We thank Chandran Sabanayagam for his help in developing the system, Joshua Edel for advice on designing the microfluidic device and the staff at the Rowland Institute at Harvard University for machining parts of the setup. We acknowledge useful discussions with members of Meller&#x2019;s group at the Rowland Institute and at Boston University, and members of T. Ha&#x2019;s group at UIUC for helpful information. Finally, we appreciate the financial support from the National Science Foundation (PHY-0701207) and the National Institute of Health (GM075893).

1 - Assembling the microfluidic cell
The microfluidic cell is made by fusing a patterned, 2 mm thick, polydimethylsiloxane (PDMS) disk to a freshly cleaned quartz cover slip. The PDMS disk contains four 30 &mu;l rectangular chambers that are accessed by inlet and outlet flexible capillaries connected to a buffer reservoir and a syringe pump, respectively, for the controlled flow of buffer. The &#xFB02;ow cell is supported on its back side by a glass window and placed on a custom-made cell holder containing a water cooled thermoelectric element to regulate temperature.
<ol>
 <li>To make the patterned PDMS disk, mix 10 parts silicone elastomer base with 1 part curing agent, mix well and leave degassing for 1 hour in vacuum.</li>
 <li>Gently pour the elastomer mix onto the microfluidic cell mold. In our case, this is a 1" silicon wafer with four strips (1x104 x 1x103 x 300 &mu;m) made from negative photoresist. Leave this to cure on a hot plate at 70&deg; C for two hours.</li>
 <li>Carefully peel the cured PDMS disk from the mold using a razor blade. Fuse the disk onto a 1'' glass window (containing 8 x 2 mm-diameter pre-drilled holes at both ends of the channels) by plasma oxidizing both surfaces for 30 seconds. Fuse the glass windows onto the flat side of the PDMS disk (the side not containing the channels).</li>
 <li>Insert a 2-inch long flexible fused silica capillary tubing through each glass window's hole until it reaches the channel. Inserting a needle first and then following with the capillary may facilitate this process. Seal the glass window holes using a fast curing silicone casting compound such as Kwik-Cast.</li>
 <li>Rinse the channels with ethanol and water, and plasma-activate the cell together with a freshly cleaned, 1-inch circular quartz cover slip for 30 seconds. We suggest cleaning this cover slip using piranha*. Fuse the cover slip to the side of the cell containing the channels, sealing the chambers.</li>
 <li>Leave the assembled cell to cure overnight at room temperature.</li>
</ol>
* Piranha is a solution of H2O2 and H2SO4 in 1:2.5 proportions. To clean the cover slips, immerse them in piranha at 90&deg; C for 20 min.
2 - Activating the surface for molecule immobilization
For nucleic acid studies, the simplest surface immobilization scheme consists of first coating a glass or quartz cover slip with biotinylated bovine serum albumin (BSA) and then with streptavidin. This allows the biotinylated sample to be immobilized with high specificity for days at room temperature. 
<ol>
 <li>Connect flexible tubing to the capillaries for easy buffer injection using a syringe and needle.</li>
 <li>Inject a solution of 0.1 mg/ml of biotinylated BSA in buffer A (10 mM TRIS-HCl, pH 8.0, 50 mM NaCl, filtered using 0.02 &mu;m filters) into the channel and incubate for at least 30 min.</li>
 <li>Flow 300-500 &mu;l of buffer A to remove any free biotinylated BSA from the channel, being careful not to trap any air bubbles inside the chamber.</li>
 <li>Inject a solution of 0.1 mg/ml of streptavidin in buffer A and incubate for 15 min. Then repeat Step 3.</li>
 <li>Flow through a 3-5 pM solution of the biotinylated sample. Perform a surface scan to check the coverage of fluorescent molecules. If necessary, increase the sample concentration to obtain better coverage. Incubate for 10 min and repeat Step 3.</li>
</ol>
The microfluidic cell is mounted on a motorized x-y stage allowing coarse (up to 25 mm) motion using optically encoded DC motors, and &#xFB01;ne (1 nm) motion using two closed-loop piezo actuators (Physik Instrumente model M-014 or similar). This can be done either before or after activating the surface for sample immobilization.
3 - Preparing the Imaging Buffer (IB)
The IB consists of an enzymatic oxygen scavenging system and a triplet quencher, such as Trolox. Since the IB is flushed through constantly overnight, at least 10 ml should be prepared in advance.
<ol>
