This work was supported by research grants DK52057 and DK107498 from the NIH to K.V.K. X.P was supported by the institutional training grant 2T32DK007201 from the NIH.

1. Handling of the Peptide
<ol>
	<li>Design and ordering of the peptide
	<ol>
		<li>Choose the desired sequence of the peptide, and add a tag, such as myc epitope (EQKLISEED) at its either N- or C-terminus.</li>
		<li>Check the predicted solubility of the peptide in water, using peptide solubility calculator http://pepcalc.com/peptide-solubility-calculator.php. If the solubility is low, try to add another water-soluble tag to change the charge balance. 
		NOTE: We used the peptide corresponding to the first luminal loop (fll) of GLUT4 tagged with the myc epitope (bold font) at the N terminus (Myc-fll-GLUT4): EQKLISEEDLNAPQKVIEQSYNATWLGRQGPGGPSSIPPGTLTTLWA that has &#34;good&#34; predicted solubility in water.</li>
		<li>Order the custom peptide with at least 75% purity which is sufficient for the assay, and ask the provider for solubility test. Separate the order into at least two aliquots in case of unforeseen problems with dissolving the peptide. 
		NOTE: Myc-fll-GLUT4 was ordered in two aliquots. Its reported solubility in ultrapure water is &#8804;10 mg/mL.</li>
	</ol>
	</li>
	<li>Dissolving the peptide 
	Note: Ultrapure water is the best solvent. If the peptide does not appear to be water soluble, refer to http://www.genscript.com/peptide_solubility_and_stablity.html for instructions.
	<ol>
		<li>Prepare 100x working solution of the peptide in ultrapure water or in buffer of choice, with the concentration of 100 &#956;g/mL.</li>
		<li>Separate the solution into 50 - 100 &#181;L aliquots, and store them at -20 &#176;C.</li>
	</ol>
	</li>
</ol>
2. Handling of Cells
<ol>
	<li>Cell culture
	<ol>
		<li>Use Wild type (WT) cells as a negative control, and cells expressing the target protein tagged with six histidines (HisP). 
		NOTE: We used 3T3 L1 pre-adipocytes stably transfected with Sortilin-myc/His5.</li>
		<li>Grow the cells in Dulbecco&#39;s Modified Eagle Medium (DMEM) high Sugar, supplemented with 10% calf bovine serum, glutamine (2 mM) and penicillin/streptomycin (5 &#956;g/mL) at 37 &#176;C in 10% CO2.</li>
		<li>Sit four 10 cm dishes of cells, transfect two of them with HisP and transfect the other two with control plasmid (WT). 48 h after transfection, cells should reach 80 - 90% confluency.</li>
	</ol>
	</li>
	<li>Preparation of the Cell Lysates
	<ol>
		<li>Prepare lysis buffer (10 mM Hepes, 30 mM NaCl, 5% glycerol, 10 mM Imidazole, 0.5% Triton X-100 and protease inhibitor cocktail without EDTA for the isolation of His-tagged proteins, pH 7.4) and keep it at 4 &#176;C:</li>
		<li>Wash cells three times with 10 mL of 1x phosphate-buffered saline (PBS) at 4 &#176;C.</li>
		<li>Put the dishes on ice and add 500 &#181;L of lysis buffer to each dish. Harvest the cell lysates using a cell scraper.</li>
		<li>Place the cell lysate from each dish into a different 1.5 mL tube. Label the tubes as WT-pH6, WT-pH8, HisP-pH6, HisP-pH8.</li>
		<li>Pass the cell lysates through a syringe with a 26G needle five times up and down, to complete lysis.</li>
		<li>Centrifuge the cell lysates at 16,000 x g for 10 min at 4 &#176;C.</li>
		<li>Transfer the supernatants to new identically labeled tubes.</li>
		<li>Analyze protein concentration using the BCA Protein Assay Kit or other kit.</li>
		<li>Using lysis buffer, equalize protein concentration of all four lysates.</li>
		<li>Separate a small (ca. 20 &#181;L) aliquot of each lysate and keep it at -20 &#176;C for possible control experiments and/or trouble shooting.</li>
	</ol>
	</li>
</ol>
3. Binding of His-tagged Proteins to the HisPur Cobalt Beads
<ol>
	<li>Use commercially available wash buffer or prepare one (50 mM Na2HPO4/NaH2PO4, 300 mM NaCl, 20 mM imidazole, pH 8) and keep it at 4 &#176;C.</li>
	<li>Prepare wash buffer with pH 6 by tittering wash buffer pH 8 with HCl. Keep it at 4 &#176;C.</li>
	<li>Gently swirl the bottle of the cobalt beads to make homogenous suspension.</li>
	<li>Dispense 40 &#181;L of the Cobalt beads suspension into four 1.5 mL tubes marked as above (step 2.2.4).</li>
	<li>Add 1 mL of lysis buffer to each tube, and centrifuge the tubes at 1000 x g for 5 s. Aspirate the supernatant, and add 40 &#956;L of lysis buffer to settled beads.</li>
	<li>Add the cell lysates to corresponding tubes.</li>
	<li>Incubate the tubes for 90 min at 4 &#176;C on a tube rotator at 20 rpm.</li>
	<li>Centrifuge the tubes at 1000 x g for 5 s and collect the supernatant. Keep the supernatant at 4 &#176;C.</li>
