The work leading to this publication has received funding from the Marie Curie COFUND programme &#8220;U-Mobility&#8221;, cofinanced by the University of Malaga and the European Union 7th Framework Programme (FP7/2007-2013) under GA No. 246550. The work in our laboratory is supported by grants from NIH, USDA/NIFA, NSF, BARD, and BSF to V.C.

1. Protein expression
<ol>
	<li>Choice of expression system 
	Select the system, i.e., vectors and vector delivery method, best suited for expression of the protein of interest in the specific model organism/cell. Note that our assay requires expression of the tested proteins in easily detectible amounts, which is best achieved by transient transformation of large numbers of cells. In plants, for example, this is best accomplished using binary plasmids as expression vectors and Agrobacterium as delivery system.</li>
	<li>Construction of binary expression vectors 
	Clone the coding sequence(s) of the protein(s) of interest into an expression vector either alone or in translational fusion to an epitope tag. Use vectors suitable for the chosen expression system and standard molecular biology procedures for gene cloning. For Agrobacterium-mediated gene delivery, employ binary vectors and introduce them, also using standard protocols, into an Agrobacterium strain, such as EHA105, for subsequent inoculation of plant tissues.</li>
	<li>Choice of plant species 
	Select the plant species in which the protein(s) of interest will be expressed. Note that, while any plant species susceptible to Agrobacterium-mediated genetic transformation can be used, our plant species of choice is Nicotiana benthamiana, which is easily grown, highly susceptible to Agrobacterium, and has large leaves which are easily inoculated.</li>
	<li>Plant growth 
	Grow one plant for 4 to 6 weeks in a pot with Pro-Mix BX in a pot (20 cm x 20 cm x 20 cm) under environmentally-controlled conditions (e.g., growth chamber) of long day (i.e., 16 h of 130-150 &#181;E m-2 s-1 light at 23&#176;C and 8 h dark at 20&#176;C) and 40-65% relative humidity. 
	1.5.2 Fertilize occasionally with commercially available products per manufacturer&#8217;s instructions. Once the plant is grown, select leaves with the size of 50 mm x 70 mm or larger (these length measurements do not include the petiole) for Agrobacterium inoculation.</li>
	<li>Inoculation with Agrobacterium 
	Grow Agrobacterium strain harboring the binary construct expressing the tested protein(s) from step 1.3 overnight at 28&#176;C in YEP medium (1% peptone, 1% yeast extract, and 0.5% NaCl) supplemented with appropriate antibiotics (e.g., 100 mg/L streptomycin and 10 mg/L rifampicin for pPZP-RSC2-based vectors24-25). 
	1.6.2 Centrifuge the cells, resuspended to OD600 =0.5 in infiltration buffer [10 mM MgCl2, 10 mM Mes (pH 5.6), 100 &#181;M acetosyringone], and incubate for 2 h at room temperature. Infiltrate the culture into the abaxial side of a leaf, using a 1-mL needleless syringe; note that the leaves are inoculated in situ, while is attached to the plant. 
	1.6.3 After infiltration, grow the plant for 72 h under the light regime of 16 h 130-150 &#181;E m-2 s-1 light at 23&#176;C/8 h dark at 20&#176;C before harvesting.</li>
	<li>Application of proteasomal inhibitors 
	To support the notion that the tested protein(s) is degraded via a proteasomal pathway, use specific proteasomal inhibitors MG132 and lactacystin28-29, which should reduce or even block degradation. Four hours before harvesting (see Step 2.1), infiltrate the Agrobacterium-inoculated leaf areas with 10 &#956;M MG132 (EMD Millipore) or 10 &#956;M lactacystin (Sigma-Aldrich) or mock-treat the leaf with the respective solvents, i.e., 0.1% DMSO or distilled water.</li>
</ol>
2. Preparation of cell-free extracts
<ol>
	<li>Leaf harvesting 
	Harvest the inoculated areas of the leaf, usually ca. 200-400 mg of fresh weight, and grind them into fine powder in liquid nitrogen. Alternatively, bead-beat the tissues, using any bead-beater (e.g., Biospec) or a dental amalgamator (e.g., TPC Advanced Technologies) directly in the degradation buffer (see Step 2.2).</li>
	<li>Protein extraction 
	Prepare total protein extract by placing the ground tissue into 500 &#956;L of degradation buffer [50mM Tris-HCL (pH 7.5), 100 mM NaCl, 10 mM MgCl2, 5 mM DTT, 5 mM adenosine 5&#180;-triphosphate, and 1x protease inhibitor cocktail (Sigma-Aldrich)]. Note that the protease inhibitor cocktail mainly affects serine, cysteine, aspartic, and metalloproteases and does not interfere with the 26S protease. Clarify the extract by two sequential centrifugations at 12,000 x g for 5 min.</li>
	<li>Protein degradation reaction 
	Transfer equal volumes of extracts, usually 20 &#956;L, to microfuge tubes and incubate them at room temperature for increasing periods of time. Typically, sample time zero, and 5, 10, 15, 20, and 30 min time points. Stop reactions by boiling in SDS gel-sample buffer, and analyze them by western blotting.</li>
</ol>
3. Detecting protein degradation by immunoblotting
<ol>
	<li>Gel electrophoresis 
	3.1.1 Resolve the protein samples by SDS-polyacrylamide gel electrophoresis and electrotransfer the resolved proteins to a nitrocellulose membrane according to the standard protocol30. 
	3.1.2 Determine protein concentration using the Bradford method (Bio-Rad), and load 50-80 &#181;g of total protein per lane. Make sure that all samples are loaded equally. For loading controls, compare intensities of a ubiquitous protein species, such as putative RuBisCo large chains which migrate as a major band with a relative electrophoretic mobility of around 50 kDa; they can be detected on Coomassie blue-stained gels or on Ponceau S- or fluorescent SYPRO Ruby-stained nitrocellulose membranes.</li>
	<li>Blocking 
	Block the membrane with 5% skim milk in TBST (10 mM Tris-HCl, 140 mM NaCl, 0.05% Tween 20, pH 7.4) for 1 h at room temperature.</li>
	<li>Primary antibody 
	Dilute primary anti-epitope antibody in 1% skim milk in TBST to the concentration recommended by the manufacturer and incubate with the blocked membrane for 1 h at room temperature or for overnight at 4&#186;C with gentle agitation.</li>
	<li>Rinsing 
	Rinse the membrane is rinsed in 20 ml TBST for 15 min once, and twice for 5 min at room temperature with gentle agitation.</li>
	<li>Secondary antibody 
	Dilute secondary antibody (e.g., anti-rabbit IgG) conjugated with horse radish peroxidase (HRP) in 1% skim milk in TBST as recommended by the manufacturer and incubate with the membrane for 1 h at room temperature with gentle agitation.</li>
	<li>Detection 
	Rinse the membrane again as described in Step 3.4. After the final rinse in TBST, visualize the proteins of interest with an HRP chemiluminescent substrate, most commonly using an ECL kit. &#160;</li>
</ol>

