This work was supported by the National Institutes of Health R35GM126934, R01AI153617, and R01CA248166 to DSL. EMB was supported through the ASERT-IRACDA program (NIH K12GM088021) and JAR by the UNM MARC program (NIH 2T34GM008751-20). We gratefully acknowledge using the University of New Mexico Comprehensive Cancer Center fluorescence microscopy shared resource, supported by NIH P30CA118100. We want to acknowledge Drs. Ankur Jain and Taekijip Ha, whose original development of SiMPull inspired this work. 
ES-C present address: Immunodynamics Group, Laboratory of Integrative Cancer Immunology, Center for Cancer Research, National Cancer Institute, Bethesda

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The authors have nothing to disclose.

The protocol described here was optimized to enable quantitative measurements of receptor phosphorylation at the single protein level. Several straightforward but important modifications to the SiMPull protocol were developed that improved the reliability of the measurement for phospho-tyrosine detection, including reduction of autofluorescence with NaBH4 treatment and postfixing of the sample to prevent antibody dissociation. Using the green channel mask to identify receptor locations for calculation of coloc...

Membrane-associated signaling is tuned by a combination of ligand-induced membrane receptor activation and recruitment of downstream accessory proteins that propagate the signal. Phosphorylation of key tyrosines in receptor cytoplasmic tails is critical to initiating the formation of signaling complexes, or signalosomes1,2. Therefore, an important question in biology is how phosphorylation patterns are created and maintained to recruit signaling partners and dictate cellular outcomes. This includes understanding the heterogeneity of receptor phosphorylation, both in abundance and in the specific phosphotyrosine patterns that can provide a means of manipulating signaling outputs by dictating the composition of the signalosome3,4,5,6,7. However, there are limitations in current methods to interrogate protein phosphorylation. Western blot analysis is excellent for describing trends of protein phosphorylation but is semi-quantitative8 and does not provide information on the heterogeneity of the system because thousands to millions of receptors are averaged together. While western blots allow probing a sample using phospho-specific antibodies to specific tyrosines, they cannot provide information on multisite phosphorylation patterns within the same protein. Quantitative phosphoproteomics report on phosphotyrosine abundance, but there are limitations to detecting multisite phosphorylation, as the residues of interest need to be located within the same peptide (typically 7-35 amino acids) that is generated by enzymatic digestion9,10,11.
To overcome the limitations mentioned above, the single-molecule pull-down (SiMPull) assay has been adapted to quantify the phosphorylation states of intact receptors at the single-molecule level. SiMPull was first demonstrated as a powerful tool for interrogating macromolecular complexes by Jain et al.12,13. In SiMPull, macromolecular complexes were immunoprecipitated (IP) on antibody-functionalized glass coverslips and then analyzed through single-molecule microscopy for protein subunit number and co-IP with complex components12. A modification by Kim et al.14, termed SiMBlot, was the first to use a variation of SiMPull to analyze phosphorylation of denatured proteins.&#160;The SiMBlot protocol relies on capturing biotinylated cell surface proteins using NeutrAvidin-coated coverslips, which are then probed for phosphorylation with phospho-specific antibody labeling14. Despite these advances, improvements were needed to make the quantification of posttranslational modification more robust and applicable to a broader range of proteins.
The present protocol describes an optimized SiMPull approach that was used to quantify phosphorylation patterns of intact epidermal growth factor receptor (EGFR) in response to a range of ligand conditions and oncogenic mutations15. While this work focuses on EGFR, this approach can be applied to any membrane receptor and cytosolic proteins of interest (POI), for which quality antibodies are available. The protocol includes steps to reduce sample autofluorescence, a sample array design that requires minimal sample volume with simultaneous preparation of up to 20 samples, and optimization of antibody labeling and fixation conditions. Data analysis algorithms have been developed for single-molecule detection and quantification of phosphorylated proteins.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>1.5 mL microcentrifuge tubes</td>
			<td>MTC Bio</td>
			<td>C2000</td>
			<td></td>
		</tr>
		<tr>
			<td>10 mM Tris-HCl pH 7.4</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>10 mM Tris-HCl pH 8.0/ 50 mM NaCl</td>
			<td></td>
			<td></td>
			<td>T50 Buffer</td>
		</tr>
		<tr>
			<td>100 mm Tissue Culture dish</td>
			<td>CELLTREAT</td>
