This research has been supported by the startup fund of Simon Fraser University. &#160;

1. Harvest Membranes
<ol>
	<li>Add hypotonic buffer (10 mM Tris-HCl, 10 mM KCl, 1.5 mM MgCl2, pH 8.0) with protease inhibitor cocktail onto a cell pellet (~108 cells) and incubate for 15-30 min on ice. Lyse the cells by passing the sample through syringe needles (5-10 passes) or homogenizing the sample by a Dounce homogenizer (15-30 strokes). Use a hemocytometer and trypan blue staining to check the efficiency of lysis.</li>
	<li>Obtain the microsomal fraction by a differential centrifugation of the lysate at 3,000 g x 15 min and then at 100,000 g x 2 hr. The first centrifugation obtains a pellet that is the nuclear fraction, and the subsequent centrifugation of the supernatant generates a second pellet, which is the microsomal fraction that contains the plasma membrane as well as membrane-bound organelles8. The final supernatant is the cytosolic fraction. Store the microsomal fractions in -80 &#176;C freezer for further analysis as indicated below.</li>
</ol>
2. Dissolve, Denature, and Digest Membrane Proteins
<ol>
	<li>Dissolve the microsomal fraction in the denaturation buffer (40 mM Tris, 10 mM EDTA, 10 mM TCEP, 0.5% Rapigest, pH 8) and then incubate the solution at 100 &#176;C for 10 min.</li>
	<li>Cool the heated solution to room temperature, add ultra-high purity urea powder into the solution to 8 M final concentration and incubate the sample at 37 &#176;C for 30 min.</li>
	<li>Add iodoacetamide stock solution to the sample to 15 mM final concentration, and incubate the solution in the dark for 30 min at room temperature to alkylate the free thiols in the sample. Add DTT stock solution to the sample to 10 mM final concentration and incubate the solution for another 10 min at room temperature to quench the excessive iodoacetamide.</li>
	<li>Dilute the obtained solution 10x&#160;with 40 mM Tris buffer at pH 8, and subsequently add trypsin to the sample at a 1:20 ratio of trypsin to total protein. Maintain the digestion reaction in a 37 &#176;C oven overnight to ensure the reaction is completed.</li>
	<li>Terminate the digestion by acidifying the sample solution to pH 1 with HCl, a condition that also breaks down the detergent, i.e. Rapigest. Degrade the Rapigest at 37 &#176;C for 1 hr, and remove the developed precipitation by centrifugation.</li>
	<li>Clean the supernatant that contains sample peptides by a C-18 solid phase extraction (SPE) cartridge and dry the obtained sample by speedvac.</li>
	<li>Perform a SDS-PAGE analysis of samples before and after trypsin digestion to confirm the digestion efficiency.</li>
</ol>
3. Glycopeptide Capture
<ol>
	<li>Dissolve the cleaned peptides in the coupling buffer (100 mM sodium acetate, pH 5.5). Add sodium periodate into the peptide solution at a 10 mM final concentration for 30 min in the dark, at room temperature. This will oxidize the cis-diol groups in the glycans to aldehydes, which allow the glycans to couple to the resin through hydrazide chemistry. Quench the excessive periodate by sodium sulphite at a 20 mM final concentration and pH 5 for 10 min at room temperature.</li>
	<li>Introduce hydrazide-derivatized resin into the peptide solution at a ratio of 1:4 (resin to solution) to couple glycopeptides to the resin. Incubate the reaction at 37 &#176;C for 1-2 days with end-to-end rotation for complete coupling.</li>
	<li>Remove the unbound peptides by washing the resin twice with 1 ml of each of the following: DI&#160;water, 1.5 M NaCl, methanol and 80% acetonitrile. Finally, wash the resin 3x&#160;with 1 ml of 100 mM NH4HCO3 at pH 8 to exchange the buffer of the system to 100 mM NH4HCO3.
	<ol>
		<li>Collect the supernatant and the washes for the analysis of unbound peptides (optional).</li>
	</ol>
	</li>
	<li>Release the N-glycopeptides from the resin by PNGase F in an overnight incubation at 37 &#176;C with an end-to-end rotation. Collect the released peptides by centrifugation and an 80% acetonitrile wash. Dry the obtained solution in the speedvac for LC-MS analysis.</li>
</ol>
4. Further Fractionation (Optional)
To further simplify sample complexity, fractionate the obtained N-glycopeptides. For example, redissolve the dried peptides into 10 mM ammonia formate, pH 3 with 20% acetonitrile and use strong&#160;cation&#160;exchange (SCX) chromatography to fractionate the peptides. Dry the obtained eluent, and then analyze the obtained peptide fractions by reverse-phase LC-MS8,17,18.
5. Cleaning of the Released N-glycopeptides (Optional)
If concerns rise for the potential contamination of the peptides, redissolve the dried peptides into 0.1% formic acid and use a MCX SPE column to further clean the peptides prior to reverse-phase LC-MS analysis.
Note: Database searching parameters.
During the selective cleavage of N-glycopeptides off the resin, PNGase F converts the N-glycan linked asparagine to an aspartic acid. Therefore, there is a 0.9840 Da mass shift of the liberated N-glycopeptides. To accurately identify these peptides, this modification needs to be added to the search parameters along with common modifications such as the carbamidomethylation of the cysteine and oxidation of the methionine.

