The authors thank Drs. Steven Chu and Joe W. Gray for helpful discussions, Henry Marr for his initial work on the BiFC-PALM project, and Alexis Shoemaker for her technical assistance. This work is supported by startup funds to X.N. from OHSU. Research in the Nan laboratory was also supported by NIH 5U54CA143836-05, the Damon Runyon Cancer Research Foundation, the M. J. Murdock Charitable Trust, and the FEI company.

<ol>
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The authors have nothing to disclose.

BiFC has been a commonly used technique for detecting and visualizing PPIs in cells, while PALM is a recent single molecule superresolution microscopy technique that enables nanoscale imaging of intact biological samples. The combination of BiFC with PALM achieved selective imaging of PPIs inside a cell with nanometer spatial resolution and single molecule sensitivity. BiFC-PALM extends the utility of both techniques, and as demonstrated in this work, shows great promise in revealing the molecular details of PPIs in thei...

Protein-protein interactions (PPIs) are fundamental to biology1 and are tightly regulated via spatiotemporal mechanisms across many time and length scales. Studies on cell signaling on the cell membrane, for example, have revealed dynamic, nanoscale spatial compartments that facilitate specific PPIs and cellular processes2. Hence, the ability to probe PPIs in biological systems with sufficient spatial and temporal resolutions, high specificity, and high sensitivity is key to achieving a mechanistic understanding of biology.
Bimolecular fluorescence complementation (BiFC) is one of the few existing tools for visualizing PPIs in a cell with subcellular resolution and live-cell compatibility3&#8211;5. The technique is relatively straightforward and involves splitting of a fluorescence protein into two non-fluorescent fragments; when genetically tagged to two interacting proteins and brought into proximity, the fragments can reconstitute to form a complete fluorescent protein, yielding fluorescent signal. When properly designed, BiFC probes should not spontaneously reconstitute in the absence of PPIs. As such, the fluorescence signal in a BiFC assay will only arise in the presence of PPIs, which enables direct visualization of PPIs with high specificity. Additional benefits of using fluorescence as the readout are high sensitivity, subcellular resolution, and compatibility with high throughput and high content screening assays, among others. For these benefits, a number of BiFC probes based on different parent fluorescent proteins have been developed. As in all other detection techniques based on conventional light microscopy, however, the spatial resolution of BiFC is limited to ~250 nm by the diffraction of light. This makes it a challenge to study the regulation of PPIs at the nanoscale, which, as alluded earlier and exemplified by lipid rafts6&#160;and Ras nanoclusters7, is a critical length scale to understanding many cellular processes such as signaling.
BiFC has been combined with photoactivated localization microscopy (PALM)8,9 to overcome this limit in spatial resolution for imaging PPIs10. PALM is a recent superresolution microscopy technique that circumvents the diffraction limit in fluorescence imaging through stochastic activation and subdiffractive localization of single fluorescent molecules. In each activation cycle, a fluorescent molecule emits a few hundred to a few thousand photons and gives rise to a single molecule image on the detector. While the image is diffraction-limited (~250 nm in width), its centroid can be determined with much higher precision, typically on the order of 10-50 nm depending on the number of photons detected. By activating and localizing each fluorescent molecule in the sample, a high resolution image can be reconstructed. Performed on living cells, single molecule tracking (smt-) PALM further permits acquisition of thousands of protein diffusion trajectories from a single cell11. Importantly, PALM uses specialized fluorescent probes such as photoactivatable fluorescent proteins (PA-FPs) to achieve stochastic activation. Since both BiFC and PALM use fluorescent proteins, they were combined by splitting PAmCherry1, a commonly used PA-FP for PALM, into two fragments between amino acids 159 and 160.
The BiFC system based on split PAmCherry1 shows low background signal from spontaneous reconstitution of the two fragments. When genetically tagged to a pair of interacting proteins, the two fragments (RN = residues 1-159; RC = Met + residues 160-236) reconstituted efficiently to form complete PAmCherry1 proteins even at 37 &#176;C and without incubating at lower temperatures, which is not the case for other BiFC pairs12 such as the parent mCherry13. Furthermore, the reconstituted PAmCherry1 protein retained the photophysical properties of the parent PAmCherry1, such as high contrast ratio, medium photon yield, and fast photoactivation, among others, which are critical for accurate single molecule localization and high-resolution PALM imaging.