	<li>Add 500 &#181;L of either pH 8 or pH 6 wash buffer to corresponding tubes and re-suspend the beads gently.</li>
	<li>Centrifuge the tubes at 1000 x g for 5 s and discard the supernatants.</li>
	<li>Repeat steps 3.9 and 3.10 for four times.</li>
</ol>
4. Binding of the Peptide to the His-tagged Protein Immobilized on the Beads
<ol>
	<li>Using 100x working solution of the peptide (step 1.2.1), prepare 1x working solutions in either pH 6 or pH 8 wash buffer (two 100 &#181;L aliquots for each, 1 &#181;g/mL).</li>
	<li>Add 100 &#956;L of 1x peptide solution to the washed beads with the corresponding pH.</li>
	<li>Incubate the beads for 30 min at 4 &#176;C on a tube rotator at 20 rpm.</li>
	<li>Centrifuge the tubes at 1,000 x g for 5 s, collect the supernatant and keep it at -20 &#176;C.</li>
	<li>Repeat steps 3.9 and 3.10 for four times. 
	NOTE: In order to decrease possible background, the following steps could replace steps 4.4 &#38; 4.5.</li>
	<li>Take four micro columns with 30 &#956;m pores, label them as in step 2.2.4, and place the columns in collection tubes.</li>
	<li>After completion of step 4.3, transfer the incubation mixture into the columns, allow the solution to pass through by gravity. Keep the flow through at -20 &#176;C.</li>
	<li>Pass 500 &#181;L of wash buffer with pH 6 or pH 8 through corresponding columns by gravity. Discard the flow through.</li>
	<li>Repeat step 4.8 four times.</li>
	<li>After the last wash, centrifuge the columns at 1,000 x g for 15 s.</li>
</ol>
5. Elution from Cobalt Beads
<ol>
	<li>Use commercial Tricine sample buffer (200 mM Tris-HCl, 40% glycerol, 2% SDS, 0.04% Coomassie Blue, pH 6.8) and add &#946;-mercaptoethanol to the final concentration of 2%.</li>
	<li>Add 40 &#181;L of Tricine sample buffer (with &#946;-mercaptoethanol) to the washed beads and vortex the sample.</li>
	<li>Heat the samples for 10 min at 100 &#176;C, vortex the samples again, and centrifuge the tubes at 1,000 x g for 5 s. 
	NOTE: In order to decrease possible nonspecific binding in the elution, the following steps could replace steps 5.1 - 5.3.</li>
	<li>Add 40 &#956;L of Elution Imidazole Buffer (50 mM Na2HPO4/NaH2PO4, 300 mM NaCl, 0.25 M imidazole) to the washed beads, vortex and incubate the tubes at room temperature for 20 min.</li>
	<li>Centrifuge the tubes at 3,000 x g for 30 s, collect the supernatant (eluted samples) and transfer it to new labeled tubes.</li>
	<li>Add 40 &#956;L of Tricine sample buffer to each tube, heat the samples for 10 min at 100 &#176;C, vortex and centrifuge the tubes at 1,000 x g for 5 s. 
	NOTE: If micro columns are used, add the elution buffer to the washed beads, incubate for 20 min&#160;and collect the elution by centrifugation at 8,000 x g for 2 min. 
	NOTE: Samples can be kept at -20 &#176;C till electrophoresis is performed.</li>
</ol>
6. Electrophoresis and Western blotting
Note: The samples are ready for the separation by SDS-PAGE and subsequent Western blotting. Follow protocol of gel electrophoresis (https://www-jove-com.bdigitaluss.remotexs.co/science-education/5065/the-western-blot) with the following modifications.
<ol>
	<li>Use commercially available 10 - 20% Tricine gradient gels.</li>
	<li>On the gel, load 20 &#181;L of the eluted samples and 20 &#181;L of lysates (step 2.2.10). For trouble shooting, it is recommended to load 20 &#181;L of unbound material (step 3.8) and 10 ng of the peptide; before loading, add equal amount of Tricine sample buffer to these samples.</li>
	<li>Carry out electrophoresis in commercial Tris/Tricine/SDS running buffer (100 mM Tris, 100 mM Tricine, and 0.1% SDS, pH 8.3).</li>
	<li>Transfer the material from the gel onto a 0.45 &#181;m nitrocellulose membrane using transfer buffer (25 mM Tris base, 192 mM glycine, pH 8.4) supplemented with 20% methanol.</li>
	<li>Block the membrane with 3% Bovine Serum Albumin (BSA) for 1 h at room temperature. Using pre-stained molecular weight markers as guides, cut the membrane horizontally, blot the top part with an antibody against HisP, and blot the bottom part with an antibody against the myc epitope to identify the myc-tagged peptide. 
	NOTE: In our particular case, cutting of the membrane is not required since both HisP and Myc-fll-GLUT4 can be detected with an anti-myc antibody.</li>
</ol>
7. Analysis of Results
<ol>
	<li>Quantify the intensity of all Western blot signals using an Image Station or ImageJ program.</li>
	<li>Normalize the signal from the myc-tagged peptide against the signal from HisP at both pH values.</li>
</ol>