<ol>
	<li>Patton, E. E., Willems, A. R., Tyers, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Combinatorial+control+in+ubiquitin-dependent+proteolysis:+don&#8217;t+Skp+the+F-box+hypothesis.">Combinatorial control in ubiquitin-dependent proteolysis: don&#8217;t Skp the F-box hypothesis.</a> Trends Genet. 14, 236-243 (1998).</li><li>Deshaies, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=SCF+and+cullin/ring+H2-based+ubiquitin+ligases.">SCF and cullin/ring H2-based ubiquitin ligases.</a> Annu. Rev. Cell Biol. 15, 435-467 (1999).</li><li>Callis, J., Vierstra, R. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+degradation+in+signaling.">Protein degradation in signaling.</a> Curr. Opin. Plant Biol. 3, 381-386 (2000).</li><li>Hellmann, H., Estelle, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plant+development:+regulation+by+protein+degradation.">Plant development: regulation by protein degradation.</a> Science. 297, 793-797 (2002).</li><li>Hershko, A., Ciechanover, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+ubiquitin+system.">The ubiquitin system.</a> Annu. Rev. Biochem. 67, 425-479 (1998).</li><li>Dharmasiri, S., Estelle, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+role+of+regulated+protein+degradation+in+auxin+response.">The role of regulated protein degradation in auxin response.</a> Plant Mol. Biol. 49, 401-409 (2002).</li><li>Devoto, A., Muskett, P. R., Shirasu, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+ubiquitination+in+the+regulation+of+plant+defence+against+pathogens.">Role of ubiquitination in the regulation of plant defence against pathogens.</a> Curr. Opin. Plant Biol. 6, 307-311 (2003).</li><li>Itoh, H., Matsuoka, M., Steber, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+role+for+the+ubiquitin-26S-proteasome+pathway+in+gibberellin+signaling.">A role for the ubiquitin-26S-proteasome pathway in gibberellin signaling.</a> Trends Plant Sci. 8, 492-497 (2003).</li><li>Wang, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+26S+proteasome+system+in+the+signaling+pathways+of+TGF-beta+superfamily.">The 26S proteasome system in the signaling pathways of TGF-beta superfamily.</a> Front. Biosci. 8, 1109-1127 (2003).</li><li>Pagano, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Control+of+DNA+synthesis+and+mitosis+by+the+Skp2-+p27-Cdk1/2+axis.">Control of DNA synthesis and mitosis by the Skp2- p27-Cdk1/2 axis.</a> Mol. Cell. 14, 414-416 (2004).</li><li>Magori, S., Citovsky, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hijacking+of+the+host+SCF+ubiquitin+ligase+machinery+by+plant+pathogens.">Hijacking of the host SCF ubiquitin ligase machinery by plant pathogens.</a> Front. Plant Sci. 2, 87 (2011).</li><li>Gelvin, S. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Agrobacterium-mediated+plant+transformation:+the+biology+behind+the+&amp;#34;gene-jockeying&amp;#34;+tool.">Agrobacterium-mediated plant transformation: the biology behind the &amp;#34;gene-jockeying&amp;#34; tool.</a> Microbiol. Mol Biol. Rev. 67, 16-37 (2003).</li><li>Lacroix, B., Citovsky, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+roles+of+bacterial+and+host+plant+factors+in+Agrobacterium-mediated+genetic+transformation.">The roles of bacterial and host plant factors in Agrobacterium-mediated genetic transformation.</a> Int. J. Dev. Biol. 57, (2013).</li><li>Schrammeijer, B., 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=Interaction+of+the+virulence+protein+VirF+of+Agrobacterium+tumefaciens+with+plant+homologs+of+the+yeast+Skp1+protein.">Interaction of the virulence protein VirF of Agrobacterium tumefaciens with plant homologs of the yeast Skp1 protein.</a> Curr. Biol. 11, 258-262 (2001).</li><li>Zaltsman, A., Krichevsky, A., Loyter, A., Citovsky, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Agrobacterium+induces+expression+of+a+plant+host+F-box+protein+required+for+tumorigenicity.">Agrobacterium induces expression of a plant host F-box protein required for tumorigenicity.</a> Cell Host Microbe. 7, 197-209 (2010).</li><li>Tzfira, T., Vaidya, M., Citovsky, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Involvement+of+targeted+proteolysis+in+plant+genetic+transformation+by+Agrobacterium.">Involvement of targeted proteolysis in plant genetic transformation by Agrobacterium.</a> Nature. 431, 87-92 (2004).</li><li>Zaltsman, A., Lacroix, B., Gafni, Y., Citovsky, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disassembly+of+synthetic+Agrobacterium+T-DNA-protein+complexes+via+the+host+SCFVBF+ubiquitin-ligase+complex+pathway.">Disassembly of synthetic Agrobacterium T-DNA-protein complexes via the host SCFVBF ubiquitin-ligase complex pathway.