 <li>Prepare 10 ml of 0.8% w/v D-glucose, at least 35 mM TRIS-HCl pH 8.0 and 50 mM NaCl in deionized water. The higher concentration of buffer is important for two reasons: dissolving Trolox will acidify the solution, and the enzymatic reaction will also lower the pH of the solution over time. Since this solution has a long shelf life it can be stored as a stock solution.</li>
 <li>Dissolve 5 mg of Trolox in the buffer prepared in Step 1 by vortexing for a couple of minutes and then shaking for &gt;10 min. Trolox is not very soluble in water at pH &gt; 7, so do not rush this step. Filter the solution using a 0.2 &mu;m filter and leave degassing in vacuum for 10 minutes.</li>
 <li>Prepare the "gloxy" enzymatic solution by mixing 60 &mu;l water, 20 &mu;l of 5X buffer A, 20 &mu;l of catalase, 1 mg glucose oxidase and 100 &mu;l of 10 mg/ml BSA. Filter the solution using a 0.22 &mu;m centrifuge filter.</li>
 <li>Gently mix the buffer from Step 2 with the "gloxy" solution from Step 3 avoiding the formation of air bubbles.</li>
 <li>Flow the IB through the channel at a constant rate of 5 &mu;l/min using a mechanical syringe pump to control the flow rate. Bubble argon gas into the IB reservoir to prevent atmospheric oxygen from re-dissolving into the buffer.</li>
</ol>
The surface scanning, molecule localization and acquisition of single molecule intensity traces is controlled by a LabView program that permits automated data collection1. The program scans 20x20 &mu;m2 areas at 0.2 &mu;m resolution, with a rate of 0.1 &mu;m/ms. Data is immediately processed to yield intensity-weighted locations of all pixels above the background counts. Once the immobilized molecules are found, they are moved one by one into the confocal volume and the intensities of the donor and acceptor fluorophore are recorded as a function of time until both fluorophores photobleach. The stage is then programmed to move to a new origin 100 &mu;m away, and the scanning process is repeated.
4 - Representative Results
Below are some images showing the assembled microfluidic cell before surface activation and after being mounted on the microscope ready for molecule immobilization. If the channel is activated successfully, surface scans should be similar to the scans depicted below showing emission of the donor dye (green) and acceptor (red) after direct donor excitation. These molecules are moved one by one into the probing volume and the fluorescence is recorded over time until both dyes photobleach, as shown in the single molecule traces. From traces like these one can obtain the FRET efficiency which informs on the instantaneous donor-acceptor separation, which in turn is used to explore the structure and dynamics of molecules of biological interest.
<img src="/files/ftp_upload/1542/1542fig1tmb.jpg" alt="Figure 1" border="0" /> Figure 1: Microfluidic cell with four channels, each with inlet/outlet capillaries. Please <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig1.jpg">click here</a> to see a larger version of figure 1.
<img src="/files/ftp_upload/1542/1542fig2tmb.jpg" alt="Figure 2" border="0" /> Figure 2: Cell mounted on the microscope ready for measurements. Please <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig2.jpg">click here</a> to see a larger version of figure 2.
<img src="/files/ftp_upload/1542/1542fig3tmb.jpg" alt="Figure 3" border="0" /> Figure 3: Setup for sm-FRET measurements. Please <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig3.jpg">click here</a> to see a larger version of figure 3.
<img src="/files/ftp_upload/1542/1542fig4tmb.jpg" alt="Figure 4" border="0" /><img src="/files/ftp_upload/1542/1542fig5tmb.jpg" alt="Figure 5" border="0" /> Figures 4 and 5: Red and green channel of a surface scan of immobilized FRET molecules excited with green laser. Please click to see a larger version of <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig4.jpg">figure 4</a>, or <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig5.jpg">figure 5</a>.
<img src="/files/ftp_upload/1542/1542fig6tmb.jpg" alt="Figure 6" border="0" /><img src="/files/ftp_upload/1542/1542fig7tmb.jpg" alt="Figure 7" border="0" /> <img src="/files/ftp_upload/1542/1542fig8tmb.jpg" alt="Figure 8" border="0" /><img src="/files/ftp_upload/1542/1542fig9tmb.jpg" alt="Figure 9" border="0" /> Figures 6, 7, 8, and 9: Intensity trajectories of donor (green) and acceptor (red) fluorophores labeled to a DNA molecule 8, 10, 16 and 18 basepairs apart. Please click to see a larger version of 
<a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig6.jpg">figure 6</a>, <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig7.jpg">figure 7</a>,<a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig8.jpg">figure 8</a>, or <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/1542/1542fig9.jpg">figure 9</a>.