In this protocol, the use of BiFC-PALM for imaging Ras-Raf interactions in U2OS cells by using split PAmCherry1 (Figure 1A) is described. The first step is to design the constructs for expressing fusion proteins between PAmCherry1 fragments (i.e., RN and RC) and the proteins of interest. In theory, for each pair of candidate proteins (A and B), there are eight pairs of fusion proteins to be tested: RN-A/RC-B; RN-A/B-RC; RC-A/RN-B; RC-A/B-RN; A-RN/RC-B; A-RN/B-RC; A-RC/RN-B; and A-RC/B-RN. This process can often be simplified by taking into account the structural or biochemical properties of the candidate proteins. In the case of Ras, the protein is post-translationally modified at the C-terminal CAAX box (C=Cys; A=aliphatic; X=any), after which the AAX motif is cleaved off. Hence, RN or RC can only be fused to the N-terminus of Ras; this reduces the number of fusion protein pairs to four (Figure 1B). For Raf, the Ras-binding domain (RBD; residues 51-131) is used and can be tagged on either end. These four fusion configurations were generated: RN-KRas/RC-Raf RBD; RN-KRas/Raf RBD-RC; RC-KRas/RN-Raf RBD; and RC-KRas/Raf RBD-RN.
Additionally, the linker between the fragments and the proteins of interest will need to be considered. A flexible linker of about ten amino acids is often used as it provides sufficient freedom for complementation to occur. One such linker is (GGGGS)x2, although there are many others that have been successfully applied, including random sequences generated from a multiple cloning site (MCS)14. The length of the linkers may need to be optimized depending on the size of the proteins of interest and their orientations when interacting.
The PAmCherry1 fragments are contained in a small cloning backbone with flanking MCSs (see Materials List). Genes of interest can be inserted via the restriction sites or with a ligation-independent method. After cloning and sequence verification, the expression cassette is transferred to an expression vector using a recombinase reaction, a cloning process with high fidelity and high efficiency.
Next, the resulting expression constructs are transfected into the target cell line or, if a stable cell line is desired, packaged into lentivirus for infecting the target cell line. Transient transfection allows for quick validation of the BiFC configurations, but potential problems must be noted. Transfection using chemicals often stresses the cells, resulting in high autofluorescence; while the use of total internal reflection fluorescence (TIRF) microscopy can mitigate this background signal by limiting the illumination volume, TIRF ideal when the PPIs take place on the cell membrane. Additionally, transient transfections often lead to high levels of protein expression, far exceeding those of the endogenous proteins, which may cause artifacts in detecting the PPIs. Hence, it is recommended that stable cell lines be established after an appropriate BiFC configuration has been determined through initial testing. Stable cell lines also have the potential for tunable expression via doxycycline induction15.
After infection, cells are selected with antibiotics, typically puromycin and neomycin, one for each construct. The subsequent steps for sample preparation, image acquisition, and data analysis will be described in detail in the protocols.
With this approach, the formation of nanoscale clusters in BiFC-PALM images of Ras-Raf RBD are routinely observed (Figure 2A). Consistently, smt-PALM trajectories of Ras-Raf RBD show a heterogeneous distribution in the diffusion states (Figure 2B-D). These results suggest that Ras-Raf complexes exist in multiple states on the cell membrane, presumably as monomers and clusters, with potential biological implications. This work demonstrates the power of BiFC-PALM in selective imaging of PPIs in cells with nanometer spatial resolution and single molecule sensitivity, which would be difficult to obtain with conventional BiFC, fluorescence co-localization, or fluorescence resonance energy transfer (FRET).
When designing BiFC experiments and interpreting the results, it is important to keep in mind that the BiFC process is irreversible in most cases, including split PAmCherry1. Once the two fragments combine and form a complete PAmCherry1 protein, the linkage between the two fragments, and thus the PPI becomes permanent. This limits the use of BiFC and BiFC-PALM for monitoring the dynamics of the PPIs (i.e., binding kinetics and not the diffusion dynamics of the protein complex once it is formed) and at times could even lead to mislocalization of the PPI complexes.
An additional factor to consider when designing BiFC-PALM experiments is the delay in chromophore maturation that is inherent in fluorescent proteins. Once two proteins interact and the FP fragments come together, they typically refold on the order of seconds (half-time of 60 sec for EYFP3). However, subsequent chromophore maturation and fluorescent signal develop on the order of minutes. While fluorescence may be detectable within 10 min&#160;after reconstitution of split-Venus, a fast-folding YFP variant, the half-time for maturation in general is often around 60 min16. Split-PAmCherry1 was observed to have similar rates. Hence, BiFC and BiFC-PALM are currently poor choices for monitoring real-time PPI kinetics; other methods, such as FRET and a recently developed dimerization dependent fluorescent protein17, may be more suited for this purpose.

