoprecipitation sequencing (ChIP-seq) experimental protocol compatible with a Gaussian mixture distribution ...

Chromatin Immunoprecipitation (ChIP) Protocol for Low-abundance Embryonic Samples

Chromatin immunoprecipitation (ChIP) is a widely-used technique for mapping the localization of post-translationally modified histones, histone variants, transcription factors, or chromatin-modifying enzymes at a given locus or on a genome-wide scale. The combination of ChIP assays with next-generation sequencing (i.e., ChIP-Seq) is a powerful approach to globally uncover gene regulatory networks and to improve the functional annotation of genomes, especially of non-coding regulatory sequences. ChIP protocols normally require large amounts of cellular material, thus precluding the applicability of this method to investigating rare cell types or small tissue biopsies. In order to make the ChIP assay compatible with the amount of biological material that can typically be obtained in vivo during early vertebrate embryogenesis, we describe here a simplified ChIP protocol in which the number of steps required to complete the assay were reduced to minimize sample loss. This ChIP protocol has been successfully used to investigate different histone modifications in various embryonic chicken and adult mouse tissues using low to medium cell numbers (5 x 104 - 5 x 105 cells). Importantly, this protocol is compatible with ChIP-seq technology using standard library preparation methods, thus providing global epigenomic maps in highly relevant embryonic tissues.

Chromatin Immunoprecipitation ChIP Protocol for Low-abundance Embryonic Samples

Here, we describe a chromatin immunoprecipitation (ChIP) and ChIP-seq library preparation protocol to generate global epigenomic profiles from low-abundance chicken embryonic samples.

Chromatin immunoprecipitation (ChIP) is a widely-used technique for mapping the localization of post-translationally modified histones, histone variants, ...

Mapping Genome-wide Accessible Chromatin in Primary Human T Lymphocytes by ATAC-Seq

Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq) is a method used for the identification of open (accessible) regions of chromatin. These regions represent regulatory DNA elements (e.g., promoters, enhancers, locus control regions, insulators) to which transcription factors bind. Mapping the accessible chromatin landscape is a powerful approach for uncovering active regulatory elements across the genome. This information serves as an unbiased approach for discovering the network of relevant transcription factors and mechanisms of chromatin structure that govern gene expression programs. ATAC-seq is a robust and sensitive alternative to DNase I hypersensitivity analysis coupled with next-generation sequencing (DNase-seq) and formaldehyde-assisted isolation of regulatory elements (FAIRE-seq) for genome-wide analysis of chromatin accessibility and to the sequencing of micrococcal nuclease-sensitive sites (MNase-seq) to determine nucleosome positioning. We present a detailed ATAC-seq protocol optimized for human primary immune cells i.e. CD4+ lymphocytes (T helper 1 (Th1) and Th2 cells). This comprehensive protocol begins with cell harvest, then describes the molecular procedure of chromatin tagmentation, sample preparation for next-generation sequencing, and also includes methods and considerations for the computational analyses used to interpret the results. Moreover, to save time and money, we introduced quality control measures to assess the ATAC-seq library prior to sequencing. Importantly, the principles presented in this protocol allow its adaptation to other human immune and non-immune primary cells and cell lines. These guidelines will also be useful for laboratories which are not proficient with next-generation sequencing methods.

Assay for Transposase-Accessible Chromatin coupled with high-throughput sequencing (ATAC-seq) is a genome-wide method to uncover accessible chromatin. This is a step-by-step ATAC-seq protocol, from molecular to the final computational analysis, optimized for human lymphocytes (Th1/Th2). This protocol can be adopted by researchers without prior experience in next-generation sequencing methods.

Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq) is a method used for the identification of open (accessible) regions ...

Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II

Chromatin spread techniques have been widely used to assess the dynamic localization of various proteins during gametogenesis, particularly for spermatogenesis. These techniques allow for visualization of protein and DNA localization patterns during meiotic events such as homologous chromosome pairing, synapsis and DNA repair. While a few protocols have been described in the literature, general chromatin spread techniques using mammalian prophase oocytes are limited and difficult due to the timing of meiosis initiation in fetal ovaries. In comparison, prophase spermatocytes can be collected from juvenile male mice with higher yields without the need for microdissection. However, it is difficult to obtain a pure synchronized population of cells at specific stages due to the heterogeneity of meiotic and post-meiotic germ cell populations in the juvenile and adult testis. For later stages of meiosis, it is advantageous to assess oocytes undergoing meiosis I (MI) or meiosis II (MII), because groups of mature oocytes can be collected from adult female mice and stimulated to resume meiosis in culture. Here, methods for meiotic chromatin spread preparations using oocytes dissected from fetal, neonatal and adult ovaries are described with accompanying video demonstrations. Chromosome missegregation events in mammalian oocytes are frequent, particularly during MI. These techniques can be used to assess and characterize the effects of different mutations or environmental exposures during various stages of oogenesis. As there are distinct differences between oogenesis and spermatogenesis, the techniques described within are invaluable to increase our understanding of mammalian oogenesis and the sexually dimorphic features of chromosome and protein dynamics during meiosis.

