Name: using scaffold liposomes to reconstitute lipid-proximal protein-protein interactions in vitro
Uploaded: 2017-01-11T12:30:00
Description: Watch this Scientific Journal Video about Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro at JoVE.com

The authors would like to acknowledge the funding received from the American Heart Association (SDG12SDG9130039).

1. Scaffold Liposome Preparation
NOTE: Ideally, initial experiments should use a relatively simple and featureless scaffold (comprised of DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine or PC) and DGS-NTA(Ni2+) (1,2-dioleoyl-sn-glycero-3-[(N-(5-amino-1-carboxypentyl)iminodiacetic acid)succinyl](nickel salt)). Building off of these experiments, lipid charge, flexibility, and curvature can be introduced as individual factors with the potential to alter membrane...

<ol>
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This protocol offers a method for investigating protein-protein interactions involving integral membrane proteins. By utilizing a modular liposome scaffold, investigators are capable of assessing the activity of one or more proteins in a lipid-proximal environment. Previous studies have demonstrated a similar method for receptor enzymes of the plasma membrane24-26. We have expanded this method to incorporate lipid cofactors and explore interactions between proteins that make up the mechan...

Studying membrane-proximal protein-protein interactions is a challenging endeavor due to difficulty in recapitulating the native environment of the integral membrane proteins involved1. This is due to the necessity of detergent solubilization and the inconsistent orientation of proteins in proteoliposomes. In order to avoid these issues, we have employed a strategy whereby soluble domains of integral membrane proteins are expressed as His-tag fusion proteins, and these soluble fragments are anchored to scaffold liposomes via interactions with NTA(Ni2+) headgroups at the lipid surface. Using these scaffolds, lipid-proximal protein interactions can be investigated over a range of lipid and protein compositions.
We have effectively applied this method to investigate the critical protein-protein interactions that govern assembly of the mitochondrial fission complex and examine lipid interactions that modulate this process2. During mitochondrial fission, a conserved membrane remodeling protein, called Dynamin-related protein 1 (Drp1)3, is recruited to the surface of the Outer Mitochondrial Membrane (OMM) in response to cellular signals that regulate energy homeostasis, apoptotic signaling, and several other integral mitochondrial processes. This large, cytosolic GTPase is recruited to the surface of mitochondria through interactions with integral OMM proteins4-8. The role of one such protein, Mitochondrial Fission Factor (Mff), has been difficult to elucidate due to an apparent weak interaction with Drp1 in vitro. Nevertheless, genetic studies have clearly demonstrated that Mff is essential for successful mitochondrial fission7,8. The method described in this manuscript was able to overcome previous shortcomings by introducing simultaneous lipid interactions that promote Drp1-Mff interactions. Overall, this novel assay revealed fundamental interactions guiding assembly of the mitochondrial fission complex and provided a new stage for ongoing structural and functional studies of this essential molecular machine.
To date, examination of interactions between Drp1 and Mff have been complicated by the inherent flexibility of Mff9, the heterogeneity of Drp1 polymers2,10, and the difficulty in purifying and reconstituting full-length Mff with an intact transmembrane domain11. We addressed these challenges by using NTA(Ni2+) scaffold liposomes to reconstitute His-tagged Mff lacking its transmembrane domain(Mff&#916;TM-His6). This strategy was advantageous because Mff&#916;TM was extremely soluble when over-expressed in E. coli, and this isolated protein was easily reconstituted on scaffold liposomes. When tethered to these lipid templates, Mff assumed an identical, outward facing orientation on the surface of the membrane. In addition to these advantages, mitochondrial lipids, such as cardiolipin, were added to stabilize Mff folding and association with the membrane11. Cardiolipin also interacts with the variable domain of Drp12,12 which may stabilize this disordered region and facilitate assembly of the fission machinery.
This robust method is widely applicable for future studies that seek to evaluate membrane-proximal protein interactions. Through the use of additional tethering/affinity interactions, the sophistication of these membrane reconstitution studies can be enhanced to mimic additional complexity found at the surface of membranes within cells. At the same time, lipid compositions can be modified to more accurately mimic the native environments of these macromolecular complexes. In summary, this method provides a means to examine the relative contributions of proteins and lipids in shaping membrane morphologies to during critical cellular processes.