<table><tbody><tr><td>Microscope frame</td><td>Nikon</td><td>Ti-U</td><td></td></tr><tr><td>60X oil immersion TIRF objective with 1.49 NA</td><td>Nikon</td><td>CFI Apo TIRF 60x H</td><td></td></tr><tr><td>EMCCD camera</td><td>Andor</td><td>iXon Ultra 897</td><td></td></tr><tr><td>561 nm laser</td><td>Opto Engine</td><td>MGL-FN-561</td><td></td></tr><tr><td>405 nm laser</td><td>Coherent</td><td>CUBE-405 50mW</td><td></td></tr><tr><td>561 nm dichroic mirror</td><td>Semrock</td><td>&amp;nbsp;Di01-R405/488/561/635-25x36</td><td></td></tr><tr><td>561 nm filter</td><td>Semrock</td><td>FF01-525/45-25</td><td></td></tr><tr><td>405/561 nm notch filter</td><td>Semrock</td><td>NF01-405/488/568-25</td><td></td></tr><tr><td>Temperature and CO&lt;sub&gt;2&lt;/sub&gt; controlled stage</td><td>Tokai Hit</td><td>INUBG2ATW-PLAM</td><td></td></tr><tr><td>pENTR-D-TOPO-PAmCherry1_1-159-MCS</td><td>Addgene</td><td>60545</td><td></td></tr><tr><td>pENTR-D-TOPO-PAmCherry160-236-MCS</td><td>Addgene</td><td>60546</td><td></td></tr><tr><td>pcDNA3.2-DEST</td><td>Life Technologies</td><td>12489-019</td><td></td></tr><tr><td>pLenti-DEST</td><td>Addgene</td><td></td><td>http://www.addgene.org/Eric_Campeau/</td></tr><tr><td>Phusion High-Fidelity DNA Polymerase</td><td>Thermo Scientific</td><td>F-531</td><td></td></tr><tr><td>In-Fusion HD Cloning</td><td>Clontech</td><td>639649</td><td></td></tr><tr><td>LR Clonase</td><td>Life Technologies</td><td>11791</td><td></td></tr><tr><td>Vira Power Lentivirus Packaging</td><td>Life Technologies</td><td>K497500</td><td></td></tr><tr><td>X-tremeGENE Transfection Reagent</td><td>Roche</td><td>13873800</td><td></td></tr><tr><td>Lab-Tek II Chambered Coverglass</td><td>Thermo Scientific</td><td>155409</td><td>#1.5 glass bottom dishes</td></tr><tr><td>U2OS cells</td><td>ATCC</td><td>HTB-96</td><td></td></tr><tr><td>293T/17 cells</td><td>ATCC</td><td>CRL-11268</td><td></td></tr><tr><td>DMEM with phenol red</td><td>Life Technologies</td><td>11995</td><td></td></tr><tr><td>DMEM no phenol red</td><td>Life Technologies</td><td>21063</td><td></td></tr><tr><td>Fetal bovine serum</td><td>Life Technologies</td><td>10082</td><td></td></tr><tr><td>Leibovit&#039;s L-15, no phenol red</td><td>Life Technologies</td><td>21083-027</td><td></td></tr><tr><td>Reduced serum medium</td><td>Life Technologies</td><td>31985</td><td></td></tr><tr><td>Phosphate Buffered Saline</td><td>Life