			<td>229620</td>
			<td>Storage of piranha etched glass/arrays</td>
		</tr>
		<tr>
			<td>15 mL conical tube</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>16% Paraformaldehyde Aqueous Solution</td>
			<td>Electron Microscopy Sciences</td>
			<td>15710</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>50 mL conical tube</td>
			<td></td>
			<td></td>
			<td>Functionalized Glass storage/ KOH reuse</td>
		</tr>
		<tr>
			<td>50 mM Tris-HCl pH 7.2/150 mM NaCl</td>
			<td></td>
			<td></td>
			<td>Lysis Buffer Component</td>
		</tr>
		<tr>
			<td>60 mm Tissue Culture dish</td>
			<td>Corning</td>
			<td>430166</td>
			<td></td>
		</tr>
		<tr>
			<td>8% Glutaraldehyde Aqueous Solution</td>
			<td>Electron Microscopy Sciences</td>
			<td>16020</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Acetone (C3H6O)</td>
			<td>Millipore Sigma</td>
			<td>270725</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Alexa Fluor 647 NHS Ester</td>
			<td>Thermo Fisher Scientific</td>
			<td>A-20006</td>
			<td></td>
		</tr>
		<tr>
			<td>Animal-Free Recombinant Human EGF</td>
			<td>Peprotech</td>
			<td>AF-100-15</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-Human EGFR (External Domain) &#8211; Biotin</td>
			<td>Leinco Technologies, Inc</td>
			<td>E101</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-p-Tyr Antibody (PY99) Alexa Fluor 647</td>
			<td>Santa Cruz Biotechnology</td>
			<td>sc-7020 AF647</td>
			<td></td>
		</tr>
		<tr>
			<td>Bath-sonicator</td>
			<td>Branson</td>
			<td>1200</td>
			<td></td>
		</tr>
		<tr>
			<td>BCA Protein Assay Kit</td>
			<td>Pierce</td>
			<td>23227</td>
			<td></td>
		</tr>
		<tr>
			<td>Biotin-PEG</td>
			<td>Laysan Bio</td>
			<td>Biotin-PEG-SVA, MW 5,000</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine serum albumin</td>
			<td>Gold Biotechnology</td>
			<td>A-420-1</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Buchner funnel</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Bunsen burner</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium Chloride (CaCl2)</td>
			<td>Millipore Sigma</td>
			<td>C4901</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Cell Scraper</td>
			<td>Bioworld</td>
			<td>30900017-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Conical Filtering Flask</td>
			<td>Fisher Scientific</td>
			<td>S15464</td>
			<td></td>
		</tr>
		<tr>
			<td>Coplin Jar</td>
			<td>WHEATON</td>
			<td>900470</td>
			<td></td>
		</tr>
		<tr>
			<td>Countess II Automated Cell Counter</td>
			<td>Thermo Fisher Scientific</td>
			<td>AMQAX1000</td>
			<td></td>
		</tr>
		<tr>
			<td>Coverslips 24 x 60 #1.5</td>
			<td>Electron Microscopy Sciences</td>
			<td>63793</td>
			<td></td>
		</tr>
		<tr>
			<td>DipImage</td>
			<td></td>
			<td></td>
			<td>https://diplib.org/</td>
		</tr>
		<tr>
			<td>DMEM</td>
			<td>Caisson Labs</td>
			<td>DML19-500</td>
			<td></td>
		</tr>
		<tr>
			<td>emCCD camera</td>
			<td>Andor iXon</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal Bovine Serum, Optima</td>
			<td>Bio-Techne</td>
			<td>S12450H</td>
			<td>Heat Inactivated</td>
		</tr>
		<tr>
			<td>Fusion 360 software</td>
			<td>Autodesk</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Geneticin G418 Disulfate</td>
			<td>Caisson Labs</td>
			<td>G030-5GM</td>
			<td></td>
		</tr>
		<tr>
			<td>Glacial Acetic Acid (CH3COOH)</td>
			<td>JT Baker</td>
			<td>JTB-9526-01</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Glass serological pipettes</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Stir Rod</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glucose (D-(+)-Glucose)</td>
			<td>Millipore Sigma</td>
			<td>D9434</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Halt Phosphotase and Protease Inhibitor Cocktail (100X)</td>
			<td>Thermo Fisher Scientific</td>
			<td>78446</td>
			<td>Lysis Buffer Component</td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Millipore Sigma</td>
			<td>H3375</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Hydrochloric Acid (HCl)</td>
			<td>VWR</td>
			<td>BDH7204-1</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Hydrogen Peroxide (H2O2) (3%)</td>
			<td></td>
			<td>HX0645</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen Peroxide (H2O2) (30%)</td>
			<td>EMD Millipore</td>
			<td>HX0635-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Ice</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>IGEPAL CA-630 (NP-40)</td>
			<td>Sigma Aldrich</td>
			<td>I8896</td>
			<td>Lysis Buffer Component</td>
		</tr>
		<tr>
			<td>ImmEdge Hydrophobic Barrier Pen</td>
			<td>Vector Laboratories</td>
			<td>H-4000</td>
			<td></td>
		</tr>
		<tr>
			<td>Immersol 518F immersion oil</td>
			<td>Zeiss</td>
			<td>444960-0000-000</td>
			<td></td>
		</tr>
		<tr>
			<td>in-house vacuum line</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>L-glutamine</td>
			<td>Thermo Fisher Scientific</td>
			<td>25030-164</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnessium Chloride Hexahydrate (MgCl2-6H2O)</td>