<table border="0" cellpadding="0" cellspacing="0" width="723">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Phosphatidylcholine (DOPC)</td>
			<td style="width:229px;">Avanti Polar Lipids</td>
			<td style="width:136px;">850375</td>
			<td style="width:72px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Phosphatidylethanolamine (DOPE)</td>
			<td style="width:229px;">Avanti Polar Lipids</td>
			<td style="width:136px;">850725</td>
			<td></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:287px;">DGS-NTA(Ni2+)</td>
			<td style="width:229px;">Avanti Polar Lipids</td>
			<td style="width:136px;">790404</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Bovine Heart Cardiolipin (CL)</td>
			<td style="width:229px;">Avanti Polar Lipids</td>
			<td style="width:136px;">840012</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Chloroform</td>
			<td style="width:229px;">Acros Organics</td>
			<td style="width:136px;">268320010</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Liposome Extruder</td>
			<td style="width:229px;">Avanti Polar Lipids</td>
			<td style="width:136px;">610023</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Cu/Rh Negative Stain Grids</td>
			<td style="width:229px;">Ted Pella</td>
			<td style="width:136px;">79712</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Microfuge Tube</td>
			<td style="width:229px;">Beckman</td>
			<td style="width:136px;">357448</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">GTP</td>
			<td style="width:229px;">Jena Biosciences</td>
			<td style="width:136px;">NU-1012</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">GMP-PCP</td>
			<td style="width:229px;">Sigma Aldrich</td>
			<td style="width:136px;">M3509</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Microtiter Plate strips</td>
			<td style="width:229px;">Thermo Scientific</td>
			<td style="width:136px;">469949</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">EDTA</td>
			<td style="width:229px;">Acros Organics</td>
			<td style="width:136px;">40993-0010</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Instant Blue Coomassie Dye</td>
			<td style="width:229px;">Expedeon</td>
			<td style="width:136px;">ISB1L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">HEPES</td>
			<td style="width:229px;">Fisher Scientific</td>
			<td style="width:136px;">BP310</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">BME</td>
			<td style="width:229px;">Sigma Aldrich</td>
			<td style="width:136px;">M6250</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">KCL</td>
			<td style="width:229px;">Fisher Scientific</td>
			<td style="width:136px;">P330</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">KOH</td>
			<td style="width:229px;">Fisher Scientific</td>
			<td style="width:136px;">P250</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Magnesium Chloride</td>
			<td style="width:229px;">Acros Organics</td>
			<td style="width:136px;">223211000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">4-20% SDS-PAGE Gel</td>
			<td style="width:229px;">Bio Rad</td>
			<td style="width:136px;">456-1096</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">4x Laemmli Loading Dye</td>
			<td style="width:229px;">Bio Rad</td>
			<td style="width:136px;">161-0747</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">HCL</td>
			<td style="width:229px;">Fisher Scientific</td>
			<td style="width:136px;">A144S</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Malachite Green Carbinol</td>
			<td style="width:229px;">Sigma Aldrich</td>
			<td style="width:136px;">229105</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Ammonium Molybdate Tetrahydrate</td>
			<td style="width:229px;">Sigma Aldrich</td>
			<td style="width:136px;">A7302</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Laboratory Film</td>
			<td style="width:229px;">Parafilm</td>
			<td style="width:136px;">PM-996</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Uranyl Acetate</td>
			<td style="width:229px;">Polysciences</td>
			<td style="width:136px;">21447</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Tecnai T12 100 keV Microscope</td>
			<td style="width:229px;">FEI</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Optima MAX</td>
			<td style="width:229px;">Beckman</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">TLA-55 Rotor</td>
			<td style="width:229px;">Beckman</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Refrigerated CentriVap Concentrator</td>
			<td style="width:229px;">Labconico</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">Mastercycler Pro Thermocycler</td>
			<td style="width:229px;">Eppendorf</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:287px;">VersaMax Microplate reader</td>
			<td style="width:229px;">Molecular Devices</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

using scaffold liposomes to reconstitute lipid-proximal protein-protein interactions in vitro