<ol>
	<li>Bogan, J. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Regulation+of+glucose+transporter+translocation+in+health+and+diabetes.">Regulation of glucose transporter translocation in health and diabetes.</a> Annu Rev Biochem. 81, 507-532 (2012).</li><li>Kandror, K. V., Pilch, P. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+sugar+is+sIRVed:+sorting+Glut4+and+its+fellow+travelers.">The sugar is sIRVed: sorting Glut4 and its fellow travelers.</a> Traffic. 12 (6), 665-671 (2011).</li><li>Pan, X., Zaarur, N., Singh, M., Morin, P., Kandror, K. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sortilin+and+retromer+mediate+retrograde+transport+of+Glut4+in+3T3-L1+adipocytes.">Sortilin and retromer mediate retrograde transport of Glut4 in 3T3-L1 adipocytes.</a> Mol Biol Cell. 28 (12), 1667-1675 (2017).</li><li>Kim, J., Kandror, K. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+first+luminal+loop+confers+insulin+responsiveness+to+the+glucose+transporter+4.">The first luminal loop confers insulin responsiveness to the glucose transporter 4.</a> Mol Biol Cell. 23 (5), 910-917 (2012).</li><li>Shi, J., Kandror, K. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sortilin+is+essential+and+sufficient+for+the+formation+of+Glut4-storage+vesicles+in+3T3-L1+adipocytes.">Sortilin is essential and sufficient for the formation of Glut4-storage vesicles in 3T3-L1 adipocytes.</a> Dev Cell. 9 (1), 99-108 (2005).</li><li>Coutinho, M. F., Prata, M. J., Alves, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+shortcut+to+the+lysosome:+the+mannose-6-phosphate-independent+pathway.">A shortcut to the lysosome: the mannose-6-phosphate-independent pathway.</a> Mol Genet Metab. 107 (3), 257-266 (2012).</li><li>Willnow, T. E., Petersen, C. M., Nykjaer, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=VPS10P-domain+receptors+-+regulators+of+neuronal+viability+and+function.">VPS10P-domain receptors - regulators of neuronal viability and function.</a> Nat Rev Neurosci. 9 (12), 899-909 (2008).</li><li>Mazella, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+100-kDa+neurotensin+receptor+is+gp95/sortilin,+a+non-G-protein-coupled+receptor.">The 100-kDa neurotensin receptor is gp95/sortilin, a non-G-protein-coupled receptor.</a> J Biol Chem. 273 (41), 26273-26276 (1998).</li><li>Ni, X., Morales, C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Lysosomal+Trafficking+of+Acid+Sphingomyelinase+is+Mediated+by+Sortilin+and+Mannose+6-phosphate+Receptor.">The Lysosomal Trafficking of Acid Sphingomyelinase is Mediated by Sortilin and Mannose 6-phosphate Receptor.</a> Traffic. 7 (7), 889-902 (2006).</li><li>Westergaard, U. B., 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=Functional+organization+of+the+sortilin+Vps10p+domain.">Functional organization of the sortilin Vps10p domain.</a> J Biol Chem. 279 (48), 50221-50229 (2004).</li><li>Nykjaer, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sortilin+is+essential+for+proNGF-induced+neuronal+cell+death.">Sortilin is essential for proNGF-induced neuronal cell death.</a> Nature. 427 (6977), 843-848 (2004).</li></ol>

The authors have nothing to disclose.

Sortilin is an evolutionary conserved multi-ligand protein receptor that is involved in both signaling at the plasma membrane and in intracellular sorting events6,7. However, the search for the authentic sortilin&#39;s ligands (some of which are luminal or integral membrane proteins) is complicated as the interaction of sortilin with some of its binding partners appears to be sensitive to detergents. Therefore, an easy and a widespread approach for studying prote...