			<td>MPBio</td>
			<td>2191421</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Matlab</td>
			<td>Mathworks</td>
			<td></td>
			<td>Curve Fitting Toolbox, Parallel Computing Toolbox, and Statistics and Machine Learning toolbox</td>
		</tr>
		<tr>
			<td>Methanol (CH3OH)</td>
			<td>IBIS Scientific</td>
			<td>MX0486-1</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Milli-Q water</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mix-n-Stain CF Dye Antibody Labeling Kits</td>
			<td>Biotium</td>
			<td>92245</td>
			<td>Suggested conjugation kit</td>
		</tr>
		<tr>
			<td>mPEG</td>
			<td>Laysan Bio</td>
			<td>mPEG-succinimidyl valerate, MW 5,000</td>
			<td></td>
		</tr>
		<tr>
			<td>N-(2-aminoethyl)-3-aminopropyltrimethoxysilane</td>
			<td>UCT United Chemical</td>
			<td>A0700</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Nanogrid</td>
			<td>Miraloma Tech</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>NeutrAvidin Biotin Binding Protein</td>
			<td>Thermo Fisher Scientific</td>
			<td>31000</td>
			<td></td>
		</tr>
		<tr>
			<td>Nitrogen (compressed gas)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>NVIDIA GPU with CUDA</td>
			<td></td>
			<td></td>
			<td>Look for compatibility at https://www.mathworks.com/help/parallel-computing/gpu-support-by-release.html</td>
		</tr>
		<tr>
			<td>Olympus iX71 Microscope</td>
			<td>Olympus</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Parafilm M Sealing Film</td>
			<td>The Lab Depot</td>
			<td>HS234526C</td>
			<td></td>
		</tr>
		<tr>
			<td>PBS pH 7.4</td>
			<td>Caisson Labs</td>
			<td>PBL06</td>
			<td></td>
		</tr>
		<tr>
			<td>PC-200 Analog Hot Plate</td>
			<td>Corning</td>
			<td>6795-200</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin (10,000 U/mL)</td>
			<td>Thermo Fisher Scientific</td>
			<td>15140-163</td>
			<td></td>
		</tr>
		<tr>
			<td>Phospho-EGF Receptor (Tyr1068) (1H12) Mouse mAb</td>
			<td>Cell Signaling Technology</td>
			<td>2236BF</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium Chloride (KCl)</td>
			<td>Millipore Sigma</td>
			<td>529552</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Potassium Hydroxide (KOH)</td>
			<td>Millipore Sigma</td>
			<td>1050330500</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Premium PLA Filament, 1.75 mm diameter</td>
			<td>Raise 3D</td>
			<td>PMS:2035U/RAL:3028</td>
			<td>Printing temperature range: 205-235 &#176;C</td>
		</tr>
		<tr>
			<td>Pro2 3D printer</td>
			<td>Raise 3D</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Pyrex 1 L beaker</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PYREX 100 mL storage bottles</td>
			<td>Corning</td>
			<td>1395-100</td>
			<td>CH3OH/C3H6O reuse</td>
		</tr>
		<tr>
			<td>Pyrex 250 mL beakers</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Pyrex 4 L beaker</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Quad-view Image Splitter</td>
			<td>Photometrics</td>
			<td>Model QV2</td>
			<td></td>
		</tr>
		<tr>
			<td>Refrigerated centrifuge</td>
			<td>Eppendorf</td>
			<td>EP-5415R</td>
			<td></td>
		</tr>
		<tr>
			<td>RevCount Cell Counters, EVE Cell Counting Slides</td>
			<td>VWR</td>
			<td>10027-446</td>
			<td></td>
		</tr>
		<tr>
			<td>Semrock emission filters: blue (445/45 nm), green (525/45 nm), red (600/37 nm), far-red (685/40 nm)</td>
			<td>Semrock</td>
			<td>LF405/488/561/635-4X4M-B-000</td>
			<td></td>
		</tr>
		<tr>
			<td>Serological pipette controller</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Serological Pipettes</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>smite single molecule analysis package</td>
			<td></td>
			<td></td>
			<td>https://github.com/LidkeLab/smite.git</td>
		</tr>
		<tr>
			<td>Sodium Bicarbonate (NaHCO3)</td>
			<td>Sigma Aldrich</td>
			<td>S6014</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Sodium Borohydride (NaBH4)</td>
			<td>Millipore Sigma</td>
			<td>452874</td>
			<td>Tyrode&#39;s Buffer Component</td>
		</tr>
		<tr>
			<td>Sodium Chloride (NaCl)</td>
			<td>Millipore Sigma</td>
			<td>S9625</td>
			<td>Activate by successive heat and pH cycling</td>
		</tr>
		<tr>
			<td>Sodium Hydroxide</td>
			<td>VWR</td>
			<td>BDH3247-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Orthovanadate (Na3VO4)</td>
			<td>Millipore Sigma</td>
			<td>S6508</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>Sulfuric Acid (H2SO4)</td>
			<td>Millipore Sigma</td>
			<td>258105</td>
			<td>Hazardous</td>
		</tr>
		<tr>
			<td>TetraSpeck Microspheres</td>
			<td>Thermo Fisher Scientific</td>
			<td>T7279</td>
			<td>multi-fluorescent beads</td>
		</tr>
		<tr>
			<td>Tris (Trizma) base</td>
			<td>Millipore Sigma</td>
			<td>T1503</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypan blue stain, 0.4%</td>
			<td>Thermo Fisher Scientific</td>
			<td>15250061</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin-EDTA 0.05%</td>
			<td>Thermo Fisher Scientific</td>
			<td>25300120</td>
			<td></td>
		</tr>
	</tbody>
</table>