Studies of integral membrane proteins in vitro are frequently complicated by the presence of a hydrophobic transmembrane domain. Further complicating these studies, reincorporation of detergent-solubilized membrane proteins into liposomes is a stochastic process where protein topology is impossible to enforce. This paper offers an alternative method to these challenging techniques that utilizes a liposome-based scaffold. Protein solubility is enhanced by deletion of the transmembrane domain, and these amino acids are replaced with a tethering moiety, such as a His-tag. This tether interacts with an anchoring group (Ni2+ coordinated by nitrilotriacetic acid (NTA(Ni2+)) for His-tagged proteins), which enforces a uniform protein topology at the surface of the liposome. An example is presented wherein the interaction between Dynamin-related protein 1 (Drp1) with an integral membrane protein, Mitochondrial Fission Factor (Mff), was investigated using this scaffold liposome method. In this work, we have demonstrated the ability of Mff to efficiently recruit soluble Drp1 to the surface of liposomes, which stimulated its GTPase activity. Moreover, Drp1 was able to tubulate the Mff-decorated lipid template in the presence of specific lipids. This example demonstrates the effectiveness of scaffold liposomes using structural and functional assays and highlights the role of Mff in regulating Drp1 activity.

While the interaction between Drp1 and Mff has been demonstrated to be important for mitochondrial fission, this interaction has been difficult to recapitulate in vitro. Our goal was to better emulate the cellular environment wherein Drp1 and Mff interact. To this end, liposomes containing limiting concentrations of NTA(Ni2+) headgroups were prepared by rehydrating a lipid film as described above. The lipid solution initially consists of unilamellar and multilamellar v...

Watch this Scientific Journal Video about Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro at JoVE.com

Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro

This paper describes a method for assessing the interactions and assemblies of integral membrane proteins in vitro with various partner factors in a lipid-proximal environment.

Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro

Studies of integral membrane proteins in vitro are frequently complicated by the presence of a hydrophobic transmembrane domain. Further complicating ...

