Name: characterization of multi-subunit protein complexes of human mxa using non-denaturing polyacrylamide gel-electrophoresis
Uploaded: 2016-10-28T15:00:00
Description: Watch this Scientific Journal Video about Characterization of Multi-subunit Protein Complexes of Human MxA Using Non-denaturing Polyacrylamide Gel-electrophoresis at JoVE.com

This work was funded by a Grant from the Swiss National Science foundation (Grant nr. 31003A_143834) to JP.

NOTE: This protocol is based on the previously published non-denaturing PAGE protocol 12. In that study, the oligomeric state of the MxA protein was assessed using either Vero cells overexpressing MxA or IFN-&#945;-stimulated A549 cells expressing endogenous MxA. The protocol described below can be used to analyze the oligomeric state of any protein in addition to MxA. However, further optimization may be required.
1. Preparation of Cell Lysate for Non-denaturing PAGE
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<ol>
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Here we describe a simple method that allows the rapid determination of the oligomeric state of proteins expressed in mammalian cells by non-denaturing PAGE followed by Western blot analysis. The major advantage of this approach is that the oligomeric state of a given protein can be determined from whole cell lysates without prior protein purification. This may be important for proteins that oligomerize or exert their function in association with auxiliary factors. In addition, the proteins are still in their native stat...

The quaternary structure of a protein plays a crucial role in many cellular processes. Signaling pathways, gene expression, and enzyme activation/deactivation all rely on the proper assembly of protein complexes 1-4. This process also known as homo- or hetero-oligomerization is due to irreversible covalent or reversible electrostatic and hydrophobic protein-protein interactions. Oligomerization not only diversifies the different cellular processes without increasing the genome size, but also provides a strategy for proteins to build stable complexes that are more resistant towards denaturation and degradation 5. Defects in oligomerization have an impact on the function of proteins and can lead to the development of diseases. For example, the enzyme phenylalanine hydroxylase forms a tetrameric complex. Some mutations within the protein complex can weaken the tetramer formation and lead to the disease phenylketonuria 6.
The human MxA protein is an interferon (IFN)-induced antiviral effector protein exerting a broad antiviral activity against various RNA as well as DNA viruses 7. It belongs to the superfamily of dynamin-like large GTPases and has the capacity to form large oligomeric structures in vitro 8. Oligomerization has been suggested to protect MxA from rapid degradation 9,10. Despite intense efforts by many research groups, the molecular mechanism of action remains largely elusive and the role of the oligomerization state of MxA for its antiviral function is under debate 9,11,12. In this regard, Gao and coworkers proposed a model where MxA exerts its antiviral activity by interacting with viral nucleoproteins in form of large ring-like oligomeric structures 11. However, more recently, we demonstrated that MxA dimers exhibit antiviral activity and interact with the nucleoprotein of influenza A virus 12. Based on the crystal structure of MxA, Gao and coworkers identified several amino acid residues in the interface regions that are critical for its oligomerization in vitro and its antiviral function 11,13. Therefore, in order to elucidate which oligomeric state of MxA exerts antiviral activity, we sought to establish a simple protocol to rapidly determine the oligmeric state of MxA interface mutants expressed in human cells as well as endogenous MxA expressed after IFN&#945; stimulation.
Although there are many techniques that are commonly used to investigate the interaction between proteins such as the split-Green Fluorescent Protein (split-GFP) complementation assay 14, surface plasmon resonance 15 and F&#246;rster resonance energy transfer (FRET) 16, they do not provide information of the exact stoichiometry of an oligomeric protein complex. For investigation of this particular aspect, techniques such as multi-angle light scattering (MALS) 17 and analytical ultracentrifugation 18 are very useful. Usually, the proteins analyzed using these methods are purified proteins. Oligomerization processes may also depend on other cellular factors. If these factors are unknown, the analysis is more difficult. Additionally, some proteins are difficult to express in E. coli and to purify. Therefore, these methods are not the optimal choice to analyze protein oligomerization in the cellular environment. In addition, these techniques require expensive instruments which are not readily available.
Non-denaturing polyacrylamide gel electrophoresis (PAGE), size exclusion chromatography or chemical crosslinking followed by conventional Sodium dodecyl sulfate (SDS)-PAGE are useful tools for the characterization of the formation of oligomers from cell lysates 2,19,20. These methods do not require specialized equipment and can be easily performed in a standard laboratory. We initially evaluated various chemical cross-linking protocols that invariantly led to non-specific aggregation and precipitation of MxA. Therefore, we next tested non-denaturing PAGE protocols. As non-denaturing PAGE excludes the use of SDS, the migration of proteins depends on their native charge. Blue-native PAGE uses coomassie brilliant blue G250 to load proteins with an overall negative charge, similar to SDS, but does not denature the protein 21. Unfortunately, coomassie brilliant blue precipitates in the presence of high salts and divalent cations (e.g. Mg2+) that are often included in lysis buffers. Depending on the buffers used, it may be difficult to analyze the sample without further optimization of steps that could have an effect on the oligomeric protein complex.
Here we present a simple protocol based on a previously published method 22 to determine oligomerization of human MxA protein derived from cellular lysates using non-denaturing PAGE.