an optimized single-molecule pull-down assay for quantification of protein phosphorylation

Phosphorylation is a necessary posttranslational modification that regulates protein function and directs cell signaling outcomes. Current methods to measure protein phosphorylation cannot preserve the heterogeneity in phosphorylation across individual proteins. The single-molecule pull-down (SiMPull) assay was developed to investigate the composition of macromolecular complexes via immunoprecipitation of proteins on a glass coverslip followed by single-molecule imaging. The current technique is an adaptation of SiMPull that provides robust quantification of the phosphorylation state of full-length membrane receptors at the single-molecule level. Imaging thousands of individual receptors in this way allows for quantifying protein phosphorylation patterns. The present protocol details the optimized SiMPull procedure, from sample preparation to imaging. Optimization of glass preparation and antibody fixation protocols further enhances data quality. The current protocol provides code for the single-molecule data analysis that calculates the fraction of receptors phosphorylated within a sample. While this work focuses on phosphorylation of the epidermal growth factor receptor (EGFR), the protocol can be generalized to other membrane receptors and cytosolic signaling molecules.

A cartoon depicting the SiMPull process is shown in Figure 1A. Coverslips are functionalized using NeutrAvidin as an anchor for biotinylated anti-EGFR antibodies to capture EGFR-GFP from total protein lysates. After washing away unbound protein, the phosphorylated receptors are labeled with an anti-phosphotyrosine (anti-PY) antibody15. Figure 1B shows an image of the hydrophobic array, where multiple samples can be prepared and imaged on ...