</a> Proc. Natl. Acad. Sci. U.S.A. 110, 169-174 (2013).</li><li>Zhou, P., Howley, P. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ubiquitination+and+degradation+of+the+substrate+recognition+subunits+of+SCF+ubiquitin-protein+ligases.">Ubiquitination and degradation of the substrate recognition subunits of SCF ubiquitin-protein ligases.</a> Mol. Cell. 2, 571-580 (1998).</li><li>Galan, J. M., Peter, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ubiquitin-dependent+degradation+of+multiple+F-box+proteins+by+an+autocatalytic+mechanism.">Ubiquitin-dependent degradation of multiple F-box proteins by an autocatalytic mechanism.</a> Proc. Natl. Acad. Sci. U.S.A. 96, 9124-9129 (1999).</li><li>Ayad, N. G., Rankin, S., Murakami, M., Jebanathirajah, J., Gygi, S., Kirschner, M. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tome-1,+a+trigger+of+mitotic+entry,+is+degraded+during+G1+via+the+APC.">Tome-1, a trigger of mitotic entry, is degraded during G1 via the APC.</a> Cell. 113, 101-113 (2003).</li><li>Guardavaccaro, D., 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=Control+of+meiotic+and+mitotic+progression+by+the+F+box+protein+b-Trcp1+in+vivo.">Control of meiotic and mitotic progression by the F box protein b-Trcp1 in vivo.</a> Dev. Cell. 4, 799-812 (2003).</li><li>Magori, S., Citovsky, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Agrobacterium+counteracts+host-induced+degradation+of+its+F-box+protein+effector.">Agrobacterium counteracts host-induced degradation of its F-box protein effector.</a> Sci. Signal. 4, (2011).</li><li>Margottin-Goguet, F., Hsu, J. Y., Loktev, A., Hsieh, H. M., Reimann, J. D., Jackson, P. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prophase+destruction+of+Emi1+by+the+SCFbTrCP/Slimb+ubiquitin+ligase+activates+the+anaphase+promoting+complex+to+allow+progression+beyond+prometaphase.">Prophase destruction of Emi1 by the SCFbTrCP/Slimb ubiquitin ligase activates the anaphase promoting complex to allow progression beyond prometaphase.</a> Dev. Cell. 4, 813-826 (2003).</li><li>Tzfira, 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=pSAT+vectors:+a+modular+series+of+plasmids+for+fluorescent+protein+tagging+and+expression+of+multiple+genes+in+plants.">pSAT vectors: a modular series of plasmids for fluorescent protein tagging and expression of multiple genes in plants.</a> Plant Mol. Biol. 57, 503-516 (2005).</li><li>Goderis, I. J., 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+set+of+modular+plant+transformation+vectors+allowing+flexible+insertion+of+up+to+six+expression+units.">A set of modular plant transformation vectors allowing flexible insertion of up to six expression units.</a> Plant Mol. Biol. 50, 17-27 (2002).</li><li>Lee, L. Y., Gelvin, S. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=T-DNA+binary+vectors+and+systems.">T-DNA binary vectors and systems.</a> Plant Physiol. 146, 325-332 (2008).</li><li>Dafny-Yelin, M., Tzfira, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Delivery+of+multiple+transgenes+to+plant+cells.">Delivery of multiple transgenes to plant cells.</a> Plant Physiol. 145, 1118-1128 (2007).</li><li>Yang, P., 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=Purification+of+the+Arabidopsis+26+S+proteasome:+biochemical+and+molecular+analyses+revealed+the+presence+of+multiple+isoforms.">Purification of the Arabidopsis 26 S proteasome: biochemical and molecular analyses revealed the presence of multiple isoforms.</a> J. Biol. Chem. 279, 6401-6413 (2004).</li><li>Fenteany, G., 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+proteasome+activities+and+subunit-specific+amino-terminal+threonine+modification+by+lactacystin.">Inhibition of proteasome activities and subunit-specific amino-terminal threonine modification by lactacystin.</a> Science. 268, 726-731 (1995).</li><li>Ausobel, F. 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=.">.</a> Current Protocols in Molecular Biology. , (1987).</li><li>Rasband, W. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> ImageJ. , (1997).</li><li>Kim, J. H., Kim, W. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Arabidopsis+RING+E3+ubiquitin+ligase+AtAIRP3/LOG2+participates+in+positive+regulation+of+high+salt+and+drought+stress+responses).">The Arabidopsis RING E3 ubiquitin ligase AtAIRP3/LOG2 participates in positive regulation of high salt and drought stress responses).</a> Plant Physiol. 162, 1733-1749 (2013).</li></ol>