<ol>
	<li>Selvin, P. R., Ha, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-Molecule+Techniques,+a+Laboratory+Manual.">Single-Molecule Techniques, a Laboratory Manual.</a> CSHL Press. , (2008).</li><li>Sabanayagam, C. R., Eid, J. S., Meller, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-throughput+scanning+confocal+microscope+for+single+molecule+analysis.">High-throughput scanning confocal microscope for single molecule analysis.</a> Applied Pys. Letters. , 84-87 (2004).</li><li>Sabanayagam, C. R., Eid, J. S., Meller, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+Fluorescence+resonance+energy+transfer+to+measure+distances+along+individual+DNA+molecules:+Corrections+due+to+nonideal+transfer.">Using Fluorescence resonance energy transfer to measure distances along individual DNA molecules: Corrections due to nonideal transfer.</a> J. Chem. Phys. 122, 61103-61105 (2004).</li><li>Echeverra, A., Aitken, C., Marshall, R. A., Puglisi, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+Oxygen+Scavenging+System+for+Improvement+of+Dye+Stability+in+Single-Molecule+Fluorescence+Experiments.">An Oxygen Scavenging System for Improvement of Dye Stability in Single-Molecule Fluorescence Experiments.</a> Biophysical Journal. 94, 1826-1835 (2008).</li><li>Rasnik, I., McKinney, S. A., Ha, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonblinking+and+long-lasting+single+molecule+fluorescence+imaging.">Nonblinking and long-lasting single molecule fluorescence imaging.</a> Nature Methods. 3 (11), 891-893 (2006).</li><li>Boukobza, E., Sonnenfeld, A., Haran, G., Puglisi, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Immobilization+in+Surface-Tethered+Lipid+Vesicles+as+a+New+Tool+for+Single+Biomolecule+Spectroscopy.">Immobilization in Surface-Tethered Lipid Vesicles as a New Tool for Single Biomolecule Spectroscopy.</a> J. Phys. Chem. 105, 12165-12170 (2001).</li><li>Di Fiori, N., Meller, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Article in preparation. , .</li></ol>

Although the above protocol has been optimized for our particular system and for a specific choice of dyes (TMR and ATTO647N), it is by no means the only proved method. Recently, an alternative oxygen scavenging system consisting of protocatechuic acid (PCA)/protocatechuate-3,4-dioxygenase (PCD) was shown to increase the photostability of certain dyes4. Likewise, other triplet-state quenchers such as &beta;-mercaptoethanol (BME) can be used instead of Trolox, although these might not suppress long-lasting blinking of some dyes, in particular Cy55.
Although immobilization via biotin-streptavidin-biotinylated bovine serum albumin (BSA) is the preferred choice for nucleic acids studies, a polyethylene glycol (PEG)-coated surface is better suited for studies of proteins since it prevents non-specific adhesion1. Additionally, sub-diffraction-limited size vesicles can be used to encapsulate the biomolecule of interest in cases where it can be affected by surface interactions6.
In this article we have used a confocal setup equipped with silicone avalanche photodiodes to acquire the fluorescence intensity traces. This protocol works equally well with Total Internal Reflection Fluorescence microscopes (TIRF) equipped with CCD cameras. Regardless of the microscopy used, sm-FRET can inform on spatial resolutions approaching the angstrom when the donor and acceptor signal are properly corrected7.

<table><tbody><tr><td>Fused silica capillary tubing</td><td>Polymicro Technologies</td><td>TSP150375</td><td></td></tr><tr><td>Fused silica cover slip</td><td>Esco Products</td><td>R425000</td><td></td></tr><tr><td>Tubing</td><td>Diba Industries</td><td></td><td></td></tr><tr><td>Biotinylated BSA</td><td>Sigma-Aldrich</td><td>A8549</td><td></td></tr><tr><td>BSA</td><td>Sigma-Aldrich</td><td>A9085</td><td></td></tr><tr><td>Streptavidin</td><td>Thermo Fisher Scientific, Inc.</td><td>0021122</td><td></td></tr><tr><td>Sylgard 184 Elastomer Kit</td><td>Dow Corning</td><td>3097366-1004</td><td>Silicone Elastomer Kit</td></tr><tr><td>Trolox</td><td>Aldrich</td><td>238813</td><td></td></tr><tr><td>Catalase</td><td>Sigma-Aldrich</td><td>C3155</td><td></td></tr><tr><td>Glucose Oxidase</td><td>Sigma-Aldrich</td><td>G2133</td><td></td></tr><tr><td>D-Glucose</td><td>Sigma-Aldrich</td><td>G7528</td><td></td></tr><tr><td>Kwik-Cast Sealant</td><td>World Precision Instruments, Inc.</td><td>Kwik-Cast</td><td>Silicone Casting Compound</td></tr></tbody></table>