Watch this Scientific Journal Video about An Optimized Single-Molecule Pull-Down Assay for Quantification of Protein Phosphorylation at JoVE.com

An Optimized Single-Molecule Pull-Down Assay for Quantification of Protein Phosphorylation

The present protocol describes sample preparation and data analysis to quantify protein phosphorylation using an improved single-molecule pull-down (SiMPull) assay.

Phosphorylation is a necessary posttranslational modification that regulates protein function and directs cell signaling outcomes. Current methods to ...

an-optimized-single-molecule-pull-down-assay-for-quantification

Research

JoVE Journal

Biochemistry

2.7K Views.  University of New Mexico School of Medicine. The present protocol describes sample preparation and data analysis to quantify protein phosphorylation using an improved single-molecule pull-down (SiMPull) assay.

Video: An Optimized Single-Molecule Pull-Down Assay for Quantification of Protein Phosphorylation

Oligopeptide Competition Assay for Phosphorylation Site Determination

Protein phosphorylation at specific sites determines its conformation and interaction with other molecules. Thus, protein phosphorylation affects biological functions and characteristics of the cell. Currently, the most common method for discovering phosphorylation sites is by liquid chromatography/mass spectrometry (LC/MS) analysis, a rapid and sensitive method. However, relatively labile phosphate moieties are often released from phosphopeptides during the fragmentation step, which often yields false-negative signals. In such cases, a traditional in vitro kinase assay using site-directed mutants would be more accurate, but this method is laborious and time-consuming. Therefore, an alternative method using peptide competition may be advantageous. The consensus recognition motif of 5&#39; adenosine monophosphate-activated protein kinase (AMPK) has been established1 and was validated using a positional scanning peptide library assay2. Thus, AMPK phosphorylation sites for a novel substrate could be predicted and confirmed by the peptide competition assays. In this report, we describe the detailed steps and procedures for the in vitro oligopeptide-competing kinase assay by illustrating AMPK-mediated nuclear factor erythroid 2-related factor 2 (Nrf2) phosphorylation. To authenticate the phosphorylation site, we carried out a sequential in vitro kinase assay using a site-specific mutant. Overall, the peptide competition assay provides a method to screen multiple potential phosphorylation sites and to identify sites for validation by the phosphorylation site mutants.

Peptide competition assays are widely used in a variety of molecular and immunological experiments. This paper describes a detailed method for an in vitro oligopeptide-competing kinase assay and the associated validation procedures, which may be useful to find specific phosphorylation sites.

Protein phosphorylation at specific sites determines its conformation and interaction with other molecules. Thus, protein phosphorylation affects ...

Assay for Phosphorylation and Microtubule Binding Along with Localization of Tau Protein in Colorectal Cancer Cells

The microtubule-associated protein tau is a neuronal protein that localizes mostly in axons. Generally tau is essential for normal neuronal functioning because it is involved in microtubule assembly and stabilization. Besides neurons, tau is expressed in human breast, prostate, gastric, colorectal, and pancreatic cancers where it shows nearly similar structure and exerts similar functions as the neuronal tau. The amount of tau and its phosphorylation can change its function as a stabilizer of microtubules, and lead to the development of paired helical filaments in different neurodegenerative disorders, such as Alzheimer&#39;s disease. Determining the phosphorylation state of tau and its microtubule-binding characteristics is important. In addition, examining the intracellular localization of tau is important in different diseases. This manuscript details standard protocols for measuring tau phosphorylation and tau binding to microtubules in colorectal cancer cells with or without curcumin and LiCl treatment. These treatments can be used to stop cancer cell proliferation and development. Intracellular localization of tau is examined by using immunohistochemistry and confocal microscopy while using low amounts of antibodies. These assays can be used repetitively for screening compounds that affect tau hyperphosphorylation or microtubule binding. Novel therapeutics used for different tauopathies or related anticancer agents can potentially be characterized using these protocols.

This manuscript describes standard protocols for measuring tau hyperphosphorylation, measuring tau binding to microtubules, and localization of intracellular tau following drug treatments. These protocols can be used repetitively for screening drugs or other compounds that target tau hyperphosphorylation or microtubule binding.