<ol>
	<li>Cho, W., Stahelin, R. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Membrane-protein+interactions+in+cell+signaling+and+membrane+trafficking.">Membrane-protein interactions in cell signaling and membrane trafficking.</a> Annual review of biophysics and biomolecular structure. 34, 119-151 (2005).</li><li>Tadevosyan, A., Vaniotis, G., Allen, B. G., Hebert, T. E., Nattel, S. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=protein-coupled+receptor+signalling+in+the+cardiac+nuclear+membrane:+evidence+and+possible+roles+in+physiological+and+pathophysiological+function.">protein-coupled receptor signalling in the cardiac nuclear membrane: evidence and possible roles in physiological and pathophysiological function.</a> The Journal of physiology. 590, 1313-1330 (2012).</li><li>White, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biophysical+dissection+of+membrane+proteins.">Biophysical dissection of membrane proteins.</a> Nature. 459, 344-346 (2009).</li><li>Heijne, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Membrane-protein+topology.+.">Membrane-protein topology. .</a> Nature reviews Molecular cell biology. 7, 909-918 (2006).</li><li>Cox, J., Mann, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+high-resolution+proteomics+for+data-driven+systems+biology.">Quantitative high-resolution proteomics for data-driven systems biology.</a> Annual review of biochemistry. 80, 273-299 (2011).</li><li>Lamond, A. I., 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=Advancing+cell+biology+through+proteomics+in+space+and+time+(PROSPECTS).">Advancing cell biology through proteomics in space and time (PROSPECTS).</a> Mol Cell Proteomics. 11, (2012).</li><li>Savas, J. N., Stein, B. D., Wu, C. C., Yates 3rd, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mass+spectrometry+accelerates+membrane+protein+analysis.">Mass spectrometry accelerates membrane protein analysis.</a> Trends in biochemical sciences. 36, 388-396 (2011).</li><li>Sun, 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=Shotgun+glycopeptide+capture+approach+coupled+with+mass+spectrometry+for+comprehensive+glycoproteomics.">Shotgun glycopeptide capture approach coupled with mass spectrometry for comprehensive glycoproteomics.</a> Mol Cell Proteomics. 6, 141-149 (2007).</li><li>Lowe, J. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glycosylation,+immunity,+and+autoimmunity.">Glycosylation, immunity, and autoimmunity.</a> Cell. 104, 809-812 (2001).</li><li>Dodds, E. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gas-phase+dissociation+of+glycosylated+peptide+ions.">Gas-phase dissociation of glycosylated peptide ions.</a> Mass spectrometry reviews. 31, 666-682 (2012).</li><li>Kaji, H., 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=Lectin+affinity+capture,+isotope-coded+tagging+and+mass+spectrometry+to+identify+N-linked+glycoproteins.">Lectin affinity capture, isotope-coded tagging and mass spectrometry to identify N-linked glycoproteins.</a> Nat Biotechnol. 21, 667-672 (2003).</li><li>Zhang, H., Li, X. J., Martin, D. B., Aebersold, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+and+quantification+of+N-linked+glycoproteins+using+hydrazide+chemistry,+stable+isotope+labeling+and+mass+spectrometry.">Identification and quantification of N-linked glycoproteins using hydrazide chemistry, stable isotope labeling and mass spectrometry.</a> Nat Biotechnol. 21, 660-666 (2003).</li><li>Hagglund, P., Bunkenborg, J., Elortza, F., Jensen, O. N., Roepstorff, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+strategy+for+identification+of+N-glycosylated+proteins+and+unambiguous+assignment+of+their+glycosylation+sites+using+HILIC+enrichment+and+partial+deglycosylation.">A new strategy for identification of N-glycosylated proteins and unambiguous assignment of their glycosylation sites using HILIC enrichment and partial deglycosylation.</a> J Proteome Res. 3, 556-566 (2004).</li><li>Bond, M. R., Kohler, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemical+methods+for+glycoprotein+discovery.">Chemical methods for glycoprotein discovery.</a> Current opinion in chemical biology. 11, 52-58 (2007).</li><li>Pasini, 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=In-depth+analysis+of+the+membrane+and+cytosolic+proteome+of+red+blood+cells.">In-depth analysis of the membrane and cytosolic proteome of red blood cells.</a> Blood. 108, 791-801 (2006).</li><li>Wollscheid, 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=Mass-spectrometric+identification+and+relative+quantification+of+N-linked+cell+surface+glycoproteins.">Mass-spectrometric identification and relative quantification of N-linked cell surface glycoproteins.</a> Nat Biotechnol. 27, 378-386 (2009).</li><li>Sun, 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=N-Glycoproteome+of+E14.Tg2a+mouse+embryonic+stem+cells.">N-Glycoproteome of E14.Tg2a mouse embryonic stem cells.</a> PLoS ONE. 8, (2013).</li><li>Sun, 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=Glycocapture-assisted+global+quantitative+proteomics+(gagQP)+reveals+multiorgan+responses+in+serum+toxicoproteome.">Glycocapture-assisted global quantitative proteomics (gagQP) reveals multiorgan responses in serum toxicoproteome.</a> J Proteome Res. 12, 2034-2044 (2013).</li></ol>