Technologies</td><td>14040</td><td></td></tr><tr><td>Syringe</td><td>BD Biosciences</td><td>309604</td><td></td></tr><tr><td>Syringe filter</td><td>Millipore</td><td>SLHV033RB</td><td></td></tr><tr><td>Lentiviral concentrator</td><td>Clontech</td><td>631231</td><td></td></tr><tr><td>Retroviral concentrator</td><td>Clontech</td><td>631455</td><td></td></tr><tr><td>10 cm culture dish</td><td>BD Biosciences</td><td>353003</td><td></td></tr><tr><td>6-well culture plate</td><td>BD Biosciences</td><td>353046</td><td></td></tr><tr><td>Polybrene</td><td>Sigma</td><td>107689</td><td></td></tr><tr><td>Puromycin</td><td>Life Technologies</td><td>A11138</td><td></td></tr><tr><td>G-418</td><td>Calbiochem</td><td>345812</td><td>Neomycin</td></tr><tr><td>Doxycyline</td><td>Fisher</td><td>BP2653</td><td></td></tr><tr><td>Tris base</td><td>Fisher</td><td>BP152</td><td></td></tr><tr><td>EDTA</td><td>Sigma</td><td>EDS</td><td></td></tr><tr><td>Sodium Hydroxide</td><td>Sigma</td><td>S5881</td><td></td></tr><tr><td>Paraformaldehyde</td><td>Sigma</td><td>158127</td><td></td></tr><tr><td>Glutaraldehyde</td><td>Sigma</td><td>G6257</td><td></td></tr><tr><td>PIPES</td><td>Sigma</td><td>P6757</td><td></td></tr><tr><td>HEPES</td><td>Sigma</td><td>H4034</td><td></td></tr><tr><td>EGTA</td><td>Sigma</td><td>3777</td><td></td></tr><tr><td>Magnesium Sulfate</td><td>Sigma</td><td>M2643</td><td></td></tr><tr><td>Potassium Hydroxide</td><td>Sigma</td><td>221473</td><td></td></tr><tr><td>Sodium chloride</td><td>Fisher</td><td>BP358</td><td></td></tr><tr><td>Magnesium chloride</td><td>Fisher</td><td>M33</td><td></td></tr><tr><td>100 nm gold particles</td><td>BBI Solutions</td><td>EM.GC100</td><td></td></tr><tr><td>Molecular grade water</td><td>Life Technologies</td><td>10977</td><td></td></tr><tr><td>Dpn1</td><td>New England Biolabs</td><td>R0176</td><td></td></tr><tr><td>PCR purification kit</td><td>Qiagen</td><td>28104</td><td></td></tr><tr><td>Miniprep kit</td><td>Qiagen</td><td>27104</td><td></td></tr><tr><td>Midiprep kit</td><td>Macherey-Nagel</td><td>740410</td><td></td></tr><tr><td>0.6 ml microcentrifuge tubes</td><td>Fisher</td><td>05-408-120</td><td></td></tr><tr><td>1.5 ml microcentrifuge tubes</td><td>Fisher</td><td>05-408-137</td><td></td></tr><tr><td>15 ml tubes</td><td>Fisher</td><td>05-539-12</td><td></td></tr><tr><td>5 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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.