The microtubule-associated protein tau is a neuronal protein that localizes mostly in axons. Generally tau is essential for normal neuronal functioning ...

Using Three-color Single-molecule FRET to Study the Correlation of Protein Interactions

Single-molecule F&#246;rster resonance energy transfer (smFRET) has become a widely used biophysical technique to study the dynamics of biomolecules. For many molecular machines in a cell proteins have to act together&#160;with interaction partners in a functional cycle to fulfill their task. The extension of two-color to multi-color smFRET makes it possible to simultaneously probe more than one interaction or conformational change. This not only adds a new dimension to smFRET experiments but it also offers the unique possibility to directly study the sequence of events and to detect correlated interactions when using an immobilized sample and a total internal reflection fluorescence microscope (TIRFM). Therefore, multi-color smFRET is a versatile tool for studying biomolecular complexes in a quantitative manner and in a previously unachievable detail.
Here, we demonstrate how to overcome the special challenges of multi-color smFRET experiments on proteins. We present detailed protocols for obtaining the data and for extracting kinetic information. This includes trace selection criteria, state separation, and the recovery of state trajectories from the noisy data using a 3D ensemble Hidden Markov Model (HMM). Compared to other methods, the kinetic information is not recovered from dwell time histograms but directly from the HMM. The maximum likelihood framework allows us to critically evaluate the kinetic model and to provide meaningful uncertainties for the rates.
By applying our method to the heat shock protein 90 (Hsp90), we are able to disentangle the nucleotide binding and the global conformational changes of the protein. This allows us to directly observe the cooperativity between the two nucleotide binding pockets of the Hsp90 dimer.

Here, we present a protocol to obtain three-color smFRET data and its analysis with a 3D ensemble Hidden Markov Model. With this approach, scientists can extract kinetic information from complex protein systems, including cooperativity or correlated interactions.

Single-molecule F&#246;rster resonance energy transfer (smFRET) has become a widely used biophysical technique to study the dynamics of biomolecules. For ...

Covalent Immobilization of Proteins for the Single Molecule Force Spectroscopy

In recent years, atomic force microscopy (AFM) based single molecule force spectroscopy (SMFS) extended our understanding of molecular properties and functions. It gave us the opportunity to explore a multiplicity of biophysical mechanisms, e.g., how bacterial adhesins bind to host surface receptors in more detail. Among other factors, the success of SMFS experiments depends on the functional and native immobilization of the biomolecules of interest on solid surfaces and AFM tips. Here, we describe a straightforward protocol for the covalent coupling of proteins to silicon surfaces using silane-PEG-carboxyls and the well-established N-hydroxysuccinimid/1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimid (EDC/NHS) chemistry in order to explore the interaction of pilus-1 adhesin RrgA from the Gram-positive bacterium Streptococcus pneumoniae (S. pneumoniae) with the extracellular matrix protein fibronectin (Fn). Our results show that the surface functionalization leads to a homogenous distribution of Fn on the glass surface and to an appropriate concentration of RrgA on the AFM cantilever tip, apparent by the target value of up to 20% of interaction events during SMFS measurements and revealed that RrgA binds to Fn with a mean force of 52 pN. The protocol can be adjusted to couple via site specific free thiol groups. This results in a predefined protein or molecule orientation and is suitable for other biophysical applications besides the SMFS.

This protocol describes the covalent immobilization of proteins with a heterobifunctional silane coupling agent to silicon-oxide surfaces designed for the atomic force microscopy based single molecule force spectroscopy which is exemplified by the interaction of RrgA (pilus-1 tip adhesin of S. pneumoniae) with fibronectin.

In recent years, atomic force microscopy (AFM) based single molecule force spectroscopy (SMFS) extended our understanding of molecular properties and ...

