This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (#19H03430 and #17K09405 to M.O., and #19J11829 to K.S.), by a Grant-in-Aid for Scientific Research on Innovative Areas (#18H05024 to M.O.), by the Research Program on Hepatitis from Japan Agency for Medical Research and Development, AMED (to M.O., #JP19fk021005), by program on the Innovative Development and the Application of New Drugs for Hepatitis B (#JP19fk0310102 to K.K.) from AMED, by grants from the Japan Foundation for Applied Enzymology and from the Kobayashi Foundation for Cancer Research (to M.O.), by GSK Japan Research Grant 2018 (to K.S.), and by a grant from the Miyakawa Memorial Research Foundation (to K.S.).

NOTE: A schematic representation of the split luciferase assay is shown in Figure 1A, and the assay process is outlined in Figure 1B. The interaction dynamics can be measured in real time without cell lysis.
1. Cell Preparation
<ol>
	<li>Maintain cultured HEK293T cells in Dulbecco&#39;s modified Eagle&#39;s medium (DMEM) supplemented with 10% v/v fetal bovine serum (FBS), 1x penicillin/streptomycin at 37 &#176;C in 20% O2 and 5% CO2.</li>
	<li>Seed 5 x 106 cells into a 100 mm dish with 10 mL of DMEM and incubate at 37 &#176;C overnight.</li>
	<li>Transiently transfect 1 &#181;g of HBx and DDB1 with split luciferases into the cells according to the following method. 
	NOTE: The amount of the plasmid DNA transfected may depend on the transfection regent used. The optimal position of the split luciferase fused to the target protein must be determined beforehand. In this case, HBx fused to LgBit at the C-terminus of HBx (HBx-LgBit) and DDB1 fused to SmBit at the N-terminus of DDB1 (SmBit-DDB1) provided the best results (i.e., the brightest luciferase signals). This process has been reported previously in detail7.
	<ol>
		<li>Dilute 1 &#181;g of HBx-LgBit expressing DNA plasmid and 1 &#181;g of SmBit-DDB1 expressing DNA plasmid (Table of Materials) in DNA condensation buffer (Table of Materials) to a total volume of 300 &#181;L.</li>
		<li>Add 16 &#181;L of enhancer solution (Table of Materials) and mix by vortexing for 1 s.</li>
		<li>Incubate the sample at room temperature for 3 min.</li>
		<li>Add 60 &#181;L of transfection reagent (Table of Materials) to the sample and mix by vortexing for 10 s.</li>
		<li>Incubate the sample at room temperature for 8 min.</li>
		<li>During incubation, aspirate the culture medium from the dish (prepared in step 1.2), and wash cells with 5 mL of phosphate-buffered saline (PBS). Remove PBS by aspiration and add 7 mL of DMEM.</li>
		<li>Add 3 mL of DMEM to the tube containing the transfection complexes. Mix by pipetting and add the transfection complexes onto the cell in the 10 cm dish.</li>
	</ol>
	</li>
	<li>Incubate the cells at 37 &#176;C in an incubator under 5% CO2 for 10 h.</li>
	<li>Reseed the cells into a white 96 well plate at 5 x 104 cells/well in 50 &#181;L of medium/well according to the following method.
	<ol>
		<li>Remove the spent cell culture medium and wash cells with 5 mL of PBS.</li>
		<li>Remove PBS by aspiration, add 1 mL of 0.25% trypsin-EDTA, and incubate at 37 &#176;C for 5 min to detach cells.</li>
		<li>Add 4 mL of DMEM and disperse the medium by pipetting over the surface of the cell layer several times. Transfer the cell suspension to a tube.</li>
		<li>Centrifuge cells at 500 x g for 5 min at room temperature.</li>
		<li>Discard supernatant and resuspend the cell pellet in 1 mL of PBS.</li>
		<li>Centrifuge the cell suspension at 500 x g for 5 min at room temperature and discard supernatant.</li>
		<li>Dilute the cell pellet with buffered cell culture medium (Table of Materials) supplemented with 10% FBS to a seeding density of 1.0 x 106 cells/mL.</li>
		<li>Pipette 50 &#181;L of cell suspension into each well of a 96 well plate and return the cells to the incubator.</li>
	</ol>
	</li>
	<li>Incubate cells at 37 &#176;C under 5% CO2 for 10 h.</li>
</ol>
2. Compound Screening
<ol>
	<li>During the incubation, dilute the screening compounds (Table of Materials) and solvent (dimethyl sulfoxide [DMSO]) at 13.5x concentration. For example, if the stock is 10 mM and the screening concentration is 10 &#181;M, add 1 &#181;L of stock solution to 73.1 &#181;L of buffered cell culture medium.</li>
	<li>Add 12.5 &#181;L of luminescent substrate (Table of Materials) to each well and incubate for 5 min at room temperature. 
	NOTE: As negative controls, the wells at both ends of the plate (i.e., columns 1 and 12) should contain no luminescent substrate.</li>
	<li>Measure the baseline luminescence using a luminometer (Table of Materials).</li>
	<li>Immediately after the initial measurement, add 5 &#181;L of compounds and control DMSO diluted in step 2.1 to each well. 
	NOTE: The final concentration will be 10 &#181;M.</li>
	<li>Measure luminescence values every 30 min for 2 h. 
	NOTE: The plate should be incubated in the dark at room temperature.</li>
	<li>Calculate the inhibitory effects by comparison with control DMSO treatment after normalization to the baseline signals. 
	NOTE: Screening each compound in duplicate or triplicate can reduce variation.</li>
</ol>

