This work is supported by the National Natural Science Foundation of China Grants 91753206, 21425204, and 21521003 and by the National Key Research and Development Project 2016YFA0501500.

<ol>
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The authors have nothing to disclose.

The maintenance of fair RNA integrity is one of the keys to successful CARIC experiments. With appropriate ligands of Cu(I) and careful operation, RNA degradation can be significantly reduced, although partial degradation was observed. The substitution ratios of EU and 4SU in experimental samples are 1.18% and 0.46%, respectively (data not shown). For intact RNAs with a length of 2,000 nt, ~90% of RNAs contain at least one EU and one 4SU. For partially degraded RNAs with a length of 1,000 nt, ~70% of RNAs contain at least one EU and one 4SU. Therefore, partial degradation of RNAs does not dramatically decrease the efficiency of CARIC, while severe degradation is not acceptable.
Another critical step is step 1.4, the preparation for the click reaction. The Cu(I)-catalyzed click reaction on RNAs is sensitive to LDS concentration. A high concentration (&#62; 0.1%) of LDS will lead to a decrease of labeling signals on EU-containing RNAs and an increase of background signals on proteins (data not shown).
In addition to EU, CARIC is also compatible with other clickable nucleosides, such as alkynyl and azido analogs of adenosine33,34,35,36. However, the application of CARIC is significantly limited by the metabolic efficiency of unnatural clickable nucleosides in a biological system of interest. Therefore, before performing CARIC using conditions other than those demonstrated in this protocol, always check the metabolic labeling efficiency (e.g., by fluorescent imaging).
Recently, a similar strategy called RICK (capture of the newly transcribed RNA interactome using click chemistry), which incorporates only EU to label total RNAs and uses 254-nm UV to cross-link RNAs and proteins, was reported37. Notably, 254-nm UV can activate all four natural nucleosides, as well as EU. Thus, 254-nm UV irradiation may cross-link free EU and its metabolites (e.g., EU phosphates) with corresponding binding proteins, which should be taken into consideration as possible false positives.
One intriguing application of CARIC is to identify RBPs in bacteria whose RNAs are mostly non-polyadenylated. The large-scale identification of RBPs will provide invaluable resources to understand the molecular basis of posttranscriptional regulations in bacteria38.

The human genome is transcribed into various types of coding and noncoding RNAs (ncRNAs), including mRNAs, rRNAs, tRNAs, small nuclear RNAs (snRNAs), small nucleolar RNAs (snoRNAs), and long non-coding RNAs (lncRNAs)1. Most of these RNAs possess clothing of RBPs and function as ribonucleoprotein particles (RNPs)2. Therefore, a comprehensive identification of RBPs is a prerequisite for understanding the regulatory network between RNAs and RBPs, which is implicated in various human diseases3,4,5.
The past&#160;few years have witnessed a great boost of RBPs discovered in various eukaryotic systems2,6, including human7,8,9,10,11, mouse12,13,14, yeast9,15,16, zebrafish17, Drosophila melanogaster18,19, Caenorhabditis elegans16, Arabidopsis thaliana20,21,22, and human parasites23,24,25. These advances have been facilitated by an RBP capture strategy developed by Castello et al.7 and Baltz et al.8 in 2012, which combines in vivo UV cross-linking of RNA and its interacting proteins, oligo(dT) capture of poly(A) RNAs, and mass spectrometry (MS)-based proteomic profiling. However, given the fact that poly(A) mostly exists on mature mRNAs, which account for only ~3% - 5% of the eukaryotic transcriptome26, this widely used strategy is not capable of capturing RBPs interacting with non-poly(A) RNAs, including most ncRNAs and pre-mRNAs.
Here, we report the detailed procedures of a recently developed strategy for the transcriptome-wide capture of both poly(A) and non-poly(A) RBPs27. Termed CARIC, this strategy combines in vivo UV cross-linking and metabolic labeling of RNAs with photoactivatable and &#34;clickable&#34; nucleoside analogs (which contain a bioorthogonal functional group that can participate in click reaction), 4-thiouridine (4SU), and 5-ethynyluridine (EU). Steps that are key to get ideal results with the CARIC strategy are efficient metabolic labeling, UV cross-linking and click reaction, and the maintenance of RNA integrity. Because Cu(I) used as the catalyst in click reaction can cause the fragmentation of RNAs, a Cu(I) ligand that can reduce RNA fragmentation is essential. We describe how to perform efficient click reactions in cell lysates without causing severe RNA degradation.
Although RBP capture and identification in HeLa cells only is described in this protocol, the CARIC strategy can be applied to various cell types and possibly to living organisms. Besides RBP capture, this protocol also provides streamlined step-by-step procedures for MS sample preparation and protein identification and quantification, which can be helpful for those who are not familiar with proteomic experiments.