The authors declare that they have no competing financial interests. &#160;

Here we introduce a glycopeptide-capture strategy for profiling cell-surface proteins. The method can be applied to study secreted proteins, such as those in blood, as well as in other body fluids or in cell culture media.
The success of the method relies on the complete digestion of samples; therefore, a SDS-PAGE characterization of the digestion efficiency is necessary, especially for the first-time analysis of a sample. A complete digestion can be challenging for membrane proteins, and can ...

Cell surface proteins interact with the extracellular environment and relay signals from the outside to the inside of a cell. Thus, these proteins,&#160;including extracellular matrix proteins,&#160;play critical roles in all aspects of cellular biology and physiology ranging from proliferation, growth, migration, differentiation to aging and so forth. Surface proteins function by interacting with other cells, proteins and small molecules1-3. Molecular characterization of cell-surface proteins is of great interest not only for biologists but also for pharmaceutical companies, as more than 60% of drugs are targeted to cell-surface proteins4.
Tandem mass spectrometry (MS), with its superior sensitivity, accuracy, and throughput for identification of proteins and peptides, has been a powerful tool for global proteomic studies5,6. Yet, surface proteins pose significant challenges to MS-based proteomics, as most surface proteins exist in low quantities and with heavy modifications. The membrane-spanning regions of the surface proteins render them hydrophobic; this is especially the case for multipass transmembrane proteins. It is thus difficult to dissolve membrane proteins in aqueous solutions without the help of a detergent; however the use of detergents generally suppresses the performance of HPLC and MS1,7,8 in protein identification. Therefore, membrane proteins have been poorly characterized in direct LC-MS based proteomics.
Glycosylation is one of the most important and abundant post-translational modifications taking place in cell-surface proteins9. The enormous complexity and heterogeneity of glycans hamper peptides&#8217; MS signal10. Nevertheless, several proteomic methods have used this unique modification to enrich surface proteins and to remove the sugar moieties from proteins prior to LC-MS analysis. These methods include lectin-based affinity capture11 and hydrazide-based or boric acid-based chemical capture12 as well as hydrophilic chromatography separations8,13. The removal of glycans transforms membrane proteins to regular proteins and drastically simplifies the MS characterization. Because glycosylation also takes place in secreted proteins that have high solubility in contrast to membrane proteins, many glycoproteomic methods are optimized for soluble proteins, and tend to have lower glycopeptide selectivity and sensitivity when being deployed to membrane proteins8,14. Other methods also exist to enrich, in particular, cell-surface proteins, such as those using ultracentrifugation15 and labeling strategies16. A detailed comparison between our method and other existing methods for characterizing membrane proteins was conducted recently17, and the results indicated that our method can perform equally well, if not better, than all the compared membrane proteomics methods, but with higher simplicity.
To help researchers use this method, we detail here a general protocol. This method integrates several advantages of existing glycoproteomics strategies and is devised specifically for membrane glycoproteins, yet the method works equally well for secreted proteins. The characteristics of this method include: 1) a complete solubilization of membrane proteins, 2) an enrichment of glycopeptides instead of glycoproteins to eliminate the potential steric hindrance when using a solid capturing substrate, 3) the use of hydrazide chemistry to form covalent bonds between glycopeptides and the capturing substrate, such that the bonded glycopeptides can tolerate stringent washes for high glycoselectivity, and 4) the capability to conduct the entire capture procedure in one tube for reduced sample loss and shortened procedure duration. After implementing this method to studying a variety of biological samples including cells and tissues, we observed a high selectivity (&#62; 90%) to glycoproteins8,17,18.