Oogenesis in mammals is known to be error-prone, particularly due to chromosome missegregation. This manuscript describes chromatin spread preparation methods for mouse prophase, metaphase I and II-staged oocytes. These fundamental techniques allow for the study of chromatin-bound proteins and chromosome morphology throughout mammalian oogenesis.

Chromatin spread techniques have been widely used to assess the dynamic localization of various proteins during gametogenesis, particularly for ...

Generation of Genome-wide Chromatin Conformation Capture Libraries from Tightly Staged Early Drosophila Embryos

Investigating the three-dimensional architecture of chromatin offers invaluable insight into the mechanisms of gene regulation. Here, we describe a protocol for performing the chromatin conformation capture technique in situ Hi-C on staged Drosophila melanogaster embryo populations. The result is a sequencing library that allows the mapping of all chromatin interactions that occur in the nucleus in a single experiment. Embryo sorting is done manually using a fluorescent stereo microscope and a transgenic fly line containing a nuclear marker. Using this technique, embryo populations from each nuclear division cycle, and with defined cell cycle status, can be obtained with very high purity. The protocol may also be adapted to sort older embryos beyond gastrulation. Sorted embryos are used as inputs for in situ Hi-C. All experiments, including sequencing library preparation, can be completed in five days. The protocol has low input requirements and works reliably using 20 blastoderm stage embryos as input material. The end result is a sequencing library for next generation sequencing. After sequencing, the data can be processed into genome-wide chromatin interaction maps that can be analyzed using a wide range of available tools to gain information about topologically associating domain (TAD) structure, chromatin loops, and chromatin compartments during Drosophila development.

Generation of Genome-wide Chromatin Conformation Capture Libraries from Tightly Staged Early Drosophila Embryos

This work describes a protocol for the generation of high resolution in situ Hi-C libraries from tightly staged pre-gastrulation Drosophila melanogaster embryos.

Investigating the three-dimensional architecture of chromatin offers invaluable insight into the mechanisms of gene regulation. Here, we describe a ...

Chromatin Immunoprecipitation (ChIP) of Histone Modifications from Saccharomyces cerevisiae

Histone post-translational modifications (PTMs), such as acetylation, methylation and phosphorylation, are dynamically regulated by a series of enzymes that add or remove these marks in response to signals received by the cell. These PTMS are key contributors to the regulation of processes such as gene expression control and DNA repair. Chromatin immunoprecipitation (chIP) has been an instrumental approach for dissecting the abundance and localization of many histone PTMs throughout the genome in response to diverse perturbations to the cell. Here, a versatile method for performing chIP of post-translationally modified histones from the budding yeast Saccharomyces cerevisiae (S. cerevisiae) is described. This method relies on crosslinking of proteins and DNA using formaldehyde treatment of yeast cultures, generation of yeast lysates by bead beating, solubilization of chromatin fragments by micrococcal nuclease, and immunoprecipitation of histone-DNA complexes. DNA associated with the histone mark of interest is purified and subjected to quantitative PCR analysis to evaluate its enrichment at multiple loci throughout the genome. Representative experiments probing the localization of the histone marks H3K4me2 and H4K16ac in wildtype and mutant yeast are discussed to demonstrate data analysis and interpretation. This method is suitable for a variety of histone PTMs and can be performed with different mutant strains or in the presence of diverse environmental stresses, making it an excellent tool for investigating changes in chromatin dynamics under different conditions.

Chromatin Immunoprecipitation ChIP of Histone Modifications from Saccharomyces cerevisiae

Here, we describe a protocol for chromatin immunoprecipitation of modified histones from the budding yeast Saccharomyces cerevisiae. Immunoprecipitated DNA is subsequently used for quantitative PCR to interrogate the abundance and localization of histone post-translational modifications throughout the genome.

Histone post-translational modifications (PTMs), such as acetylation, methylation and phosphorylation, are dynamically regulated by a series of enzymes ...