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			<td height="84" style="height:84px;width:244px;">Slide-A-Lyzer MINI Dialysis Units, 10K MWCO, 0.5 mL</td>
			<td style="width:140px;">Thermo Fisher Scientific</td>
			<td style="width:131px;">69570</td>
			<td style="width:268px;">Pre-equilibrate in dialysis buffer ( if Glycerol removal is desired) 
			Can be self-made according to Fiala et al. 2011</td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:244px;">4&#8211;15% Mini-PROTEAN TGX Precast Protein Gels, 10-well,</td>
			<td style="width:140px;">Bio-Rad</td>
			<td style="width:131px;">456-1083</td>
			<td style="width:268px;">Pre-run in running buffer to adjust buffer system</td>
		</tr>
		<tr height="26">
			<td height="26" style="height:27px;width:244px;">cOmplete, Mini, EDTA-free</td>
			<td style="width:140px;">Roche&#160;</td>
			<td style="width:131px;">11836170001</td>
			<td>use 1 tablet per 50 ml</td>
		</tr>
		<tr height="50">
			<td height="50" style="height:50px;width:244px;">PBS, pH 7.4&#160; bottle a 500ml Gibco</td>
			<td style="width:140px;">Thermo Fisher Scientific</td>
			<td style="width:131px;">14190-094</td>
			<td></td>
		</tr>
		<tr height="52">
			<td height="52" style="height:52px;width:244px;">Ponceau S solution</td>
			<td style="width:140px;">Sigma-Aldrich</td>
			<td style="width:131px;">P7170</td>
			<td style="width:268px;">toxic! wear gloves and protect eyes!</td>
		</tr>
		<tr height="54">
			<td height="54" style="height:54px;width:244px;">NativeMark Unstained Protein Standard&#160; 50ul</td>
			<td style="width:140px;">Invitrogen</td>
			<td style="width:131px;">P/N 57030</td>
			<td>load 5 ul/well</td>
		</tr>
		<tr height="26">
			<td height="26" style="height:27px;width:244px;">A549 cells</td>
			<td style="width:140px;">ATCC</td>
			<td style="width:131px;">ATCC CCL185</td>
			<td>Grow in growth medium (see Table 1)</td>
		</tr>
		<tr height="26">
			<td height="26" style="height:27px;width:244px;">Vero cells</td>
			<td style="width:140px;">ATCC</td>
			<td style="width:131px;">ATCC CCL81</td>
			<td>Grow in growth medium (see Table 1)</td>
		</tr>
		<tr height="26">
			<td height="26" style="height:27px;width:244px;">anti-Mx1 antibody</td>
			<td style="width:140px;">Novus Biologicals</td>
			<td style="width:131px;">H00004599_D01P</td>
			<td>Use at a 1:1000 dilution</td>
		</tr>
		<tr height="66">
			<td height="66" style="height:66px;width:244px;">ECL Anti-rabbit IgG, Horseradish Peroxidase linked whole antibody (from donkey)</td>
			<td style="width:140px;">GE-Healthcare</td>
			<td style="width:131px;">NA934V</td>
			<td>Use at a 1:10000 dilution</td>
		</tr>
		<tr height="49">
			<td height="49" style="height:49px;width:244px;">0.5% Trypsin-EDTA (1x)&#160;&#160;&#160;&#160;&#160;&#160;&#160; Life Technologies</td>
			<td style="width:140px;">Thermo Fisher</td>
			<td style="width:131px;">15400-054</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Iodoacetamide&#160;&#160; 5g</td>
			<td style="width:140px;">Sigma-Aldrich</td>
			<td style="width:131px;">I-6125</td>
			<td>stock&#160; 100mM</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Bromphenolblue</td>
			<td style="width:140px;">Sigma-Aldrich</td>
			<td>B0126-25G</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:244px;">DMEM +4.5g/l Gluc,+L-Glut,+Pyruvate life technologies</td>
			<td style="width:140px;">Thermo Fisher Scientific</td>
			<td style="width:131px;">41966-029</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:244px;">Pen&#160; Strep 100 x&#160;&#160;&#160;&#160; 100ml&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; life technologies</td>
			<td style="width:140px;">Thermo Fisher Scientific</td>
			<td style="width:131px;">15140 - 130</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:244px;">Glutamax 100xStock, 100ml&#160;&#160;&#160;&#160; life technologies</td>
			<td style="width:140px;">Thermo Fisher Scientific</td>
			<td style="width:131px;">350500-038</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:244px;">Fetal Bovine Serum, Dialyzed , US Origin 500ml Gibco Lot:42G9552K</td>
			<td style="width:140px;">Thermo Fisher Scientific</td>
			<td style="width:131px;">10270-106</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Cellulose filter paper</td>
			<td style="width:140px;">Bio-Rad</td>
			<td style="width:131px;">1703965</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">PVDV blotting&#160; membrane</td>
			<td style="width:140px;">GE-Healthcare</td>
			<td style="width:131px;">10600022</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Tris(hydroxymethyl)aminomethane</td>
			<td style="width:140px;">Biosolve</td>
			<td style="width:131px;">0020092391BS</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">sodium fluoride (NaF)</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">S-7920</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">NP-40</td>
			<td style="width:140px;">Calbiochem</td>
			<td style="width:131px;">492015</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">cOmplete, Mini, EDTA-free</td>
			<td style="width:140px;">Roche&#160;</td>
			<td style="width:131px;">11836170001</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Tween 20</td>
			<td style="width:140px;">Calbiochem</td>
			<td style="width:131px;">6555204</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">CHAPS 10% solution</td>
			<td style="width:140px;">Amresco</td>
			<td style="width:131px;">N907</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">DL-Dithiothreitol (DTT)</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">43819</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Glycine</td>
			<td style="width:140px;">Biosolve</td>
			<td style="width:131px;">0007132391BS</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:244px;">sodium orthovanadate (Na3VO4)</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">450243</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Glycerol</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">G7757</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">b-Glycerophospate</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">G9422</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Milk powder</td>
			<td style="width:140px;">Migros/Switzerland</td>
			<td style="width:131px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Methanol</td>
			<td style="width:140px;">Millipore</td>
			<td style="width:131px;">1.06009</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">sodium cloride (NaCl)</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">71380</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:244px;">magnesium chloride (MgCl2)</td>
			<td style="width:140px;">Amresco</td>
			<td style="width:131px;">288</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Sodium dodecyl sulphate (SDS)</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">L4509</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">sodium hydroxide (NaOH)</td>
			<td style="width:140px;">Sigma Aldrich</td>
			<td style="width:131px;">S-8045</td>
			<td></td>
		</tr>
	</tbody>
</table>