<table border="0" cellpadding="0" cellspacing="0" width="701">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">DTT</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">646563</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">TCEP</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">646547</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Iodoacetamide</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">A3221</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Rapigest SF</td>
			<td style="width:199px;">Waters</td>
			<td style="width:119px;">186001860</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium periodate</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">311448</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PNGase F</td>
			<td style="width:199px;">New England Biolabs</td>
			<td style="width:119px;">P0704S</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Affi-Gel Hz Hydrazide gel</td>
			<td style="width:199px;">Bio-Rad</td>
			<td style="width:119px;">153-6047</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Trypsin</td>
			<td style="width:199px;">Worthington Biomedical</td>
			<td style="width:119px;">LS02115</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sep-Pak C-18 cartridge</td>
			<td style="width:199px;">Waters</td>
			<td style="width:119px;">WAT054955</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Oasis MCX cartridge</td>
			<td style="width:199px;">Waters</td>
			<td style="width:119px;">186000252</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Protease inhibitor coctail</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">P8340</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Urea</td>
			<td style="width:199px;">Amersco</td>
			<td style="width:119px;">568</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium sulphite</td>
			<td style="width:199px;">Caledon</td>
			<td style="width:119px;">8360-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Invertase</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">I0408</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Alpha-1 trypsin</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">F2006</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ribonulease B</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">R7884</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Avidin</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">A9275</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ovalbumin</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">A5503</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Conalbumin</td>
			<td style="width:199px;">Sigma</td>
			<td style="width:119px;">C0755</td>
			<td></td>
		</tr>
	</tbody>
</table>

glycopeptide capture for cell surface proteomics

Cell surface proteins, including extracellular matrix proteins, participate in all major cellular processes and functions, such as growth, differentiation, and proliferation. A comprehensive characterization of these proteins provides rich information for biomarker discovery, cell-type identification, and drug-target selection, as well as helping to advance our understanding of cellular biology and physiology. Surface proteins, however, pose significant analytical challenges, because of their inherently low abundance, high hydrophobicity, and heavy post-translational modifications. Taking advantage of the prevalent glycosylation on surface proteins, we introduce here a high-throughput glycopeptide-capture approach that integrates the advantages of several existing N-glycoproteomics means. Our method can enrich the glycopeptides derived from surface proteins and remove their glycans for facile proteomics using LC-MS. The resolved N-glycoproteome comprises the information of protein identity and quantity as well as their sites of glycosylation. This method has been applied to a series of studies in areas including cancer, stem cells, and drug toxicity. The limitation of the method lies in the low abundance of surface membrane proteins, such that a relatively large quantity of samples is required for this analysis compared to studies centered on cytosolic proteins.

A representative flow chart of the experimental procedure is summarized in Figure 1. The labeling and further fractionation steps are optional and details are described in a recent publication18. Another option is to analyze the unmodified peptides, which do not react with the resin. The advantages of analyzing the unmodified peptides include the potential identification of non-glycosylated peptides and proteins, such as claudins in tight junctions; an additional advantage is more accurate qua...

