This research was financially supported by the Academy of Finland (project numbers 307335 and 303757) and Solar Energy into Biomass (SE2B) Marie Sk&#322;odowska-Curie grant agreement (675006). The protocol is based on reference3.

1. Preparing BN Gel1,2,3
<ol>
	<li>Set up the gel caster with 8 cm x 10 cm plates (rectangular glass and notched alumina plate) according to manufacturer&#39;s instructions using 0.75 mm spacers.</li>
	<li>Place a gradient mixer on a stir plate and connect it with the peristaltic pump by a tubing. Attach a syringe needle to the other end of the tubing and place the needle between the glass and aluminum plate. Place magnetic stirrer to the &#34;heavy&#34; (H)-chamber.</li>
	<li>Prepare the 3.5% (v/v) and 12.5% (v/v) acrylamide (AA) solutions in 15 mL conical centrifuge tubes for the separation gel gradient (see recipes in Table 1). To prevent untimely polymerization, keep the centrifuge tubes on ice while preparing the solutions. 
	CAUTION: Acrylamide is neurotoxic and carcinogenic, wear protective clothes and gloves.</li>
	<li>Add 5% APS and TEMED right before pipetting the solutions to the gradient mixer. Pipet the 12.5% solution to the H-chamber.
	<ol>
		<li>Remove air bubbles from the channel connecting the &#34;light&#34; (L) and H-chamber by opening the valve connecting the two chambers allowing solution to enter to the L-chamber. Close the valve and pipet the traces of solution back to H-chamber.</li>
		<li>Finally, pipet the 3.5% solution to the L-chamber.</li>
	</ol>
	</li>
	<li>Switch on the magnetic stirrer (the speed of the stir is not critical, but it should ensure proper mixing of the heavy and light solutions), open the valves and switch on the peristaltic pump. Allow the gel solutions to flow between the glass and aluminum plate, the flowrate should be roughly 0.5 mL/min. The needle must be above the liquid all the time, it can be attached to the upper part of the glass plate with a tape.</li>
	<li>When the H- and L-chambers have emptied, fill them with ultrapure water and allow water&#160;to gently overlay the gel surface. The gel polymerization takes around 1-2 hours at RT.</li>
	<li>Prepare the 3% acrylamide solution (see recipe in Table 1) for the stacking gel at RT. Pipet the stacking gel on top of the polymerized separation gel (before casting the stacking gel, remove the water overlaying the gel surface) and place a sample gel comb between the glass and aluminum plate avoiding air bubbles.
	<ol>
		<li>Allow to polymerize 30-60 min at RT. Remove the comb gently under ultrapure water. Store the gel at +4 &#730;C. 
		Pause point. The gel can be stored at +4 &#730;C for a few days. The gel should be kept in moist conditions to avoid drying of the wells and the gel surface.</li>
	</ol>
	</li>
</ol>
2. Thylakoid Solubilization1,2,3
NOTE: All steps should be performed under very dim light. Keep samples and buffers on ice.
<ol>
	<li>Dilute isolated thylakoids with ice-cold 25BTH20G buffer to a final chlorophyll concentration of 1 mg/mL. For 2D-BN-SDS-PAGE analysis roughly 4-8 &#181;g of chlorophyll/ sample is suitable. 
	NOTE: The thylakoids used in the experiments must be isolated from fresh leaves (for protocol of thylakoid isolation, see3)</li>
	<li>Add an equal volume of detergent buffer, i.e., 2% &#946;-DM (w/v) or 2% digitonin (w/v). Mix the detergent to the thylakoid sample gently with the pipet tip and avoid making air bubbles. The final concentration of the detergent is 1% and that of the thylakoids 0.5 mg Chl/mL.
	<ol>
		<li>Solubilize the thylakoids for 2 min on ice (&#946;-DM) or 10 minutes at RT with continuous gentle mixing on a rocker/shaker (digitonin). 
		NOTE: Digitonin and &#946;-DM are generally used for the solubilization of thylakoid protein complexes. If other non-ionic detergents are used, the detergent concentration and the solubilization time must be first optimized. Usually the detergent concentration range from 0.5%-5% (v/v). 
		CAUTION: Digitonin is toxic, wear protective clothes and gloves</li>
	</ol>
	</li>
	<li>Remove the insolubilized material by centrifugation at 18,000 x g for 20 min, at +4 &#730;C.</li>
	<li>Transfer the supernatant to a new 1.5 mL tube and add 1/10 (v/v) of CBB buffer to the sample. 
	NOTE: When the overall composition of the thylakoid membrane protein complexes is examined, determining the yield and the represented thylakoid domain of solubilized fraction is recommended. To determine the yield of the solubilized material, take 5 &#181;L of the supernatant to a new tube (before adding CBB) and measure the Chl content and Chl a/b ratio5.</li>
</ol>
3. BN-PAGE1,2,3
<ol>
	<li>Set up the gel to a vertical electrophoresis system (e.g., Hoefer SE 250). Fill the upper buffer chamber with blue cathode buffer (see Table 1) and pour anode buffer to the lower buffer chamber.</li>
	<li>Load thylakoid sample (e.g., 5 &#181;g of chlorophyll) into the wells.</li>
	<li>Start the electrophoresis and gradually increase the voltage: 75 V for 30 min, 100 V for 30 min, 125 V for 30 min, 150 V for 1 h and 175 V until the complexes have been separated completely. Run the gel at +4&#730;C, either using a cold room or adjusting the gel temperature with a cooling system. 
	NOTE: Change the blue cathode buffer to a clear cathode buffer when the sample front has migrated about one third of the gel.</li>
	<li>After the electrophoretic run, scan the gel with a photoscanner for image archiving.</li>
</ol>
4. 2D-SDS-PAGE
<ol>
	<li>Assemble the vertical electrophoresis system (gel size 16 cm x 20 cm). Use 1 mm spacers.</li>
	<li>Prepare standard SDS gel (12% acrylamide, 6 M Urea, see recipe in Table 2) with a 2D-comb (single large well for the strip and one standard well for molecular weight marker).</li>
	<li>Cut the lane from BN-gel and place it in a (5 mL) tube. Add 2 mL of Laemmli buffer (containing 5% &#946;-mercaptoethanol) and incubate the strip for 45 min with gentle shaking at RT.</li>
	<li>Place the lane, with a help of e.g., a spacer, on top of the gel avoiding air bubbles.</li>
	<li>Pipet 5 &#181;L of molecular weight marker on a narrow piece of filter paper and place the paper to the standard well.</li>
	<li>To seal the BN-gel strip and the marker paper, pour 0.5% agarose (in running buffer) on top of the gel strip and allow to solidify.</li>
	<li>Perform electrophoresis according to standard protocols. After the electrophoretic run, visualize the proteins with e.g., Sypro Ruby stain or silver staining according to6.</li>
</ol>