automated system for single molecule fluorescence measurements of surface-immobilized biomolecules

Fluorescence Resonance Energy Transfer (FRET) microscopy has been widely used to study the structure and dynamics of molecules of biological interest, such as nucleic acids and proteins. Single molecule FRET (sm-FRET) measurements on immobilized molecules permit long observations of the system -effectively until both dyes photobleach- resulting in time-traces that report on biomolecular dynamics with a broad range of timescales from milliseconds to minutes. To facilitate the acquisition of large number of traces for statistical analyses, the process must be automated and the sample environment should be tightly controlled over the entire measurement time (~12 hours). This is accomplished using an automated scanning confocal microscope that allows the interrogation of thousands of single molecules overnight, and a microfluidic cell that permits the controlled exchange of buffer, with restricted oxygen content and maintains a constant temperature throughout the entire measuring period. Here we show how to assemble the microfluidic device and how to activate its surface for DNA immobilization. Then we explain how to prepare a buffer to maximize the photostability and lifetime of the fluorophores. Finally, we show the steps involved in preparing the setup for the automated acquisition of time-resolved single molecule FRET traces of DNA molecules.

Watch this Scientific Journal Video about Automated System for Single Molecule Fluorescence Measurements of Surface-immobilized Biomolecules at JoVE.com

Automated System for Single Molecule Fluorescence Measurements of Surface-immobilized Biomolecules

In this article we describe how we obtain FRET traces from individual DNA molecules immobilized to a surface using an automated scanning confocal microscope.

Fluorescence Resonance Energy Transfer (FRET) microscopy has been widely used to study the structure and dynamics of molecules of biological interest, ...

automated-system-for-single-molecule-fluorescence-measurements

Universidad de Cartagena UDEC

Research

JoVE Journal

Biology

12.8K Views.  Boston University. In this article we describe how we obtain FRET traces from individual DNA molecules immobilized to a surface using an automated scanning confocal microscope. 

Video: Automated System for Single Molecule Fluorescence Measurements of Surface-immobilized Biomolecules

Structural Information from Single-molecule FRET Experiments Using the Fast Nano-positioning System

Single-molecule F&#246;rster Resonance Energy Transfer (smFRET) can be used to obtain structural information on biomolecular complexes in real-time. Thereby, multiple smFRET measurements are used to localize an unknown dye position inside a protein complex by means of trilateration. In order to obtain quantitative information, the Nano-Positioning System (NPS) uses probabilistic data analysis to combine structural information from X-ray crystallography with single-molecule fluorescence data to calculate not only the most probable position but the complete three-dimensional probability distribution, termed posterior, which indicates the experimental uncertainty. The concept was generalized for the analysis of smFRET networks containing numerous dye molecules. The latest version of NPS, Fast-NPS, features a new algorithm using Bayesian parameter estimation based on Markov Chain Monte Carlo sampling and parallel tempering that allows for the analysis of large smFRET networks in a comparably short time. Moreover, Fast-NPS allows the calculation of the posterior by choosing one of five different models for each dye, that account for the different spatial and orientational behavior exhibited by the dye molecules due to their local environment.
Here we present a detailed protocol for obtaining smFRET data and applying the Fast-NPS. We provide detailed instructions for the acquisition of the three input parameters of Fast-NPS: the smFRET values, as well as the quantum yield and anisotropy of the dye molecules. Recently, the NPS has been used to elucidate the architecture of an archaeal open promotor complex. This data is used to demonstrate the influence of the five different dye models on the posterior distribution.

We present the setup and experimental procedure to obtain smFRET data from large donor-acceptor networks with a TIRF microscope. The step-by-step analysis of these measurements with the Bayesian inference software Fast-NPS yields high-resolved structural information via the application of adapted dye models.