<table border="0" cellpadding="0" cellspacing="0" width="867">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">HeLa</td>
			<td style="width:175px;">ATCC</td>
			<td style="width:175px;"></td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:351px;">DMEM (Dulbecco&#39;s Modified Eagle Medium)</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">11995065</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">FBS (Fetal Bovine Serum)</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">10099141</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Penicillin &#38; Streptomycin</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">15140122</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">EU (5-ethynyl uridine)</td>
			<td style="width:175px;">Wuhu Huaren Co.</td>
			<td style="width:175px;">CAS:69075-42-9</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">4SU (4-thiouridine)</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">T4509</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">10&#215;PBS (Phosphate-Buffered Saline)</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">AM9625</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">UV cross-linker</td>
			<td style="width:175px;">UVP</td>
			<td style="width:175px;">CL-1000</td>
			<td>Equiped with 365-nm UV lamp</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">DEPC (Diethyl pyrocarbonate)</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">D5758</td>
			<td>To treat water. Highly toxic!</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Tris&#183;HCl, pH 7.5</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">15567027</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">LiCl</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">62476</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Nonidet P-40</td>
			<td style="width:175px;">Biodee</td>
			<td style="width:175px;">74385</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">EDTA-free protease inhibitor cocktail</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">88265</td>
			<td>One tablet for 50 mL lysis buffer.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">LDS (Lithium dodecyl sulfate)</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">L9781</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">15-mL ultrafiltration tube (10 kDa cutoff)</td>
			<td style="width:175px;">Millipore</td>
			<td style="width:175px;">UFC901024</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">0.5-mL ultrafiltration tube (10 kDa cutoff)</td>
			<td style="width:175px;">Millipore</td>
			<td style="width:175px;">UFC501096</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Streptavidin magnetic beads</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">88816</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">DMSO (Dimethyl sulfoxide)</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">41639</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Azide-biotin</td>
			<td style="width:175px;">Click Chemistry Tools</td>
			<td style="width:175px;">AZ104</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Copper(II) sulfate</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">C1297</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">THPTA [Tris(3-hydroxypropyltriazolylmethyl)amine]</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">762342</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Sodium ascorbate</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">11140</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Azide-Cy5</td>
			<td style="width:175px;">Click Chemistry Tools</td>
			<td style="width:175px;">AZ118</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">LDS sample buffer (4&#215;)</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">NP0008</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">10% bis-Tris gel</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">NP0301BOX</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">EDTA</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">AM9260G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">RNase A</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">R6513</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">SDS (Sodium dodecyl sulfate)</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">15525017</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">NaCl</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">S3014</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Brij-97 [Polyoxyethylene (20) oleyl ether]</td>
			<td style="width:175px;">J&#38;K</td>
			<td style="width:175px;">315442</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Triethanolamine</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">V900257</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Streptavidin agarose</td>
			<td style="width:175px;">Thermo Fisher Scientific</td>
			<td style="width:175px;">20353</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Urea</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">U5378</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Sarkosyl (N-Lauroylsarcosine sodium salt)</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">61743</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Biotin</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">B4501</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">Sodium deoxycholate</td>
			<td style="width:175px;">Sigma Aldrich</td>
			<td style="width:175px;">30970</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:351px;">MaxQuant</td>
			<td style="width:175px;"></td>
			<td style="width:175px;"></td>
			<td>Version: 1.5.5.1</td>
		</tr>
	</tbody>
</table>

capture and identification of rna-binding proteins by using click chemistry-assisted rna-interactome capture (caric) strategy