Proteome-wide Quantification of Labeling Homogeneity at the Single Molecule Level

Cell proteomes are often characterized using electrophoresis assays, where all species of proteins in the cells are non-specifically labeled with a fluorescent dye and are spotted by a photodetector following their separation. Single molecule fluorescence imaging can provide ultrasensitive protein detection with its ability for visualizing individual fluorescent molecules. However, the application of this powerful imaging method to electrophoresis assays is hampered by the lack of ways to characterize the homogeneity of fluorescent labeling of each protein species across the proteome. Here, we developed a method to evaluate the labeling homogeneity across the proteome based on a single molecule fluorescence imaging assay. In our measurement using a HeLa cell sample, the proportion of proteins labeled with at least one dye, which we termed &#8216;labeling occupancy&#8217; (LO), was determined to range from 50% to 90%, supporting the high potential of the application of single molecule imaging to sensitive and precise proteome analysis.

Here, we present a protocol to assess the labeling homogeneity for each protein species in a complex protein sample at the single molecule level.

Cell proteomes are often characterized using electrophoresis assays, where all species of proteins in the cells are non-specifically labeled with a ...

OaAEP1-Mediated Enzymatic Synthesis and Immobilization of Polymerized Protein for Single-Molecule Force Spectroscopy

Chemical and bio-conjugation techniques have been developed rapidly in recent years and allow the building of protein polymers. However, a controlled protein polymerization process is always a challenge. Here, we have developed an enzymatic methodology for constructing polymerized protein step by step in a rationally-controlled sequence. In this method, the C-terminus of a protein monomer is NGL for protein conjugation using OaAEP1 (Oldenlandia affinis asparaginyl endopeptidases) 1) while the N-terminus was a cleavable TEV (tobacco etch virus) cleavage site plus an L (ENLYFQ/GL) for temporary N-terminal protecting. Consequently, OaAEP1 was able to add only one protein monomer at a time, and then the TEV protease cleaved the N-terminus between Q and G to expose the NH2-Gly-Leu. Then the unit is ready for next OaAEP1 ligation. The engineered polyprotein is examined by unfolding individual protein domain using atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS). Therefore, this study provides a useful strategy for polyprotein engineering and immobilization.

Here, we present a protocol to conjugate protein monomer by enzymes forming protein polymer with a controlled sequence and immobilize it on the surface for single-molecule force spectroscopy studies.

Chemical and bio-conjugation techniques have been developed rapidly in recent years and allow the building of protein polymers. However, a controlled ...

An Electrochemiluminescence-Based Assay for MeCP2 Protein Variants

The ECLIA is a versatile method which is able to quantify endogenous and recombinant protein amounts in a 96-well format. To demonstrate ECLIA efficiency, this assay was used to analyze intrinsic levels of MeCP2 in mouse brain tissue and the uptake of TAT-MeCP2 in human dermal fibroblasts. The MeCP2-ECLIA produces highly accurate and reproducible measurements with low intra- and inter-assay error. In summary, we developed a quantitative method for the evaluation of MeCP2 protein variants that can be utilized in high-throughput screens.

The electrochemiluminescence immunoassay (ECLIA) is a novel approach for quantitative detection of endogenous and exogenously applied MeCP2 protein variants, which produces highly quantitative, accurate and reproducible measurements with low intra- and inter-assay error over a wide working range. Here, the protocol for the MeCP2-ECLIA in a 96-well format is described.

The ECLIA is a versatile method which is able to quantify endogenous and recombinant protein amounts in a 96-well format. To demonstrate ECLIA efficiency, ...

An In Vitro Single-Molecule Imaging Assay for the Analysis of Cap-Dependent Translation Kinetics

Cap-dependent protein synthesis is the predominant translation pathway in eukaryotic cells. While various biochemical and genetic approaches have allowed extensive studies of cap-dependent translation and its regulation, high resolution kinetic characterization of this translation pathway is still lacking. Recently, we developed an in vitro assay to measure cap-dependent translation kinetics with single-molecule resolution. The assay is based on fluorescently labeled antibody binding to nascent epitope-tagged polypeptide. By imaging the binding and dissociation of antibodies to and from nascent peptide&#8211;ribosome&#8211;mRNA complexes, the translation progression on individual mRNAs can be tracked. Here, we present a protocol for establishing this assay, including mRNA and PEGylated slide preparations, real-time imaging of translation, and analysis of single molecule trajectories. This assay enables tracking of individual cap-dependent translation events and resolves key translation kinetics, such as initiation and elongation rates. The assay can be widely applied to distinct translation systems and should broadly benefit in vitro studies of cap-dependent translation kinetics and translational control mechanisms.