<ol>
	<li>Sch&#228;gger, H., von Jagow, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Blue+native+electrophoresis+for+isolation+of+membrane+protein+complexes+in+enzymatically+active+form.">Blue native electrophoresis for isolation of membrane protein complexes in enzymatically active form.</a> Analytical Biochemistry. 199, 223-231 (1991).</li><li>K&#252;gler, M., J&#228;nsch, L., Kruft, V., Schmitz, U. K., Braun, H. -. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+the+chloroplast+protein+complexes+by+blue-native+polyacrylamide+gel+electrophoresis+(BN-PAGE).">Analysis of the chloroplast protein complexes by blue-native polyacrylamide gel electrophoresis (BN-PAGE).</a> Photosynthesis Research. 53, 35-44 (1997).</li><li>J&#228;rvi, S., Suorsa, M., Paakkarinen, V., Aro, E. -. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimized+native+gel+systems+for+separation+of+thylakoid+protein+complexes:+novel+super-+and+mega-complexes.">Optimized native gel systems for separation of thylakoid protein complexes: novel super- and mega-complexes.</a> Biochemical Journal. 439, 207-214 (2011).</li><li>Strecker, V., Wumaier, Z., Wittig, I., Sch&#228;gger, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large+pore+gels+to+separate+mega+protein+complexes+larger+than+10+MDa+by+blue+native+electrophoresis:+Isolation+of+putative+respiratory+strings+or+patches.">Large pore gels to separate mega protein complexes larger than 10 MDa by blue native electrophoresis: Isolation of putative respiratory strings or patches.</a> Proteomics. 10, 3379-3387 (2010).</li><li>Porra, R. J., Thompson, W. A., Kriedemann, P. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+accurate+extinction+coefficients+and+simultaneous+equations+for+assaying+chlorophylls+a+and+b+extracted+with+four+different+solvents:+verification+of+the+concentration+of+chlorophyll+standards+by+atomic+absorption+spectroscopy.">Determination of accurate extinction coefficients and simultaneous equations for assaying chlorophylls a and b extracted with four different solvents: verification of the concentration of chlorophyll standards by atomic absorption spectroscopy.</a> Biochimica et Biophysica Acta (BBA) - Bioenergetics. , 384-394 (1989).</li><li>Blum, H., Beier, H., Gross, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improved+silver+staining+of+plant+proteins,+RNA+and+DNA+in+polyacrylamide+gels.">Improved silver staining of plant proteins, RNA and DNA in polyacrylamide gels.</a> Electrophoresis. 8, 93-99 (1987).</li><li>Aro, E. -. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamics+of+photosystem+II:+a+proteomic+approach+to+thylakoid+protein+complexes.">Dynamics of photosystem II: a proteomic approach to thylakoid protein complexes.</a> Journal of Experimental Botany. 56, 347-356 (2005).</li><li>Suorsa, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light+acclimation+involves+dynamic+re-organization+of+the+pigment-protein+megacomplexes+in+non-appressed+thylakoid+domains.">Light acclimation involves dynamic re-organization of the pigment-protein megacomplexes in non-appressed thylakoid domains.</a> The plant journal for cell and molecular biology. 84, 360-373 (2015).</li><li>Laemmli, U. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cleavage+of+Structural+Proteins+during+the+Assembly+of+the+Head+of+Bacteriophage+T4.">Cleavage of Structural Proteins during the Assembly of the Head of Bacteriophage T4.</a> Nature. 227, 680-685 (1970).</li><li>Sch&#228;gger, H., Pfeiffer, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Supercomplexes+in+the+respiratory+chains+of+yeast+and+mammalian+mitochondria.">Supercomplexes in the respiratory chains of yeast and mammalian mitochondria.</a> The EMBO Journal. 19, 1777-1783 (2000).</li><li>Rantala, S., Tikkanen, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phosphorylation-induced+lateral+rearrangements+of+thylakoid+protein+complexes+upon+light+acclimation.">Phosphorylation-induced lateral rearrangements of thylakoid protein complexes upon light acclimation.</a> Plant Direct. 2, 1-12 (2018).</li><li>Rantala, M., Tikkanen, M., Aro, E. -. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Proteomic+characterization+of+hierarchical+megacomplex+formation+in+Arabidopsis+thylakoid+membrane.">Proteomic characterization of hierarchical megacomplex formation in Arabidopsis thylakoid membrane.</a> Plant Journal. 92, 951-962 (2017).</li></ol>