Single-molecule F&#246;rster Resonance Energy Transfer (smFRET) can be used to obtain structural information on biomolecular complexes in real-time. ...

Biochemistry

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 ...

From Fast Fluorescence Imaging to Molecular Diffusion Law on Live Cell Membranes in a Commercial Microscope

It has become increasingly evident that the spatial distribution and the motion of membrane components like lipids and proteins are key factors in the regulation of many cellular functions. However, due to the fast dynamics and the tiny structures involved, a very high spatio-temporal resolution is required to catch the real behavior of molecules. Here we present the experimental protocol for studying the dynamics of fluorescently-labeled plasma-membrane proteins and lipids in live cells with high spatiotemporal resolution. Notably, this approach doesn&#8217;t need to track each molecule, but it calculates population behavior using all molecules in a given region of the membrane. The starting point is a fast imaging of a given region on the membrane. Afterwards, a complete spatio-temporal autocorrelation function is calculated correlating acquired images at increasing time delays, for example each 2, 3, n repetitions. It is possible to demonstrate that the width of the peak of the spatial autocorrelation function increases at increasing time delay as a function of particle movement due to diffusion. Therefore, fitting of the series of autocorrelation functions enables to extract the actual protein mean square displacement from imaging (iMSD), here presented in the form of apparent diffusivity vs average displacement. This yields a quantitative view of the average dynamics of single molecules with nanometer accuracy. By using a GFP-tagged variant of the Transferrin Receptor (TfR) and an ATTO488 labeled 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine (PPE) it is possible to observe the spatiotemporal regulation of protein and lipid diffusion on &#181;m-sized membrane regions in the micro-to-milli-second time range.

Spatial distribution and temporal dynamics of plasma membrane proteins and lipids is a hot topic in biology. Here this issue is addressed by a spatio-temporal image fluctuation analysis that provides conceptually the same physical quantities of single particle tracking, but it uses small molecular labels and standard microscopy setups.

It has become increasingly evident that the spatial distribution and the motion of membrane components like lipids and proteins are key factors in the ...

Bioengineering

Fluorescence Biomembrane Force Probe: Concurrent Quantitation of Receptor-ligand Kinetics and Binding-induced Intracellular Signaling on a Single Cell

Membrane receptor-ligand interactions mediate many cellular functions. Binding kinetics and downstream signaling triggered by these molecular interactions are likely affected by the mechanical environment in which binding and signaling take place. A recent study demonstrated that mechanical force can regulate antigen recognition by and triggering of the T-cell receptor (TCR). This was made possible by a new technology we developed and termed fluorescence biomembrane force probe (fBFP), which combines single-molecule force spectroscopy with fluorescence microscopy. Using an ultra-soft human red blood cell as the sensitive force sensor, a high-speed camera and real-time imaging tracking techniques, the fBFP is of ~1 pN (10-12 N), ~3 nm and ~0.5 msec in force, spatial and temporal resolution. With the fBFP, one can precisely measure single receptor-ligand binding kinetics under force regulation and simultaneously image binding-triggered intracellular calcium signaling on a single live cell. This new technology can be used to study other membrane receptor-ligand interaction and signaling in other cells under mechanical regulation.

We describe a technique for concurrently measuring force-regulated single receptor-ligand binding kinetics and real-time imaging of calcium signaling in a single T lymphocyte.

Membrane receptor-ligand interactions mediate many cellular functions. Binding kinetics and downstream signaling triggered by these molecular interactions ...

Photoactivated Localization Microscopy with Bimolecular Fluorescence Complementation (BiFC-PALM)

Protein-protein interactions (PPIs) are key molecular events to biology. However, it remains a challenge to visualize PPIs with sufficient resolution and sensitivity in cells because the resolution of conventional light microscopy is diffraction-limited to ~250 nm. By combining bimolecular fluorescence complementation (BiFC) with photoactivated localization microscopy (PALM), PPIs can be visualized in cells with single molecule sensitivity and nanometer spatial resolution. BiFC is a commonly used technique for visualizing PPIs with fluorescence contrast, which involves splitting of a fluorescent protein into two non-fluorescent fragments. PALM is a recent superresolution microscopy technique for imaging biological samples at the nanometer and single molecule scales, which uses phototransformable fluorescent probes such as photoactivatable fluorescent proteins (PA-FPs). BiFC-PALM was demonstrated by splitting PAmCherry1, a PA-FP compatible with PALM, for its monomeric nature, good single molecule brightness, high contrast ratio, and utility for stoichiometry measurements. When split between amino acids 159 and 160, PAmCherry1 can be made into a BiFC probe that reconstitutes efficiently at 37 &#176;C with high specificity to PPIs and low non-specific reconstitution. Ras-Raf interaction is used as an example to show how BiFC-PALM helps to probe interactions at the nanometer scale and with single molecule resolution. Their diffusion can also be tracked in live cells using single molecule tracking (smt-) PALM. In this protocol, factors to consider when designing the fusion proteins for BiFC-PALM are discussed, sample preparation, image acquisition, and data analysis steps are explained, and a few exemplary results are showcased. Providing high spatial resolution, specificity, and sensitivity, BiFC-PALM is a useful tool for studying PPIs in intact biological samples.