<ol>
	<li>Tang, L. S. Y., Covert, E., Wilson, E., Kottilil, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chronic+hepatitis+B+infection:+a+review.">Chronic hepatitis B infection: a review.</a> The Journal of the American Medical Association. 319, 1802-1813 (2018).</li><li>Sekiba, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hepatitis+B+virus+pathogenesis:+Fresh+insights+into+hepatitis+B+virus+RNA.">Hepatitis B virus pathogenesis: Fresh insights into hepatitis B virus RNA.</a> World Journal of Gastroenterology. 24, 2261-2268 (2018).</li><li>Slagle, B. L., Bouchard, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hepatitis+B+virus+X+and+regulation+of+viral+gene+expression.">Hepatitis B virus X and regulation of viral gene expression.</a> Cold Spring Harbor Perspectives in Medicine. 6, a021402 (2016).</li><li>Murphy, C. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hepatitis+B+Virus+X+Protein+Promotes+Degradation+of+SMC5/6+to+Enhance+HBV+Replication.">Hepatitis B Virus X Protein Promotes Degradation of SMC5/6 to Enhance HBV Replication.</a> Cell Reports. 16, 2846-2854 (2016).</li><li>Decorsi&#232;re, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hepatitis+B+virus+X+protein+identifies+the+Smc5/6+complex+as+a+host+restriction+factor.">Hepatitis B virus X protein identifies the Smc5/6 complex as a host restriction factor.</a> Nature. 531, 386-389 (2016).</li><li>Niu, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Smc5/6+complex+restricts+HBV+when+localized+to+ND10+without+inducing+an+innate+immune+response+and+is+counteracted+by+the+HBV+X+protein+shortly+after+infection.">The Smc5/6 complex restricts HBV when localized to ND10 without inducing an innate immune response and is counteracted by the HBV X protein shortly after infection.</a> PLoS One. 12, e0169648 (2017).</li><li>Sekiba, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inhibition+of+HBV+Transcription+From+cccDNA+With+Nitazoxanide+by+Targeting+the+HBx-DDB1+Interaction.">Inhibition of HBV Transcription From cccDNA With Nitazoxanide by Targeting the HBx-DDB1 Interaction.</a> Cellular and Molecular Gastroenterology and Hepatology. 7, 297-312 (2019).</li><li>Eggers, C. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NanoLuc+Complementation+Reporter+Optimized+for+Accurate+Measurement+of+Protein+Interactions+in+Cells.">NanoLuc Complementation Reporter Optimized for Accurate Measurement of Protein Interactions in Cells.</a> ACS Chemical Biology. 11, 400-408 (2015).</li><li>Zhang, J. H., Chung, T. D. Y., Oldenburg, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+statistical+parameter+for+use+in+evaluation+and+validation+of+high+throughput+screening+assays.">A simple statistical parameter for use in evaluation and validation of high throughput screening assays.</a> Journal of Biomolecular Screening. 4, 67-73 (1999).</li><li>Rao, V. S., Srinivas, K., Sujini, G. N., Kumar, G. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein-protein+interaction+detection:+Methods+and+analysis.">Protein-protein interaction detection: Methods and analysis.</a> International Journal of Proteomics. 2014. 2014, 147648 (2014).</li><li>Michael, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Robotic+Platform+for+Quantitative+High-Throughput+Screening.">A Robotic Platform for Quantitative High-Throughput Screening.</a> ASSAY and Drug Development Technologies. 6, 637-657 (2008).</li><li>Skwarczynska, M., Ottmann, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein-protein+interactions+as+drug+targets.">Protein-protein interactions as drug targets.</a> Future Medicinal Chemistry. 7, 2195-2219 (2015).</li><li>de Chassey, B., Meyniel-Schicklin, L., Vonderscher, J., Andr&#233;, P., Lotteau, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Virus-host+interactomics:+New+insights+and+opportunities+for+antiviral+drug+discovery.">Virus-host interactomics: New insights and opportunities for antiviral drug discovery.</a> Genome Medicine. 6, 115 (2014).</li><li>Prasad, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Virus-Host+Interactions:+New+Insights+and+Advances+in+Drug+Development+Against+Viral+Pathogens.">Virus-Host Interactions: New Insights and Advances in Drug Development Against Viral Pathogens.</a> Current Drug Metabolism. 18, 942-970 (2017).</li></ol>