characterization of multi-subunit protein complexes of human mxa using non-denaturing polyacrylamide gel-electrophoresis

The formation of oligomeric complexes is a crucial prerequisite for the proper structure and function of many proteins. The interferon-induced antiviral effector protein MxA exerts a broad antiviral activity against many viruses. MxA is a dynamin-like GTPase and has the capacity to form oligomeric structures of higher order. However, whether oligomerization of MxA is required for its antiviral activity is an issue of debate. We describe here a simple protocol to assess the oligomeric state of endogenously or ectopically expressed MxA in the cytoplasmic fraction of human cell lines by non-denaturing polyacrylamide gel electrophoresis (PAGE) in combination with Western blot analysis. A critical step of the protocol is the choice of detergents to prevent aggregation and/or precipitation of proteins particularly associated with cellular membranes such as MxA, without interfering with its enzymatic activity. Another crucial aspect of the protocol is the irreversible protection of the free thiol groups of cysteine residues by iodoacetamide to prevent artificial interactions of the protein. This protocol is suitable for a simple assessment of the oligomeric state of MxA and furthermore allows a direct correlation of the antiviral activity of MxA interface mutants with their respective oligomeric states.

Using non-denaturing PAGE, we analyzed the oligomeric state of the human wild type MxA, the dimeric interface mutants MxA(R640A) and MxA(L617D) as well as the monomeric interface mutant MxA(M527D) from cell lysates 12. Cells were lysed in a buffer containing 1% octylphenoxypolyethoxyethanol (NP-40) and iodoacetamide to ensure protein solubilization and protection of free thiol groups (see Figure 1). As described before, salt and small metabolites were removed by dialysis 19. Protein...

Watch this Scientific Journal Video about Characterization of Multi-subunit Protein Complexes of Human MxA Using Non-denaturing Polyacrylamide Gel-electrophoresis at JoVE.com

Characterization of Multi-subunit Protein Complexes of Human MxA Using Non-denaturing Polyacrylamide Gel-electrophoresis

This article describes a simple and rapid protocol to evaluate the oligomeric state of the dynamin-like GTPase MxA protein from lysates of human cells using a combination of non-denaturing PAGE with western blot analysis.

The formation of oligomeric complexes is a crucial prerequisite for the proper structure and function of many proteins. The interferon-induced antiviral ...

