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Method Article
Here we present a modified CLIP-seq protocol called FbioCLIP-seq with FLAG-biotin tandem purification to determine the RNA targets of RNA-binding proteins (RBPs) in mammalian cells.
RNA and RNA-binding proteins (RBPs) control multiple biological processes. The spatial and temporal arrangement of RNAs and RBPs underlies the delicate regulation of these processes. A strategy called CLIP-seq (cross-linking and immunoprecipitation) has been developed to capture endogenous protein-RNA interactions with UV cross-linking followed by immunoprecipitation. Despite the wide use of conventional CLIP-seq method in RBP study, the CLIP method is limited by the availability of high-quality antibodies, potential contaminants from the copurified RBPs, requirement of isotope manipulation, and potential loss of information during a tedious experimental procedure. Here we describe a modified CLIP-seq method called FbioCLIP-seq using the FLAG-biotin tag tandem purification. Through tandem purification and stringent wash conditions, almost all the interacting RNA-binding proteins are removed. Thus, the RNAs interacting indirectly mediated by these copurified RBPs are also reduced. Our FbioCLIP-seq method allows efficient detection of direct protein-bound RNAs without SDS-PAGE and membrane transfer procedures in an isotope-free and protein-specific antibody-free manner.
RNAs and RNA-binding proteins (RBPs) control diverse cellular processes including splicing, translation, ribosome biogenesis, epigenetic regulation, and cell fate transition1,2,3,4,5,6. The delicate mechanisms of these processes depend on the unique spatial and temporal arrangement of RNAs and RBPs. Therefore, an important step towards understanding RNA regulation at the molecular level is to reveal the positional information about the binding sites of RBPs.
A strategy referred to as cross-linking and immunoprecipitation (CLIP-seq) has been developed to capture protein-RNA interactions with UV cross-linking followed by immunoprecipitation of the protein of interest7. The key feature of the methodology is the induction of covalent cross-links between an RNA-binding protein and its directly bound RNA molecules (within ~1 Å) by UV irradiation8. The RBP footprints can be determined by CLIP tag clustering and peak calling, which usually have a resolution of 30−60 nt. Alternatively, the reverse transcription step of CLIP can lead to indels (insertions or deletions) or substitutions to the cross-linking sites, which allows identification of protein binding sites on the RNAs at a single-nucleotide resolution. Pipelines like Novoalign and CIMS have been developed for the analysis of the high-throughput sequencing results of CLIP-seq8. Several modified CLIP-seq methods have also been proposed, including individual-nucleotide resolution cross-linking and immunoprecipitation (iCLIP), enhanced CLIP (eCLIP), irCLIP, and photoactivatable ribonucleoside-enhanced cross-linking and immunoprecipitation (PAR-CLIP)9,10,11,12.
Despite the wide use of traditional CLIP-seq methods in the study of RBPs, the CLIP methods have several drawbacks. First, the tedious denatured gel electrophoresis and membrane transfer procedure may lead to loss of information, and cause limited sequence complexity. Second, the protein specific antibody-based CLIP method may pull down a protein complex instead of a single target protein, which may lead to false positive protein-RNA interactions from the copurified RBPs. Third, the antibody-based strategy requires a large amount of high-quality antibodies, which makes the application of these methods inadequate for the study of RBPs without high-quality antibodies available. Fourth, the traditional CLIP method requires radiolabeled ATP to label the protein-bound RNAs.
The high affinity of streptavidin to biotinylated proteins makes it a very powerful approach to purify specific proteins or protein complexes. The efficient biotinylation of proteins carrying an artificial peptide sequence by ectopically expressed bacterial BirA biotin ligase in mammalian cells makes it an efficient strategy to perform biotin purification in vivo13. We developed a modified CLIP-seq method called FbioCLIP-seq (FLAG-Biotin-mediated Cross-linking and Immunoprecipitation followed by high-throughput sequencing) using FLAG-biotin tag tandem purification14 (Figure 1). Through tandem purification and stringent wash conditions, almost all the interacting RBPs are removed (Figure 2). The stringent wash conditions also allow circumventing the SDS-PAGE and membrane transfer, which is labor intensive and technically challenging. And similar to eCLIP and irCLIP, the FbioCLIP-seq method is isotope-free. Skipping the gel running and transfer steps avoids the loss of information, keeps authentic protein-RNA interactions intact, and increases the library complexity. Moreover, the high efficiency of the tagging system makes it a good choice for RBPs without high-quality antibodies available.
Here we provide a step-by-step description of the FbioCLIP-seq protocol for mammalian cells. Briefly, cells are cross-linked by 254 nm UV, followed by cell lysis and FLAG immunoprecipitation (FLAG-IP). Next, the protein-RNA complexes are further purified by biotin affinity capture and RNAs are fragmented by partial digestion with MNase. Then, the protein-bound RNA is dephosphorylated and ligated with a 3’ linker. A 5’ RNA linker is added after the RNA is phosphorylated with PNK and eluted by proteinase K digestion. After reverse transcription, the protein-bound RNA signals are amplified by PCR and purified by agarose gel purification. Two RBPs were chosen to exemplify the FbioCLIP-seq result. LIN28 is a well-characterized RNA-binding protein involved in microRNA maturation, protein translation, and cell reprogramming15,16,17. WDR43 is a WD40 domain-containing protein thought to coordinate ribosome biogenesis, eukaryotic transcription, and embryonic stem cell pluripotency control14,18. Consistent with previously reported results for LIN28 with CLIP-seq, FbioCLIP-seq reveals binding sites of LIN28 on “GGAG” motifs in the microRNA mir-let7g and mRNAs16,19 (Figure 3). WDR43 FbioCLIP-seq also identified the binding preference of WDR43 with 5' external transcribed spacers (5'-ETS) of pre-rRNAs20 (Figure 4). These results validate the reliability of the FbioCLIP-seq method.