GLUT4 is a glucose transporter protein which is expressed predominantly in fat and skeletal muscle cells where it mediates the effect of insulin on post-prandial blood glucose clearance1. Being a very stable protein, GLUT4 is regulated at a post-translational level. In the absence of insulin, GLUT4 is largely excluded from the plasma membrane (hence low basal permeability for glucose) and is localized mainly inside the cell in small insulin-responsive vesicles (IRVs) and trans-Golgi network (TGN) that is likely to represent the IRV donor compartment. Upon insulin administration, the IRVs fuse with the plasma membrane and deliver GLUT4 to the site of its functioning. This increases the permeability of the plasma membrane for glucose, so that glucose uptake from blood into adipocytes and skeletal myocytes rises 10 to 40-fold. After insulin withdrawal, GLUT4 is internalized into early/sorting endosomes and then retrieved to TGN where the IRVs are re-formed. Both sorting steps in the GLUT4 pathway, i.e. retrieval from the peripheral early endosomes to the perinuclear TGN, and the formation of the IRVs on the TGN donor membranes are enabled by the Vps10p family member, sortilin, which represents a type I transmembrane protein and a sorting receptor. According to one model, sortilin works as a transmembrane scaffold protein: it binds GLUT4 in the lumen of endosomes and TGN, and recruits retromer or clathrin adaptors to the cytoplasmic side of the donor membrane via its C-terminus2,3. This facilitates the distribution of GLUT4 into vesicular carriers that translocate GLUT4 between intracellular compartments.
The interaction of the cytoplasmic tail of sortilin with retromer and various adaptor proteins has been well documented. However, the binding of sortilin to GLUT4 (and to several of its other protein ligands) has been challenging to prove. In particular, attempts to co-immunoprecipitate sortilin and GLUT4 have not been successful probably due to the detergent-sensitive nature of the interaction between these two proteins. In addition, as a typical transporter protein, GLUT4 has 12 transmembrane domains and 6 luminal loops any combination of which may potentially represent a sortilin-binding site. At the same time, a large body of indirect evidence, such as substantial co-localization in the cell, cross-linking with membrane-permeable DSP, and the interaction in yeast two hybrid system suggest that sortilin can bind to GLUT4. Furthermore, using the latter approach in a combination with the alanine scanning mutagenesis, we have previously determined that the Vps10p domain of sortilin binds primarily to the first luminal loop of GLUT4. However, the proof of such an interaction in mammalian cells has been missing. Here, we have isolated His-tagged sortilin from transfected 3T3-L1 cells using cobalt resin and demonstrated that it can interact with chemically synthesized peptide corresponding to the first luminal loop of GLUT4 at pH 6 and pH 8 that resemble acidic milieu in the endosomal lumen and neutral environment in the lumen of the TGN membranes. No peptide binding was detected in control experiments where extract prepared from non-transfected cells was loaded on the same beads.

<table><tbody><tr><td>Protease inhibitor cocktail</td><td>Sigma</td><td>P8849</td><td>for use in purification of histidine tag protein</td></tr><tr><td>Phosphate-Buffered Saline&amp;nbsp;</td><td>Corning</td><td>21-040-CV</td><td>PBS</td></tr><tr><td>1mL Insulin Syringe U-100</td><td>BD</td><td>329652</td><td>26G x 1/2</td></tr><tr><td>BCA Protein Assay Kit</td><td>Pierce</td><td>23228</td><td></td></tr><tr><td>Wash buffer</td><td>Boston BioProducts</td><td>BP-234</td><td>For His-tag protein purification</td></tr><tr><td>HisPur Cobalt Resin</td><td>Thermo Scientific</td><td>89964</td><td></td></tr><tr><td>Tricine sample Buffer</td><td>Bio-Rad</td><td>161-0739</td><td>with SDS, bMercaptaethanol should be added</td></tr><tr><td>Elution&amp;nbsp; Buffer</td><td>Boston BioProducts</td><td>BP-236</td><td>For His-tag protein purificationwith 250mM Imidazole</td></tr><tr><td>Mini-Protean Tris-Tricine precast gels 10-20%&amp;nbsp;</td><td>Bio-Rad</td><td>456-3115</td><td>For seperation of peptide and small protein</td></tr><tr><td>Tris/Tricine/SDS running buffer&amp;nbsp;</td><td>Bio-Rad</td><td>161-0744</td><td></td></tr><tr><td>Transfer Buffer</td><td>Boston BioProducts</td><td>BP-190</td><td>Add 20% methanol</td></tr><tr><td>Nitrocellulose membrane&amp;nbsp; 0.45 mm</td><td>Bio-Rad</td><td>1620115</td><td></td></tr><tr><td>Bovine Serum Albumin</td><td>Sigma</td><td>A9647</td><td>BSA</td></tr><tr><td>Anti Myc antibody</td><td>Cell Signaling</td><td>2272</td><td>Rabbit</td></tr><tr><td>Peptide</td><td>Genscipt</td><td></td><td>customize ordering</td></tr><tr><td>Precision Plus Protein Dual color Standarts</td><td>BioRad</td><td>161-0374</td><td></td></tr></tbody></table>