The BiFC-PALM example shown is KRas G12D mutant interacting with the Ras binding domain (RBD) of CRaf (Figure 1A). As discussed, the RN or RC fragments were not tagged to the C-terminus of Ras because it would disrupt the membrane localization and hence the biological activity of Ras. This reduced the possible combinations from eight to four (Figure 1B). Each of these four combinations was introduced into U2OS cells using lentiviral infection. The cells were fixed as detailed in the prot...

Watch this Scientific Journal Video about Photoactivated Localization Microscopy with Bimolecular Fluorescence Complementation BiFC-PALM at JoVE.com

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.

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 ...

photoactivated-localization-microscopy-with-bimolecular-fluorescence

Research

JoVE Journal

Bioengineering

9.9K Views.  Oregon Health and Science University. 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. 

Video: Photoactivated Localization Microscopy with Bimolecular Fluorescence Complementation BiFC-PALM

Visualization of Cortex Organization and Dynamics in Microorganisms, using Total Internal Reflection Fluorescence Microscopy

TIRF microscopy has emerged as a powerful imaging technology to study spatio-temporal dynamics of fluorescent molecules in vitro and in living cells1. The optical phenomenon of total internal reflection occurs when light passes from a medium with high refractive index into a medium with low refractive index at an angle larger than a characteristic critical angle (i.e. closer to being parallel with the boundary). Although all light is reflected back under such conditions, an evanescent wave is created that propagates across and along the boundary, which decays exponentially with distance, and only penetrates sample areas that are 100-200 nm near the interface2. In addition to providing superior axial resolution, the reduced excitation of out of focus fluorophores creates a very high signal to noise ratios and minimizes damaging effects of photobleaching2,3. Being a widefield technique, TIRF also allows faster image acquisition than most scanning based confocal setups.
At first glance, the low penetration depth of TIRF seems to be incompatible with imaging of bacterial and fungal cells, which are often surrounded by thick cell walls. On the contrary, we have found that the cell walls of yeast and bacterial cells actually improve the usability of TIRF and increase the range of observable structures4-8. Many cellular processes can therefore be directly accessed by TIRF in small, single-cell microorganisms, which often offer powerful genetic manipulation techniques. This allows us to perform in vivo biochemistry experiments, where kinetics of protein interactions and activities can be directly assessed in living cells.
We describe here the individual steps required to obtain high quality TIRF images for Saccharomyces cerevisiae or Bacillus subtilis cells. We point out various problems that can affect TIRF visualization of fluorescent probes in cells and illustrate the procedure with several application examples. Finally, we demonstrate how TIRF images can be further improved using established image restoration techniques.

Total Internal Reflection Fluorescence (TIRF) microscopy is a powerful approach to observe structures close to the cell surface at high contrast and temporal resolution. We demonstrate how TIRF can be employed to study protein dynamics at the cortex of cell wall-enclosed bacterial and fungal cells.

TIRF microscopy has emerged as  a powerful imaging technology to study spatio-temporal dynamics of fluorescent molecules  in vitro and in living cells1. ...

Nanotopology of Cell Adhesion upon Variable-Angle Total Internal Reflection Fluorescence Microscopy (VA-TIRFM)

Surface topology, e.g. of cells growing on a substrate, is determined with nanometer precision by Variable-Angle Total Internal Reflection Fluorescence Microscopy (VA-TIRFM). Cells are cultivated on transparent slides and incubated with a fluorescent marker homogeneously distributed in their plasma membrane. Illumination occurs by a parallel laser beam under variable angles of total internal reflection (TIR) with different penetration depths of the evanescent electromagnetic field. Recording of fluorescence images upon irradiation at about 10 different angles permits to calculate cell-substrate distances with a precision of a few nanometers. Differences of adhesion between various cell lines, e.g. cancer cells and less malignant cells, are thus determined. In addition, possible changes of cell adhesion upon chemical or photodynamic treatment can be examined. In comparison with other methods of super-resolution microscopy light exposure is kept very small, and no damage of living cells is expected to occur.

Nanotopology of Cell Adhesion upon Variable-Angle Total Internal Reflection Fluorescence Microscopy VA-TIRFM

Topology of cell adhesion on a substrate is measured with nanometre precision by variable-angle total internal reflection fluorescence microscopy (VA-TIRFM).

Surface topology, e.g. of cells growing on a substrate, is determined with nanometer precision by Variable-Angle Total Internal Reflection Fluorescence ...