The overall goal of this technique is to capture lipid-proximal protein-protein interactions in vitro. This method can help answer questions in the mitochondrial dynamics field, such as how protein-protein interactions at the mitochondrial outer membrane can be affected by accessory lipids. The main advantage of this technique is that tethering soluble protein domains to lipid templates ensures that a more native confirmation can be achieved.Moreover, complex interactions between soluble proteins and their tethered partners can be assessed in the presence of defined lipid cofactors. Though we use this method to investigate protein interactions in the mitochondrial fission machinery, it can also be applied to other protein complexes that localize to diverse membrane compartments. Demonstrating this procedure will be Ryan, a grad student from my laboratory.First, combine chloroform solutions of the desired lipids in a clean glass test tube. Evaporate the solvent with dry nitrogen gas while rotating the tube to form a thin lipid film. Then remove the residual solvent with the centrifugal evaporator for one hour at 37 degrees Celsius.Following this, add pre-heated buffer A to give a final lipid concentration of one to two millimolar. Incubate for 30 minutes at 37 degrees Celsius with occasional vortexing to fully resuspend the lipid mixture. After incubation, transfer the lipid mixture to a plastic test tube and place the tube in liquid nitrogen for approximately 30 seconds until completely frozen.Then place the tube in a 37 degree Celsius waterbath for one to two minutes until fully thawed. Next prepare a lipid extruder by soaking four filter supports and a polycarbonate filter in buffer A and assembling the extruder according to the manufacturer's instructions. Extrude the lipid solution through the filter 21 times using gentle constant pressure to ensure a homogenous size distribution.While extruding, use slow constant pressure to ensure liposome size homogeneity. With a one micron filter, this is less important. But with smaller filter sizes, can introduce more apparent heterogeneity.Store the extruded liposomes at four degrees Celsius. Incubate His-tagged mitochondrial fission factor or Mff with scaffold liposomes for at least 15 minutes at room temperature in buffer A and BME. For an Mff-free control, incubate the liposomes with a His-tagged control protein, such as GFP, to bind and shield exposed Nickel-NTA.Add dynamin-related protein one or Drp1 to the liposome mixture and incubate for one hour at room temperature. Next, transfer five microliters of the sample to a sheet of laboratory film and lay a carbon-coated copper-rhodium grid on top of the sample. After incubating for one minute, remove the excess liquid with filter paper and add a drop of 2%uranyl acetate.After incubating for another minute, remove the excess stain with filter paper, and transfer the sample to a grid box. On the following day, image the samples using a transmission electron microscope at the 18, 500 to 30, 000X magnification to observe ultrastructural changes in protein and liposome morphologies. Incubate His-tagged Mff and Fis1 with the scaffold liposomes for 15 minutes at room temperature in buffer A and BME.Add Drp1 and incubate each mixture for an additional 15 minutes at room temperature. Transfer the tubes to the thermocycler set to 37 degrees Celsius and initiate the reactions by adding GTP and magnesium chloride. At the desired time points, transfer 20 microliters of each reaction mixture to wells of a microtiter plate containing five microliters of 0.5 molar EDTA to chelate the magnesium and stop the reaction.Following this, prepare a set of phosphate standards by diluting potassium phosphate in buffer A and BME to calibrate the results. Add 20 microliters of each standard to the wells containing five microliters of 0.5 molar EDTA. Add 150 microliters of malachite green reagent to each well.Finally, read the OD 650 five minutes after each addition. GTP will hydrolyze slowly in the presence of the acidic malachite green reagent. So this reagent should be added to all sample wells within 10 minutes of measuring the absorbance to maximize signal to noise.In the absence in Drp1, neither Mff nor GFP resulted in membrane deformation. And only featureless liposomes were observed for GFP-decorated enriched scaffold liposomes or ESL. However, when Drp1 was added to Mff-decorated ESL templates remodeling of the liposomes was evident.Decoration of the SL with Mff enhanced GTPase activity. Conversely, when the exposed NTA head groups were blocked with His-tagged GFP, this augmented GTPase activity was ablated. Tethering of Fis1 lacking its transmembrane domain to the SL, also failed to elicit a stimulation of Drp1's GTPase activity.Similar to the SL, addition of SL/Cl to Drp1 resulted in a slight stimulation of GTPase activity that was reversed by tethering His-tagged Fis1 or GFP to the liposomes. A synergy between Mff and cardiolipin was observed as the GTPase activity of Drp1 was stimulated 2.6-fold when it was incubated with Mff-decorated SL/CL. When SL/PE/CL templates were decorated with Mff, Drp1 activity was enhanced.The ability of Drp1 to remodel liposomes into lipid tubules was enhanced by the addition of PE to the scaffold liposomes. Once mastered, these scaffolds can be prepared and used in hours if the procedure is performed properly. Following this procedure, other protein and lipid interaction methods like sedimentation and light-scattering assays can be performed in order to characterize functional interactions.The application of this technique allows people examining protein translocation to a defined biological membrane to explore transient protein interactions and the impact of specific lipid cofactors in the formation and stabilization of essential cellular assemblies.

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Research

JoVE Journal

Biochemistry

Fluorescence Anisotropy as a Tool to Study Protein-protein Interactions

Protein-protein interactions play an essential role in the function of a living organism. Once an interaction has been identified and validated it is necessary to characterize it at the structural and mechanistic level. Several biochemical and biophysical methods exist for such purpose. Among them, fluorescence anisotropy is a powerful technique particularly used when the fluorescence intensity of a fluorophore-labeled protein remains constant upon protein-protein interaction. In this technique, a fluorophore-labeled protein is excited with vertically polarized light of an appropriate wavelength that selectively excites a subset of the fluorophores according to their relative orientation with the incoming beam. The resulting emission also has a directionality whose relationship in the vertical and horizontal planes defines anisotropy (r) as follows: r=(IVV-IVH)/(IVV+2IVH), where IVV and IVH are the fluorescence intensities of the vertical and horizontal components, respectively. Fluorescence anisotropy is sensitive to the rotational diffusion of a fluorophore, namely the apparent molecular size of a fluorophore attached to a protein, which is altered upon protein-protein interaction. In the present text, the use of fluorescence anisotropy as a tool to study protein-protein interactions was exemplified to address the binding between the protein mutated in the Shwachman-Diamond Syndrome (SBDS) and the Elongation factor like-1 GTPase (EFL1). Conventionally, labeling of a protein with a fluorophore is carried out on the thiol groups (cysteine) or in the amino groups (the N-terminal amine or lysine) of the protein. However, SBDS possesses several cysteines and lysines that did not allow site directed labeling of it. As an alternative technique, the dye 4&#39;,5&#39;-bis(1,3,2 dithioarsolan-2-yl) fluorescein was used to specifically label a tetracysteine motif, Cys-Cys-Pro-Gly-Cys-Cys, genetically engineered in the C-terminus of the recombinant SBDS protein. Fitting of the experimental data provided quantitative and mechanistic information on the binding mode between these proteins.