A comprehensive identification of RNA-binding proteins (RBPs) is key to understanding the posttranscriptional regulatory network in cells. A widely used strategy for RBP capture exploits the polyadenylation [poly(A)] of target RNAs, which mostly occurs on eukaryotic mature mRNAs, leaving most binding proteins of non-poly(A) RNAs unidentified. Here we describe the detailed procedures of a recently reported method termed click chemistry-assisted RNA-interactome capture (CARIC), which enables the transcriptome-wide capture of both poly(A) and non-poly(A) RBPs by combining the metabolic labeling of RNAs, in vivo UV cross-linking, and bioorthogonal tagging.

The representative results of quality control steps are presented. The results include figures of the in-gel fluorescence analysis described in step 2.3.2 (Figure 1), the western blot analysis described in step 4.1.3 (Figure 2A), and the silver-staining analysis described in step 4.2.2 (Figure 2B). The quality control steps are critical for the optimization of CARIC protocols. Always include quality controls in the preparation of large-scale RBP identification experiments.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58580/58580fig1.jpg" /> 
Figure 1: In-gel fluorescence analysis of the click-labeled samples described in step 2.3.2. (A) This panel shows a typical in-gel fluorescence pattern of click-labeled samples. Only the doubly labeled sample shows a strong smear band at a high molecular weight (&#62; 250 kDa), which represents the signal of cross-linked RNPs. To abolish the RNP signal, omit either 4SU or EU or digest with RNase A. The background sharp bands at a lower molecular weight represent the signals of non-specific labeled proteins. (B) In some occasions, a strong smeared band (~130 - 250 kDa) can be observed in the no4SU-control sample. This band represents the signal of labeled uncross-linked RNAs, which will be degraded during the heat denaturation, for most cases. It will not interfere with the subsequent procedures. CBB = Coomassie brilliant blue. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58580/58580fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58580/58580fig2.jpg" /> 
Figure 2: Quality control of affinity pulldown efficiency and the captured RBPs. (A) This panel shows a western blot analysis of the biotin signals in samples before pulldown (input) and in samples after pulldown (supernatant). Estimate the ratio of the remaining signals and optimize the bead amount used in step 3.1.1. (B) This panel shows a silver-staining analysis of captured RBPs compared to 0.1% input total proteins. For HeLa cells, the general total capture efficiency is ~0.05% - 0.1% of input proteins. This value can vary significantly due to the variance of the metabolic labeling efficiency of different cell types. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58580/58580fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/58580/58580fig3.jpg" /> 
Figure 3: Representative MS results of CARIC. (A) This panel shows a volcano plot displaying the averaged Log2-fold change and adjusted P values of quantified proteins, calculated by the limma package. 597 of proteins with a Log2-fold change of &#62; 2 and an adjusted P value of &#60; 0.01 were classified as &#34;CARIC RBPs&#34;. (B) This panel shows the overlap of the CARIC proteins with previously identified human poly(A) RBPs7,8,9,10,11. The overlapped proteins are mostly coding RBPs, while the rest of the CARIC RBPs are more likely to be non-coding RBPs. This figure is a reprinted from previously published work with permission from the National Academy of Sciences27. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58580/58580fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Capture and Identification of RNA-binding Proteins by Using Click Chemistry-assisted RNA-interactome Capture CARIC Strategy at JoVE.com

Capture and Identification of RNA-binding Proteins by Using Click Chemistry-assisted RNA-interactome Capture CARIC Strategy

A detailed protocol for applying the click chemistry-assisted RNA-interactome capture (CARIC) strategy to identify proteins binding to both coding and noncoding RNAs is presented.

Capture and Identification of RNA-binding Proteins by Using Click Chemistry-assisted RNA-interactome Capture (CARIC) Strategy

A comprehensive identification of RNA-binding proteins (RBPs) is key to understanding the posttranscriptional regulatory network in cells. A widely used ...

capture-identification-rna-binding-proteins-using-click-chemistry

Nanyang Technological University

Research

JoVE Journal

Biochemistry

9.3K Views.  Peking University. A detailed protocol for applying the click chemistry-assisted RNA-interactome capture (CARIC) strategy to identify proteins binding to both coding and noncoding RNAs is presented.