Visualizing Protein-DNA Interactions in Live Bacterial Cells Using Photoactivated Single-molecule Tracking

Protein-DNA interactions are at the heart of many fundamental cellular processes. For example, DNA replication, transcription, repair, and chromosome organization are governed by DNA-binding proteins that recognize specific DNA structures or sequences. In vitro experiments have helped to generate detailed models for the function of many types of DNA-binding proteins, yet, the exact mechanisms of these processes and their organization in the complex environment of the living cell remain far less understood. We recently introduced a method for quantifying DNA-repair activities in live Escherichia coli cells using Photoactivated Localization Microscopy (PALM) combined with single-molecule tracking. Our general approach identifies individual DNA-binding events by the change in the mobility of a single protein upon association with the chromosome. The fraction of bound molecules provides a direct quantitative measure for the protein activity and abundance of substrates or binding sites at the single-cell level. Here, we describe the concept of the method and demonstrate sample preparation, data acquisition, and data analysis procedures.

Photoactivated localization microscopy (PALM) combined with single-molecule tracking allows direct observation and quantification of protein-DNA interactions in live Escherichia coli cells.

Protein-DNA interactions are at the heart of many fundamental cellular processes. For example, DNA replication, transcription, repair, and chromosome ...

Immunology and Infection

Conventional BODIPY Conjugates for Live-Cell Super-Resolution Microscopy and Single-Molecule Tracking

Single molecule localization microscopy (SMLM) techniques overcome the optical diffraction limit of conventional fluorescence microscopy and can resolve intracellular structures and the dynamics of biomolecules with ~20 nm precision. A prerequisite for SMLM are fluorophores that transition from a dark to a fluorescent state in order to avoid spatio-temporal overlap of their point spread functions in each of the thousands of data acquisition frames. BODIPYs are well-established dyes with numerous conjugates used in conventional microscopy. The transient formation of red-shifted BODIPY ground-state dimers (DII) results in bright single molecule emission enabling single molecule localization microscopy (SMLM). Here we present a simple but versatile protocol for SMLM with conventional BODIPY conjugates in living yeast and mammalian cells. This procedure can be used to acquire super-resolution images and to track single BODIPY-DII states to extract spatio-temporal information of BODIPY conjugates. We apply this procedure to resolve lipid droplets (LDs), fatty acids, and lysosomes in living yeast and mammalian cells at the nanoscopic length scale. Furthermore, we demonstrate the multi-color imaging capability with BODIPY dyes when used in conjunction with other fluorescent&#160;probes. Our representative results show the differential spatial distribution and mobility of BODIPY-fatty acids and neutral lipids in yeast under fed and fasted conditions. This optimized protocol for SMLM can be used with hundreds of commercially available BODIPY conjugates and is a useful resource to study biological processes at the nanoscale far beyond the applications of this work.&#160;

Conventional BODIPY conjugates can be used for live-cell single-molecule localization microscopy (SMLM) through exploitation of their transiently forming, red-shifted ground state dimers. We present an optimized SMLM protocol to track and resolve subcellular neutral lipids and fatty acids in living mammalian and yeast cells at the nanoscopic length scale.

Single molecule localization microscopy (SMLM) techniques overcome the optical diffraction limit of conventional fluorescence microscopy and can resolve ...

Biochemistry

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 ...

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.

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, ...