1. Cell line construction
2. Cross-linking
3. Cell lysate preparation
4. FLAG beads preparation
5. Immunoprecipitation
6. Elution with 3x FLAG peptide
7. Streptavidin beads preparation
8. Biotin affinity purification
9. Partial RNA digestion
10. Dephosphorylation of RNA
11. 3’ linker ligation
12. PNK treatment
13. RNA isolation
14. 5’ RNA linker ligation
15. Reverse transcription
16. PCR amplification
17. Bioinformatics analysis
The schematic representation of the FbioCLIP-seq procedure is shown in Figure 1. Compared with FLAG-mediated or streptavidin-mediated one-step affinity purification, FLAG-biotin tandem purification removed almost all the copurified proteins, avoiding the contamination of indirect protein-RNA interactions (Figure 2). Representative results for FbioCLIP-seq for LIN28 and WDR43 are depicted in Figure 3 and
Here we introduce a modified CLIP-seq method called FbioCLIP-seq, taking advantage of the FLAG-biotin double tagging system to perform tandem purification of protein-RNA complexes. The FLAG-biotin double tagging system has been shown to be powerful in identifying protein-protein and protein-DNA interactions13,21. Here we demonstrate the high specificity and convenience of this system in identifying the RNAs interacting with proteins. Through tandem pur...
The authors have nothing to disclose.
Grant support is from the National Basic Research Program of China (2017YFA0504204, 2018YFA0107604), the National Natural Science Foundation of China (31630095), and the Center for Life Sciences at Tsinghua University.
Name | Company | Catalog Number | Comments |
Equipment | |||
UV crosslinker | UVP | HL-2000 HybrilLinker | |
Affinity Purification Beads | |||
ANTI-FLAG beads | Sigma-Aldrich | A2220 | |
Streptavidin beads | Invitrogen | 112.06D | |
Reagents | |||
10x PBS | Gibco | 70013032 | |
3 M NaOAc | Ambion | AM9740 | |
3 x FLAG peptide | Sigma-Aldrich | F4799 | |
ATP | Sigma-Aldrich | A6559 | |
Calcium chloride (CaCl2) | Sigma-Aldrich | C1016 | |
CIP | NEB | M0290S | CIP buffer is in the same package. |
DTT | Sigma-Aldrich | D0632 | |
EDTA | Sigma-Aldrich | E9884 | |
EGTA | Sigma-Aldrich | E3889 | |
Gel purification kit | QIAGEN | 28704 | |
Glycogen | Ambion | AM9510 | |
Magnesium chloride (MgCl2) | Sigma-Aldrich | 449172 | |
MNase | NEB | M0247S | |
NP-40 | Amresco | M158-500ML | |
PMSF | Sigma-Aldrich | 10837091001 | |
Porteinase K | TAKARA | 9033 | |
Protease inhibitor cocktail | Sigma-Aldrich | P8340 | |
Q5 High-Fidelity 2X Master Mix | NEB | 0492S | |
reverse trancriptase (SupperScriptIII) | Invitrogen | 18080093 | |
RNA isolation reagent (Trizol) | Invitrogen | 15596018 | |
RNase Inhibitor | ThermoFisher | EO0381 | |
RNaseOUT | Invitrogen | 10777019 | |
RQ1 Dnase | Promega | M6101 | |
SDS | Sigma-Aldrich | 1614363 | |
Sodium chloride | Sigma-Aldrich | S9888 | |
Sodium deoxycholate | Sigma-Aldrich | D6750 | |
T4 PNK | NEB | M0201S | PNK buffer is in the same package. |
T4 RNA ligaes | ThermoFisher | EL0021 | T4 RNA ligase buffer and BSA are in the same package. |
T4 RNA ligase2, truncated | NEB | M0242S | T4 RNA ligase buffer and 50% PEG are in the same package. |
Trypsin-EDTA | ThermoFisher | 25200072 | |
Urea | Sigma-Aldrich | 208884 | |
mESC culture medium | |||
DMEM (80%) | Gibco | 11965126 | |
2-Mercaptoethanol | Gibco | 21985023 | |
FCS (15%) | Hyclone | ||
Glutamax (1%) | Gibco | 35050061 | |
LIF | purified recombinant protein; 10,000 fold dilution | ||
NEAA (1%) | Gibco | 11140050 | |
Nucleoside mix (1%) | Millipore | ES-008-D | |
Penicillin-Streptomycin (1%) | Gibco | 15140122 | |
Kit | |||
DNA gel extraction kit | QIAGEN | 28704 |
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