Adaptation of Hybridization Capture of Chromatin-associated Proteins for Proteomics to Mammalian Cells

The hybridization capture of chromatin-associated proteins for proteomics (HyCCAPP) technology was initially developed to uncover novel DNA-protein interactions in yeast. It allows analysis of a target region of interest without the need for prior knowledge about likely proteins bound to the target region. This, in theory, allows HyCCAPP to be used to analyze any genomic region of interest, and it provides sufficient flexibility to work in different cell systems. This method is not meant to study binding sites of known transcription factors, a task better suited for Chromatin Immunoprecipitation (ChIP) and ChIP-like methods. The strength of HyCCAPP lies in its ability to explore DNA regions for which there is limited or no knowledge about the proteins bound to it. It can also be a convenient method to avoid biases (present in ChIP-like methods) introduced by protein-based chromatin enrichment using antibodies. Potentially, HyCCAPP can be a powerful tool to uncover truly novel DNA-protein interactions. To date, the technology has been predominantly applied to yeast cells or to high copy repeat sequences in mammalian cells. In order to become the powerful tool we envision, HyCCAPP approaches need to be optimized to efficiently capture single-copy loci in mammalian cells. Here, we present our adaptation of the initial yeast HyCCAPP capture protocol to human cell lines, and show that single-copy chromatin regions can be efficiently isolated with this modified protocol.

This is a method to identify novel DNA-interacting proteins at specific target loci, relying on sequence-specific capture of crosslinked chromatin for subsequent proteomic analyses. No prior knowledge about potential binding proteins, nor cell modifications are required. Initially developed for yeast, the technology has now been adapted for mammalian cells.

The hybridization capture of chromatin-associated proteins for proteomics (HyCCAPP) technology was initially developed to uncover novel DNA-protein ...

Formaldehyde-assisted Isolation of Regulatory Elements to Measure Chromatin Accessibility in Mammalian Cells

Appropriate gene expression in response to extracellular cues, that is, tissue- and lineage-specific gene transcription, critically depends on highly defined states of chromatin organization. The dynamic architecture of the nucleus is controlled by multiple mechanisms and shapes the transcriptional output programs. It is, therefore, important to determine locus-specific chromatin accessibility in a reliable fashion that is preferably independent from antibodies, which can be a potentially confounding source of experimental variability. Chromatin accessibility can be measured by various methods, including the Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE) assay, that allow the determination of general chromatin accessibility in a relatively low number of cells. Here we describe a FAIRE protocol that allows simple, reliable, and fast identification of genomic regions with a low protein occupancy. In this method, the DNA is covalently bound to the chromatin proteins using formaldehyde as a crosslinking agent and sheared to small pieces. The free DNA is afterwards enriched using phenol:chloroform extraction. The ratio of free DNA is determined by quantitative polymerase chain reaction (qPCR) or DNA sequencing (DNA-seq) compared to a control sample representing total DNA. The regions with a looser chromatin structure are enriched in the free DNA sample, thus allowing the identification of genomic regions with lower chromatin compaction.

The plasticity of nuclear architecture is controlled by dynamic epigenetic mechanisms including non-coding RNAs, DNA methylation, nucleosome repositioning as well as histone composition and modification. Here we describe a Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE) protocol which allows the determination of chromatin accessibility in a reproducible, inexpensive, and straightforward manner.

Appropriate gene expression in response to extracellular cues, that is, tissue- and lineage-specific gene transcription, critically depends on highly ...

CRISPR-Mediated Reorganization of Chromatin Loop Structure

Recent studies have clearly shown that long-range, three-dimensional chromatin looping interactions play a significant role in the regulation of gene expression, but whether looping is responsible for or a result of alterations in gene expression is still unknown. Until recently, how chromatin looping affects the regulation of gene activity and cellular function has been relatively ambiguous, and limitations in existing methods to manipulate these structures prevented in-depth exploration of these interactions. To resolve this uncertainty, we engineered a method for selective and reversible chromatin loop re-organization using CRISPR-dCas9 (CLOuD9). The dynamism of the CLOuD9 system has been demonstrated by successful localization of CLOuD9 constructs to target genomic loci to modulate local chromatin conformation. Importantly, the ability to reverse the induced contact and restore the endogenous chromatin conformation has also been confirmed. Modulation of gene expression with this method establishes the capacity to regulate cellular gene expression and underscores the great potential for applications of this technology in creating stable de novo chromatin loops that markedly affect gene expression in the contexts of cancer and development.

Chromatin looping plays a significant role in gene regulation; however, there have been no technological advances that allow for selective and reversible modification of chromatin loops. Here we describe a powerful system for chromatin loop re-organization using CRISPR-dCas9 (CLOuD9), demonstrated to selectively and reversibly modulate gene expression at targeted loci.

Recent studies have clearly shown that long-range, three-dimensional chromatin looping interactions play a significant role in the regulation of gene ...