Biology

Bimolecular Fluorescence Complementation

Defining the subcellular distribution of signaling complexes is imperative to understanding the output from that complex. Conventional methods such as immunoprecipitation do not provide information on the spatial localization of complexes. In contrast, BiFC monitors the interaction and subcellular compartmentalization of protein complexes. In this method, a fluororescent protein is split into amino- and carboxy-terminal non-fluorescent fragments which are then fused to two proteins of interest. Interaction of the proteins results in reconstitution of the fluorophore (Figure 1)1,2. A limitation of BiFC is that once the fragmented fluorophore is reconstituted the complex is irreversible3. This limitation is advantageous in detecting transient or weak interactions, but precludes a kinetic analysis of complex dynamics. An additional caveat is that the reconstituted flourophore requires 30min to mature and fluoresce, again precluding the observation of real time interactions4. BiFC is a specific example of the protein fragment complementation assay (PCA) which employs reporter proteins such as green fluorescent protein variants (BiFC), dihydrofolate reductase, b-lactamase, and luciferase to measure protein:protein interactions5,6. Alternative methods to study protein:protein interactions in cells include fluorescence co-localization and F&ouml;rster resonance energy transfer (FRET)7. For co-localization, two proteins are individually tagged either directly with a fluorophore or by indirect immunofluorescence. However, this approach leads to high background of non-interacting proteins making it difficult to interpret co-localization data. In addition, due to the limits of resolution of confocal microscopy, two proteins may appear co-localized without necessarily interacting. With BiFC, fluorescence is only observed when the two proteins of interest interact. FRET is another excellent method for studying protein:protein interactions, but can be technically challenging. FRET experiments require the donor and acceptor to be of similar brightness and stoichiometry in the cell. In addition, one must account for bleed through of the donor into the acceptor channel and vice versa. Unlike FRET, BiFC has little background fluorescence, little post processing of image data, does not require high overexpression, and can detect weak or transient interactions. Bioluminescence resonance energy transfer (BRET) is a method similar to FRET except the donor is an enzyme (e.g. luciferase) that catalyzes a substrate to become bioluminescent thereby exciting an acceptor. BRET lacks the technical problems of bleed through and high background fluorescence but lacks the ability to provide spatial information due to the lack of substrate localization to specific compartments8. Overall, BiFC is an excellent method for visualizing subcellular localization of protein complexes to gain insight into compartmentalized signaling.

The subcellular localization of proteins is important in determining the spatio-temporal regulation of cell signaling. Here, we describe bimolecular fluorescence complementation (BiFC) as a straightforward method for monitoring the spatial interactions of proteins in the cell.

Defining the subcellular distribution of signaling complexes is imperative to understanding the output from that complex. Conventional methods such as ...

Simultaneous Multicolor Imaging of Biological Structures with Fluorescence Photoactivation Localization Microscopy

Localization-based super resolution microscopy can be applied to obtain a spatial map (image) of the distribution of individual fluorescently labeled single molecules within a sample with a spatial resolution of tens of nanometers. Using either photoactivatable (PAFP) or&#160;photoswitchable (PSFP) fluorescent proteins fused to proteins of interest, or organic dyes conjugated to antibodies or other molecules of interest, fluorescence photoactivation localization microscopy (FPALM) can simultaneously image multiple species of molecules within single cells. By using the following approach, populations of large numbers (thousands to hundreds of thousands) of individual molecules are imaged in single cells and localized with a precision of ~10-30 nm. Data obtained can be applied to understanding the nanoscale spatial distributions of multiple protein types within a cell. One primary advantage of this technique is the dramatic increase in spatial resolution: while diffraction limits resolution to ~200-250 nm in conventional light microscopy, FPALM can image length scales more than an order of magnitude smaller. As many biological hypotheses concern the spatial relationships among different biomolecules, the improved resolution of FPALM can provide insight into questions of cellular organization which have previously been inaccessible to conventional fluorescence microscopy. In addition to detailing the methods for sample preparation and data acquisition, we here describe the optical setup for FPALM. One additional consideration for researchers wishing to do super-resolution microscopy is cost: in-house setups are significantly cheaper than most commercially available imaging machines. Limitations of this technique include the need for optimizing the labeling of molecules of interest within cell samples, and the need for post-processing software to visualize results. We here describe the use of PAFP and PSFP expression to image two protein species in fixed cells. Extension of the technique to living cells is also described.

We demonstrate the use of fluorescence photo activation localization microscopy (FPALM) to simultaneously image multiple types of fluorescently labeled molecules&#160;within cells. The techniques described yield the localization of thousands to hundreds of thousands of individual fluorescent labeled proteins, with a precision of tens of nanometers&#160;within single cells.

Localization-based super resolution microscopy can be applied to obtain a spatial map (image) of the distribution of individual fluorescently labeled ...