Protein interactions are at the heart of a cell&#39;s function. Calorimetric and spectroscopic techniques are commonly used to characterize them. Here we describe fluorescence anisotropy as a tool to study the interaction between the protein mutated in the Shwachman-Diamond Syndrome (SBDS) and the Elongation factor-like 1 GTPase (EFL1).

Protein-protein interactions play an essential role in the function of a living organism. Once an interaction has been identified and validated it is ...

Time-resolved ElectroSpray Ionization Hydrogen-deuterium Exchange Mass Spectrometry for Studying Protein Structure and Dynamics

Intrinsically disordered proteins (IDPs) have long been a challenge to structural biologists due to their lack of stable secondary structure elements. Hydrogen-Deuterium Exchange (HDX) measured at rapid time scales is uniquely suited to detect structures and hydrogen bonding networks that are briefly populated, allowing for the characterization of transient conformers in native ensembles. Coupling of HDX to mass spectrometry offers several key advantages, including high sensitivity, low sample consumption and no restriction on protein size. This technique has advanced greatly in the last several decades, including the ability to monitor HDX labeling times on the millisecond time scale. In addition, by incorporating the HDX workflow onto a microfluidic platform housing an acidic protease microreactor, we are able to localize dynamic properties at the peptide level. In this study, Time-Resolved ElectroSpray Ionization Mass Spectrometry (TRESI-MS) coupled to HDX was used to provide a detailed picture of residual structure in the tau protein, as well as the conformational shifts induced upon hyperphosphorylation.

Conformational flexibility plays a critical role in protein function. Herein, we describe the use of time-resolved electrospray ionization mass spectrometry coupled to hydrogen-deuterium exchange for probing the rapid structural changes that drive function in ordered and disordered proteins.

Intrinsically disordered proteins (IDPs) have long been a challenge to structural biologists due to their lack of stable secondary structure elements. ...

Biotinylated Cell-penetrating Peptides to Study Intracellular Protein-protein Interactions

Here we present a protocol to study intracellular protein-protein interactions that is based on the widely used biotin-avidin pull-down system. The modification presented includes the combination of this technique with cell-penetrating sequences. We propose to design cell-penetrating baits that can be incubated with living cells instead of cell lysates and therefore the interactions found will reflect those that occur within the intracellular context. Connexin43 (Cx43), a protein that forms gap junction channels and hemichannels is down-regulated in high-grade gliomas. The Cx43 region comprising amino acids 266-283 is responsible for the inhibition of the oncogenic activity of c-Src in glioma cells. Here we use TAT as the cell-penetrating sequence, biotin as the pull-down tag and the region of Cx43 comprised between amino acids 266-283 as the target to find intracellular interactions in the hard-to-transfect human glioma stem cells. One of the limitations of the proposed method is that the molecule used as bait could fail to fold properly and, consequently, the interactions found could not be associated with the effect. However, this method can be especially interesting for the interactions involved in signal transduction pathways because they are usually carried out by intrinsically disordered regions and, therefore, they do not require an ordered folding. In addition, one of the advantages of the proposed method is that the relevance of each residue on the interaction can be easily studied. This is a modular system; therefore, other cell-penetrating sequences, other tags, and other intracellular targets can be employed. Finally, the scope of this protocol is far beyond protein-protein interaction because this system can be applied to other bioactive cargoes such as RNA sequences, nanoparticles, viruses or any molecule that can be transduced with cell-penetrating sequences and fused to pull-down tags to study their intracellular mechanism of action.

This is a protocol to study intracellular protein-protein interactions based on the biotin-avidin pull-down system with the novelty of combining cell-penetrating sequences. The main advantage is that the target sequence is incubated with living cells instead of cell lysates and therefore the interactions will occur within the cellular context.

Here we present a protocol to study intracellular protein-protein interactions that is based on the widely used biotin-avidin pull-down system. The ...

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 ...