No conflicts of interest declared.

This assay relies on the expression of the tested proteins in plant tissues; thus, the potential proteasomal degradation process obviously occurs already within the living tissues. We assay protein destabilization, however, only in the extracts, with the time zero sample serving as the initial reference point. Hence, we define it as a cell-free assay.
One important aspect for the success of this assay is the correct choice of the expression vector from which the tested protein(s) will be produ...

The ubiquitin/26S proteasome pathway is emerging as a widespread mechanism for control of diverse biological reactions, including transcriptional regulation, cell-cycle progression and signal transduction, receptor down-regulation or endocytosis, among others processes1-4. In this pathway, the target protein is tagged by ubiquitin residues which are first attached via a thiolester bond to ubiquitin-activating enzyme E1 and then translocated to a cysteine amino acid residue of ubiquitin-conjugation enzyme E2; finally, E2 interacts with ubiquitin ligase E3, resulting in polyubiquitination of the protein substrate. Ultimately, the polyubiquitinated proteins are recognized and degraded by the 26S proteasome. In this mechanism, the E3 enzyme specifies the substrate and acts as the key regulatory component of the ubiquitin/26S proteasome system (UPS). E3 ligases can act independently, such as RING domain ligases, or as part of a multisubunit SCF (Skp1/Cullin/F-box protein) complex, such as F-box domain ligases. SCF-mediated proteasomal degradation pathways are involved in regulation of transcription, cell cycle, signal transduction5-10 and many other major cellular functions.
Besides these critical roles in regulation of cellular processes, UPS takes the central stage in many plant-pathogen interactions. For example, increasing evidence suggests that several plant pathogens, including Agrobacterium tumefaciens, rely on the host UPS for to facilitate the infection process11. Agrobacterium elicits neoplastic growths on plants, which represent its natural hosts, and it can also transform a wide range of other eukaryotes, from fungi1,2 to human cells12,13. During its infection, Agrobacterium exports a DNA element (T-DNA) and several virulence (Vir) proteins into the host cell12-13. One of these proteins is VirF, the first F-box protein found to be encoded by a prokaryotic genome14. As part of the SCF ubiquitin ligase complex, VirF, and its functional host homolog VBF15, facilitate Agrobacterium infection via the UPS-mediated protein degradation, which presumably facilitates uncoating of the invading bacterial T-DNA from its accompanying bacterial and host proteins, VirE2 and VIP1, respectively16,17. Interestingly, many F-box proteins, including VirF, are intrinsically unstable due to their own proteolysis, which is mediated by autoubiquitination activity18,19 or by other E3 ligases for which F-box proteins may serve as substrates20-23.

When studying biochemical activities of F-box proteins, other ubiquitin ligases, and/or their substrates, it would be very useful to employ a simple and reliable assay for proteasomal degradation. Here we describe one such protocol for analyzing protein stability in a cell-free system. In this assay, the stability of the UPS substrate is analyzed in the presence or absence of one of the essential components of the proteasomal degradation pathway, such as an F-box protein, in a cell-free system. Generally, we express the tested protein(s) in plant tissues, prepare cell-free extracts from these tissues and monitor the amounts of the protein(s) of interest by western blotting. The UPS-dependent mechanism of protein degradation is demonstrated by inclusion of specific proteasomal inhibitors and/or using coexpression of dominant-negative form of an SCF component, Cullin. Whereas we illustrate this assay using proteasomal degradation of the Arabidopsis VIP117 protein by the F-box protein VBF15, it may be employed to investigate the stability of any other proteasomal substrates.