Watch this Scientific Journal Video about Glycopeptide Capture for Cell Surface Proteomics at JoVE.com

Glycopeptide Capture for Cell Surface Proteomics

Cell surface proteins are biologically important and widely glycosylated. We introduce here a glycopeptide-capture approach to solubilize, enrich, and deglycosylate these proteins for facile LC-MS based proteomic analyses.

Cell surface proteins, including extracellular matrix proteins, participate in all major cellular processes and functions, such as growth, ...

glycopeptide-capture-for-cell-surface-proteomics

Research

JoVE Journal

Biology

11.2K Views.  Simon Fraser University. Cell surface proteins are biologically important and widely glycosylated. We introduce here a glycopeptide-capture approach to solubilize, enrich, and deglycosylate these proteins for facile LC-MS based proteomic analyses.

Video: Glycopeptide Capture for Cell Surface Proteomics

Cell Capture Using a Microfluidic Device

Avidity-based Extracellular Interaction Screening (AVEXIS) for the Scalable Detection of Low-affinity Extracellular Receptor-Ligand Interactions

Extracellular protein:protein interactions between secreted or membrane-tethered proteins are critical for both initiating intercellular communication and ensuring cohesion within multicellular organisms. Proteins predicted to form extracellular interactions are encoded by approximately a quarter of human genes1, but despite their importance and abundance, the majority of these proteins have no documented binding partner. Primarily, this is due to their biochemical intractability: membrane-embedded proteins are difficult to solubilise in their native conformation and contain structurally-important posttranslational modifications. Also, the interaction affinities between receptor proteins are often characterised by extremely low interaction strengths (half-lives &lt; 1 second) precluding their detection with many commonly-used high throughput methods2. 
Here, we describe an assay, AVEXIS (AVidity-based EXtracellular Interaction Screen) that overcomes these technical challenges enabling the detection of very weak protein interactions (t1/2 &le; 0.1 sec) with a low false positive rate3. The assay is usually implemented in a high throughput format to enable the systematic screening of many thousands of interactions in a convenient microtitre plate format (Fig. 1). It relies on the production of soluble recombinant protein libraries that contain the ectodomain fragments of cell surface receptors or secreted proteins within which to screen for interactions; therefore, this approach is suitable for type I, type II, GPI-linked cell surface receptors and secreted proteins but not for multipass membrane proteins such as ion channels or transporters.
The recombinant protein libraries are produced using a convenient and high-level mammalian expression system4, to ensure that important posttranslational modifications such as glycosylation and disulphide bonds are added. Expressed recombinant proteins are secreted into the medium and produced in two forms: a biotinylated bait which can be captured on a streptavidin-coated solid phase suitable for screening, and a pentamerised enzyme-tagged (&beta;-lactamase) prey. The bait and prey proteins are presented to each other in a binary fashion to detect direct interactions between them, similar to a conventional ELISA (Fig. 1). The pentamerisation of the proteins in the prey is achieved through a peptide sequence from the cartilage oligomeric matrix protein (COMP) and increases the local concentration of the ectodomains thereby providing significant avidity gains to enable even very transient interactions to be detected. By normalising the activities of both the bait and prey to predetermined levels prior to screening, we have shown that interactions having monomeric half-lives of 0.1 sec can be detected with low false positive rates3.

Avidity-based Extracellular Interaction Screening AVEXIS for the Scalable Detection of Low-affinity Extracellular Receptor-Ligand Interactions

AVEXIS is a high throughput protein interaction assay developed to systematically screen for novel extracellular receptor-ligand pairs involved in cellular recognition processes. It is specifically designed to detect transient protein interactions that are difficult to identify using other high throughput approaches.

Extracellular protein:protein interactions between secreted or membrane-tethered proteins are critical for both initiating intercellular communication and ...