detection of detergent-sensitive interactions between membrane proteins

Our ability to explore protein-protein interactions is the key to understanding regulatory connections in the cell. However, detection of protein-protein interactions in many cases is associated with significant experimental challenges. In particular, sorting receptors interact with their protein cargo in the lumen of the membrane compartments often in a detergent-sensitive fashion, making co-immunoprecipitation of these proteins unusable. Binding of the sorting receptor sortilin to glucose transporter GLUT4 may serve as an example of weak luminal interactions between membrane proteins. Here, we describe a fast, simple, and inexpensive assay to validate the interaction between sortilin and GLUT4. For that, we have designed and chemically synthesized the myc-tagged peptide corresponding to the potential sortilin-binding epitope in the luminal part of GLUT4. Sortilin tagged with six histidines was expressed in mammalian cells, and isolated from cell lysates using Cobalt beads. Sortilin immobilized on the beads was incubated with the peptide solution at different pH values, and the eluted material was analyzed by Western blotting. This assay can be easily adapted to study other detergent-sensitive protein-protein interactions.

Lysates were prepared from 3T3 L1 cells stably transfected with Sortilin-myc/His5 and from WT 3T3 L1 cells, used as a negative control. Both lysates were incubated with cobalt beads at pH 6 or pH 8 and thoroughly washed. Beads with immobilized proteins were then incubated in the solution of Myc-fll-Glut4. After careful washes, proteins bound to the beads were eluted with 0.25 M Imidazole. Samples were subjected, along with the original lysates, to SDS-PAGE in a 10-...

Watch this Scientific Journal Video about Detection of Detergent-sensitive Interactions Between Membrane Proteins at JoVE.com

Detection of Detergent-sensitive Interactions Between Membrane Proteins

We describe a protocol for detection of detergent-sensitive interactions between membrane proteins using binding of the sorting receptor, sortilin, to the first luminal loop of the glucose transporter protein, GLUT4, as an example.

Our ability to explore protein-protein interactions is the key to understanding regulatory connections in the cell. However, detection of protein-protein ...

detection-detergent-sensitive-interactions-between-membrane

Universidad San Sebastian

Research

JoVE Journal

Biochemistry

5.9K Views.  Boston University School of Medicine. We describe a protocol for detection of detergent-sensitive interactions between membrane proteins using binding of the sorting receptor, sortilin, to the first luminal loop of the glucose transporter protein, GLUT4, as an example. 

Video: Detection of Detergent-sensitive Interactions Between Membrane Proteins

Method to Visualize and Analyze Membrane Interacting Proteins by Transmission Electron Microscopy

Monotopic proteins exert their function when attached to a membrane surface, and such interactions depend on the specific lipid composition and on the availability of enough area to perform the function. Nanodiscs are used to provide a membrane surface of controlled size and lipid content. In the absence of bound extrinsic proteins, sodium phosphotungstate-stained nanodiscs appear as stacks of coins when viewed from the side by transmission electron microscopy (TEM). This protocol is therefore designed to intentionally promote stacking; consequently, the prevention of stacking can be interpreted as the binding of the membrane-binding protein to the nanodisc. In a further step, the TEM images of the protein-nanodisc complexes can be processed with standard single-particle methods to yield low-resolution structures as a basis for higher resolution cryoEM work. Furthermore, the nanodiscs provide samples suitable for either TEM or non-denaturing gel electrophoresis. To illustrate the method, Ca2+-induced binding of 5-lipoxygenase on nanodiscs is presented.

Many proteins perform their function when attached to membrane surfaces. The binding of extrinsic proteins on nanodisc membranes can be indirectly imaged by transmission electron microscopy. We show that the characteristic stacking (rouleau) of nanodiscs induced by the negative stain sodium phosphotungstate is prevented by the binding of extrinsic protein.

Monotopic proteins exert their function when attached to a membrane surface, and such interactions depend on the specific lipid composition and on the ...