The overall goal of this experiment is to determine the number of subunits in homooligomeric protein complexes in cell lysates using non-denaturing sodium dodecosulfate-polyacrylamide gel electrophoresis. This method can help answer key questions in the field of cell biology, such as the oligomeric status of proteins involved in fundamental cellular processes. The main advantage of this technique is that it does not require purification of the homooligomeric protein complexes or specialized, expensive equipment.Therefore, it can be performed in most laboratories. This experiment utilizes two types of cells. A549 cells that express endogenous Myxovirus resistance A protein, or MxA;and virocells that overexpress MxA.Seed the A549 cells and virocells into six well dishes with two milliliters of growth medium per well. Incubate the cells overnight in a cell culture incubator. On the following day, harvest the cells.Wash each well with one milliliter of PBS and attach the cells by adding 0.5 milliliters of 0.25%Trypsin-EDTA solution for approximately five minutes at room temperature. As soon as the cells detach from the dish, add 0.5 milliliters of growth medium, and carefully mix by pipetting up and down. Transfer the cells of each well into a two milliliter tube, and pellet them using a table top centrifuge.Carefully remove the supernatant by pipetting without disturbing the cell pellet. Wash the cells by adding one milliliter of ice cold PBS and carefully pipetting the cell suspension up and down. Pellet the cells in a table top centrifuge.Carefully remove the supernatant by pipetting without detaching the cell pellet. Add 200 microliters of ice cold lysis buffer, resuspend the cells by pipetting up and down, and then place on ice. Immediately protect the lysate from light by covering the tubes with tinfoil because the iodoacetamide in the lysis buffer is light-sensitive.Incubate on ice for 30 minutes. The use of iodoacetamide is crucial because iodoacetamide irreversibly protects the thiol group of free cysteines by forming a thioether bond and prevents artificial protein aggregation. Next, remove cell debris by centrifugation in a pre-chilled table top centrifuge.During the centrifugation, prepare dialysis columns with a molecular weight cutoff of 10, 000. Attach the dialysis columns to a float buoy and place them into a beaker filled with pre-chilled dialysis buffer to equilibrate the columns. Do not touch the membrane.Place a magnetic stirrer in a beaker and transfer the beaker into the cold room. Using the magnetic stirrer to ensure gentle stirring, equilibrate for 20 minutes at four degrees Celsius. After 20 minutes, remove the columns from the dialysis buffer and the float buoy.Transfer the cleared lysates into the prepared dialysis columns by pipetting without touching the membrane. Attach the columns to a float buoy. Put them back into the beaker filled with dialysis buffer, and then transfer the beaker to the cold room.Dialyze the lysate for at least four hours at four degrees Celsius while carefully stirring with a magnetic stirrer to remove small metabolites and salts that could interfere with non-denaturing polyacrylamide gel-electrophoresis. When the dialysis is complete, transfer each dialyzed sample to a 1.5 milliliter tube. Remove precipitates by centrifugation in a table top centrifuge.Electrophoresis must be performed immediately after dialysis of the lysates to prevent the dissociation of the oligomeric protein complexes. Fill the inner and outer gel chamber with pre-chilled running buffer. Pre-run the non-denaturing polyacrylamide gel with pre-chilled running buffer at 25 milliamps per gel for 15 minutes at four degrees Celsius.Mix 15 microliters of the prepared lysate with five microliters of 4X sample buffer. Load a native protein standard of choice and 15 microliters of sample on the gel. Run the gel at 25 milliamps for four hours at four degrees Celsius.When the non-denaturing polyacrylamide gel electrophoresis is complete, disassemble the gel and carefully transfer into SDS buffer. Incubate for 10 minutes at room temperature with gentle shaking. Prepare two sponges, four cellulose filter paper sheets, and a blotting membrane per gel and soak them in blotting