A Fluorescence Fluctuation Spectroscopy Assay of Protein-Protein Interactions at Cell-Cell Contacts

A variety of biological processes involves cell-cell interactions, typically mediated by proteins that interact at the interface between neighboring cells. Of interest, only few assays are capable of specifically probing such interactions directly in living cells. Here, we present an assay to measure the binding of proteins expressed at the surfaces of neighboring cells, at cell-cell contacts. This assay consists of two steps: mixing of cells expressing the proteins of interest fused to different fluorescent proteins, followed by fluorescence fluctuation spectroscopy measurements at cell-cell contacts using a confocal laser scanning microscope. We demonstrate the feasibility of this assay in a biologically relevant context by measuring the interactions of the amyloid precursor-like protein 1 (APLP1) across cell-cell junctions. We provide detailed protocols on the data acquisition using fluorescence-based techniques (scanning fluorescence cross-correlation spectroscopy, cross-correlation number and brightness analysis) and the required instrument calibrations. Further, we discuss critical steps in the data analysis and how to identify and correct external, spurious signal variations, such as those due to photobleaching or cell movement.
In general, the presented assay is applicable to any homo- or heterotypic protein-protein interaction at cell-cell contacts, between cells of the same or different types and can be implemented on a commercial confocal laser scanning microscope. An important requirement is the stability of the system, which needs to be sufficient to probe diffusive dynamics of the proteins of interest over several minutes.

This protocol describes a fluorescence fluctuation spectroscopy-based approach to investigate interactions among proteins mediating cell-cell interactions, i.e. proteins localized in cell junctions, directly in living cells. We provide detailed guidelines on instrument calibration, data acquisition and analysis, including corrections to possible artefact sources.

A variety of biological processes involves cell-cell interactions, typically mediated by proteins that interact at the interface between neighboring ...

Site-Directed Mutagenesis for In Vitro and In Vivo Experiments Exemplified with RNA Interactions in Escherichia Coli

Site-directed mutagenesis is a technique used to introduce specific mutations in DNA to investigate the interaction between small non-coding ribonucleic acid (sRNA) molecules and target messenger RNAs (mRNAs). In addition, site-directed mutagenesis is used to map specific protein binding sites to RNA. A 2-step and 3-step PCR based introduction of mutations is described. The approach is relevant to all protein-RNA and RNA-RNA interaction studies. In short, the technique relies on designing primers with the desired mutation(s), and through 2 or 3 steps of PCR synthesizing a PCR product with the mutation. The PCR product is then used for cloning. Here, we describe how to perform site-directed mutagenesis with both the 2- and 3-step approach to introduce mutations to the sRNA, McaS, and the mRNA, csgD, to investigate RNA-RNA and RNA-protein interactions. We apply this technique to investigate RNA interactions; however, the technique is applicable to all mutagenesis studies (e.g., DNA-protein interactions, amino-acid substitution/deletion/addition). It is possible to introduce any kind of mutation except for non-natural bases but the technique is only applicable if a PCR product can be used for downstream application (e.g., cloning and template for further PCR).

Site-Directed Mutagenesis for In Vitro and In Vivo Experiments Exemplified with RNA Interactions in Escherichia Coli

Site-directed mutagenesis is a technique used to introduce specific mutations in deoxyribonucleic acid (DNA). This protocol describes how to do site-directed mutagenesis with a 2-step and 3-step polymerase chain reaction (PCR) based approach, which is applicable to any DNA fragment of interest.

Site-directed mutagenesis is a technique used to introduce specific mutations in DNA to investigate the interaction between small non-coding ribonucleic ...

Application of Biolayer Interferometry (BLI) for Studying Protein-Protein Interactions in Transcription

A transcription factor&#160;(TF) is a protein that regulates gene expression by interacting with the RNA polymerase, another TF, and/or template DNA. GrgA is a novel transcription activator found specifically in the obligate intracellular bacterial pathogen Chlamydia. Protein pulldown assays using affinity beads have revealed that GrgA binds two &#963; factors, namely &#963;66 and &#963;28, which recognize different sets of promoters for genes whose products are differentially required at developmental stages. We have used BLI to confirm and further characterize the interactions. BLI demonstrates several advantages over pulldown: 1) It reveals real-time association and dissociation between binding partners, 2) It generates quantitative kinetic parameters, and 3) It can detect bindings that pulldown assays often fail to detect. These characteristics have enabled us to deduce the physiological roles of GrgA in gene expression regulation in Chlamydia, and possible detailed interaction mechanisms. We envision that this relatively affordable technology can be extremely useful for studying transcription and other biological processes.