Photoactivated Localization Microscopy with Bimolecular Fluorescence Complementation BiFC-PALM

Protein-protein interactions are visualized in cells with nanometer spatial resolution by combining bimolecular fluorescence complementation (BiFC) with photoactivated localization microscopy (PALM). Described here is the use of BiFC-PALM for imaging Ras-Raf interactions in U2OS cells for visualizing the nanoscale clustering and diffusion of individual Ras-Raf complexes.

Protein-protein interactions (PPIs) are key molecular events to biology. However, it remains a challenge to visualize PPIs with sufficient resolution and ...

Single Molecule Fluorescence Microscopy on Planar Supported Bilayers

In the course of a single decade single molecule microscopy has changed from being a secluded domain shared merely by physicists with a strong background in optics and laser physics to a discipline that is now enjoying vivid attention by life-scientists of all venues 1. This is because single molecule imaging has the unique potential to reveal protein behavior in situ in living cells and uncover cellular organization with unprecedented resolution below the diffraction limit of visible light 2. Glass-supported planar lipid bilayers (SLBs) are a powerful tool to bring cells otherwise growing in suspension in close enough proximity to the glass slide so that they can be readily imaged in noise-reduced Total Internal Reflection illumination mode 3,4. They are very useful to study the protein dynamics in plasma membrane-associated events as diverse as cell-cell contact formation, endocytosis, exocytosis and immune recognition. Simple procedures are presented how to generate highly mobile protein-functionalized SLBs in a reproducible manner, how to determine protein mobility within and how to measure protein densities with the use of single molecule detection. It is shown how to construct a cost-efficient single molecule microscopy system with TIRF illumination capabilities and how to operate it in the experiment.

Preparation of protein-functionalized planar glass-supported lipid bilayers, determination of protein mobility within and measurement of protein densities is shown here. A roadmap to building a noise-reduced Total Internal Reflection microscope is outlined, which allows visualizing single bilayer-resident fluorochromes with high spatiotemporal resolution.

In the course of a single decade single molecule microscopy has changed from being a secluded domain shared merely by physicists with a strong background ...

Making Precise and Accurate Single-Molecule FRET Measurements using the Open-Source smfBox

The smfBox is a recently developed cost-effective, open-source instrument for single-molecule F&#246;rster Resonance Energy Transfer (smFRET), which makes measurements on freely diffusing biomolecules more accessible. This overview includes a step-by-step protocol for using this instrument to make measurements of precise FRET efficiencies in duplex DNA samples, including details of the sample preparation, instrument setup and alignment, data acquisition, and complete analysis routines. The presented approach, which includes how to determine all the correction factors required for accurate FRET-derived distance measurements, builds on a large body of recent collaborative work across the FRET Community, which aims to establish standard protocols and analysis approaches. This protocol, which is easily adaptable to a range of biomolecular systems, adds to the growing efforts in democratising smFRET for the wider scientific community.

This article provides step-by-step instructions for making fully-corrected accurate FRET measurements on individual, freely diffusing biomolecules using the open-source, inexpensive smfBox, from switch on, through alignment and focusing, to data collection and analysis.

The smfBox is a recently developed cost-effective, open-source instrument for single-molecule F&#246;rster Resonance Energy Transfer (smFRET), which makes ...