buffer.Assemble the sandwich as follows from bottom to top. One sponge, two cellulose filter paper sheets, a blotting membrane, the gel, two cellulose filter paper sheets, and one sponge. Put the sandwich into the blotting tank, making sure the membrane faces the plus pole, while the gel faces the minus pole.Fill the blotting tank with pre-chilled blotting buffer. For the best protein transfer results, blot at 90 milliamps overnight at four degrees Celsius. On the following day, disassemble the sandwich.Incubate the membrane in Ponceau S solution for five minutes at room temperature to visualize the protein standard. Destain the membrane by placing it in deionized water and washing it. After transferring the membrane to a tray, mark the bands of the protein standard using a pen.Block the membrane with blocking buffer for at least one hour at room temperature or overnight at four degrees Celsius. Subsequently, the proteins of interest are visualized by immunostaining using a rabbit polyclonal antibody directed against MxA as described in the text protocol. Virocells lacking endogenous MxA were transfected with recombinant wild type MxA and monomeric and dimeric MxA variance.The migration of the recombinant MxA variance at their expected molecular weight confirmed their oligomeric state. The ectopically expressed wild type MxA migrated as a tetramer. These recombinant proteins were then used to assess the oligomeric state of endogenous human MxA protein derived from interferon-alpha stimulated A549 cells.The result revealed that the size of MxA in lysates of interferon-alpha stimulated A549 cells corresponds to a tetramer. Once mastered, this technique can be done in one and a half days if it is performed properly. While attempting this procedure, it's important to remember not to freeze and thaw the samples during the entire procedure, since this could destroy protein interactions or cause protein aggregations.Additionally, when working with light-sensitive reagents, like iodoacetamide, it is important to protect the samples from light during the lysis step. Following the non-denaturing SDS-PAGE, other methods like protein gel extraction and enzymatic activity AsiS or a second dimension PAGE can be performed in order to answer additional questions like enzymatic activity and the characterization of potential hetero-oligomeric complexes. After its development, this technique paved the way for researchers in the field of cell biology to explore the composition of oligomeric protein complexes in cultured cells.After watching this video, you should have a good understanding of how to prepare cell lysates from cell cultures to study oligomeric proteins in non-denaturing PAGE analysis. Don't forget that working with iodoacetamide can be extremely hazardous and precautions such as wearing gloves and eye protection should always be taken.

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Research

JoVE Journal

Biochemistry

Isolation of Cognate RNA-protein Complexes from Cells Using Oligonucleotide-directed Elution

Ribonucleoprotein particles direct the biogenesis and post-transcriptional regulation of all mRNAs through distinct combinations of RNA binding proteins. They are composed of position-dependent, cis-acting RNA elements and unique combinations of RNA binding proteins. Defining the composition of a specific RNP is essential to achieving a fundamental understanding of gene regulation. The isolation of a select RNP is akin to finding a needle in a haystack. Here, we demonstrate an approach to isolate RNPs associated at the 5' untranslated region of a select mRNA in asynchronous, transfected cells. This cognate RNP has been demonstrated to be necessary for the translation of select viruses and cellular stress-response genes.
The demonstrated RNA-protein co-precipitation protocol is suitable for the downstream analysis of protein components through proteomic analyses, immunoblots, or suitable biochemical identification assays. This experimental protocol demonstrates that DHX9/RNA helicase A is enriched at the 5' terminus of cognate retroviral RNA and provides preliminary information for the identification of its association with cell stress-associated huR and junD cognate mRNAs.