Video: Capture and Identification of RNA-binding Proteins by Using Click Chemistry-assisted RNA-interactome Capture CARIC Strategy

Protein Complex Affinity Capture from Cryomilled Mammalian Cells

Affinity capture is an effective technique for isolating endogenous protein complexes for further study. When used in conjunction with an antibody, this technique is also frequently referred to as immunoprecipitation. Affinity capture can be applied in a bench-scale and in a high-throughput context. When coupled with protein mass spectrometry, affinity capture has proven to be a workhorse of interactome analysis. Although there are potentially many ways to execute the numerous steps involved, the following protocols implement our favored methods. Two features are distinctive: the use of cryomilled cell powder to produce cell extracts, and antibody-coupled paramagnetic beads as the affinity medium. In many cases, we have obtained superior results to those obtained with more conventional affinity capture practices. Cryomilling avoids numerous problems associated with other forms of cell breakage. It provides efficient breakage of the material, while avoiding denaturation issues associated with heating or foaming. It retains the native protein concentration up to the point of extraction, mitigating macromolecular dissociation. It reduces the time extracted proteins spend in solution, limiting deleterious enzymatic activities, and it may reduce the non-specific adsorption of proteins by the affinity medium. Micron-scale magnetic affinity media have become more commonplace over the last several years, increasingly replacing the traditional agarose- and Sepharose-based media. Primary benefits of magnetic media include typically lower non-specific protein adsorption; no size exclusion limit because protein complex binding occurs on the bead surface rather than within pores; and ease of manipulation and handling using magnets.

Here we describe protocols to disrupt mammalian cells by solid-state milling at a cryogenic temperature, produce a cell extract from the resulting cell powder, and isolate protein complexes of interest by affinity capture upon antibody-coupled micron-scale paramagnetic beads.

Affinity capture is an effective technique for isolating endogenous protein complexes for further study. When used in conjunction with an antibody, this ...

Identification of Small Molecule-binding Proteins in a Native Cellular Environment by Live-cell Photoaffinity Labeling

Identifying the molecular target(s) of small molecules is a challenging but necessary step towards understanding their mechanism of action. While several target identification methods have been developed and used to successfully elucidate the binding proteins of a variety of small molecules, these techniques have drawbacks that make them unsuitable for detecting certain types of small molecule-target interactions. In particular, non-covalent interactions that depend on native cellular conditions, such as those of membrane proteins whose structures may be perturbed upon cell lysis, are often not amenable to affinity-based target identification methods. Here, we demonstrate a method wherein a probe containing a photolabile group is used to covalently crosslink to the small molecule binding protein within the environment of the live cell, allowing the detection and isolation of the target protein without the need for maintenance of the interaction after cell lysis. This technique is a valuable tool for studying biologically interesting small molecules with unknown mechanisms, both in the context of basic biology as well as drug discovery.

We describe here a method for identification of small molecule-binding proteins using photoaffinity labeling. The advantage of this technique is that binding and covalent labeling of the target proteins occurs within the live cellular environment, removing the risk of disrupting native protein structure and binding conditions upon cell lysis.

Identifying the molecular target(s) of small molecules is a challenging but necessary step towards understanding their mechanism of action. While several ...

Identification of Plant Ice-binding Proteins Through Assessment of Ice-recrystallization Inhibition and Isolation Using Ice-affinity Purification