Contrast-Matching Detergent in Small-Angle Neutron Scattering Experiments for Membrane Protein Structural Analysis and Ab Initio Modeling

The biological small-angle neutron scattering instrument at the High-Flux Isotope Reactor of Oak Ridge National Laboratory is dedicated to the investigation of biological materials, biofuel processing, and bio-inspired materials covering nanometer to micrometer length scales. The methods presented here for investigating physical properties (i.e., size and shape) of membrane proteins (here, MmIAP, an intramembrane aspartyl protease from Methanoculleus marisnigri) in solutions of micelle-forming detergents are well-suited for this small-angle neutron scattering instrument, among others. Other biophysical characterization techniques are hindered by their inability to address the detergent contributions in a protein-detergent complex structure. Additionally, access to the Bio-Deuteration Lab provides unique capabilities for preparing large-scale cultivations and expressing deuterium-labeled proteins for enhanced scattering signal from the protein. While this technique does not provide structural details at high-resolution, the structural knowledge gap for membrane proteins contains many addressable areas of research without requiring near-atomic resolution. For example, these areas include determination of oligomeric states, complex formation, conformational changes during perturbation, and folding/unfolding events. These investigations can be readily accomplished through applications of this method.

Contrast-Matching Detergent in Small-Angle Neutron Scattering Experiments for Membrane Protein Structural Analysis and Ab Initio Modeling

This protocol demonstrates how to obtain a low-resolution ab initio model and structural details of a detergent-solubilized membrane protein in solution using small-angle neutron scattering with contrast-matching of the detergent.

The biological small-angle neutron scattering instrument at the High-Flux Isotope Reactor of Oak Ridge National Laboratory is dedicated to the ...

Membrane-SPINE: A Biochemical Tool to Identify Protein-protein Interactions of Membrane Proteins In Vivo

Membrane proteins are essential for cell viability and are therefore important therapeutic targets1-3. Since they function in complexes4, methods to identify and characterize their interactions are necessary5. To this end, we developed the Membrane Strep-protein interaction experiment, called Membrane-SPINE6. This technique combines in vivo cross-linking using the reversible cross-linker formaldehyde with affinity purification of a Strep-tagged membrane bait protein. During the procedure, cross-linked prey proteins are co-purified with the membrane bait protein and subsequently separated by boiling. Hence, two major tasks can be executed when analyzing protein-protein interactions (PPIs) of membrane proteins using Membrane-SPINE: first, the confirmation of a proposed interaction partner by immunoblotting, and second, the identification of new interaction partners by mass spectrometry analysis. Moreover, even low affinity, transient PPIs are detectable by this technique. Finally, Membrane-SPINE is adaptable to almost any cell type, making it applicable as a powerful screening tool to identify PPIs of membrane proteins.

Membrane-SPINE: A Biochemical Tool to Identify Protein-protein Interactions of Membrane Proteins In Vivo

A biochemical approach is described to identify in vivo protein-protein interactions (PPI) of membrane proteins. The method combines protein cross-linking, affinity purification and mass spectrometry, and is adaptable to almost any cell type or organism. With this approach, even the identification of transient PPIs becomes possible.

Membrane proteins are essential for cell viability and are therefore important therapeutic targets1-3. Since they function in complexes4, methods to ...

Bioengineering

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

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

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

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

Use of Microscale Thermophoresis to Measure Protein-Lipid Interactions

The ability to determine the binding affinity of lipids to proteins is an essential part of understanding protein-lipid interactions in membrane trafficking, signal transduction and cytoskeletal remodeling. Classic tools for measuring such interactions include surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). While powerful tools, these approaches have setbacks. ITC requires large amounts of purified protein as well as lipids, which can be costly and difficult to produce. Furthermore, ITC as well as SPR are very time consuming, which could add significantly to the cost of performing these experiments. One way to bypass these restrictions is to use the relatively new technique of microscale thermophoresis (MST). MST is fast and cost effective using small amounts of sample to obtain a saturation curve for a given binding event. There currently are two types of MST systems available. One type of MST requires labeling with a fluorophore in the blue or red spectrum. The second system relies on the intrinsic fluorescence of aromatic amino acids in the UV range. Both systems detect the movement of molecules in response to localized induction of heat from an infrared laser. Each approach has its advantages and disadvantages. Label-free MST can use untagged native proteins; however, many analytes, including pharmaceuticals, fluoresce in the UV range, which can interfere with determination of accurate KD values. In comparison, labeled MST allows for a greater diversity of measurable pairwise interactions utilizing fluorescently labeled probes attached to ligands with measurable absorbances in the visible range as opposed to UV, limiting the potential for interfering signals from analytes.

Microscale thermophoresis obtains binding constants quickly at low material cost. Either labeled or label free microscale thermophoresis is commercially available; however, label free thermophoresis is not capable of the diversity of interaction measurements that can be performed using fluorescent labels. We provide a protocol for labeled thermophoresis measurements.

The ability to determine the binding affinity of lipids to proteins is an essential part of understanding protein-lipid interactions in membrane ...