Metabolic Labeling and Membrane Fractionation for Comparative Proteomic Analysis of Arabidopsis thaliana Suspension Cell Cultures

Plasma membrane microdomains are features based on the physical properties of the lipid and sterol environment and have particular roles in signaling processes. Extracting sterol-enriched membrane microdomains from plant cells for proteomic analysis is a difficult task mainly due to multiple preparation steps and sources for contaminations from other cellular compartments. The plasma membrane constitutes only about 5-20% of all the membranes in a plant cell, and therefore isolation of highly purified plasma membrane fraction is challenging. A frequently used method involves aqueous two-phase partitioning in polyethylene glycol and dextran, which yields plasma membrane vesicles with a purity of 95% 1. Sterol-rich membrane microdomains within the plasma membrane are insoluble upon treatment with cold nonionic detergents at alkaline pH. This detergent-resistant membrane fraction can be separated from the bulk plasma membrane by ultracentrifugation in a sucrose gradient 2. Subsequently, proteins can be extracted from the low density band of the sucrose gradient by methanol/chloroform precipitation. Extracted protein will then be trypsin digested, desalted and finally analyzed by LC-MS/MS. Our extraction protocol for sterol-rich microdomains is optimized for the preparation of clean detergent-resistant membrane fractions from Arabidopsis thaliana cell cultures.
We use full metabolic labeling of Arabidopsis thaliana suspension cell cultures with K15NO3 as the only nitrogen source for quantitative comparative proteomic studies following biological treatment of interest 3. By mixing equal ratios of labeled and unlabeled cell cultures for joint protein extraction the influence of preparation steps on final quantitative result is kept at a minimum. Also loss of material during extraction will affect both control and treatment samples in the same way, and therefore the ratio of light and heave peptide will remain constant. In the proposed method either labeled or unlabeled cell culture undergoes a biological treatment, while the other serves as control 4.

Metabolic Labeling and Membrane Fractionation for Comparative Proteomic Analysis of Arabidopsis thaliana Suspension Cell Cultures

Here we describe a robust method for the fractionation of plant plasma membranes into detergent resistant and detergent soluble membranes based on a mixture of unlabeled and in vivo fully 15N labeled Arabidopsis thaliana cell cultures. The procedure is applied for comparative proteomic studies to understand signaling processes.

Plasma membrane microdomains are features based on the physical properties of the lipid and sterol environment and have particular roles in signaling ...

Proteomic Sample Preparation from Formalin Fixed and Paraffin Embedded Tissue

Preserved clinical material is a unique source for proteomic investigation of human disorders. Here we describe an optimized protocol allowing large scale quantitative analysis of formalin fixed and paraffin embedded (FFPE) tissue. The procedure comprises four distinct steps. The first one is the preparation of sections from the FFPE material and microdissection of cells of interest. In the second step the isolated cells are lysed and processed using 'filter aided sample preparation' (FASP) technique. In this step, proteins are depleted from reagents used for the sample lysis and are digested in two-steps using endoproteinase LysC and trypsin. 
After each digestion, the peptides are collected in separate fractions and their content is determined using a highly sensitive fluorescence measurement. Finally, the peptides are fractionated on 'pipette-tip' microcolumns. The LysC-peptides are separated into 4 fractions whereas the tryptic peptides are separated into 2 fractions. In this way prepared samples allow analysis of proteomes from minute amounts of material to a depth of 10,000 proteins. Thus, the described workflow is a powerful technique for studying diseases in a system-wide-fashion as well as for identification of potential biomarkers and drug targets.

Archival formalin fixed and paraffin embedded (FFPE) clinical samples are valuable material for investigation of diseases. Here we demonstrate a sample preparation workflow allowing in-depth proteomic analysis of microdissected FFPE tissue.

Preserved clinical material is a unique source for proteomic investigation of human disorders. Here we describe an optimized protocol allowing large scale ...

Capture Compound Mass Spectrometry - A Powerful Tool to Identify Novel c-di-GMP Effector Proteins

Considerable progress has been made during the last decade towards the identification and characterization of enzymes involved in the synthesis (diguanylate cyclases) and degradation (phosphodiesterases) of the second messenger c-di-GMP. In contrast, little information is available regarding the molecular mechanisms and cellular components through which this signaling molecule regulates a diverse range of cellular processes. Most of the known effector proteins belong to the PilZ family or are degenerated diguanylate cyclases or phosphodiesterases that have given up on catalysis and have adopted effector function. Thus, to better define the cellular c-di-GMP network in a wide range of bacteria experimental methods are required to identify and validate novel effectors for which reliable in silico predictions fail.
We have recently developed a novel Capture Compound Mass Spectrometry (CCMS) based technology as a powerful tool to biochemically identify and characterize c-di-GMP binding proteins. This technique has previously been reported to be applicable to a wide range of organisms1. Here we give a detailed description of the protocol that we utilize to probe such signaling components. As an example, we use Pseudomonas aeruginosa, an opportunistic pathogen in which c-di-GMP plays a critical role in virulence and biofilm control. CCMS identified 74% (38/51) of the known or predicted components of the c-di-GMP network. This study explains the CCMS procedure in detail, and establishes it as a powerful and versatile tool to identify novel components involved in small molecule signaling.