Application of Biolayer Interferometry BLI for Studying Protein-Protein Interactions in Transcription

Interactions of transcription factors (TFs) with the RNA polymerase are usually studied using pulldown assays. We apply a Biolayer Interferometry (BLI) technology to characterize the interaction of GrgA with the chlamydial RNA polymerase. Compared to pulldown assays, BLI detects real-time association and dissociation, offers higher sensitivity, and is highly quantitative.

A transcription factor&#160;(TF) is a protein that regulates gene expression by interacting with the RNA polymerase, another TF, and/or template DNA. GrgA ...

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

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

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

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

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

Dual-Color Fluorescence Cross-Correlation Spectroscopy to Study Protein-Protein Interaction and Protein Dynamics in Live Cells

We present a protocol and workflow to perform live cell dual-color fluorescence cross-correlation spectroscopy (FCCS) combined with F&#246;rster Resonance Energy transfer (FRET) to study membrane receptor dynamics in live cells using modern fluorescence labeling techniques. In dual-color FCCS, where the fluctuations in fluorescence intensity represent the dynamic &#34;fingerprint&#34; of the respective fluorescent biomolecule, we can probe co-diffusion or binding of the receptors. FRET, with its high sensitivity to molecular distances, serves as a well-known &#34;nanoruler&#34; to monitor intramolecular changes. Taken together, conformational changes and key parameters such as local receptor concentrations and mobility constants become accessible in cellular settings.
Quantitative fluorescence approaches are challenging in cells due to high noise levels and the vulnerability of the sample. Here we show how to perform this experiment, including the calibration steps using dual-color labeled &#946;2-adrenergic receptor (&#946;2AR) labeled with eGFP and SNAP-tag-TAMRA. A step-by-step data analysis procedure is provided using open-source software and templates that are easy to customize.
Our guideline enables researchers to unravel molecular interactions of biomolecules in live cells in situ with high reliability despite the limited signal-to-noise levels in live cell experiments. The operational window of FRET and particularly FCCS at low concentrations allows quantitative analysis at near-physiological conditions.

We present an experimental protocol and data analysis workflow to perform live cell dual-color fluorescence cross correlation spectroscopy (FCCS) combined with F&#246;rster Resonance Energy transfer (FRET) to study membrane receptor dynamics in live cells using modern fluorescence labeling techniques.

We present a protocol and workflow to perform live cell dual-color fluorescence cross-correlation spectroscopy (FCCS) combined with F&#246;rster Resonance ...

Pulldown Assay Coupled with Co-Expression in Bacteria Cells as a Time-Efficient Tool for Testing Challenging Protein-Protein Interactions

Pulldown is an easy and widely used protein-protein interaction assay. However, it has limitations in studying protein complexes that do not assemble effectively in vitro. Such complexes may require co-translational assembly and the presence of molecular chaperones; either they form stable oligomers which cannot dissociate and re-associate in vitro or are unstable without a binding partner. To overcome these problems, it is possible to use a method based on the bacterial co-expression of differentially tagged proteins using a set of compatible vectors followed by the conventional pulldown techniques. The workflow is more time-efficient compared to traditional pulldown because it lacks the time-consuming steps of separate purification of interacting proteins and their following incubation. Another advantage is a higher reproducibility due to a significantly smaller number of steps and a shorter period of time in which proteins that exist within the in vitro environment are exposed to proteolysis and oxidation. The method was successfully applied for studying a number of protein-protein interactions when other in vitro techniques were found to be unsuitable. The method can be used for batch testing protein-protein interactions. Representative results are shown for studies of interactions between BTB domain and intrinsically disordered proteins, and of heterodimers of zinc-finger-associated domains.

Here, we describe a method for the bacterial co-expression of differentially tagged proteins using a set of compatible vectors, followed by the conventional pulldown techniques to study protein complexes that cannot assemble in vitro.

Pulldown is an easy and widely used protein-protein interaction assay. However, it has limitations in studying protein complexes that do not assemble ...