The authors have nothing to disclose.

We developed a convenient screening method using a split luciferase assay to find HBx-DDB1 binding inhibitors. The interaction dynamics can be detected in real time in living cells without the need for cell lysis. Inhibition of the HBx-DDB1 interaction leads to restoration of Smc5/6, which results in suppression of viral transcription, protein expression, and cccDNA production7. This novel mechanism of antiviral action may overcome the inadequacies of current HBV therapies.
<p class="jove_cont...

Hepatitis B virus (HBV) infection is a major public health concern worldwide, with annual estimates of 240 million people chronically infected with HBV and 90,000 deaths due to complications from the infection, including cirrhosis and hepatocellular carcinoma (HCC)1. Although the current anti-HBV therapeutic agents, nucleos(t)ide analogues, sufficiently inhibit viral reverse transcription, they rarely achieve elimination of viral proteins, which is the long-term clinical goal. Their poor effect on viral protein elimination is due to their lack of direct effect on viral transcription from episomal viral covalently closed circular DNA (cccDNA) minichromosomes in the hepatocyte nucleus2.
HBV transcription is activated by HBV regulatory X (HBx) protein3. Recent studies revealed that HBx degrades structural maintenance of chromosomes 5/6 (Smc5/6), a host restriction factor that blocks HBV transcription from cccDNA, via hijacking a DDB1-CUL4-ROC1 E3 ubiquitin ligase complex4,5,6. Therefore, a crucial step in promoting viral transcription from cccDNA is thought to be the HBx-DDB1 interaction. Compounds capable of inhibiting the binding between HBx and DDB1 may block viral transcription, and indeed nitazoxanide was identified as an inhibitor of the HBx-DDB1 interaction via a screening system developed in our laboratory7.
Here, we present our convenient screening system used to identify inhibitors of the HBx-DDB1 interaction, which utilizes a split luciferase complementary assay7,8. Split luciferase subunits are fused to HBx and DDB1, and the HBx-DDB1 interaction brings the subunits into close proximity to form a functional enzyme that generates a bright luminescent signal. As the interaction between the subunits is reversible, this system can detect rapidly dissociating HBx-DDB1 proteins (Figure 1). Using this system, a large compound library can be easily screened, which may result in the discovery of novel compounds capable of efficiently inhibiting the HBx-DDB1 interaction.