The ubiquitous second messenger c-di-GMP controls growth and behavior of many bacteria. We have developed a novel Capture Compound Mass Spectrometry based technology to biochemically identify and characterize c-di-GMP binding proteins in virtually any bacterial species.

Considerable progress has been made during the last decade towards the identification and characterization of enzymes involved in the synthesis ...

Non-enzymatic, Serum-free Tissue Culture of Pre-invasive Breast Lesions for Spontaneous Generation of Mammospheres

Breast ductal carcinoma in situ (DCIS), by definition, is proliferation of neoplastic epithelial cells within the confines of the breast duct, without breaching the collagenous basement membrane. While DCIS is a non-obligate precursor to invasive breast cancers, the molecular mechanisms and cell populations that permit progression to invasive cancer are not fully known. To determine if progenitor cells capable of invasion existed within the DCIS cell population, we developed a methodology for collecting and culturing sterile human breast tissue at the time of surgery, without enzymatic disruption of tissue.
Sterile breast tissue containing ductal segments is harvested from surgically excised breast tissue following routine pathological examination. Tissue containing DCIS is placed in nutrient rich, antibiotic-containing, serum free medium, and transported to the tissue culture laboratory. The breast tissue is further dissected to isolate the calcified areas. Multiple breast tissue pieces (organoids) are placed in a minimal volume of serum free medium in a flask with a removable lid and cultured in a humidified CO2 incubator. Epithelial and fibroblast cell populations emerge from the organoid after 10 - 14 days. Mammospheres spontaneously form on and around the epithelial cell monolayer. Specific cell populations can be harvested directly from the flask without disrupting neighboring cells. Our non-enzymatic tissue culture system reliably reveals cytogenetically abnormal, invasive progenitor cells from fresh human DCIS lesions.

Primary cell culture using intact tissue organoids provides a model system that mimics the multi-cellular in vivo microenvironment. We developed a serum-free primary breast epithelium tissue culture model that perpetuates mixed cell culture lineages and exhibits differentiated morphology, without enzymatic tissue disruption. Breast organoids remain viable for &#62;6 months.

Breast ductal carcinoma in situ (DCIS), by definition, is proliferation of neoplastic epithelial cells within the confines of the breast duct, without ...

Selected Reaction Monitoring Mass Spectrometry for Absolute Protein Quantification

Absolute quantification of target proteins within complex biological samples is critical to a wide range of research and clinical applications. This protocol provides step-by-step instructions for the development and application of quantitative assays using selected reaction monitoring (SRM) mass spectrometry (MS). First, likely quantotypic target peptides are identified based on numerous criteria. This includes identifying proteotypic peptides, avoiding sites of posttranslational modification, and analyzing the uniqueness of the target peptide to the target protein. Next, crude external peptide standards are synthesized and used to develop SRM assays, and the resulting assays are used to perform qualitative analyses of the biological samples. Finally, purified, quantified, heavy isotope labeled internal peptide standards are prepared and used to perform isotope dilution series SRM assays. Analysis of all of the resulting MS data is presented. This protocol was used to accurately assay the absolute abundance of proteins of the chemotaxis signaling pathway within RAW 264.7 cells (a mouse monocyte/macrophage cell line). The quantification of Gi2 (a heterotrimeric G-protein &#945;-subunit) is described in detail.

This protocol describes how to perform absolute quantification assays of target proteins within complex biological samples using selected reaction monitoring. It was used to accurately quantify proteins of the mouse macrophage chemotaxis signaling pathway. Target peptide selection, assay development, and qualitative and quantitative assays are described in detail.

Absolute quantification of target proteins within complex biological samples is critical to a wide range of research and clinical applications. This ...