Multi-color Localization Microscopy of Single Membrane Proteins in Organelles of Live Mammalian Cells

Knowledge about the localization of proteins in cellular subcompartments is crucial to understand their specific function. Here, we present a super-resolution technique that allows for the determination of the microcompartments that are accessible for proteins by generating localization and tracking maps of these proteins. Moreover, by multi-color localization microscopy, the localization and tracking profiles of proteins in different subcompartments are obtained simultaneously. The technique is specific for live cells and is based on the repetitive imaging of single mobile membrane proteins. Proteins of interest are genetically fused with specific, so-called self-labeling tags. These tags are enzymes that react with a substrate in a covalent manner. Conjugated to these substrates are fluorescent dyes. Reaction of the enzyme-tagged proteins with the fluorescence labeled substrates results in labeled proteins. Here, Tetramethylrhodamine (TMR) and Silicon Rhodamine (SiR) are used as fluorescent dyes attached to the substrates of the enzymes. By using substrate concentrations in the pM to nM range, sub-stoichiometric labeling is achieved that results in distinct signals. These signals are localized with ~15&#8211;27 nm precision. The technique allows for multi-color imaging of single molecules, whereby the number of colors is limited by the available membrane-permeable dyes and the repertoire of self-labeling enzymes. We show the feasibility of the technique by determining the localization of the quality control enzyme (Pten)-induced kinase 1 (PINK1) in different mitochondrial compartments during its processing in relation to other membrane proteins. The test for true physical interactions between differently labeled single proteins by single molecule FRET or co-tracking is restricted, though, because the low labeling degrees decrease the probability for having two adjacent proteins labeled at the same time. While the technique is strong for imaging proteins in membrane compartments, in most cases it is not appropriate to determine the localization of highly mobile soluble proteins.

Here, we present a protocol for multi-color localization of single membrane proteins in organelles of live cells. To attach fluorophores, self-labeling proteins are used. Proteins, located in different membranes compartments of the same organelle, can be localized with a precision of ~18 nm.

Knowledge about the localization of proteins in cellular subcompartments is crucial to understand their specific function. Here, we present a ...

Bimolecular Fluorescence Complementation-Coupled Photoactivated Localization Microscopy

To prepare a sample for imaging, plate about 5.5 x 104 stable expression cells per well of an 8-well glass-bottomed chamber slide, in 350 microliters of phenol red-free DMEM. Use fresh paraformaldehyde with glutaraldehyde to fix the cells, and after replacing the fixative with PBS or imaging buffer, vortex 100-nanometer gold particles, and add 35 microliters per well for tracking stage drift during imaging.
Approach the microscope and power on the 405 and 561-nanometer lasers. Keeping the shutters closed at this point, ensure the 561-nanometer dichroic mirror and notch filter are in place. Open the image acquisition software.
Turn on the EMCCD camera and allow it to cool down, and set the exposure time to 100 milliseconds and the EMCCD gain to an appropriate value. Add immersion oil to the objective, and secure the sample on the microscope stage. Next, with either brightfield or the 561-nanometer laser, bring the sample into focus.
For imaging Ras and other membrane proteins, use a 60X apochromat TIRF objective with a 1.49 numerical aperture, and bring the microscope into TIRF configuration.
Adjust the excitation laser so that it is off-centered when hitting the back aperture of the TIRF objective, which will cause the laser to deflect upon reaching the coverslip buffer interface. Keep adjusting the laser incident position until the critical angle is reached and the laser is being reflected back.
Search for a cell to image with several gold particles in view. Then, set a region of interest that encloses the cell or a region of it and the gold particles.
In case sufficient activation is already occurring because of high expression levels, begin acquisition with the 405-nanometer laser off and the 561-laser on. Otherwise, turn on the 405-nanometer laser at the lowest factory power setting, and increase it gradually until there are tens of molecules per frame or so that single molecules are well-separated.
As data acquisition continues, gradually increase the 405-nanometer laser power to keep the spot density roughly constant. Continue image acquisition until high 405 power does not initiate any more activation events.

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

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