The authors have nothing to disclose.

The photosynthetic energy conversion machinery is composed of large multisubunit protein complexes, which are embedded in the thylakoid membrane. This protocol describes a basic method for analysis of the plant thylakoid protein complexes from Arabidopsis thaliana with BN-PAGE combined with 2D-SDS-PAGE. The protocol is also suitable for the analysis of thylakoid protein complexes from tobacco and spinach thylakoids, but might require small adjustments.
For the solubilization of membrane protein complexes, nonionic detergents are commonly used for their ability to preserve the complexes in their native form. Here, two commonly used detergents &#946;-DM and digitonin were applied. Dodecyl maltoside solubilizes individual protein complexes, whereas digitonin can be used for the analysis of larger protein complex assemblies10. The bulky structured digitonin is unable to fit to the tightly appressed grana partitions and therefore solubilizes only the non-appressed regions of the thylakoid membrane3,11. It is therefore suited for the analysis of stroma thylakoids and grana margins. However, when digitonin is used together with aminocaproic acid (ACA), the combination solubilizes the entire thylakoid membrane, including also the appressed grana thylakoids12. Through an unknown mechanism, ACA allows digitonin to have access to the partition gap between adjacent grana membrane layers. Importantly, digitonin preserves labile interactions between protein complexes and can therefore be used for the analysis of labile protein super and megacomplexes, which are most abundant in the non-appressed thylakoid regions8. It must be noted that detergents always interfere with some of the labile interactions between the protein complexes and therefore it is not possible to isolate completely intact network of protein complexes. Some of the complexes are dissociation products that have been disconnected from larger protein complex associations during thylakoid solubilization and the electrophoresis. The quality of the BN-PAGE separation depends not only on the sample preparation (protein complex solubilization), but also on the quality of the thylakoid isolation, which must be done from fresh leaves. If special care is not taken, the PSII-LHCII supercomplexes typically degrade during &#946;-DM solubilization.
BN-PAGE maintains the integrity of the solubilized protein complexes. The separation capacity of the BN gel depends on the acrylamide gradient, and the gradient should be optimized based on the protein complexes of interest. The pore-size of the polyacrylamide gel can be modified by changing the concentration gradient (total acrylamide concentration, T) or by adjusting the bis-acrylamide concentration (C) relative to the total amount of acrylamide monomers4. The BN-PA gel gradient used here is optimized and well suited for the analysis of large protein super- and megacomplexes3. Importantly, pore-size of the stacking gel must be large to allow all complexes to enter to the separation gel. After protein complex separation with BN-PAGE, the composition or structure of each individual protein complex band can be further analyzed. For structural analysis the protein complex of interest must be eluted from the gel and most labile complexes may be destroyed during the elution. Therefore, sucrose density gradient is more often used for the protein complex purification for structural analysis. For the analysis of spectroscopic properties of the protein complexes in the gel, the clear-native PAGE must be used instead of BN-PAGE, since the Coomassie dye interferes with such measurements. For analysis of the subunit composition of the protein complexes, a denaturing 2D-SDS-PAGE is described here. The subunits of each complex are separated in a vertical line and can be easily identified. It has to be noted that several proteins may be present in a single spot and the subunits in the same vertical line may belong to separate complexes co-migrating in BN-PAGE.