<table border="0" cellpadding="0" cellspacing="0" width="662">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Protein assay kit</td>
			<td style="width:129px;">Bio-Rad</td>
			<td style="width:119px;">500-0001</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Proteinase inhibitor cocktail&#160;</td>
			<td style="width:129px;">Sigma-Aldrich</td>
			<td style="width:119px;">S8820</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Mini-Protean system</td>
			<td style="width:129px;">Bio-Rad</td>
			<td style="width:119px;">165-8000</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Semi-dry western blotting SD electrotransfer system</td>
			<td style="width:129px;">Bio-Rad</td>
			<td style="width:119px;">170-3940</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Affinity Purified Rabbit Anti-Ha</td>
			<td style="width:129px;">icllab</td>
			<td>RHGT-45A-Z</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Goat anti-Rabbit IgG Peroxidase Conjugate</td>
			<td style="width:129px;">Thermo Scientific</td>
			<td style="width:119px;">31460</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">BioTrace, NT nitrocellulose transfer membrane</td>
			<td style="width:129px;">Pall Corportation</td>
			<td style="width:119px;">27377-000</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Immobilon western chemiluminescent HRP substrate</td>
			<td style="width:129px;">EMD Millipore</td>
			<td style="width:119px;">WBKL S0 050</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">MG132</td>
			<td style="width:129px;">EMD Millipore</td>
			<td>474790-1MG</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Lactacystin</td>
			<td style="width:129px;">Sigma-Aldrich</td>
			<td>L6785</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Thermo Scientific Pierce Fast Western Blot Kit, ECL Substrate</td>
			<td style="width:129px;">Pierce</td>
			<td style="width:119px;">35055</td>
			<td></td>
		</tr>
	</tbody>
</table>

assaying proteasomal degradation in a cell-free system in plants

The ubiquitin-proteasome pathway for protein degradation has emerged as one of the most important mechanisms for regulation of a wide spectrum of cellular functions in virtually all eukaryotic organisms. Specifically, in plants, the ubiquitin/26S proteasome system (UPS) regulates protein degradation and contributes significantly to development of a wide range of processes, including immune response, development and programmed cell death. Moreover, increasing evidence suggests that numerous plant pathogens, such as Agrobacterium, exploit the host UPS for efficient infection, emphasizing the importance of UPS in plant-pathogen interactions.
The substrate specificity of UPS is achieved by the E3 ubiquitin ligase that acts in concert with the E1 and E2 ligases to recognize and mark specific protein molecules destined for degradation by attaching to them chains of ubiquitin molecules. One class of the E3 ligases is the SCF (Skp1/Cullin/F-box protein) complex, which specifically recognizes the UPS substrates and targets them for ubiquitination via its F-box protein component. To investigate a potential role of UPS in a biological process of interest, it is important to devise a simple and reliable assay for UPS-mediated protein degradation. Here, we describe one such assay using a plant cell-free system. This assay can be adapted for studies of the roles of regulated protein degradation in diverse cellular processes, with a special focus on the F-box protein-substrate interactions.

Figure 1, adapted from Zaltsman et al.17, illustrates representative experiments for detection of proteasomal degradation in a cell-free system. Specifically, we demonstrate destabilization of a plant defense-related protein VIP1 by the VBF F-box protein via the SCFVBF pathway in N. benthamiana. Arabidopsis VBF and HA-tagged VIP1 (HA-VIP1) proteins were transiently coexpressed, and HA-VIP1 content protein within extracts of the expressing leaves was analyz...

Watch this Scientific Journal Video about Assaying Proteasomal Degradation in a Cell-free System in Plants at JoVE.com

Assaying Proteasomal Degradation in a Cell-free System in Plants

Targeted protein degradation represents a major regulatory mechanism for cell function. It occurs via a conserved ubiquitin-proteasome pathway, which attaches polyubiquitin chains to the target protein that then serve as molecular &#8220;tags&#8221; for the 26S proteasome. Here, we describe a simple and reliable cell-free assay for proteasomal degradation of proteins.

The ubiquitin-proteasome pathway for protein degradation has emerged as one of the most important mechanisms for regulation of a wide spectrum of cellular ...

assaying-proteasomal-degradation-in-a-cell-free-system-in-plants

Research

JoVE Journal

Biology

14.3K Views.  Stony Brook University, State University of New York. Targeted protein degradation represents a major regulatory mechanism for cell function. It occurs via a conserved ubiquitin-proteasome pathway, which attaches polyubiquitin chains to the target protein that then serve as molecular “tags” for the 26S proteasome. Here, we describe a simple and reliable cell-free assay for proteasomal degradation of proteins.