This manuscript describes an approach to isolate select cognate RNPs formed in eukaryotic cells via a specific oligonucleotide-directed enrichment. We demonstrate the applicability of this approach by isolating a cognate RNP bound to the retroviral 5' untranslated region that is composed of DHX9/RNA helicase A.

Ribonucleoprotein particles direct the biogenesis and post-transcriptional regulation of all mRNAs through distinct combinations of RNA binding proteins. ...

Tandem Affinity Purification of Protein Complexes from Eukaryotic Cells

The purification of active protein-protein and protein-nucleic acid complexes is crucial for the characterization of enzymatic activities and de novo identification of novel subunits and post-translational modifications. Bacterial systems allow for the expression and purification of a wide variety of single polypeptides and protein complexes. However, this system does not enable the purification of protein subunits that contain post-translational modifications (e.g., phosphorylation and acetylation), and the identification of novel regulatory subunits that are only present/expressed in the eukaryotic system. Here, we provide a detailed description of a novel, robust, and efficient tandem affinity purification (TAP) method using STREP- and FLAG-tagged proteins that facilitates the purification of protein complexes with transiently or stably expressed epitope-tagged proteins from eukaryotic cells. This protocol can be applied to characterize protein complex functionality, to discover post-translational modifications on complex subunits, and to identify novel regulatory complex components by mass spectrometry. Notably, this TAP method can be applied to study protein complexes formed by eukaryotic or pathogenic (viral and bacterial) components, thus yielding a wide array of downstream experimental opportunities. We propose that researchers working with protein complexes could utilize this approach in many different ways.

We describe here a novel, robust, and efficient tandem affinity purification (TAP) method for the expression, isolation, and characterization of protein complexes from eukaryotic cells. This protocol could be utilized for the biochemical characterization of discrete complexes as well as the identification of novel interactors and post-translational modifications that regulate their function.

The purification of active protein-protein and protein-nucleic acid complexes is crucial for the characterization of enzymatic activities and de novo ...

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 ...

Detection and Visualization of DNA Damage-induced Protein Complexes in Suspension Cell Cultures Using the Proximity Ligation Assay

The DNA damage response orchestrates the repair of DNA lesions that occur spontaneously, are caused by genotoxic stress, or appear in the context of programmed DNA breaks in lymphocytes. The Ataxia-Telangiectasia Mutated kinase (ATM), ATM- and Rad3-Related kinase (ATR) and the catalytic subunit of DNA-dependent Protein Kinase (DNA-PKcs) are among the first that are activated upon induction of DNA damage, and are central regulators of a network that controls DNA repair, apoptosis and cell survival. As part of a tumor-suppressive pathway, ATM and ATR activate p53 through phosphorylation, thereby regulating the transcriptional activity of p53. DNA damage also results in the formation of so-called ionizing radiation-induced foci (IRIF) that represent complexes of DNA damage sensor and repair proteins that accumulate at the sites of DNA damage, which are visualized by fluorescence microscopy. Co-localization of proteins in IRIFs, however, does not necessarily imply direct protein-protein interactions, as the resolution of fluorescence microscopy is limited. 
In situ Proximity Ligation Assay (PLA) is a novel technique that allows the direct visualization of protein-protein interactions in cells and tissues with unprecedented specificity and sensitivity. This technique is based on the spatial proximity of specific antibodies binding to the proteins of interest. When the interrogated proteins are within ~40 nm an amplification reaction is triggered by oligonucleotides that are conjugated to the antibodies, and the amplification product is visualized by fluorescent labeling, yielding a signal that corresponds to the subcellular location of the interacting proteins. Using the established functional interaction between ATM and p53 as an example, it is demonstrated here how PLA can be used in suspension cell cultures to study the direct interactions between proteins that are integral parts of the DNA damage response.

Here, it is demonstrated how the in situ Proximity Ligation Assay (PLA) can be used to detect and visualize the direct protein-protein interactions between ATM and p53 in suspension cell cultures exposed to genotoxic stress.

The DNA damage response orchestrates the repair of DNA lesions that occur spontaneously, are caused by genotoxic stress, or appear in the context of ...