Ice-binding proteins (IBPs) belong to a family of stress-induced proteins that are synthesized by certain organisms exposed to subzero temperatures. In plants, freeze damage occurs when extracellular ice crystals grow, resulting in the rupture of plasma membranes and possible cell death. Adsorption of IBPs to ice crystals restricts further growth by a process known as ice-recrystallization inhibition (IRI), thereby reducing cellular damage. IBPs also demonstrate the ability to depress the freezing point of a solution below the equilibrium melting point, a property known as thermal hysteresis (TH) activity. These protective properties have raised interest in the identification of novel IBPs due to their potential use in industrial, medical and agricultural applications. This paper describes the identification of plant IBPs through 1) the induction and extraction of IBPs in plant tissue, 2) the screening of extracts for IRI activity, and 3) the isolation and purification of IBPs. Following the induction of IBPs by low temperature exposure, extracts are tested for IRI activity using a &#39;splat assay&#39;, which allows the observation of ice crystal growth using a standard light microscope. This assay requires a low protein concentration and generates results that are quickly obtained and easily interpreted, providing an initial screen for ice binding activity. IBPs can then be isolated from contaminating proteins by utilizing the property of IBPs to adsorb to ice, through a technique called &#39;ice-affinity purification&#39;. Using cell lysates collected from plant extracts, an ice hemisphere can be slowly grown on a brass probe. This incorporates IBPs into the crystalline structure of the polycrystalline ice. Requiring no a priori biochemical or structural knowledge of the IBP, this method allows for recovery of active protein. Ice-purified protein fractions can be used for downstream applications including the identification of peptide sequences by mass spectrometry and the biochemical analysis of native proteins.

This paper outlines the identification of ice-binding proteins from freeze-tolerant plants through the assessment of ice-recrystallization inhibition activity and subsequent isolation of native IBPs using ice-affinity purification.

Ice-binding proteins (IBPs) belong to a family of stress-induced proteins that are synthesized by certain organisms exposed to subzero temperatures. In ...

Sample Preparation for Mass Spectrometry-based Identification of RNA-binding Regions

Noncoding RNAs play important roles in several nuclear processes, including regulating gene expression, chromatin structure, and DNA repair. In most cases, the action of noncoding RNAs is mediated by proteins whose functions are in turn regulated by these interactions with noncoding RNAs. Consistent with this, a growing number of proteins involved in nuclear functions have been reported to bind RNA and in a few cases the RNA-binding regions of these proteins have been mapped, often through laborious, candidate-based methods.
Here, we report a detailed protocol to perform a high-throughput, proteome-wide unbiased identification of RNA-binding proteins and their RNA-binding regions. The methodology relies on the incorporation of a photoreactive uridine analog in the cellular RNA, followed by UV-mediated protein-RNA crosslinking, and mass spectrometry analyses to reveal RNA-crosslinked peptides within the proteome. Although we describe the procedure for mouse embryonic stem cells, the protocol should be easily adapted to a variety of cultured cells.

We describe a protocol to identify RNA-binding proteins and map their RNA-binding regions in live cells using UV-mediated photocrosslinking and mass spectrometry.

Noncoding RNAs play important roles in several nuclear processes, including regulating gene expression, chromatin structure, and DNA repair. In most ...

mRNA Interactome Capture from Plant Protoplasts

RNA-binding proteins (RBPs) determine the fates of RNAs. They participate in all RNA biogenesis pathways and especially contribute to post-transcriptional gene regulation (PTGR) of messenger RNAs (mRNAs). In the past few years, a number of mRNA-bound proteomes from yeast and mammalian cell lines have been successfully isolated through the use of a novel method called &#34;mRNA interactome capture,&#34; which allows for the identification of mRNA-binding proteins (mRBPs) directly from a physiological environment. The method is composed of in vivo ultraviolet (UV) crosslinking, pull-down and purification of messenger ribonucleoprotein complexes (mRNPs) by oligo(dT) beads, and the subsequent identification of the crosslinked proteins by mass spectrometry (MS). Very recently, by applying the same method, several plant mRNA-bound proteomes have been reported simultaneously from different Arabidopsis tissue sources: etiolated seedlings, leaf tissue, leaf mesophyll protoplasts, and cultured root cells. Here, we present the optimized mRNA interactome capture method for Arabidopsis thaliana leaf mesophyll protoplasts, a cell type that serves as a versatile tool for experiments that include various cellular assays. The conditions for optimal protein yield include the amount of starting tissue and the duration of UV irradiation. In the mRNA-bound proteome obtained from a medium-scale experiment (107 cells), RBPs noted to have RNA-binding capacity were found to be overrepresented, and many novel RBPs were identified. The experiment can be scaled up (109 cells), and the optimized method can be applied to other plant cell types and species to broadly isolate, catalog, and compare mRNA-bound proteomes in plants.