<table><tbody><tr><td>Cell culture microplate, 96 well, PS, F-BOTTOM</td><td>Greiner-Bio-One GmbH</td><td>655098</td><td></td></tr><tr><td>DMEM</td><td>Sigma Aldrich</td><td>D6046</td><td></td></tr><tr><td>DMSO</td><td>Tocris Bioscience</td><td>3176</td><td></td></tr><tr><td>Effectene transfection reagent</td><td>Qiagen</td><td>301425</td><td>Includes DNA-condensation buffer, enhancer solution and transfection reagent</td></tr><tr><td>FBS</td><td>Nichirei</td><td>175012</td><td></td></tr><tr><td>GloMax 96 microplate luminometer</td><td>Promega</td><td>E6521</td><td></td></tr><tr><td>HBx&amp;ndash;LgBit expressing DNA plasmid</td><td>Our laboratory</td><td></td><td>Available upon request</td></tr><tr><td>HEK293T cells</td><td>American Type Culture Collection</td><td>CRL-11268</td><td></td></tr><tr><td>NanoBiT PPI starter systems</td><td>Promega</td><td>N2015</td><td>Includes Nano-Glo Live Cell Reagent</td></tr><tr><td>Opti-MEM</td><td>Thermo Fisher Scientific</td><td>11058021</td><td>Described as &quot;buffered cell culture medium&quot; in the manuscript</td></tr><tr><td>PBS</td><td>Takara</td><td>T900</td><td></td></tr><tr><td>Penicillin-Streptomycin</td><td>Sigma Aldrich</td><td>P0781</td><td></td></tr><tr><td>Screen-Well FDA-approved drug library V2 version 1.0</td><td>Enzo Life Sciences</td><td>BML-2841</td><td>Compounds used here were as follows: mequinol, mercaptopurine hydrate, mesna, mestranol, metaproterenol hemisulfate, metaraminol bitartrate, metaxalone, methacholine chloride, methazolamide, methenamine hippurate, methocarbamol, methotrexate, methoxsalen, methscopolamine bromide, methsuximide, methyclothiazide, methyl aminolevulinate&amp;middot;HCl, methylergonovine maleate, metolazone, metyrapone, mexiletine&amp;middot;HCl, micafungin, miconazole, midodrine&amp;middot;HCl, miglitol, milnacipran&amp;middot;HCl, mirtazapine, mitotane, moexipril&amp;middot;HCl, mometasone furoate, mupirocin, nadolol, nafcillin&amp;middot;Na, naftifine&amp;middot;HCl, naratriptan&amp;middot;HCl, natamycin, nebivolol&amp;middot;HCl, nelarabine, nepafenac, nevirapine, niacin, nicotine, nilotinib, nilutamide, nitazoxanide, nitisinone, nitrofurantoin, nizatidine, nortriptyline&amp;middot;HCl, olsalazine&amp;middot;Na, orlistat, oxaprozin, oxtriphylline, oxybutynin Chloride, oxytetracycline&amp;middot;HCl, paliperidone, palonosetron&amp;middot;HCl, paromomycin sulfate, pazopanib&amp;middot;HCl, pemetrexed disodium, pemirolast potassium, penicillamine, penicillin G potassium, pentamidine isethionate, pentostatin, perindopril erbumine, permethrin, perphenazine, phenelzine sulfate, phenylephrine, phytonadione, pimecrolimus, pitavastatin calcium, and podofilox</td></tr><tr><td>SmBit&amp;ndash;DDB1 expressing DNA plasmid</td><td>Our laboratory</td><td></td><td>Available upon request</td></tr><tr><td>Trypsin-EDTA</td><td>Sigma Aldrich</td><td>T4049</td><td></td></tr></tbody></table>

identifying inhibitors of the hbx-ddb1 interaction using a split luciferase assay system

There is an urgent need for novel therapeutic agents for hepatitis B virus (HBV) infection. Although currently available nucleos(t)ide analogs potently inhibit viral replication, they have no direct effect on the expression of viral proteins transcribed from a viral covalently closed circular DNA (cccDNA). As high viral antigen load may play a role in this chronic and HBV-related carcinogenesis, the goal of HBV treatment is to eradicate viral proteins. HBV regulatory protein X (HBx) binds to the host DNA damage-binding protein 1 (DDB1) protein to degrade structural maintenance of chromosomes 5/6 (Smc5/6), resulting in activation of viral transcription from cccDNA. Here, using a split luciferase complementation assay system, we present a comprehensive compound screening system to identify inhibitors of the HBx-DDB1 interaction. Our protocol enables easy detection of interaction dynamics in real time within living cells. This technique may become a key assay to discover novel therapeutic agents for treatment of HBV infection.

Representative outcomes following the use of this protocol are shown in Figure 2A,B. The signal-to-background ratio was greater than 80 and the Z&#39; factor9 (the gold standard quality index for high-throughput screening) was greater than 0.5, indicating that this assay system was acceptable for high-throughput screening. With the threshold set to &#62;40% inhibition compared with the control (DMSO only), we identifie...

Watch this Scientific Journal Video about Identifying Inhibitors of the HBx-DDB1 Interaction Using a Split Luciferase Assay System at JoVE.com

Identifying Inhibitors of the HBx-DDB1 Interaction Using a Split Luciferase Assay System

Here, we present a method for screening anti-hepatitis B viral agents that inhibit the HBx-DDB1 interaction using a split luciferase assay system. This system allows easy detection of protein-protein interactions and is suitable for identifying inhibitors of such interactions.