Enrichment of Extracellular Matrix Proteins from Tissues and Digestion into Peptides for Mass Spectrometry Analysis

The extracellular matrix (ECM) is a complex meshwork of cross-linked proteins that provides biophysical and biochemical cues that are major regulators of cell proliferation, survival, migration, etc. The ECM plays important roles in development and in diverse pathologies including cardio-vascular and musculo-skeletal diseases, fibrosis, and cancer. Thus, characterizing the composition of ECMs of normal and diseased tissues could lead to the identification of novel prognostic and diagnostic biomarkers and potential novel therapeutic targets. However, the very nature of ECM proteins (large in size, cross-linked and covalently bound, heavily glycosylated) has rendered biochemical analyses of ECMs challenging. To overcome this challenge, we developed a method to enrich ECMs from fresh or frozen tissues and tumors that takes advantage of the insolubility of ECM proteins. We describe here in detail the decellularization procedure that consists of sequential incubations in buffers of different pH and salt and detergent concentrations and that results in 1) the extraction of intracellular (cytosolic, nuclear, membrane and cytoskeletal) proteins and 2) the enrichment of ECM proteins. We then describe how to deglycosylate and digest ECM-enriched protein preparations into peptides for subsequent analysis by mass spectrometry.

This protocol describes a procedure for enriching ECM proteins from tissues or tumors and deglycosylating and digesting the ECM-enriched preparations into peptides to analyze their protein composition by mass spectrometry.

The extracellular matrix (ECM) is a complex meshwork of cross-linked proteins that provides biophysical and biochemical cues that are major regulators of ...

Surface Spreading and Immunostaining of Yeast Chromosomes

The small size of nuclei of the budding yeast Saccharomyces cerevisiae limits the utility of light microscopy for analysis of the subnuclear distribution of chromatin-bound proteins. Surface spreading of yeast nuclei results in expansion of chromatin without loss of bound proteins. A method for surface spreading balances fixation of DNA bound proteins with detergent treatment. The method demonstrated is slightly modified from that described by Josef Loidl and Franz Klein1,2. The method has been used to characterize the localization of many chromatin-bound proteins at various stages of the mitotic cell cycle, but is especially useful for the study of meiotic chromosome structures such as meiotic recombinosomes and the synaptonemal complex. We also describe a modification that does not require use of Lipsol, a proprietary detergent, which was called for in the original procedure, but no longer commercially available. An immunostaining protocol that is compatible with the chromosome spreading method is also described.

A method for surface-spreading chromosomes from budding yeast is presented. This method is derived from a method previously described by Loidl and Klein. In addition, we demonstrate a procedure for immunostaining of spread chromosomes.

The small size of nuclei of the budding yeast Saccharomyces cerevisiae limits the utility of light microscopy for analysis of the subnuclear distribution ...

Analyzing Cell Surface Adhesion Remodeling in Response to Mechanical Tension Using Magnetic Beads

Mechanosensitive cell surface adhesion complexes allow cells to sense the mechanical properties of their surroundings. Recent studies have identified both force-sensing molecules at adhesion sites, and force-dependent transcription factors that regulate lineage-specific gene expression and drive phenotypic outputs. However, the signaling networks converting mechanical tension into biochemical pathways have remained elusive. To explore the signaling pathways engaged upon mechanical tension applied to cell surface receptor, superparamagnetic microbeads can be used. Here we present a protocol for using magnetic beads to apply forces to cell surface adhesion proteins. Using this approach, it is possible to investigate not only force-dependent cytoplasmic signaling pathways by various biochemical approaches, but also adhesion remodeling by magnetic isolation of adhesion complexes attached to the ligand-coated beads. This protocol includes the preparation of ligand-coated superparamagnetic beads, and the application of define tensile forces followed by biochemical analyses. Additionally, we provide a representative sample of data demonstrating that tension applied to integrin-based adhesion triggers adhesion remodeling and alters protein tyrosine phosphorylation.

Cell surface adhesions are central in mechanotransduction, as they transmit mechanical tension and initiate the signaling pathways involved in tissue homeostasis and development. Here, we present a protocol for dissecting the biochemical pathways that are activated in response to tension, using ligand-coated magnetic microbeads and force application to adhesion receptors.

Mechanosensitive cell surface adhesion complexes allow cells to sense the mechanical properties of their surroundings. Recent studies have identified both ...