Horizontal Gel Electrophoresis for Enhanced Detection of Protein-RNA Complexes

Native polyacrylamide gel electrophoresis is a fundamental tool of molecular biology that has been used extensively for the biochemical analysis of RNA-protein interactions. These interactions have been traditionally analyzed with polyacrylamide gels generated between two glass plates and samples electrophoresed vertically. However, polyacrylamide gels cast in trays and electrophoresed horizontally offers several advantages. For example, horizontal gels used to analyze complexes between fluorescent RNA substrates and specific proteins can be imaged multiple times as electrophoresis progresses. This provides the unique opportunity to monitor RNA-protein complexes at several points during the experiment. In addition, horizontal gel electrophoresis makes it possible to analyze many samples in parallel. This can greatly facilitate time course experiments as well as analyzing multiple reactions simultaneously to compare different components and conditions. Here we provide a detailed protocol for generating and using horizontal native gel electrophoresis for analyzing RNA-Protein interactions.

Native polyacrylamide gel electrophoresis is a fundamental tool for analyzing RNA-protein interactions. Traditionally most experiments have used vertical gels. However, horizontal gels provide several advantages, such as the opportunity to monitor complexes during electrophoresis. We provide a detailed protocol for generating and using horizontal native gel electrophoresis.

Native polyacrylamide gel electrophoresis is a fundamental tool of molecular biology that has been used extensively for the biochemical analysis of ...

DNA Polymerase Activity Assay Using Near-infrared Fluorescent Labeled DNA Visualized by Acrylamide Gel Electrophoresis

For any enzyme, robust, quantitative methods are required for characterization of both native and engineered enzymes. For DNA polymerases, DNA synthesis can be characterized using an in vitro DNA synthesis assay followed by polyacrylamide gel electrophoresis. The goal of this assay is to quantify synthesis of both natural DNA and modified DNA (M-DNA). These approaches are particularly useful for resolving oligonucleotides with single nucleotide resolution, enabling observation of individual steps during enzymatic oligonucleotide synthesis. These methods have been applied to the evaluation of an array of biochemical and biophysical properties such as the measurement of steady-state rate constants of individual steps of DNA synthesis, the error rate of DNA synthesis, and DNA binding affinity. By using modified components including, but not limited to, modified nucleoside triphosphates (NTP), M-DNA, and/or mutant DNA polymerases, the relative utility of substrate-DNA polymerase pairs can be effectively evaluated. Here, we detail the assay itself, including the changes that must be made to accommodate nontraditional primer DNA labeling strategies such as near-infrared fluorescently labeled DNA. Additionally, we have detailed crucial technical steps for acrylamide gel pouring and running, which can often be technically challenging.

This protocol describes the characterization of DNA polymerase synthesis of modified DNA through observation of changes to near-infrared fluorescently labeled DNA using gel electrophoresis and gel imaging. Acrylamide gels are used for high resolution imaging of the separation of short nucleic acids, which migrate at different rates depending on size.

For any enzyme, robust, quantitative methods are required for characterization of both native and engineered enzymes. For DNA polymerases, DNA synthesis ...

Use of Two Dimensional Semi-denaturing Detergent Agarose Gel Electrophoresis to Confirm Size Heterogeneity of Amyloid or Amyloid-like Fibers

Amyloid or amyloid-like fibers have been associated with many human diseases, and are now being discovered to be important for many signaling pathways. The ability to readily detect the formation of these fibers under various experimental conditions is essential for understanding their potential function. Many methods have been used to detect the fibers, but not without some drawbacks. For example, electron microscopy (EM), or staining with Congo Red or Thioflavin T often requires purification of the fibers. On the other hand, semi-denaturing detergent agarose gel electrophoresis (SDD-AGE) allows detection of the SDS-resistant amyloid-like fibers in the cell extracts without purification. In addition, it allows the comparison of the size difference of the fibers. More importantly, it can be used to identify specific proteins within the fibers by Western blotting. It is less time consuming and more easily accessible to a wider number of labs. SDD-AGE results often show variable degree of heterogeneity. It raises the question whether part of the heterogeneity results from the dissociation of the protein complex during the electrophoresis in the presence of SDS. For this reason, we have employed a second dimension of SDD-AGE to determine if the size heterogeneity seen in SDD-AGE is truly a result of fiber heterogeneity in vivo and not a result of either degradation or dissociation of some of the proteins during electrophoresis. This method allows fast, qualitative confirmation that the amyloid or amyloid-like fibers are not partially dissociating during the SDD-AGE process.