There is an urgent need for novel therapeutic agents for hepatitis B virus (HBV) infection. Although currently available nucleos(t)ide analogs potently ...

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6.7K Views.  The University of Tokyo. Here, we present a method for screening anti-hepatitis B viral agents that inhibit the HBx-DDB1 interaction using a split luciferase assay system. This system allows easy detection of protein-protein interactions and is suitable for identifying inhibitors of such interactions.

Video: Identifying Inhibitors of the HBx-DDB1 Interaction Using a Split Luciferase Assay System

Monitoring Hippo Signaling Pathway Activity Using a Luciferase-based Large Tumor Suppressor (LATS) Biosensor

The Hippo signaling pathway is a conserved regulator of organ size and has important roles in the development and cancer biology. Due to technical challenges, it remains difficult to assess the activity of this signaling pathway and interpret it within a biological context. The existing literature on large tumor suppressor (LATS) relies on methods that are qualitative and cannot easily be scaled-up for screening. Recently, we have developed a bioluminescence-based biosensor to monitor the kinase activity of LATS-a core component of the Hippo kinase cascade. Here, we describe procedures for how this LATS biosensor (LATS-BS) can be used to characterize Hippo pathway regulators. First, we provide a detailed protocol for investigating the effect of an overexpressed protein candidate (e.g., VEGFR2) on LATS activity using the LATS-BS. Then, we show how the LATS-BS can be used for a small-scale kinase inhibitor screen. This protocol can feasibly be scaled-up to perform larger screens, which undoubtedly will identify novel regulators of the Hippo pathway.

Monitoring Hippo Signaling Pathway Activity Using a Luciferase-based Large Tumor Suppressor LATS Biosensor

Here we present a luciferase-based biosensor to quantify the kinase activity of large tumor suppressor (LATS)-a central kinase in the Hippo signaling pathway. This biosensor has diverse applications in basic and translational research aimed at investigating Hippo pathway regulators in vitro and in vivo.

The Hippo signaling pathway is a conserved regulator of organ size and has important roles in the development and cancer biology. Due to technical ...

Cancer Research

Split-BioID &#8212; Proteomic Analysis of Context-specific Protein Complexes in Their Native Cellular Environment

To complement existing affinity purification (AP) approaches for the identification of protein-protein interactions (PPI), enzymes have been introduced that allow the proximity-dependent labeling of proteins in living cells. One such enzyme, BirA* (used in the BioID approach), mediates the biotinylation of proteins within a range of approximately 10 nm. Hence, when fused to a protein of interest and expressed in cells, it allows the labeling of proximal proteins in their native environment. As opposed to AP that relies on the purification of assembled protein complexes, BioID detects proteins that have been marked within cells no matter whether they are still interacting with the protein of interest when they are isolated. Since it biotinylates proximal proteins, one can moreover capitalize on the exceptional affinity of streptavidin for biotin to very efficiently isolate them. While BioID performs better than AP for identifying transient or weak interactions, both AP- and BioID-mass spectrometry approaches provide an overview of all possible interactions a given protein may have. However, they do not provide information on the context of each identified PPI. Indeed, most proteins are typically part of several complexes, corresponding to distinct maturation steps or different functional units. To address this common limitation of both methods, we have engineered a protein-fragments complementation assay based on the BirA* enzyme. In this assay, two inactive fragments of BirA* can reassemble into an active enzyme when brought in close proximity by two interacting proteins to which they are fused. The resulting split-BioID assay thus allows the labeling of proteins that assemble around a pair of interacting proteins. Provided these two only interact in a given context, split-BioID then allows the analysis of specific context-dependent functional units in their native cellular environment. Here, we provide a step-by-step protocol to test and apply split-BioID to a pair of interacting proteins.

We provide a step-by-step protocol for split-BioID, a protein fragments-complementation assay based on the proximity-labeling technique BioID. Activated on the interaction of two given proteins, it allows the proteomics analysis of context-dependent protein complexes in their native cellular environment. The method is simple, cost-effective and only requires standard laboratory equipment.

To complement existing affinity purification (AP) approaches for the identification of protein-protein interactions (PPI), enzymes have been introduced ...