PeptiQuick, a One-Step Incorporation of Membrane Proteins into Biotinylated Peptidiscs for Streamlined Protein Binding Assays

Membrane proteins, including transporters, channels, and receptors, constitute nearly one-fourth of the cellular proteome and over half of current drug targets. Yet, a major barrier to their characterization and exploitation in academic or industrial settings is that most biochemical, biophysical, and drug screening strategies require these proteins to be in a water-soluble state. Our laboratory recently developed the peptidisc, a membrane mimetic offering a &#8220;one-size-fits-all&#8221; approach to the problem of membrane protein solubility. We present here a streamlined protocol that combines protein purification and peptidisc reconstitution in a single chromatographic step. This workflow, termed PeptiQuick, allows for bypassing dialysis and incubation with polystyrene beads, thereby greatly reducing exposure to detergent, protein denaturation, and sample loss. When PeptiQuick is performed with biotinylated scaffolds, the preparation can be directly attached to streptavidin-coated surfaces. There is no need to biotinylate or modify the membrane protein target. PeptiQuick is showcased here with the membrane receptor FhuA and antimicrobial ligand colicin M, using biolayer interferometry to determine the precise kinetics of their interaction. It is concluded that PeptiQuick is a convenient way to prepare and analyze membrane protein-ligand interactions within one&#160;day in a detergent-free environment.

We present a method that combines membrane protein purification and reconstitution into peptidiscs in a single chromatographic step. Biotinylated scaffolds are used for direct surface attachment and measurement of protein-ligand interactions via biolayer interferometry.

Membrane proteins, including transporters, channels, and receptors, constitute nearly one-fourth of the cellular proteome and over half of current drug ...

Native Cell Membrane Nanoparticles System for Membrane Protein-Protein Interaction Analysis

Protein-protein interactions in cell membrane systems play crucial roles in a wide range of biological processes- from cell-to-cell interactions to signal transduction; from sensing environmental signals to biological response; from metabolic regulation to developmental control. Accurate structural information of protein-protein interactions is crucial for understanding the molecular mechanisms of membrane protein complexes and for the design of highly specific molecules to modulate these proteins. Many in vivo and in vitro approaches have been developed for the detection and analysis of protein-protein interactions. Among them the structural biology approach is unique in that it can provide direct structural information of protein-protein interactions at the atomic level. However, current membrane protein structural biology is still largely limited to detergent-based methods. The major drawback of detergent-based methods is that they often dissociate or denature membrane protein complexes once their native lipid bilayer environment is removed by detergent molecules. We have been developing a&#160;native cell membrane nanoparticle system for membrane protein structural biology. Here, we demonstrate the use of this system in the analysis of protein-protein interactions on the cell membrane with a case study of the oligomeric state of AcrB.

Presented here is a protocol for the determination of oligomeric state of membrane proteins that utilizes a native cell membrane nanoparticle system in conjunction with electron microscopy.

Protein-protein interactions in cell membrane systems play crucial roles in a wide range of biological processes- from cell-to-cell interactions to signal ...

In-vivo Detection of Protein-protein Interactions on Micro-patterned Surfaces

Unraveling the interaction network of molecules in-vivo is key to understanding the mechanisms that regulate cell function and metabolism. A multitude of methodological options for addressing molecular interactions in cells have been developed, but most of these methods suffer from being rather indirect and therefore hardly quantitative. On the contrary, a few high-end quantitative approaches were introduced, which however are difficult to extend to high throughput. To combine high throughput capabilities with the possibility to extract quantitative information, we recently developed a new concept for identifying protein-protein interactions (Schwarzenbacher et al., 2008). Here, we describe a detailed protocol for the design and the construction of this system which allows for analyzing interactions between a fluorophore-labeled protein ("prey") and a membrane protein ("bait") in-vivo. Cells are plated on micropatterned surfaces functionalized with antibodies against the bait exoplasmic domain. Bait-prey interactions are assayed via the redistribution of the fluorescent prey. The method is characterized by high sensitivity down to the level of single molecules, the capability to detect weak interactions, and high throughput capability, making it applicable as screening tool.

In-vivo Detection of Protein-protein Interactions on Micro-patterned Surfaces

This video shows experiments with subsequent analysis of protein-protein interactions by the use of micro-patterned surfaces. The approach offers the possibility to detect protein interactions in living cells and combines high throughput capabilities with the possibility to extract quantitative information.

Unraveling the interaction network of molecules in-vivo is key to understanding the mechanisms that regulate cell function and metabolism. A multitude of ...