Here, we present an interactome capture protocol applied to Arabidopsis thaliana leaf mesophyll protoplasts. This method critically relies on in vivo UV crosslinking and allows for the isolation and identification of plant mRNA-binding proteins from a physiological environment.

RNA-binding proteins (RBPs) determine the fates of RNAs. They participate in all RNA biogenesis pathways and especially contribute to post-transcriptional ...

Nitropeptide Profiling and Identification Illustrated by Angiotensin II

Protein nitration is one of the most important post-translational modifications (PTM) on tyrosine residues and it can be induced by chemical actions of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in eukaryotic cells. Precise identification of nitration sites on proteins is crucial for understanding the physiological and pathological processes related to protein nitration, such as inflammation, aging, and cancer. Since the nitrated proteins are of low abundance in cells even under induced conditions, no universal and efficient methods have been developed for the profiling and identification of protein nitration sites. Here we describe a protocol for nitropeptide enrichment by using a chemical reduction reaction and biotin labeling, followed by high resolution mass spectrometry. In our method, nitropeptide derivatives can be identified with high accuracy. Our method exhibits two advantages compared to the previously reported methods. First, dimethyl labeling is used to block the primary amine on nitropeptides, which can be used to generate quantitative results. Second, a disulfide bond containing NHS-biotin reagent is used for the enrichment, which can be further reduced and alkylated to enhance the detection signal on a mass spectrometer. This protocol has been successfully applied to the model peptide Angiotensin II in the current paper.

Proteomic profiling of tyrosine-nitrated proteins has been a challenging technique due to the low abundance of the 3-nitrotyrosine modification. Here we describe a novel approach for nitropeptide enrichment and profiling by using Angiotensin II as the model. This method can be extended for other in vitro or in vivo systems.

Protein nitration is one of the most important post-translational modifications (PTM) on tyrosine residues and it can be induced by chemical actions of ...

Identification of Inositol Phosphate or Phosphoinositide Interacting Proteins by Affinity Chromatography Coupled to Western Blot or Mass Spectrometry

Inositol phosphates and phosphoinositides regulate several cellular processes in eukaryotes, including gene expression, vesicle trafficking, signal transduction, metabolism, and development. These metabolites perform this regulatory activity by binding to proteins, thereby changing protein conformation, catalytic activity, and/or interactions. The method described here uses affinity chromatography coupled to mass spectrometry or Western blotting to identify proteins that interact with inositol phosphates or phosphoinositides. Inositol phosphates or phosphoinositides are chemically tagged with biotin, which is then captured via streptavidin conjugated to agarose or magnetic beads. Proteins are isolated by their affinity of binding to the metabolite, then eluted and identified by mass spectrometry or Western blotting. The method has a simple workflow that is sensitive, non-radioactive, liposome-free, and customizable, supporting the analysis of protein and metabolite interaction with precision. This approach can be used in label-free or in amino acid-labelled quantitative mass spectrometry methods to identify protein-metabolite interactions in complex biological samples or using purified proteins. This protocol is optimized for the analysis of proteins from Trypanosoma brucei, but it can be adapted to related protozoan parasites, yeast or mammalian cells.

This protocol focuses on the identification of proteins that bind to inositol phosphates or phosphoinositides. It uses affinity chromatography with biotinylated inositol phosphates or phosphoinositides that are immobilized via streptavidin to agarose or magnetic beads. Inositol phosphate or phosphoinositide binding proteins are identified by Western blotting or mass spectrometry.

Inositol phosphates and phosphoinositides regulate several cellular processes in eukaryotes, including gene expression, vesicle trafficking, signal ...