Biochemistry

Characterize Disease-related Mutants of RAF Family Kinases by Using a Set of Practical and Feasible Methods

The rapidly accelerated fibrosarcoma (RAF) family kinases play a central role in cell biology and their dysfunction leads to cancers and developmental disorders. A characterization of disease-related RAF mutants will help us select appropriate therapeutic strategies for treating these diseases. Recent studies have shown that RAF family kinases have both catalytic and allosteric activities, which are tightly regulated by dimerization. Here, we constructed a set of practical and feasible methods to determine the catalytic and allosteric activities and the relative dimer affinity/stability of RAF family kinases and their mutants. Firstly, we amended the classical in vitro kinase assay by reducing the detergent concentration in buffers, utilizing a gentle quick wash procedure, and employing a glutathione S-transferase (GST) fusion to prevent RAF dimers from dissociating during purification. This enables us to measure the catalytic activity of constitutively active RAF mutants appropriately. Secondly, we developed a novel RAF co-activation assay to evaluate the allosteric activity of kinase-dead RAF mutants by using N-terminal truncated RAF proteins, eliminating the requirement of active Ras in current protocols and thereby achieving a higher sensitivity. Lastly, we generated a unique complementary split luciferase assay to quantitatively measure the relative dimer affinity/stability of various RAF mutants, which is more reliable and sensitive compared to the traditional co-immunoprecipitation assay. In summary, these methods have the following advantages: (1) user-friendly; (2) able to carry out effectively without advanced equipment; (3) cost-effective; (4) highly sensitive and reproducible.

In this article, we presented a set of practical and feasible methods for characterizing disease-related mutants of RAF family kinases, which include in vitro kinase assay, RAF co-activation assay, and complementary split luciferase assay.

The rapidly accelerated fibrosarcoma (RAF) family kinases play a central role in cell biology and their dysfunction leads to cancers and developmental ...

A Multiplexed Luciferase-based Screening Platform for Interrogating Cancer-associated Signal Transduction in Cultured Cells

Genome-scale interrogation of gene function using RNA interference (RNAi) holds tremendous promise for the rapid identification of chemically tractable cancer cell vulnerabilities. Limiting the potential of this technology is the inability to rapidly delineate the mechanistic basis of phenotypic outcomes and thus inform the development of molecularly targeted therapeutic strategies. We outline here methods to deconstruct cellular phenotypes induced by RNAi-mediated gene targeting using multiplexed reporter systems that allow monitoring of key cancer cell-associated processes. This high-content screening methodology is versatile and can be readily adapted for the screening of other types of large molecular libraries.

Achieving a systems level understanding of cellular processes is a goal of modern-day cell biology. We describe here strategies for multiplexing luciferase reporters of various cellular function end-points to interrogate gene function using genome-scale RNAi libraries.

Genome-scale interrogation of gene function using RNA interference (RNAi)  holds tremendous promise for the rapid identification of chemically tractable  ...

A Simple Assay to Measure ER-Mitochondria Contacts for High-Throughput Screens

Split-Luciferase Reassembly Assay to Measure Endoplasmic Reticulum-Mitochondria Contacts in Live Cells

Endoplasmic reticulum (ER)-mitochondria contact sites play a critical role in cell health and homeostasis, such as the regulation of Ca2+ and lipid homeostasis, mitochondrial dynamics, autophagosome and mitophagosome biogenesis, and apoptosis. Failure to maintain normal ER-mitochondrial coupling is implicated in many neurodegenerative diseases such as Alzheimer&#39;s disease, Parkinson&#39;s disease, amyotrophic lateral sclerosis, and hereditary spastic paraplegia. It is of considerable significance to explore how the dysregulation of ER-mitochondrial contacts could lead to cell death and whether repairing these contacts to the normal level could ameliorate neurodegenerative conditions. Thus, improved assays that measure the level of these contacts could help to illuminate the pathogenic mechanisms of these diseases. Ultimately, establishing simple and reliable assays will facilitate the development of new therapeutic strategies. Here we describe&#160;a split-luciferase assay to quantitatively measure the level of ER-mitochondria contacts in live cells. This assay can be used to study the pathophysiological role of these contacts as well as to identify their modulators in high-throughput screening.

Author Spotlight: Regulation and Dysregulation of ER-Mitochondria Contacts &#8212; Implications for Neurodegenerative Disease Pathogenesis

We have established a split-luciferase reassembly assay to monitor the endoplasmic reticulum-mitochondria contacts in live cells. Using this assay, we describe a protocol to quantitatively measure the level of these inter-organelle couplings in HEK293T cells, under the condition of chemical treatment.