Single-Molecule Diffusion and Assembly on Polymer-Crowded Lipid Membranes

Cellular membranes are highly crowded environments for biomolecular reactions and signaling. Yet, most in vitro experiments probing protein interaction with lipids employ naked bilayer membranes. Such systems lack the complexities of crowding by membrane-embedded proteins and glycans and exclude the associated volume effects encountered on cellular membrane surfaces. Also, the negatively charged glass surface onto which the lipid bilayers are formed prevents the free diffusion of transmembrane biomolecules. Here, we present a well-characterized polymer-lipid membrane as a mimic for crowded lipid membranes. This protocol utilizes polyethylene glycol (PEG)-conjugated lipids as a generalized approach for incorporating crowders into the supported lipid bilayer&#160;(SLB). First, a cleaning procedure of the microscopic slides and coverslips for performing single-molecule experiments is presented. Next, methods for characterizing the PEG-SLBs and performing single-molecule experiments of the binding, diffusion, and assembly of biomolecules using single-molecule tracking and photobleaching are discussed. Finally, this protocol demonstrates how to monitor the nanopore assembly of bacterial pore-forming toxin Cytolysin A (ClyA) on crowded lipid membranes with single-molecule photobleaching analysis. MATLAB codes with example datasets are also included to perform some of the common analyses such as particle tracking, extracting diffusive behavior, and subunit counting.

Here, a protocol to perform and analyze the binding, mobility, and assembly of single molecules on artificial crowded lipid membranes using single-molecule total internal reflection fluorescence (smTIRF) microscopy is presented.

Cellular membranes are highly crowded environments for biomolecular reactions and signaling. Yet, most in vitro experiments probing protein interaction ...

Conducting Multiple Imaging Modes with One Fluorescence Microscope

Fluorescence microscopy is a powerful tool to detect biological molecules in situ and monitor their dynamics and interactions in real-time. In addition to conventional epi-fluorescence microscopy, various imaging techniques have been developed to achieve specific experimental goals. Some of the widely used techniques include single-molecule fluorescence resonance energy transfer (smFRET), which can report conformational changes and molecular interactions with angstrom resolution, and single-molecule detection-based super-resolution (SR) imaging, which can enhance the spatial resolution approximately ten&#160;to twentyfold compared to diffraction-limited microscopy. Here we present a customer-designed integrated system, which merges multiple imaging methods in one microscope, including conventional epi-fluorescent imaging, single-molecule detection-based SR imaging, and multi-color single-molecule detection, including smFRET imaging. Different imaging methods can be achieved easily and reproducibly by switching optical elements. This set-up is easy to adopt by any research laboratory in biological sciences with a need for routine and diverse imaging experiments at a reduced cost and space relative to building separate microscopes for individual purposes.

Here we present a practical guide of building an integrated microscopy system, which merges conventional epi-fluorescent imaging, single-molecule detection-based super-resolution imaging, and multi-color single-molecule detection, including single-molecule fluorescence resonance energy transfer imaging, into one set-up in a cost-efficient way.

Fluorescence microscopy is a powerful tool to detect biological molecules in situ and monitor their dynamics and interactions in real-time. In addition to ...

Automated Two-dimensional Spatiotemporal Analysis of Mobile Single-molecule FRET Probes

Single-molecule F&#246;rster resonance energy transfer (smFRET) is a versatile technique reporting on distances in the sub-nanometer to nanometer range. It has been used in a wide range of biophysical and molecular biological experiments, including the measurement of molecular forces, characterization of conformational dynamics of biomolecules, observation of intracellular colocalization of proteins, and determination of receptor-ligand interaction times. In a widefield microscopy configuration, experiments are typically performed using surface-immobilized probes. Here, a method combining single-molecule tracking with alternating excitation (ALEX) smFRET experiments is presented, permitting the acquisition of smFRET time traces of surface-bound, yet mobile probes in plasma membranes or glass-supported lipid bilayers. For the analysis of recorded data, an automated, open-source software collection was developed supporting (i) the localization of fluorescent signals, (ii) single-particle tracking, (iii) determination of FRET-related quantities including correction factors, (iv) stringent verification of smFRET traces, and (v) intuitive presentation of the results. The generated data can conveniently be used as input for further exploration via specialized software, e.g., for the assessment of the diffusional behavior of probes or the investigation of FRET transitions.

This article presents a method for spatiotemporal analysis of mobile, single-molecule F&#246;rster resonance energy transfer (smFRET)-based probes using widefield fluorescence microscopy. The newly developed software toolkit allows the determination of smFRET time traces of moving probes, including the correct FRET efficiency and the molecular positions, as functions of time.

Single-molecule F&#246;rster resonance energy transfer (smFRET) is a versatile technique reporting on distances in the sub-nanometer to nanometer range. ...