Video: Assaying Proteasomal Degradation in a Cell-free System in Plants

Maintaining Wolbachia in Cell-free Medium

In this video protocol, procedures are demonstrated to (1) purify Wolbachia symbionts out of cultured mosquito cells, (2) use a fluorescent assay to ascertain the viability of the purified Wolbachia and (3) maintain the now extracellular Wolbachia in cell-free medium.  Purified Wolbachia remain alive in the extracellular phase but do not replicate until re-inoculated into eukaryotic cells.  Extracellular Wolbachia purified in this manner will remain viable for at least a week at room temperature, and possibly longer.  Purified Wolbachia are suitable for micro-injection, DNA extraction and other applications. 


This video describes a method for purifying Wolbachia pipientis from an Anopheles gambiae cell line and then culturing the endosymbiont in cell-free medium. An assay for viability of the bacterium is demonstrated.

In this video protocol, procedures are demonstrated to (1) purify Wolbachia symbionts out of cultured mosquito cells, (2) use a fluorescent assay to ...

Assaying the Kinase Activity of LRRK2 in vitro

Leucine Rich Repeat Kinase 2 (LRRK2) is a 2527 amino acid member of the ROCO family of proteins, possessing a complex, multidomain structure including a GTPase domain (termed ROC, for Ras of Complex proteins) and a kinase domain1. The discovery in 2004 of mutations in LRRK2 that cause Parkinson's disease (PD) resulted in LRRK2 being the focus of a huge volume of research into its normal function and how the protein goes awry in the disease state2,3. Initial investigations into the function of LRRK2 focused on its enzymatic activities4-6. Although a clear picture has yet to emerge of a consistent alteration in these due to mutations, data from a number of groups has highlighted the importance of the kinase activity of LRRK2 in cell death linked to mutations7,8. Recent publications have reported inhibitors targeting the kinase activity of LRRK2, providing a key experimental tool9-11. In light of these data, it is likely that the enzymatic properties of LRRK2 afford us an important window into the biology of this protein, although whether they are potential drug targets for Parkinson's is open to debate.
A number of different approaches have been used to assay the kinase activity of LRRK2. Initially, assays were carried out using epitope tagged protein overexpressed in mammalian cell lines and immunoprecipitated, with the assays carried out using this protein immobilised on agarose beads4,5,7. Subsequently, purified recombinant fragments of LRRK2 in solution have also been used, for example a GST tagged fragment purified from insect cells containing residues 970 to 2527 of LRRK212. Recently, Dani&euml;ls et al. reported the isolation of full length LRRK2 in solution from human embryonic kidney cells, however this protein is not widely available13. In contrast, the GST fusion truncated form of LRRK2 is commercially available (from Invitrogen, see table 1 for details), and provides a convenient tool for demonstrating an assay for LRRK2 kinase activity. Several different outputs for LRRK2 kinase activity have been reported. Autophosphorylation of LRRK2 itself, phosphorylation of Myelin Basic Protein (MBP) as a generic kinase substrate and phosphorylation of an artificial substrate - dubbed LRRKtide, based upon phosphorylation of threonine 558 in Moesin - have all been used, as have a series of putative physiological substrates including &alpha;-synuclein, Moesin and 4-EBP14-17. The status of these proteins as substrates for LRRK2 remains unclear, and as such the protocol described below will focus on using MBP as a generic substrate, noting the utility of this system to assay LRRK2 kinase activity directed against a range of potential substrates.

Assaying the Kinase Activity of LRRK2 in vitro

Leucine Rich Repeat Kinase 2 is a large multidomain kinase, mutations in which are the most common genetic cause of Parkinson's disease. Analysis of the kinase activity of this protein has proven to be a crucial tool in understanding the biology and dysfunction of this protein. In this paper, in vitro assaying of the kinase activity of LRRK2 and a selection of its mutants is described, providing an experimental system to examine phosphorylation of putative substrates and potential dysfunction of LRRK2 in disease.

Leucine Rich Repeat Kinase 2 (LRRK2) is a 2527 amino acid member of the ROCO family of proteins, possessing a complex, multidomain structure including a ...

Using Caenorhabditis elegans as a Model System to Study Protein Homeostasis in a Multicellular Organism

The folding and assembly of proteins is essential for protein function, the long-term health of the cell, and longevity of the organism. Historically, the function and regulation of protein folding was studied in vitro, in isolated tissue culture cells and in unicellular organisms. Recent studies have uncovered links between protein homeostasis (proteostasis), metabolism, development, aging, and temperature-sensing. These findings have led to the development of new tools for monitoring protein folding in the model metazoan organism Caenorhabditis elegans. In our laboratory, we combine behavioral assays, imaging and biochemical approaches using temperature-sensitive or naturally occurring metastable proteins as sensors of the folding environment to monitor protein misfolding. Behavioral assays that are associated with the misfolding of a specific protein provide a simple and powerful readout for protein folding, allowing for the fast screening of genes and conditions that modulate folding. Likewise, such misfolding can be associated with protein mislocalization in the cell. Monitoring protein localization can, therefore, highlight changes in cellular folding capacity occurring in different tissues, at various stages of development and in the face of changing conditions. Finally, using biochemical tools ex vivo, we can directly monitor protein stability and conformation. Thus, by combining behavioral assays, imaging and biochemical techniques, we are able to monitor protein misfolding at the resolution of the organism, the cell, and the protein, respectively.