Endoplasmic reticulum (ER)-mitochondria contact sites play a critical role in cell health and homeostasis, such as the regulation of Ca2+ and lipid ...

Identifying Targets of Human microRNAs with the LightSwitch Luciferase Assay System using 3'UTR-reporter Constructs and a microRNA Mimic in Adherent Cells

MicroRNAs (miRNAs) are important regulators of gene expression and play a role in many biological processes. More than 700 human miRNAs have been identified so far with each having up to hundreds of unique target mRNAs. Computational tools, expression and proteomics assays, and chromatin-immunoprecipitation-based techniques provide important clues for identifying mRNAs that are direct targets of a particular miRNA. In addition, 3'UTR-reporter assays have become an important component of thorough miRNA target studies because they provide functional evidence for and quantitate the effects of specific miRNA-3'UTR interactions in a cell-based system. To enable more researchers to leverage 3'UTR-reporter assays and to support the scale-up of such assays to high-throughput levels, we have created a genome-wide collection of human 3'UTR luciferase reporters in the highly-optimized LightSwitch Luciferase Assay System. The system also includes synthetic miRNA target reporter constructs for use as positive controls, various endogenous 3'UTR reporter constructs, and a series of standardized experimental protocols.
Here we describe a method for co-transfection of individual 3'UTR-reporter constructs along with a miRNA mimic that is efficient, reproducible, and amenable to high-throughput analysis.

MicroRNAs (miRNAs) are important regulators of gene expression and have been shown to play a role in numerous biological processes. To better understand miRNA-UTR interactions, we have created a genome-wide collection of 3 UTR luciferase reporters paired with a novel luciferase gene and assay reagent, the LightSwitch system.

MicroRNAs (miRNAs) are important regulators of gene expression and play a role in many biological processes.  More than 700 human miRNAs have been ...

Biology

Reconstitution Of &#946;-catenin Degradation In Xenopus Egg Extract

Xenopus laevis egg extract is a well-characterized, robust system for studying the biochemistry of diverse cellular processes. Xenopus egg extract has been used to study protein turnover in many cellular contexts, including the cell cycle and signal transduction pathways1-3. Herein, a method is described for isolating Xenopus egg extract that has been optimized to promote the degradation of the critical Wnt pathway component, &beta;-catenin. Two different methods are described to assess &beta;-catenin protein degradation in Xenopus egg extract. One method is visually informative ([35S]-radiolabeled proteins), while the other is more readily scaled for high-throughput assays (firefly luciferase-tagged fusion proteins). The techniques described can be used to, but are not limited to, assess &beta;-catenin protein turnover and identify molecular components contributing to its turnover. Additionally, the ability to purify large volumes of homogenous Xenopus egg extract combined with the quantitative and facile readout of luciferase-tagged proteins allows this system to be easily adapted for high-throughput screening for modulators of &beta;-catenin degradation.

Reconstitution Of &#946;-catenin Degradation In Xenopus Egg Extract

A method is described for analyzing protein degradation using radiolabeled and luciferase-fusion proteins in Xenopus egg extract and its adaptation for high-throughput screening for small molecule modulators of protein degradation.

Xenopus laevis egg extract is a well-characterized, robust system for studying the biochemistry of diverse cellular processes. Xenopus egg extract has ...

Investigating Protein-protein Interactions in Live Cells Using Bioluminescence Resonance Energy Transfer

Assays based on Bioluminescence Resonance Energy Transfer (BRET) provide a sensitive and reliable means to monitor protein-protein interactions in live cells. BRET is the non-radiative transfer of energy from a 'donor' luciferase enzyme to an 'acceptor' fluorescent protein. In the most common configuration of this assay, the donor is Renilla reniformis luciferase and the acceptor is Yellow Fluorescent Protein (YFP). Because the efficiency of energy transfer is strongly distance-dependent, observation of the BRET phenomenon requires that the donor and acceptor be in close proximity. To test for an interaction between two proteins of interest in cultured mammalian cells, one protein is expressed as a fusion with luciferase and the second as a fusion with YFP. An interaction between the two proteins of interest may bring the donor and acceptor sufficiently close for energy transfer to occur. Compared to other techniques for investigating protein-protein interactions, the BRET assay is sensitive, requires little hands-on time and few reagents, and is able to detect interactions which are weak, transient, or dependent on the biochemical environment found within a live cell. It is therefore an ideal approach for confirming putative interactions suggested by yeast two-hybrid or mass spectrometry proteomics studies, and in addition it is well-suited for mapping interacting regions, assessing the effect of post-translational modifications on protein-protein interactions, and evaluating the impact of mutations identified in patient DNA.