Large multisubunit protein complexes photosystem PSI and PSII, Cyt b6f and ATPase coordinate the production of NADPH and ATP in photosynthetic light reactions. In higher plant chloroplasts, the complexes are located in the thylakoid membrane, which is a structurally heterogeneous membrane structure, comprising appressed grana and non-appressed stroma thylakoids. Blue native polyacrylamide gel electrophoresis (BN-PAGE) is an extensively used method in the analysis of large multisubunit protein complexes in their native and biologically active form. The method was established for the dissection of mitochondrial membrane protein complexes1, but has later been customized for the separation of protein complexes from the thylakoid membrane network3. The method is suitable (i) for the purification of individual thylakoid protein complexes for structural analysis, (ii) for determining native interactions between protein complexes and (iii) for the analysis of overall organization of the protein complexes upon changing environmental cues.
Prior to the separation, protein complexes are isolated from the membrane with carefully chosen nonionic detergents, which are generally mild and preserve the native structure of the protein complexes. Detergents contain hydrophobic and hydrophilic sites and form stable micelles above a certain concentration, called a critical micellar concentration (CMC). Increasing the detergent concentration above the CMC results in disruption of the lipid-lipid interactions and in the solubilization of protein complexes. The choice of detergent depends on the stability of the protein complex of interest and on the solubilization capacity of the detergent. Routinely used detergents include &#945;/&#946;-dodecyl-maltoside and digitonin. Following the solubilization of protein complexes in their native state, insoluble material is removed by centrifugation. In higher plants, the thylakoid membrane is highly heterogenic in structure and some detergents (e.g., digitonin) selectively solubilize only a specific fraction of the membrane3. Therefore, to characterize the protein complex organization or the interactions between the protein complexes, it is crucial to always determine the solubilization capacity of the chosen detergent by determining the chlorophyll content and the chlorophyll a/b ratio of supernatant to assess the yield and the represented thylakoid (sub)domain, respectively, of the solubilized fraction. The chlorophyll a/b ratio in intact thylakoids of growth-light acclimated plants is typically around 3, whereas the chl a/b value of thylakoid fractions enriched either in grana or stroma thylakoids falls below (~2.5) or exceeds (~4.5) the value of the total thylakoids, respectively.
To provide negative charge to the protein complexes, Coomassie brilliant blue (CBB) dye is added to the solubilized sample. Due to the charge shift, protein complexes migrate towards the anode and are separated on an acrylamide (AA) gradient according to their molecular mass and shape. Effective and high-resolution separation is achieved by using a linear acrylamide concentration gradient. During the electrophoresis, the protein complexes migrate towards the anode until they reach their size-dependent pore-size limit. The pore-size of polyacrylamide gel depends on (i) the total acrylamide/bis-acrylamide concentration (T) and (ii) on the cross-linker bis-acrylamide monomer concentration (C) relative to the total monomers4. After the separation with BN-PAGE, the protein complexes can be further subdivided into their individual protein subunits by second-dimension (2D)-SDS-PAGE. Here, we describe a detailed protocol for the analysis of thylakoid membrane protein complexes by BN-PAGE/2D-SDS-PAGE.