Using Caenorhabditis elegans as a Model System to Study Protein Homeostasis in a Multicellular Organism

To study the relationship between protein homeostasis, stress and aging, we monitored changes in protein folding by following protein dysfunction, protein localization in the cell and protein stability at the organismal, cellular and protein levels, using the genetically tractable metazoan Caenorhabditis elegans as a model system.

The folding and assembly of proteins is essential for protein function, the long-term health of the cell, and longevity of the organism. Historically, the ...

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 &amp;#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 ...

Measuring Diurnal Rhythms in Autophagic and Proteasomal Flux

Cells employ several methods for recycling unwanted proteins and other material, including lysosomal and non-lysosomal pathways. The main lysosome-dependent pathway is called autophagy, while the primary non-lysosomal method for protein catabolism is the ubiquitin-proteasome system. Recent studies in model organisms suggest that the activity of both autophagy and the ubiquitin-proteasome system is not constant across the day but instead varies according to a daily (circadian) rhythm. The ability to measure biological rhythms in protein turnover is important for understanding how cellular quality control is achieved and for understanding the dynamics of specific proteins of interest. Here we present a standardized protocol for quantifying autophagic and proteasomal flux in vivo that captures the circadian component of protein turnover. Our protocol includes details for mouse handling, tissue processing, fractionation, and autophagic flux quantification using mouse liver as the starting material.

We describe our protocol for measuring biological rhythms in protein catabolism via autophagy and the proteasome in mouse liver.

Cells employ several methods for recycling unwanted proteins and other material, including lysosomal and non-lysosomal pathways. The main ...

A Simple Protocol for Mapping the Plant Root System Architecture Traits

Comprehensive knowledge of plant root system architecture (RSA) development is critical for improving nutrient use efficiency and increasing crop cultivar tolerance to environmental challenges. An experimental protocol is presented for setting up the hydroponic system, plantlet growth, RSA spreading, and imaging. The approach used a magenta box-based hydroponic system containing polypropylene mesh supported by polycarbonate wedges. Experimental settings are exemplified by assessing the RSA of the plantlets under varying nutrient (phosphate [Pi]) supply. The system was established to examine the RSA of Arabidopsis, but it is readily adaptable to study other plants like Medicago sativa (Alfalfa). Arabidopsis thaliana (Col-0) plantlets are used in this investigation as an example to understand the plant RSA. Seeds are surface sterilized by treating ethanol and diluted commercial bleach, and kept at 4 &#176;C for stratification. The seeds are germinated and grown on a liquid half-MS medium on a polypropylene mesh supported by polycarbonate wedges. The plantlets are grown under standard growth conditions for the desired number days, gently picked out from the mesh, and submersed in water-containing agar plates. Each root system of the plantlets is spread gently on the water-filled plate with the help of a round art brush. These Petri plates are photographed or scanned at high resolution to document the RSA traits. The root traits, such as primary root, lateral roots, and branching zone, are measured using the freely available ImageJ software. This study provides techniques for measuring plant root characteristics in controlled environmental settings. We discuss how to (1) grow the plantlets, and collect and spread root samples, (2) obtain pictures of spread RSA samples, (3) capture the images, and (4) use image analysis software to quantify root attributes. The advantage of the present method is the versatile, easy, and efficient measurement of the RSA traits.

We use simple laboratory tools to examine the root system architecture (RSA) of Arabidopsis and Medicago. The plantlets are grown hydroponically over mesh and spread using an art brush to reveal the RSA. Images are taken using scanning or a high-resolution camera, then analyzed with ImageJ to map traits.

Comprehensive knowledge of plant root system architecture (RSA) development is critical for improving nutrient use efficiency and increasing crop cultivar ...

Quantification of Skeletal Muscle Fatty Infiltration via Decellularization

Decellularization-Based Quantification of Skeletal Muscle Fatty Infiltration

Fatty infiltration is the accumulation of adipocytes between myofibers in skeletal muscle and is a prominent feature of many myopathies, metabolic disorders, and dystrophies. Clinically in human populations, fatty infiltration is assessed using noninvasive methods, including computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). Although some studies have used CT or MRI to quantify fatty infiltration in mouse muscle, costs and insufficient spatial resolution remain challenging. Other small animal methods utilize histology to visualize individual adipocytes; however, this methodology suffers from sampling bias in heterogeneous pathology. This protocol describes the methodology to qualitatively view and quantitatively measure fatty infiltration comprehensively throughout intact mouse muscle and at the level of individual adipocytes using decellularization. The protocol is not limited to specific muscles or specific species and can be extended to human biopsy. Additionally, gross qualitative and quantitative assessments can be made with standard laboratory equipment for little cost, making this procedure more accessible across research laboratories.