Here we use two-dimensional semi-denaturing agarose gel electrophoresis to confirm the presence of amyloid-like fibers of heterogeneous size and exclude the possibility that the size heterogeneity is due to dissociation of the amyloid fibers during the gel running process.

Amyloid or amyloid-like fibers have been associated with many human diseases, and are now being discovered to be important for many signaling pathways. ...

2 in 1: One-step Affinity Purification for the Parallel Analysis of Protein-Protein and Protein-Metabolite Complexes

Cellular processes are regulated by interactions between biological molecules such as proteins, metabolites, and nucleic acids. While the investigation of protein-protein interactions (PPI) is no novelty, experimental approaches aiming to characterize endogenous protein-metabolite interactions (PMI) constitute a rather recent development. Herein, we present a protocol that allows simultaneous characterization of the PPI and PMI of a protein of choice, referred to as bait. Our protocol was optimized for Arabidopsis cell cultures and combines affinity purification (AP) with mass spectrometry (MS)-based protein and metabolite detection. In short, transgenic Arabidopsis lines, expressing bait protein fused to an affinity tag, are first lysed to obtain a native cellular extract. Anti-tag antibodies are used to pull down protein and metabolite partners of the bait protein. The affinity-purified complexes are extracted using a one-step methyl tert-butyl ether (MTBE)/methanol/water method. Whilst metabolites separate into either the polar or the hydrophobic phase, proteins can be found in the pellet. Both metabolites and proteins are then analyzed by ultra-performance liquid chromatography-mass spectrometry (UPLC-MS or UPLC-MS/MS). Empty-vector (EV) control lines are used to exclude false positives. The major advantage of our protocol is that it enables identification of protein and metabolite partners of a target protein in parallel in near-physiological conditions (cellular lysate). The presented method is straightforward, fast, and can be easily adapted to biological systems other than plant cell cultures.

Protein-protein and protein-metabolite interactions are crucial for all cellular functions. Herein, we describe a protocol that allows parallel analysis of these interactions with a protein of choice. Our protocol was optimized for plant cell cultures and combines affinity purification with mass spectrometry-based protein and metabolite detection.

Cellular processes are regulated by interactions between biological molecules such as proteins, metabolites, and nucleic acids. While the investigation of ...

Analysis of AtHIRD11 Intrinsic Disorder and Binding Towards Metal Ions by Capillary Gel Electrophoresis and Affinity Capillary Electrophoresis

Plants are strongly dependent on their environment. In order to adjust to stressful changes (e.g., drought and high salinity), higher plants evolve classes of intrinsically disordered proteins (IDPs) to reduce oxidative and osmotic stress. This article uses a combination of capillary gel electrophoresis (CGE) and mobility shift affinity electrophoresis (ACE) in order to describe the binding behavior of different conformers of the IDP AtHIRD11 from Arabidopsis thaliana. CGE is used to confirm the purity of AtHIRD11 and to exclude fragments, posttranslational modifications, and other impurities as reasons for complex peak patterns. In this part of the experiment, the different sample components are separated by a viscous gel inside a capillary by their different masses and detected with a diode array detector. Afterward, the binding behavior of the sample towards various metal ions is investigated by ACE. In this case, the ligand is added to the buffer solution and the shift in migration time is measured in order to determine whether a binding event has occurred or not. One of the advantages of using the combination of CGE and ACE to determine the binding behavior of an IDP is the possibility to automate the gel electrophoresis and the binding assay. Furthermore, CGE shows a lower limit of detection than the classical gel electrophoresis and ACE is able to determine the manner of binding a ligand in a fast manner. In addition, ACE can also be applied to other charged species than metal ions. However, the use of this method for binding experiments is limited in its ability to determine the number of binding sites. Nevertheless, the combination of CGE and ACE can be adapted for characterizing the binding behavior of any protein sample towards numerous charged ligands.

This protocol combines the characterization of a protein sample by capillary gel electrophoresis and a fast-binding screening for charged ligands by affinity capillary electrophoresis. It is recommended for proteins with a flexible structure, such as intrinsically disordered proteins, to determine any differences in binding for different conformers.

Plants are strongly dependent on their environment. In order to adjust to stressful changes (e.g., drought and high salinity), higher plants evolve ...

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 ...