<table>
	<tbody>
		<tr>
			<td>6-aminocaproic acid (ACA)</td>
			<td>Sigma-Aldrich</td>
			<td>A2504</td>
			<td></td>
		</tr>
		<tr>
			<td>BisTris</td>
			<td>Sigma-Aldrich</td>
			<td>B4429</td>
			<td></td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma-Aldrich</td>
			<td>S0389</td>
			<td></td>
		</tr>
		<tr>
			<td>Acrylamide (AA)</td>
			<td>Sigma-Aldrich</td>
			<td>A9099</td>
			<td>Caution: Neurotoxic!</td>
		</tr>
		<tr>
			<td>n-dodecyl-&#946;-D-maltoside</td>
			<td>Sigma-Aldrich</td>
			<td>D4641</td>
			<td></td>
		</tr>
		<tr>
			<td>Tricine</td>
			<td>Sigma-Aldrich</td>
			<td>T0377</td>
			<td></td>
		</tr>
		<tr>
			<td>Tris</td>
			<td>Sigma-Aldrich</td>
			<td>T1503</td>
			<td></td>
		</tr>
		<tr>
			<td>SDS</td>
			<td>VWR</td>
			<td>442444H</td>
			<td></td>
		</tr>
		<tr>
			<td>Urea</td>
			<td>VWR</td>
			<td>28877.292</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>J.T. Baker</td>
			<td>7044</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Fluoride (NaF)</td>
			<td>J.T. Baker</td>
			<td>3688</td>
			<td></td>
		</tr>
		<tr>
			<td>EDTA disodium salt</td>
			<td>J.T. Baker</td>
			<td>1073</td>
			<td></td>
		</tr>
		<tr>
			<td>Digitonin</td>
			<td>Calbiochem</td>
			<td>300410</td>
			<td>Caution:Toxic!</td>
		</tr>
		<tr>
			<td>Pefabloc SC</td>
			<td>Roche</td>
			<td>11585916001</td>
			<td></td>
		</tr>
		<tr>
			<td>Serva Coomassie Blue G</td>
			<td>Serva</td>
			<td>35050</td>
			<td></td>
		</tr>
		<tr>
			<td>&#946;-mercaptoethanol</td>
			<td>Bio-Rad</td>
			<td>1610710</td>
			<td></td>
		</tr>
		<tr>
			<td>APS (Ammonium persulfate)</td>
			<td>Bio-Rad</td>
			<td>161-0700</td>
			<td></td>
		</tr>
		<tr>
			<td>TEMED (Tetramethylethylenediamine)</td>
			<td>Bio-Rad</td>
			<td>1610801</td>
			<td></td>
		</tr>
		<tr>
			<td>(N,N&#39;-Methylene)-Bis-Acrylamide</td>
			<td>Omnipur</td>
			<td>2610</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycine</td>
			<td>Fisher</td>
			<td>G0800</td>
			<td></td>
		</tr>
		<tr>
			<td>Prestained Protein Marker, Broad Range (7-175 kDa)</td>
			<td>New England Biolabs</td>
			<td>P7708</td>
			<td></td>
		</tr>
		<tr>
			<td>Falcon, Conical Centrifuge Tubes 15 ml</td>
			<td>Corning</td>
			<td>352093</td>
			<td></td>
		</tr>
		<tr>
			<td>Dual gel caster with 10 x 8 cm plates</td>
			<td>Hoefer</td>
			<td>SE215</td>
			<td></td>
		</tr>
		<tr>
			<td>Gradient maker SG5</td>
			<td>Hoefer</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>0.75 mm T-spacers</td>
			<td>Hoefer</td>
			<td>SE2119T-2-.75</td>
			<td></td>
		</tr>
		<tr>
			<td>Sample gel comb, 0.75 mm</td>
			<td>Hoefer</td>
			<td>SE211A-10-.75</td>
			<td></td>
		</tr>
		<tr>
			<td>Mighty Small SE250 vertical electrophoresis system</td>
			<td>Hoefer</td>
			<td>SE250</td>
			<td></td>
		</tr>
		<tr>
			<td>IPC-pump</td>
			<td>Ismatec</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Power supply, PowerPac HV</td>
			<td>Bio-Rad</td>
			<td>164-5097</td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Eppendorf</td>
			<td>5424R</td>
			<td></td>
		</tr>
		<tr>
			<td>Rocker-Shaker</td>
			<td>Biosan</td>
			<td>BS-010130-AAI</td>
			<td></td>
		</tr>
		<tr>
			<td> 
			PROTEAN II xi Cell</td>
			<td>Bio-Rad</td>
			<td>1651813</td>
			<td></td>
		</tr>
	</tbody>
</table>

analysis of thylakoid membrane protein complexes by blue native gel electrophoresis

Photosynthetic electron transfer chain (ETC) converts solar energy to chemical energy in the form of NADPH and ATP. Four large protein complexes embedded in the thylakoid membrane harvest solar energy to drive electrons from water to NADP+ via two photosystems, and use the created proton gradient for production of ATP. Photosystem PSII, PSI, cytochrome b6f (Cyt b6f) and ATPase are all multiprotein complexes with distinct orientation and dynamics in the thylakoid membrane. Valuable information about the composition and interactions of the protein complexes in the thylakoid membrane can be obtained by solubilizing the complexes from the membrane integrity by mild detergents followed by native gel electrophoretic separation of the complexes. Blue native polyacrylamide gel electrophoresis (BN-PAGE) is an analytical method used for the separation of protein complexes in their native and functional form. The method can be used for protein complex purification for more detailed structural analysis, but it also provides a tool to dissect the dynamic interactions between the protein complexes. The method was developed for the analysis of mitochondrial respiratory protein complexes, but has since been optimized and improved for the dissection of the thylakoid protein complexes. Here, we provide a detailed up-to-date protocol for analysis of labile photosynthetic protein complexes and their interactions in Arabidopsis thaliana.