Interactions between proteins are fundamental to all cellular processes. Using Bioluminescence Resonance Energy Transfer, the interaction between a pair of proteins can be monitored in live cells and in real time. Furthermore, the effects of potentially pathogenic mutations can be assessed.

Assays based on Bioluminescence Resonance Energy Transfer (BRET) provide a sensitive and reliable means to monitor protein-protein interactions in live ...

Genome-wide Protein-protein Interaction Screening by Protein-fragment Complementation Assay (PCA) in Living Cells

Proteins are the building blocks, effectors and signal mediators of cellular processes. A protein&#8217;s function, regulation and localization often depend on its interactions with other proteins. Here, we describe a protocol for the yeast protein-fragment complementation assay (PCA), a powerful method to detect direct and proximal associations between proteins in living cells. The interaction between two proteins, each fused to a dihydrofolate reductase (DHFR) protein fragment, translates into growth of yeast strains in presence of the drug methotrexate (MTX). Differential fitness, resulting from different amounts of reconstituted DHFR enzyme, can be quantified on high-density colony arrays, allowing to differentiate interacting from non-interacting bait-prey pairs. The high-throughput protocol presented here is performed using a robotic platform that parallelizes mating of bait and prey strains carrying complementary DHFR-fragment fusion proteins and the survival assay on MTX. This protocol allows to systematically test for thousands of protein-protein interactions (PPIs) involving bait proteins of interest and offers several advantages over other PPI detection assays, including the study of proteins expressed from their endogenous promoters without the need for modifying protein localization and for the assembly of complex reporter constructs.

Genome-wide Protein-protein Interaction Screening by Protein-fragment Complementation Assay PCA in Living Cells

Proteins interact with each other and these interactions determine in a large part their functions. Protein interaction partners can be identified at high-throughput in vivo using a yeast fitness assay based on the dihydrofolate reductase protein-fragment complementation assay (DHFR-PCA).

Proteins are the building blocks, effectors and signal mediators of cellular processes. A protein&#8217;s function, regulation and localization often ...

Investigating Interactions Between Histone Modifying Enzymes and Transcription Factors in vivo by Fluorescence Resonance Energy Transfer

Epigenetic regulation of gene expression is commonly affected by histone modifying enzymes (HMEs) that generate heterochromatic or euchromatic histone marks for transcriptional repression or activation, respectively. HMEs are recruited to their target chromatin by transcription factors (TFs). Thus, detecting and characterizing direct interactions between HMEs and TFs are critical for understanding their function and specificity better. These studies would be more biologically relevant if performed in vivo within living tissues. Here, a protocol is described for visualizing interactions in plant leaves between a plant histone deubiquitinase and a plant transcription factor using fluorescence resonance energy transfer (FRET), which allows the detection of complexes between protein molecules that are within &#60;10 nm from each other. Two variations of the FRET technique are presented: SE-FRET (sensitized emission) and AB-FRET (acceptor bleaching), in which the energy is transferred non-radiatively from the donor to the acceptor or emitted radiatively by the donor upon photobleaching of the acceptor. Both SE-FRET and AB-FRET approaches can be adapted easily to discover other interactions between other proteins in planta.

Investigating Interactions Between Histone Modifying Enzymes and Transcription Factors in vivo by Fluorescence Resonance Energy Transfer

Fluorescence resonance energy transfer (FRET) is an imaging technique for detecting protein interactions in living cells. Here, a FRET protocol is presented to study the association of histone-modifying enzymes with transcription factors that recruit them to the target promoters for epigenetic regulation of gene expression in plant tissues.

Epigenetic regulation of gene expression is commonly affected by histone modifying enzymes (HMEs) that generate heterochromatic or euchromatic histone ...

Identifying Inhibitors of the HBx-DDB1 Interaction Using a Split Luciferase Assay System

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Reconstitution Of β-catenin Degradation In Xenopus Egg Extract

Investigating Protein-protein Interactions in Live Cells Using Bioluminescence Resonance Energy Transfer

Genome-wide Protein-protein Interaction Screening by Protein-fragment Complementation Assay (PCA) in Living Cells

Investigating Interactions Between Histone Modifying Enzymes and Transcription Factors in vivo by Fluorescence Resonance Energy Transfer