Activated Cross-linked Agarose for the Rapid Development of Affinity Chromatography Resins - Antibody Capture as a Case Study

The purification of monoclonal antibodies (mAbs) is commonly achieved by Protein A affinity chromatography, which can account for up to 25% of the overall process costs. Alternative, cost-effective capture steps are therefore valuable for industrial-scale manufacturing, where large quantities of a single mAb are produced. Here we present a method for the immobilization of a DsRed-based epitope ligand to a cross-linked agarose resin allowing the selective capture of the HIV-neutralizing antibody 2F5 from crude plant extracts without using Protein A. The linear epitope ELDKWA was first genetically fused to the fluorescent protein DsRed and the fusion protein was expressed in transgenic tobacco (Nicotiana tabacum) plants before purification by immobilized metal-ion affinity chromatography. Furthermore, a method based on activated cross-linked agarose was optimized for high ligand density, efficient coupling and low costs. The pH and buffer composition and the soluble ligand concentration were the most important parameters during the coupling procedure, which was improved using a design-of-experiments approach. The resulting affinity resin was tested for its ability to selectively bind the target mAb in a crude plant extract and the elution buffer was optimized for high mAb recovery, product activity and affinity resin stability. The method can easily be adapted to other antibodies with linear epitopes. The new resins allow gentler elution conditions than Protein A and could also reduce the costs of an initial capture step for mAb production.

In this procedure, a DsRed-based epitope ligand is immobilized to produce a highly selective affinity resin for the capture of monoclonal antibodies from crude plant extracts or cell culture supernatants, as an alternative to Protein A.

The purification of monoclonal antibodies (mAbs) is commonly achieved by Protein A affinity chromatography, which can account for up to 25% of the overall ...

Detection of Protein S-Acylation using Acyl-Resin Assisted Capture

Protein S-acylation, also referred to as S-palmitoylation, is a reversible post-translational modification of cysteine residues with long-chain fatty acids via a labile thioester bond. S-acylation, which is emerging as a widespread regulatory mechanism, can modulate almost all aspects of the biological activity of proteins, from complex formation to protein trafficking and protein stability. The recent progress in understanding of the biological function of protein S-acylation was achieved largely due to the development of novel biochemical tools allowing robust and sensitive detection of protein S-acylation in a variety of biological samples. Here, we describe acyl resin-assisted capture (Acyl-RAC), a recently developed method based on selective capture of endogenously S-acylated proteins by thiol-reactive Sepharose beads. Compared to existing approaches, Acyl-RAC requires fewer steps and can yield more reliable results when coupled with mass spectrometry for identification of novel S-acylation targets. A major limitation in this technique is the lack of ability to discriminate between fatty acid species attached to cysteines via the same thioester bond.

Acyl-RAC (Acyl-Resin Assisted Capture) is a highly sensitive, reliable and easy to perform method to detect reversible lipid modification of cysteine residues (S-acylation) in a variety of biological samples.

Protein S-acylation, also referred to as S-palmitoylation, is a reversible post-translational modification of cysteine residues with long-chain fatty ...

Transverse Sectioning of Mature Rice (Oryza sativa L.) Kernels for Scanning Electron Microscopy Imaging Using Pipette Tips as Immobilization Support

Starch granules (SGs) exhibit different morphologies depending on the plant species, especially in the endosperm of the Poaceae family. Endosperm phenotyping can be used to classify genotypes based on SG morphotype using scanning electron microscopic (SEM) analysis. SGs can be visualized using SEM by slicing through the kernel (pericarp, aleurone layers, and endosperm) and exposing the organellar contents. Current methods require the rice kernel to be embedded in plastic resin and sectioned using a microtome or embedded in a truncated pipette tip and sectioned by hand using a razor blade. The former method requires specialized equipment and is time-consuming, while the latter introduces a new host of problems depending on rice genotype. Chalky rice varieties, particularly, pose a problem for this type of sectioning due to the friable nature of their endosperm tissue. Presented here is a technique for preparing translucent and chalky rice kernel sections for microscopy, requiring only pipette tips and a scalpel blade. Preparing the sections within the confines of a pipette tip prevents rice kernel endosperm from shattering (for translucent or &#8216;vitreous&#8217; phenotypes) and crumbling (for chalky phenotypes). Using this technique, endosperm cell patterning and the structure of intact SGs can be observed.

Transverse Sectioning of Mature Rice Oryza sativa L. Kernels for Scanning Electron Microscopy Imaging Using Pipette Tips as Immobilization Support

This protocol allows for the preparation of transverse sections of cereal seeds (e.g., rice) for the analysis of endosperm and starch granule morphology using scanning electron microscopy.