A representative 2D-BN/SDS-PAGE system in Figure 1 demonstrates the separation of digitonin and &#946;-DM-solubilized thylakoid protein complexes and their detailed protein subunit composition. The protein complex pattern of digitonin solubilized thylakoids (horizontal gel on the top on the top of Figure 1A) contains the PSII-LHCII-PSI megacomplex, two large PSII-LHCII supercomplexes (sc), PSI-LHCII supercomplex, PSI monomer (m), PSII m/Cyt b6f, loosely bound (L)-LHCII trimer (Figure 1A). The slightly stronger detergent, &#946;-DM, solubilizes the entire thylakoid membrane (horizontal gel on the top of Figure 1B), but is unable to preserve weak interactions between protein complexes. Thylakoid solubilization with &#946;-DM typically produces four PSII-LHCII supercomplexes (with different amount of LHCII antenna attached), PSII dimer (d) and PSI m, ATPase, PSII m and Cytb6f, M-LHCII, L-LHCII and LHCII monomer (Figure 1B). Unlike &#946;-DM, digitonin produces only minor amount of LHCII monomer. The protein complex pattern may differ depending on plant exposure to different light conditions and might also differ between mutant lines, since the protein complex interactions are dynamic and dependent on i.e., protein phosphorylation. SDS-PA gels below the native gel strips in Figure 1 A and B represent the polypeptide composition of each protein complex initially solubilized with digitonin or &#946;-DM.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58369/58369fig1.jpg" /> 
Figure 1. A two-dimensional BN-PAGE/SDS-PAGE of Arabidopsis thylakoid. Thylakoid protein complexes solubilized with (A) 1% digitonin and (B) 1% &#946;-DM and separated first by 1D-BN-PAGE (the lanes on top) and subsequently on 2D-SDS-PAGE to demonstrate the individual protein composition of each complex. Due to the incubation of BN-strips with denaturing Laemmli buffer, the protein subunits of each complex (in the BN strip) dissociate and are separated in a vertical line during the 2D-SDS-PAGE. The protein identification is based on mass spectrometry analysis presented in references7,8. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58369/58369fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Buffer</td>
			<td>Content </td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>3.5% (T) Acrylamide (AA) BN Separation gel </td>
			<td>48% AA, 1.5% bis-AA: 148 &#181;L 
			3xGel Buffer: 700 &#181;L 
			75% (w/v) glycerol: 140 &#181;L 
			Ultrapure H2O: 1092 &#181;L 
			5% APS: 15 &#181;L 
			TEMED 3 &#181;L</td>
			<td>NOTE: Acrylamide is neurotoxic. Add APS and TEMED immediately before use. The recipe is sufficient for casting one small BN gel.</td>
		</tr>
		<tr>
			<td>12.5% (T) Acrylamide (AA) BN Separation gel </td>
			<td>48% AA, 1,5% bis-AA: 530 &#181;L 
			3xGel Buffer: 700 &#181;L 
			75% (w/v) glycerol: 560 &#181;L 
			Ultrapure H2O: 290 &#181;L 
			5% APS: 11 &#181;L 
			TEMED 2&#181;L</td>
			<td>NOTE: Acrylamide is neurotoxic. Add APS and TEMED immediately before use. The recipe is sufficient for casting one small BN gel.</td>
		</tr>
		<tr>
			<td>3 % (T) Acrylamide (AA) BN Stacking gel </td>
			<td>20% AA, 5% bis-AA: 180 &#181;L 
			3xGel Buffer: 500 &#181;L 
			Ultrapure H2O: 800 &#181;L 
			5% APS: 30 &#181;L 
			TEMED 3 &#181;L</td>
			<td>NOTE: Acrylamide is neurotoxic. Add APS and TEMED immediately before use. The recipe is sufficient for one small BN gel.</td>
		</tr>
		<tr>
			<td>3x Gel buffer </td>
			<td>1.5M ACA 
			150mM BisTris/HCl (pH 7.0)</td>
			<td>Store at + 4 &#730;C.</td>
		</tr>
		<tr>
			<td>CBB buffer </td>
			<td>100 mM BisTris/HCl (pH 7.0) 0.5 M ACA 
			30% (w/v) sucrose 
			50 mg/ml Serva Blue G</td>
			<td>Store at + 4 &#730;C</td>
		</tr>
		<tr>
			<td>Anode buffer </td>
			<td>50 mM BisTris/HCl (pH 7.0)</td>
			<td>Store at + 4 &#730;C. Buffer can be prepared as 10x stock solution</td>
		</tr>
		<tr>
			<td>Cathode buffer </td>
			<td>50 mM Tricine 
			15 mM BisTris 
			0.01% Serva Blue G</td>
			<td>Store at + 4 &#730;C. Buffer can be prepared as 10x stock solution, add the dye to the 1x solution</td>
		</tr>
		<tr>
			<td>25BTH20G </td>
			<td>25 mM BisTris/HCl (pH 7.0) 
			20% (w/v) glycerol 
			0.25 mg/ml Pefabloc (add freshly) 
			10 mM NaF (add freshly)</td>
			<td>Buffer can be prepared as 2X stock solution (store at + 4 &#730;C) , but add Pefabloc and NaF freshly</td>
		</tr>
		<tr>
			<td>Detergent buffer </td>
			<td>2% &#946;-dodecyl maltoside/Digitonin (w/v) 
			25 mM BisTris/HCl (pH 7.0) 
			20% (w/v) glycerol and 
			0.25 mg/ml Pefabloc (add freshly from the stock solution) 
			10 mM NaF (add freshly)</td>
			<td>Detergents can be prepared as 5-10% stock solutions (in water). If other detergents are used, the final detergent concentration has to be optimized.</td>
		</tr>
	</tbody>
</table>
Table 1. Buffers and solutions for native gel electrophoresis
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Buffer</td>
			<td>Content </td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Laemmli buffer </td>
			<td>138 mM Tris/HCL pH 6.8 
			6M Urea 
			22.2 % (v/v) glycerol 
			4.4 % SDS</td>
			<td>Reference: 9</td>
		</tr>
		<tr>
			<td>12% Acrylamide, 6M Urea SDS Separation gel </td>
			<td>50% AA, 1,33% bis-AA: 10.5 mL 
			20% SDS: 0.7 mL 
			1.5 M Tris-HCl (pH 8.8): 8.05mL 
			Urea: 12.6 g 
			MQ-H2O: 6.16 mL 
			10% APS: 200 &#181;L 
			TEMED 28 &#181;L</td>
			<td>NOTE: Acrylamide is neurotoxic.The recipe is suitable for casting one big SDS-gel.</td>
		</tr>
		<tr>
			<td>6% Acrylamide, 6M Urea SDS Stacking gel </td>
			<td>50% AA, 1.33% bis-AA: 1.2 mL 
			20% SDS: 0.2 mL 
			0.5 M Tris-HCl (pH 6.8): 2.5 mL 
			Urea: 3.6 g 
			MQ-H2O: 3.45 mL 
			10% APS: 100 &#181;L 
			TEMED 10 &#181;L</td>
			<td>NOTE: Acrylamide is neurotoxic.</td>
		</tr>
		<tr>
			<td>SDS Running buffer </td>
			<td>19 mM Tris 
			2.5 mM Glycine 
			0.01 % SDS</td>
			<td></td>
		</tr>
	</tbody>
</table>
Table 2. Buffers for 2D-SDS-PAGE