An In Vitro Single-Molecule Imaging Assay for the Analysis of Cap-Dependent Translation Kinetics

Tracking individual translation events allows for high-resolution kinetic studies of cap-dependent translation mechanisms. Here we demonstrate an in vitro single-molecule assay based on imaging interactions between fluorescently labeled antibodies and epitope-tagged nascent peptides. This method enables single-molecule characterization of initiation and peptide elongation kinetics during active in vitro cap-dependent translation.

Cap-dependent protein synthesis is the predominant translation pathway in eukaryotic cells. While various biochemical and genetic approaches have allowed ...

Single Molecule Fluorescence Energy Transfer Study of Ribosome Protein Synthesis

The ribosome is a large ribonucleoprotein complex that assembles proteins processively along mRNA templates. The diameter of the ribosome is approximately 20 nm to accommodate large tRNA substrates at the A-, P- and E-sites. Consequently, the ribosome dynamics are naturally de-phased quickly. Single molecule method can detect each ribosome separately and distinguish inhomogeneous populations, which is essential to reveal the complicated mechanisms of multi-component systems. We report the details of a smFRET method based on the Nikon Ti2 inverted microscope to probe the ribosome dynamics between the ribosomal protein L27 and tRNAs. The L27 is labeled at its unique Cys 53 position and reconstituted into a ribosome that is engineered to lack L27. The tRNA is labeled at its elbow region. As the tRNA moves to different locations inside the ribosome during the elongation cycle, such as pre- and post- translocation, the FRET efficiencies and dynamics exhibit differences, which have suggested multiple subpopulations. These subpopulations are not detectable by ensemble methods. The TIRF-based smFRET microscope is built on a manual or motorized inverted microscope, with home-built laser illumination. The ribosome samples are purified by ultracentrifugation, loaded into a home-built multi-channel sample cell and then illuminated via an evanescent laser field. The reflection laser spot can be used to achieve feedback control of perfect focus. The fluorescence signals are separated by a motorized filter-turret and collected by two digital CMOS cameras. The intensities are retrieved via the NIS-Elements software.

Single molecule fluorescence energy transfer is a method that tracks the tRNA dynamics during ribosomal protein synthesis. By tracking individual ribosomes, inhomogeneous populations are identified, which shed light on mechanisms. This method can be used to track biological conformational changes in general to reveal dynamic-function relationships in many other complexed biosystems. Single molecule methods can observe non-rate limiting steps and low-populated key intermediates, which are not accessible by conventional ensemble methods due to the average effect.

The ribosome is a large ribonucleoprotein complex that assembles proteins processively along mRNA templates. The diameter of the ribosome is approximately ...

In Vitro Characterization of Histone Chaperones using Analytical, Pull-Down and Chaperoning Assays

Histone proteins associate with DNA to form the eukaryotic chromatin. The basic unit of chromatin is a nucleosome, made up of a histone octamer consisting of two copies of the core histones H2A, H2B, H3, and H4, wrapped around by the DNA. The octamer is composed of two copies of an H2A/H2B dimer and a single copy of an H3/H4 tetramer. The highly charged core histones are prone to non-specific interactions with several proteins in the cellular cytoplasm and the nucleus. Histone chaperones form a diverse class of proteins that shuttle histones from the cytoplasm into the nucleus and aid their deposition onto the DNA, thus assisting the nucleosome assembly process. Some histone chaperones are specific for either H2A/H2B or H3/H4, and some function as chaperones for both. This protocol describes how in vitro laboratory techniques such as pull-down assays, analytical size-exclusion chromatography, analytical ultra-centrifugation, and histone chaperoning assay could be used in tandem to confirm whether a given protein is functional as a histone chaperone.

In Vitro Characterization of Histone Chaperones using Analytical, Pull-Down and Chaperoning Assays

This protocol describes a battery of methods that includes analytical size-exclusion chromatography to study histone chaperone oligomerization and stability, pull-down assay to unravel histone chaperone-histone interactions, AUC to analyze the stoichiometry of the protein complexes, and histone chaperoning assay to functionally characterize a putative histone chaperone in vitro.

Histone proteins associate with DNA to form the eukaryotic chromatin. The basic unit of chromatin is a nucleosome, made up of a histone octamer consisting ...