Biology

Protein Membrane Overlay Assay: A Protocol to Test Interaction Between Soluble and Insoluble Proteins in vitro

Validating interactions between different proteins is vital for investigation of their biological functions on the molecular level. There are several methods, both in vitro and in vivo, to evaluate protein binding, and at least two methods that complement the shortcomings of each other should be conducted to obtain reliable insights. 
For an in vivo assay, the bimolecular fluorescence complementation (BiFC) assay represents the most popular and least invasive approach that enables to detect protein-protein interaction within living cells, as well as identify the intracellular localization of the interacting proteins 1,2. In this assay, non-fluorescent N- and C-terminal halves of GFP or its variants are fused to tested proteins, and when the two fusion proteins are brought together due to the tested proteins&#x2019; interactions, the fluorescent signal is reconstituted3-6. Because its signal is readily detectable by epifluorescence or confocal microscopy, BiFC has emerged as a powerful tool of choice among cell biologists for studying about protein-protein interactions in living cells 3. This assay, however, can sometimes produce false positive results. For example, the fluorescent signal can be reconstituted by two GFP fragments arranged as far as 7 nm from each other due to close packing in a small subcellular compartment, rather that due to specific interactions7.
Due to these limitations, the results obtained from live cell imaging technologies should be confirmed by an independent approach based on a different principle for detecting protein interactions. Co-immunoprecipitation (Co-IP) or glutathione transferase (GST) pull-down assays represent such alternative methods that are commonly used to analyze protein-protein interactions in vitro. However, iIn these assays, however, the tested proteins must be readily soluble in the buffer that supportsused for the binding reaction. Therefore, specific interactions involving an insoluble protein cannot be assessed by these techniques.
 Here, we illustrate the protocol for the protein membrane overlay binding assay, which circumvents this difficulty. In this technique, interaction between soluble and insoluble proteins can be reliably tested because one of the proteins is immobilized on a membrane matrix. This method, in combination with in vivo experiments, such as BiFC, provides a reliable approach to investigate and characterize interactions faithfully between soluble and insoluble proteins. In this article, binding between Tobacco mosaic virus (TMV) movement protein (MP), which exerts multiple functions during viral cell-to-cell transport8-14, and a recently identified plant cellular interactor, tobacco ankyrin repeat-containing protein (ANK) 15, is demonstrated using this technique.

Protein Membrane Overlay Assay: A Protocol to Test Interaction Between Soluble and Insoluble Proteins in vitro

Testing protein-protein interaction is indispensable for dissection of protein functionality. Here, we introduce an in vitro protein-protein binding assay to probe a membrane-immobilized protein with a soluble protein. This assay provides a reliable method to test interaction between an insoluble protein and a protein in solution.

Validating interactions between different proteins is vital for investigation of their biological functions on the molecular level.  There are several ...

Fluorescence-Detection Size-Exclusion Chromatography: A Technique to Identify the Integrity of Fluorescent Membrane Proteins upon Detergent Solubilization

Source: Bird, L. E., et al. <a href="https://www-jove-com.bdigitaluss.remotexs.co/v/52357">Green Fluorescent Protein-based Expression Screening of Membrane Proteins in Escherichia coli</a>. J. Vis. Exp. (2015)
In this video, we demonstrate a fluorescence-detection size-exclusion chromatography technique to screen for green fluorescent protein (GFP)-fused membrane protein stability in Escherichia coli following detergent solubilization. Proteins exhibiting a symmetrical peak in the size-exclusion profile with little or no free GFP indicate purified proteins with minimum degradation and aggregation, which can subsequently be selected for structure-function analysis.

Source: Bird, L. E., et al. Green Fluorescent Protein-based Expression Screening of Membrane Proteins in Escherichia coli. J. Vis. Exp. (2015)
In this ...

Detection of Detergent-sensitive Interactions Between Membrane Proteins

Summary

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Chapters in this video

Method to Visualize and Analyze Membrane Interacting Proteins by Transmission Electron Microscopy

Contrast-Matching Detergent in Small-Angle Neutron Scattering Experiments for Membrane Protein Structural Analysis and Ab Initio Modeling

Membrane-SPINE: A Biochemical Tool to Identify Protein-protein Interactions of Membrane Proteins In Vivo

Evaluation of Protein–Protein Interactions using an On-Membrane Digestion Technique

Use of Microscale Thermophoresis to Measure Protein-Lipid Interactions

PeptiQuick, a One-Step Incorporation of Membrane Proteins into Biotinylated Peptidiscs for Streamlined Protein Binding Assays

Native Cell Membrane Nanoparticles System for Membrane Protein-Protein Interaction Analysis

In-vivo Detection of Protein-protein Interactions on Micro-patterned Surfaces

Protein Membrane Overlay Assay: A Protocol to Test Interaction Between Soluble and Insoluble Proteins in vitro

Fluorescence-Detection Size-Exclusion Chromatography: A Technique to Identify the Integrity of Fluorescent Membrane Proteins upon Detergent Solubilization