Watch this Scientific Journal Video about Analysis of Thylakoid Membrane Protein Complexes by Blue Native Gel Electrophoresis at JoVE.com

Analysis of Thylakoid Membrane Protein Complexes by Blue Native Gel Electrophoresis

A protocol for the elucidation of plant thylakoid protein complex organization and composition with blue native polyacrylamide gel electrophoresis (BN-PAGE) and 2D-SDS-PAGE is described. The protocol is optimized for Arabidopsis thaliana, but can be used for other plant species with minor modifications.

Photosynthetic electron transfer chain (ETC) converts solar energy to chemical energy in the form of NADPH and ATP. Four large protein complexes embedded ...

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Biochemistry

12.8K Views.  University of Turku. A protocol for the elucidation of plant thylakoid protein complex organization and composition with blue native polyacrylamide gel electrophoresis (BN-PAGE) and 2D-SDS-PAGE is described. The protocol is optimized for Arabidopsis thaliana, but can be used for other plant species with minor modifications.

Video: Analysis of Thylakoid Membrane Protein Complexes by Blue Native Gel Electrophoresis

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.

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 ...

Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro

Studies of integral membrane proteins in vitro are frequently complicated by the presence of a hydrophobic transmembrane domain. Further complicating these studies, reincorporation of detergent-solubilized membrane proteins into liposomes is a stochastic process where protein topology is impossible to enforce. This paper offers an alternative method to these challenging techniques that utilizes a liposome-based scaffold. Protein solubility is enhanced by deletion of the transmembrane domain, and these amino acids are replaced with a tethering moiety, such as a His-tag. This tether interacts with an anchoring group (Ni2+ coordinated by nitrilotriacetic acid (NTA(Ni2+)) for His-tagged proteins), which enforces a uniform protein topology at the surface of the liposome. An example is presented wherein the interaction between Dynamin-related protein 1 (Drp1) with an integral membrane protein, Mitochondrial Fission Factor (Mff), was investigated using this scaffold liposome method. In this work, we have demonstrated the ability of Mff to efficiently recruit soluble Drp1 to the surface of liposomes, which stimulated its GTPase activity. Moreover, Drp1 was able to tubulate the Mff-decorated lipid template in the presence of specific lipids. This example demonstrates the effectiveness of scaffold liposomes using structural and functional assays and highlights the role of Mff in regulating Drp1 activity.

Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro

This paper describes a method for assessing the interactions and assemblies of integral membrane proteins in vitro with various partner factors in a lipid-proximal environment.

Studies of integral membrane proteins in vitro are frequently complicated by the presence of a hydrophobic transmembrane domain. Further complicating ...

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

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

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

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

Analyzing Supercomplexes of the Mitochondrial Electron Transport Chain with Native Electrophoresis, In-gel Assays, and Electroelution

The mitochondrial electron transport chain (ETC) transduces the energy derived from the breakdown of various fuels into the bioenergetic currency of the cell, ATP. The ETC is composed of 5 massive protein complexes, which also assemble into supercomplexes called respirasomes (C-I, C-III, and C-IV) and synthasomes (C-V) that increase the efficiency of electron transport and ATP production. Various methods have been used for over 50 years to measure ETC function, but these protocols do not provide information on the assembly of individual complexes and supercomplexes. This protocol describes the technique of native gel polyacrylamide gel electrophoresis (PAGE), a method that was modified more than 20 years ago to study ETC complex structure. Native electrophoresis permits the separation of ETC complexes into their active forms, and these complexes can then be studied using immunoblotting, in-gel assays (IGA), and purification by electroelution. By combining the results of native gel PAGE with those of other mitochondrial assays, it is possible to obtain a completer picture of ETC activity, its dynamic assembly and disassembly, and how this regulates mitochondrial structure and function. This work will also discuss limitations of these techniques. In summary, the technique of native PAGE, followed by immunoblotting, IGA, and electroelution, presented below, is a powerful way to investigate the functionality and composition of mitochondrial ETC supercomplexes.