Author Spotlight: Decellularization-Based Quantification of Skeletal Muscle Fatty Infiltration  

The present study describes decellularization-based methodologies for visualizing and quantifying intramuscular adipose tissue (IMAT) deposition through intact muscle volume, as well as quantifying metrics of individual adipocytes that comprise IMAT.

Fatty infiltration is the accumulation of adipocytes between myofibers in skeletal muscle and is a prominent feature of many myopathies, metabolic ...

Embedding CFPS Reactions in a Variety of Hydrogel Matrices

Methods for Embedding Cell-Free Protein Synthesis Reactions in Macro-Scale Hydrogels

Synthetic gene networks provide a platform for scientists and engineers to design and build novel systems with functionality encoded at a genetic level. While the dominant paradigm for the deployment of gene networks is within a cellular chassis, synthetic gene networks may also be deployed in cell-free environments. Promising applications of cell-free gene networks include biosensors, as these devices have been demonstrated against biotic (Ebola, Zika, and SARS-CoV-2 viruses) and abiotic (heavy metals, sulfides, pesticides, and other organic contaminants) targets. Cell-free systems are typically deployed in liquid form within a reaction vessel. Being able to embed such reactions in a physical matrix, however, may facilitate their broader application in a wider set of environments. To this end, methods for embedding cell-free protein synthesis (CFPS) reactions in a variety of hydrogel matrices have been developed. One of the key properties of hydrogels conducive to this work is the high-water reconstitution capacity of hydrogel materials. Additionally, hydrogels possess physical and chemical characteristics that are functionally beneficial. Hydrogels can be freeze-dried for storage and rehydrated for use later. Two step-by-step protocols for the inclusion and assay of CFPS reactions in hydrogels are presented. First, a CFPS system can be incorporated into a hydrogel via rehydration with a cell lysate. The system within the hydrogel can then be induced or expressed constitutively for complete protein expression through the hydrogel. Second, cell lysate can be introduced to a hydrogel at the point of polymerization, and the entire system can be freeze-dried and rehydrated at a later point with an aqueous solution containing the inducer for the expression system encoded within the hydrogel. These methods have the potential to allow for cell-free gene networks that confer sensory capabilities to hydrogel materials, with the potential for deployment beyond the laboratory.

Author Spotlight: A Novel Approach for Embedding Cell-Free Protein Synthesis Reactions in Hydrogels 

Here, we present two protocols for embedding cell-free protein synthesis reactions in macro-scale hydrogel matrices without the need for an external liquid phase.

Synthetic gene networks provide a platform for scientists and engineers to design and build novel systems with functionality encoded at a genetic level. ...

Quantitative Analysis of Mitophagy in Murine Pancreatic &#946;-Cells

Complementary Approaches to Interrogate Mitophagy Flux in Pancreatic &#946;-Cells

Mitophagy is a quality control mechanism necessary to maintain optimal mitochondrial function. Dysfunctional &#946;-cell mitophagy results in insufficient insulin release. Advanced quantitative assessments of mitophagy often require the use of genetic reporters. The mt-Keima mouse model, which expresses a mitochondria-targeted pH-sensitive dual-excitation ratiometric probe for quantifying mitophagy via flow cytometry, has been optimized in &#946;-cells. The ratio of acidic-to-neutral mt-Keima wavelength emissions can be used to robustly quantify mitophagy. However, using genetic mitophagy reporters can be challenging when working with complex genetic mouse models or difficult-to-transfect cells, such as primary human islets. This protocol describes a novel complementary dye-based method to quantify &#946;-cell mitophagy in primary islets using MtPhagy. MtPhagy is a pH-sensitive, cell-permeable dye that accumulates in the mitochondria and increases its fluorescence intensity when mitochondria are in low pH environments, such as lysosomes during mitophagy. By combining the MtPhagy dye with Fluozin-3-AM, a Zn2+ indicator that selects for &#946;-cells, and Tetramethylrhodamine, ethyl ester (TMRE) to assess mitochondrial membrane potential, mitophagy flux can be quantified specifically in &#946;-cells via flow cytometry. These two approaches are highly complementary, allowing for flexibility and precision in assessing mitochondrial quality control in numerous &#946;-cell models.

Author Spotlight: Assessment of Mitophagy Flux in Pancreatic &#946;-Cells Using Effective and Robust Complementary Approaches 

This protocol outlines two methods for the quantitative analysis of mitophagy in pancreatic &#946;-cells: first, a combination of cell-permeable mitochondria-specific dyes, and second, a genetically encoded mitophagy reporter. These two techniques are complementary and can be deployed based on specific needs, allowing for flexibility and precision in quantitatively addressing mitochondrial quality control.

Mitophagy is a quality control mechanism necessary to maintain optimal mitochondrial function. Dysfunctional &#946;-cell mitophagy results in insufficient ...