This protocol describes the separation of functional mitochondrial electron transport chain complexes (Cx) I-V and supercomplexes thereof using native electrophoresis to reveal information about their assembly and structure. The native gel can be subjected to immunoblotting, in-gel assays, and purification by electroelution to further characterize individual complexes.

The mitochondrial electron transport chain (ETC) transduces the energy derived from the breakdown of various fuels into the bioenergetic currency of the ...

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 ...

Split-BioID &#8212; Proteomic Analysis of Context-specific Protein Complexes in Their Native Cellular Environment

To complement existing affinity purification (AP) approaches for the identification of protein-protein interactions (PPI), enzymes have been introduced that allow the proximity-dependent labeling of proteins in living cells. One such enzyme, BirA* (used in the BioID approach), mediates the biotinylation of proteins within a range of approximately 10 nm. Hence, when fused to a protein of interest and expressed in cells, it allows the labeling of proximal proteins in their native environment. As opposed to AP that relies on the purification of assembled protein complexes, BioID detects proteins that have been marked within cells no matter whether they are still interacting with the protein of interest when they are isolated. Since it biotinylates proximal proteins, one can moreover capitalize on the exceptional affinity of streptavidin for biotin to very efficiently isolate them. While BioID performs better than AP for identifying transient or weak interactions, both AP- and BioID-mass spectrometry approaches provide an overview of all possible interactions a given protein may have. However, they do not provide information on the context of each identified PPI. Indeed, most proteins are typically part of several complexes, corresponding to distinct maturation steps or different functional units. To address this common limitation of both methods, we have engineered a protein-fragments complementation assay based on the BirA* enzyme. In this assay, two inactive fragments of BirA* can reassemble into an active enzyme when brought in close proximity by two interacting proteins to which they are fused. The resulting split-BioID assay thus allows the labeling of proteins that assemble around a pair of interacting proteins. Provided these two only interact in a given context, split-BioID then allows the analysis of specific context-dependent functional units in their native cellular environment. Here, we provide a step-by-step protocol to test and apply split-BioID to a pair of interacting proteins.

We provide a step-by-step protocol for split-BioID, a protein fragments-complementation assay based on the proximity-labeling technique BioID. Activated on the interaction of two given proteins, it allows the proteomics analysis of context-dependent protein complexes in their native cellular environment. The method is simple, cost-effective and only requires standard laboratory equipment.

To complement existing affinity purification (AP) approaches for the identification of protein-protein interactions (PPI), enzymes have been introduced ...

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 ...

A Fluorescence Fluctuation Spectroscopy Assay of Protein-Protein Interactions at Cell-Cell Contacts

A variety of biological processes involves cell-cell interactions, typically mediated by proteins that interact at the interface between neighboring cells. Of interest, only few assays are capable of specifically probing such interactions directly in living cells. Here, we present an assay to measure the binding of proteins expressed at the surfaces of neighboring cells, at cell-cell contacts. This assay consists of two steps: mixing of cells expressing the proteins of interest fused to different fluorescent proteins, followed by fluorescence fluctuation spectroscopy measurements at cell-cell contacts using a confocal laser scanning microscope. We demonstrate the feasibility of this assay in a biologically relevant context by measuring the interactions of the amyloid precursor-like protein 1 (APLP1) across cell-cell junctions. We provide detailed protocols on the data acquisition using fluorescence-based techniques (scanning fluorescence cross-correlation spectroscopy, cross-correlation number and brightness analysis) and the required instrument calibrations. Further, we discuss critical steps in the data analysis and how to identify and correct external, spurious signal variations, such as those due to photobleaching or cell movement.
In general, the presented assay is applicable to any homo- or heterotypic protein-protein interaction at cell-cell contacts, between cells of the same or different types and can be implemented on a commercial confocal laser scanning microscope. An important requirement is the stability of the system, which needs to be sufficient to probe diffusive dynamics of the proteins of interest over several minutes.

This protocol describes a fluorescence fluctuation spectroscopy-based approach to investigate interactions among proteins mediating cell-cell interactions, i.e. proteins localized in cell junctions, directly in living cells. We provide detailed guidelines on instrument calibration, data acquisition and analysis, including corrections to possible artefact sources.

A variety of biological processes involves cell-cell interactions, typically mediated by proteins that interact at the interface between neighboring ...